Avr Libc User Manual

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avr-libc Reference Manual
1.4.6
Generated by Doxygen 1.4.7
Tue May 15 14:56:11 2007

CONTENTS

i

Contents
1

2

3

4

5

6

AVR Libc

1

1.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1.2

General information about this library . . . . . . . . . . . . . . . . .

1

1.3

Supported Devices . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1.4

avr-libc License . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

avr-libc Module Index

7

2.1

7

avr-libc Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . .

avr-libc Data Structure Index

8

3.1

8

avr-libc Data Structures . . . . . . . . . . . . . . . . . . . . . . . . .

avr-libc File Index

9

4.1

9

avr-libc File List . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

avr-libc Page Index

12

5.1

12

avr-libc Related Pages . . . . . . . . . . . . . . . . . . . . . . . . . .

avr-libc Module Documentation
6.1

6.2

6.3

6.4

13

: Diagnostics . . . . . . . . . . . . . . . . . . . . . . . .

13

6.1.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

13

6.1.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

13

: Character Operations . . . . . . . . . . . . . . . . . . .

14

6.2.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

14

6.2.2

Function Documentation . . . . . . . . . . . . . . . . . . . .

15

: System Errors . . . . . . . . . . . . . . . . . . . . . . .

17

6.3.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

17

6.3.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

17

: Integer Type conversions . . . . . . . . . . . . . . . .

17

6.4.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

17

6.4.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

20

6.4.3

Typedef Documentation . . . . . . . . . . . . . . . . . . . .

29

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: Mathematics . . . . . . . . . . . . . . . . . . . . . . . .

29

6.5.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

29

6.5.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

30

6.5.3

Function Documentation . . . . . . . . . . . . . . . . . . . .

31

: Non-local goto . . . . . . . . . . . . . . . . . . . . . .

34

6.6.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

34

6.6.2

Function Documentation . . . . . . . . . . . . . . . . . . . .

35

: Standard Integer Types . . . . . . . . . . . . . . . . . .

36

6.7.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

36

6.7.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

39

6.7.3

Typedef Documentation . . . . . . . . . . . . . . . . . . . .

45

: Standard IO facilities . . . . . . . . . . . . . . . . . . . .

48

6.8.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

48

6.8.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

52

6.8.3

Function Documentation . . . . . . . . . . . . . . . . . . . .

55

: General utilities . . . . . . . . . . . . . . . . . . . . . .

66

6.9.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

66

6.9.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

68

6.9.3

Typedef Documentation . . . . . . . . . . . . . . . . . . . .

68

6.9.4

Function Documentation . . . . . . . . . . . . . . . . . . . .

68

6.9.5

Variable Documentation . . . . . . . . . . . . . . . . . . . .

77

6.10 : Strings . . . . . . . . . . . . . . . . . . . . . . . . . . .

77

6.10.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

77

6.10.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

78

6.10.3 Function Documentation . . . . . . . . . . . . . . . . . . . .

79

6.11 : Bootloader Support Utilities . . . . . . . . . . . . . .

88

6.11.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

88

6.11.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

90

6.12 : EEPROM handling . . . . . . . . . . . . . . . . .

94

6.12.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

94

6.12.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

95

6.5

6.6

6.7

6.8

6.9

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6.12.3 Function Documentation . . . . . . . . . . . . . . . . . . . .

96

6.13 : Interrupts . . . . . . . . . . . . . . . . . . . . . .

97

6.13.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

97

6.13.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

114

6.14 : Program Space Utilities . . . . . . . . . . . . . 116
6.14.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

116

6.14.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

118

6.14.3 Typedef Documentation . . . . . . . . . . . . . . . . . . . .

120

6.14.4 Function Documentation . . . . . . . . . . . . . . . . . . . .

122

6.15 : Power Reduction Management . . . . . . . . . . . . 128
6.16 Additional notes from  . . . . . . . . . . . . . . . .

131

6.17 : Special function registers . . . . . . . . . . . . . . 133
6.17.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

133

6.17.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

134

6.18 : Power Management and Sleep Modes . . . . . . . . .

136

6.18.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

136

6.18.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

137

6.18.3 Function Documentation . . . . . . . . . . . . . . . . . . . .

138

6.19 : avr-libc version macros . . . . . . . . . . . . . . .

138

6.19.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

138

6.19.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

139

6.20 : Watchdog timer handling . . . . . . . . . . . . . . . .

140

6.20.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

140

6.20.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

141

6.21 : CRC Computations . . . . . . . . . . . . . . . . . .

143

6.21.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

143

6.21.2 Function Documentation . . . . . . . . . . . . . . . . . . . .

144

6.22 : Convenience functions for busy-wait delay loops . . .

147

6.22.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

147

6.22.2 Function Documentation . . . . . . . . . . . . . . . . . . . .

147

6.23 : Basic busy-wait delay loops . . . . . . . . . .

148

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6.23.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

148

6.23.2 Function Documentation . . . . . . . . . . . . . . . . . . . .

148

6.24 : Parity bit generation . . . . . . . . . . . . . . . . . .

149

6.24.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

149

6.24.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

149

6.25 : TWI bit mask definitions . . . . . . . . . . . . . . . .

150

6.25.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

150

6.25.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

151

6.26 : Deprecated items . . . . . . . . . . . . . . 154
6.26.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

154

6.26.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

155

6.26.3 Function Documentation . . . . . . . . . . . . . . . . . . . .

157

6.27 : Compatibility with IAR EWB 3.x . . . . . . . .

157

6.28 Demo projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

158

6.28.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

158

6.29 Combining C and assembly source files . . . . . . . . . . . . . . . .

159

6.29.1 Hardware setup . . . . . . . . . . . . . . . . . . . . . . . . .

160

6.29.2 A code walkthrough . . . . . . . . . . . . . . . . . . . . . .

160

6.29.3 The source code . . . . . . . . . . . . . . . . . . . . . . . .

162

6.30 A simple project . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

162

6.30.1 The Project . . . . . . . . . . . . . . . . . . . . . . . . . . .

162

6.30.2 The Source Code . . . . . . . . . . . . . . . . . . . . . . . .

165

6.30.3 Compiling and Linking . . . . . . . . . . . . . . . . . . . . .

166

6.30.4 Examining the Object File . . . . . . . . . . . . . . . . . . .

167

6.30.5 Linker Map Files . . . . . . . . . . . . . . . . . . . . . . . .

171

6.30.6 Generating Intel Hex Files . . . . . . . . . . . . . . . . . . .

173

6.30.7 Letting Make Build the Project . . . . . . . . . . . . . . . . .

174

6.30.8 Reference to the source code . . . . . . . . . . . . . . . . . .

176

6.31 A more sophisticated project . . . . . . . . . . . . . . . . . . . . . .

177

6.31.1 Hardware setup . . . . . . . . . . . . . . . . . . . . . . . . .

177

6.31.2 Functional overview . . . . . . . . . . . . . . . . . . . . . .

180

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6.31.3 A code walkthrough . . . . . . . . . . . . . . . . . . . . . .

181

6.31.4 The source code . . . . . . . . . . . . . . . . . . . . . . . .

184

6.32 Using the standard IO facilities . . . . . . . . . . . . . . . . . . . . .

184

6.32.1 Hardware setup . . . . . . . . . . . . . . . . . . . . . . . . .

184

6.32.2 Functional overview . . . . . . . . . . . . . . . . . . . . . .

186

6.32.3 A code walkthrough . . . . . . . . . . . . . . . . . . . . . .

186

6.32.4 The source code . . . . . . . . . . . . . . . . . . . . . . . .

191

6.33 Example using the two-wire interface (TWI) . . . . . . . . . . . . . . 192

7

192

6.33.2 The TWI example project . . . . . . . . . . . . . . . . . . .

192

6.33.3 The Source Code . . . . . . . . . . . . . . . . . . . . . . . .

193

avr-libc Data Structure Documentation

197

7.1

div_t Struct Reference . . . . . . . . . . . . . . . . . . . . . . . . .

197

7.1.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

197

7.1.2

Field Documentation . . . . . . . . . . . . . . . . . . . . . .

197

ldiv_t Struct Reference . . . . . . . . . . . . . . . . . . . . . . . . .

197

7.2.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

197

7.2.2

Field Documentation . . . . . . . . . . . . . . . . . . . . . .

197

7.2

8

6.33.1 Introduction into TWI . . . . . . . . . . . . . . . . . . . . .

avr-libc File Documentation

198

8.1

assert.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

198

8.1.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

198

atoi.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

198

8.2.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

198

atol.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

198

8.3.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

198

boot.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

199

8.4.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

199

8.4.2

Define Documentation . . . . . . . . . . . . . . . . . . . . .

200

crc16.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

205

8.5.1

205

8.2

8.3

8.4

8.5

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

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vi

ctype.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

205

8.6.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

205

delay.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

206

8.7.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

206

delay_basic.h File Reference . . . . . . . . . . . . . . . . . . . . . .

206

8.8.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

206

eeprom.h File Reference . . . . . . . . . . . . . . . . . . . . . . . .

207

8.9.1

Detailed Description . . . . . . . . . . . . . . . . . . . . . .

207

8.10 errno.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

208

8.10.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

208

8.11 fdevopen.c File Reference . . . . . . . . . . . . . . . . . . . . . . .

208

8.11.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

208

8.12 ffs.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . . .

209

8.12.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

209

8.13 ffsl.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

209

8.13.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

209

8.14 ffsll.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

209

8.14.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

209

8.15 interrupt.h File Reference . . . . . . . . . . . . . . . . . . . . . . . .

209

8.15.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

209

8.16 inttypes.h File Reference . . . . . . . . . . . . . . . . . . . . . . . .

209

8.16.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

209

8.17 math.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

212

8.17.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

212

8.18 memccpy.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

213

8.18.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

213

8.19 memchr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

213

8.19.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

213

8.20 memchr_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

214

8.20.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

214

8.21 memcmp.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

214

8.7

8.8

8.9

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8.21.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

214

8.22 memcmp_P.S File Reference . . . . . . . . . . . . . . . . . . . . . .

214

8.22.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

214

8.23 memcpy.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

214

8.23.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

214

8.24 memcpy_P.S File Reference . . . . . . . . . . . . . . . . . . . . . .

214

8.24.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

214

8.25 memmem.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

215

8.25.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

215

8.26 memmove.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

215

8.26.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

215

8.27 memrchr.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

215

8.27.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

215

8.28 memrchr_P.S File Reference . . . . . . . . . . . . . . . . . . . . . .

215

8.28.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

215

8.29 memset.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

215

8.29.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

215

8.30 parity.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

216

8.30.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

216

8.31 pgmspace.h File Reference . . . . . . . . . . . . . . . . . . . . . . .

216

8.31.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

216

8.31.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

218

8.32 power.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

223

8.32.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

223

8.32.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

223

8.33 setjmp.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

223

8.33.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

223

8.34 sleep.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

224

8.34.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

224

8.35 stdint.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

225

8.35.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

225

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8.36 stdio.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

228

8.36.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

228

8.37 stdlib.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

230

8.37.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

230

8.38 strcasecmp.S File Reference . . . . . . . . . . . . . . . . . . . . . .

232

8.38.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

232

8.39 strcasecmp_P.S File Reference . . . . . . . . . . . . . . . . . . . . .

232

8.39.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

232

8.40 strcasestr.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

232

8.40.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

232

8.41 strcat.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

232

8.41.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

232

8.42 strcat_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

233

8.42.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.43 strchr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

233

8.43.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.44 strchr_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

233

8.44.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.45 strchrnul.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

233

8.45.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.46 strchrnul_P.S File Reference . . . . . . . . . . . . . . . . . . . . . .

233

8.46.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.47 strcmp.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

233

8.47.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.48 strcmp_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

233

8.48.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.49 strcpy.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

233

8.49.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

233

8.50 strcpy_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

234

8.50.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

234

8.51 strcspn.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

234

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8.51.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

234

8.52 strcspn_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

234

8.52.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

234

8.53 string.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

234

8.53.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

234

8.54 strlcat.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

236

8.54.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.55 strlcat_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

236

8.55.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.56 strlcpy.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

236

8.56.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.57 strlcpy_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

236

8.57.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.58 strlen.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

236

8.58.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.59 strlen_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

236

8.59.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

236

8.60 strlwr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

237

8.60.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

237

8.61 strncasecmp.S File Reference . . . . . . . . . . . . . . . . . . . . . .

237

8.61.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

237

8.62 strncasecmp_P.S File Reference . . . . . . . . . . . . . . . . . . . .

237

8.62.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

237

8.63 strncat.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

237

8.63.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

237

8.64 strncat_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

237

8.64.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

237

8.65 strncmp.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

238

8.65.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

238

8.66 strncmp_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

238

8.66.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

238

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8.67 strncpy.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

238

8.67.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

238

8.68 strncpy_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

239

8.68.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

239

8.69 strnlen.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

239

8.69.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

239

8.70 strnlen_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

239

8.70.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

239

8.71 strpbrk.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

240

8.71.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

240

8.72 strpbrk_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

240

8.72.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

240

8.73 strrchr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

240

8.73.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

240

8.74 strrchr_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . .

242

8.74.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.75 strrev.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.75.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.76 strsep.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.76.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.77 strsep_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

242

8.77.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.78 strspn.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.78.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.79 strspn_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

242

8.79.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.80 strstr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.80.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.81 strstr_P.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

242

8.81.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.82 strtok_r.S File Reference . . . . . . . . . . . . . . . . . . . . . . . .

242

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9

xi

8.82.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.83 strupr.S File Reference . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.83.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.84 twi.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . . .

242

8.84.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

242

8.85 wdt.h File Reference . . . . . . . . . . . . . . . . . . . . . . . . . .

244

8.85.1 Detailed Description . . . . . . . . . . . . . . . . . . . . . .

244

8.85.2 Define Documentation . . . . . . . . . . . . . . . . . . . . .

244

avr-libc Page Documentation

245

9.1

Toolchain Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .

245

9.1.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

245

9.1.2

FSF and GNU . . . . . . . . . . . . . . . . . . . . . . . . .

245

9.1.3

GCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245

9.1.4

GNU Binutils . . . . . . . . . . . . . . . . . . . . . . . . . .

246

9.1.5

avr-libc . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

248

9.1.6

Building Software . . . . . . . . . . . . . . . . . . . . . . .

248

9.1.7

AVRDUDE . . . . . . . . . . . . . . . . . . . . . . . . . . .

248

9.1.8

GDB / Insight / DDD . . . . . . . . . . . . . . . . . . . . . .

248

9.1.9

AVaRICE . . . . . . . . . . . . . . . . . . . . . . . . . . . .

249

9.1.10 SimulAVR . . . . . . . . . . . . . . . . . . . . . . . . . . .

249

9.1.11 Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

249

9.1.12 Toolchain Distributions (Distros) . . . . . . . . . . . . . . . .

249

9.1.13 Open Source . . . . . . . . . . . . . . . . . . . . . . . . . .

250

Memory Areas and Using malloc() . . . . . . . . . . . . . . . . . . .

250

9.2.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

250

9.2.2

Internal vs. external RAM . . . . . . . . . . . . . . . . . . .

251

9.2.3

Tunables for malloc() . . . . . . . . . . . . . . . . . . . . . .

252

9.2.4

Implementation details . . . . . . . . . . . . . . . . . . . . .

253

Memory Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . .

255

9.3.1

255

9.2

9.3

The .text Section . . . . . . . . . . . . . . . . . . . . . . . .

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9.4

9.5

9.6

9.7

xii

9.3.2

The .data Section . . . . . . . . . . . . . . . . . . . . . . . .

255

9.3.3

The .bss Section . . . . . . . . . . . . . . . . . . . . . . . .

256

9.3.4

The .eeprom Section . . . . . . . . . . . . . . . . . . . . . .

256

9.3.5

The .noinit Section . . . . . . . . . . . . . . . . . . . . . . .

256

9.3.6

The .initN Sections . . . . . . . . . . . . . . . . . . . . . . .

256

9.3.7

The .finiN Sections . . . . . . . . . . . . . . . . . . . . . . .

258

9.3.8

Using Sections in Assembler Code . . . . . . . . . . . . . . .

258

9.3.9

Using Sections in C Code . . . . . . . . . . . . . . . . . . .

259

Data in Program Space . . . . . . . . . . . . . . . . . . . . . . . . .

259

9.4.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

259

9.4.2

A Note On const . . . . . . . . . . . . . . . . . . . . . . . .

260

9.4.3

Storing and Retrieving Data in the Program Space . . . . . .

260

9.4.4

Storing and Retrieving Strings in the Program Space . . . . .

262

9.4.5

Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

264

avr-libc and assembler programs . . . . . . . . . . . . . . . . . . . .

264

9.5.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

264

9.5.2

Invoking the compiler . . . . . . . . . . . . . . . . . . . . .

265

9.5.3

Example program . . . . . . . . . . . . . . . . . . . . . . . .

265

9.5.4

Pseudo-ops and operators . . . . . . . . . . . . . . . . . . .

269

Inline Assembler Cookbook . . . . . . . . . . . . . . . . . . . . . .

270

9.6.1

GCC asm Statement . . . . . . . . . . . . . . . . . . . . . .

271

9.6.2

Assembler Code . . . . . . . . . . . . . . . . . . . . . . . .

273

9.6.3

Input and Output Operands . . . . . . . . . . . . . . . . . . .

273

9.6.4

Clobbers . . . . . . . . . . . . . . . . . . . . . . . . . . . .

277

9.6.5

Assembler Macros . . . . . . . . . . . . . . . . . . . . . . .

279

9.6.6

C Stub Functions . . . . . . . . . . . . . . . . . . . . . . . .

280

9.6.7

C Names Used in Assembler Code . . . . . . . . . . . . . . .

281

9.6.8

Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

282

How to Build a Library . . . . . . . . . . . . . . . . . . . . . . . . .

282

9.7.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

282

9.7.2

How the Linker Works . . . . . . . . . . . . . . . . . . . . .

282

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9.8

9.9

xiii

9.7.3

How to Design a Library . . . . . . . . . . . . . . . . . . . .

283

9.7.4

Creating a Library . . . . . . . . . . . . . . . . . . . . . . .

283

9.7.5

Using a Library . . . . . . . . . . . . . . . . . . . . . . . . .

284

Porting From IAR to AVR GCC . . . . . . . . . . . . . . . . . . . .

285

9.8.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

285

9.8.2

Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . .

285

9.8.3

Interrupt Service Routines (ISRs) . . . . . . . . . . . . . . .

286

9.8.4

Intrinsic Routines . . . . . . . . . . . . . . . . . . . . . . . .

287

9.8.5

Flash Variables . . . . . . . . . . . . . . . . . . . . . . . . .

287

9.8.6

Non-Returning main() . . . . . . . . . . . . . . . . . . . . .

288

9.8.7

Locking Registers . . . . . . . . . . . . . . . . . . . . . . .

289

Frequently Asked Questions . . . . . . . . . . . . . . . . . . . . . .

289

9.9.1

FAQ Index . . . . . . . . . . . . . . . . . . . . . . . . . . .

289

9.9.2

My program doesn’t recognize a variable updated within an
interrupt routine . . . . . . . . . . . . . . . . . . . . . . . .

291

9.9.3

I get "undefined reference to..." for functions like "sin()" . . .

291

9.9.4

How to permanently bind a variable to a register? . . . . . . .

291

9.9.5

How to modify MCUCR or WDTCR early? . . . . . . . . . .

292

9.9.6

What is all this _BV() stuff about? . . . . . . . . . . . . . . .

292

9.9.7

Can I use C++ on the AVR? . . . . . . . . . . . . . . . . . .

293

9.9.8

Shouldn’t I initialize all my variables? . . . . . . . . . . . . . 294

9.9.9

Why do some 16-bit timer registers sometimes get trashed? .

295

9.9.10 How do I use a #define’d constant in an asm statement? . . . .

296

9.9.11 Why does the PC randomly jump around when single-stepping
through my program in avr-gdb? . . . . . . . . . . . . . . . .

296

9.9.12 How do I trace an assembler file in avr-gdb? . . . . . . . . . .

297

9.9.13 How do I pass an IO port as a parameter to a function? . . . .

298

9.9.14 What registers are used by the C compiler? . . . . . . . . . .

300

9.9.15 How do I put an array of strings completely in ROM? . . . . .

302

9.9.16 How to use external RAM? . . . . . . . . . . . . . . . . . . .

304

9.9.17 Which -O flag to use? . . . . . . . . . . . . . . . . . . . . .

304

9.9.18 How do I relocate code to a fixed address? . . . . . . . . . . .

305

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9.9.19 My UART is generating nonsense! My ATmega128 keeps
crashing! Port F is completely broken! . . . . . . . . . . . . . 306
9.9.20 Why do all my "foo...bar" strings eat up the SRAM? . . . . .

306

9.9.21 Why does the compiler compile an 8-bit operation that uses
bitwise operators into a 16-bit operation in assembly? . . . . .

307

9.9.22 How to detect RAM memory and variable overlap problems? .

308

9.9.23 Is it really impossible to program the ATtinyXX in C? . . . .

308

9.9.24 What is this "clock skew detected" messsage? . . . . . . . . .

309

9.9.25 Why are (many) interrupt flags cleared by writing a logical 1?

309

9.9.26 Why have "programmed" fuses the bit value 0? . . . . . . . .

310

9.9.27 Which AVR-specific assembler operators are available? . . . .

310

9.9.28 Why are interrupts re-enabled in the middle of writing the stack
pointer? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

310

9.9.29 Why are there five different linker scripts? . . . . . . . . . . .

311

9.9.30 How to add a raw binary image to linker output? . . . . . . .

311

9.10 Installing the GNU Tool Chain . . . . . . . . . . . . . . . . . . . . .

312

9.10.1 Required Tools . . . . . . . . . . . . . . . . . . . . . . . . .

313

9.10.2 Optional Tools . . . . . . . . . . . . . . . . . . . . . . . . .

313

9.10.3 GNU Binutils for the AVR target . . . . . . . . . . . . . . . .

314

9.10.4 GCC for the AVR target . . . . . . . . . . . . . . . . . . . .

315

9.10.5 AVR Libc . . . . . . . . . . . . . . . . . . . . . . . . . . . .

316

9.10.6 Avrdude . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

316

9.10.7 GDB for the AVR target . . . . . . . . . . . . . . . . . . . .

317

9.10.8 Simulavr . . . . . . . . . . . . . . . . . . . . . . . . . . . .

317

9.10.9 AVaRice . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

318

9.11 Using the GNU tools . . . . . . . . . . . . . . . . . . . . . . . . . .

318

9.11.1 Options for the C compiler avr-gcc . . . . . . . . . . . . . . .

318

9.11.2 Options for the assembler avr-as . . . . . . . . . . . . . . . .

324

9.11.3 Controlling the linker avr-ld . . . . . . . . . . . . . . . . . .

326

9.12 Using the avrdude program . . . . . . . . . . . . . . . . . . . . . . .

328

9.13 Release Numbering and Methodology . . . . . . . . . . . . . . . . .

330

9.13.1 Release Version Numbering Scheme . . . . . . . . . . . . . . 330

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1 AVR Libc

1

1

9.13.2 Releasing AVR Libc . . . . . . . . . . . . . . . . . . . . . .

331

9.14 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . .

333

9.15 Todo List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

334

9.16 Deprecated List . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

335

AVR Libc

1.1

Introduction

The latest version of this document
http://savannah.nongnu.org/projects/avr-libc/

is

always

available

from

The AVR Libc package provides a subset of the standard C library for Atmel AVR
8-bit RISC microcontrollers. In addition, the library provides the basic
startup code needed by most applications.
There is a wealth of information in this document which goes beyond simply describing the interfaces and routines provided by the library. We hope that this document
provides enough information to get a new AVR developer up to speed quickly using
the freely available development tools: binutils, gcc avr-libc and many others.
If you find yourself stuck on a problem which this document doesn’t quite address, you
may wish to post a message to the avr-gcc mailing list. Most of the developers of the
AVR binutils and gcc ports in addition to the devleopers of avr-libc subscribe to the list,
so you will usually be able to get your problem resolved. You can subscribe to the list
at http://lists.nongnu.org/mailman/listinfo/avr-gcc-list . Before posting to the list, you
might want to try reading the Frequently Asked Questions chapter of this document.
Note:
If you think you’ve found a bug, or have a suggestion for an improvement, either in this documentation or in the library itself, please use the bug tracker
at https://savannah.nongnu.org/bugs/?group=avr-libc to ensure the issue won’t be
forgotten.

1.2

General information about this library

In general, it has been the goal to stick as best as possible to established standards
while implementing this library. Commonly, this refers to the C library as described by
the ANSI X3.159-1989 and ISO/IEC 9899:1990 ("ANSI-C") standard, as well as parts
of their successor ISO/IEC 9899:1999 ("C99"). Some additions have been inspired by
other standards like IEEE Std 1003.1-1988 ("POSIX.1"), while other extensions are
purely AVR-specific (like the entire program-space string interface).

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1.3

Supported Devices

2

Unless otherwise noted, functions of this library are not guarenteed to be reentrant. In
particular, any functions that store local state are known to be non-reentrant, as well
as functions that manipulate IO registers like the EEPROM access routines. If these
functions are used within both, standard and interrupt context, undefined behaviour
will result.

1.3

Supported Devices

The following is a list of AVR devices currently supported by the library. Note that
actual support for some newer devices depends on the ability of the compiler/assembler
to support these devices at library compile-time.
AT90S Type Devices:

• at90s1200 [1]
• at90s2313
• at90s2323
• at90s2333
• at90s2343
• at90s4414
• at90s4433
• at90s4434
• at90s8515
• at90c8534
• at90s8535
ATmega Type Devices:

• atmega8
• atmega103
• atmega128
• atmega1280

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1.3

Supported Devices

• atmega1281
• atmega16
• atmega161
• atmega162
• atmega163
• atmega164p
• atmega165
• atmega165p
• atmega168
• atmega169
• atmega169p
• atmega2560
• atmega2561
• atmega32
• atmega323
• atmega324p
• atmega325
• atmega325p
• atmega3250
• atmega3250p
• atmega329
• atmega329p
• atmega3290
• atmega3290p
• atmega48
• atmega64
• atmega640
• atmega644
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3

1.3

Supported Devices

• atmega644p
• atmega645
• atmega6450
• atmega649
• atmega6490
• atmega8515
• atmega8535
• atmega88
ATtiny Type Devices:

• attiny11 [1]
• attiny12 [1]
• attiny13
• attiny15 [1]
• attiny22
• attiny24
• attiny25
• attiny26
• attiny261
• attiny28 [1]
• attiny2313
• attiny44
• attiny45
• attiny461
• attiny84
• attiny85
• attiny861

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4

1.3

Supported Devices

5

Misc Devices:

• at94K [2]
• at76c711 [3]
• at43usb320
• at43usb355
• at86rf401
• at90can32
• at90can64
• at90can128
• at90pwm1
• at90pwm2
• at90pwm3
• at90usb82
• at90usb162
• at90usb646
• at90usb647
• at90usb1286
• at90usb1287
• atmega8hva
• atmega16hva
• atmega406

Note:
[1] Assembly only. There is no direct support for these devices to be programmed
in C since they do not have a RAM based stack. Still, it could be possible to
program them in C, see the FAQ for an option.

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1.4

avr-libc License

6

Note:
[2] The at94K devices are a combination of FPGA and AVR microcontroller.
[TRoth-2002/11/12: Not sure of the level of support for these. More information
would be welcomed.]

Note:
[3] The at76c711 is a USB to fast serial interface bridge chip using an AVR core.

1.4

avr-libc License

avr-libc can be freely used and redistributed, provided the following license conditions
are met.
Portions of avr-libc are Copyright (c) 1999-2007
Keith Gudger,
Bjoern Haase,
Steinar Haugen,
Peter Jansen,
Reinhard Jessich,
Magnus Johansson,
Artur Lipowski,
Marek Michalkiewicz,
Colin O’Flynn,
Bob Paddock,
Reiner Patommel,
Michael Rickman,
Theodore A. Roth,
Juergen Schilling,
Philip Soeberg,
Anatoly Sokolov,
Nils Kristian Strom,
Michael Stumpf,
Stefan Swanepoel,
Eric B. Weddington,
Joerg Wunsch,
Dmitry Xmelkov,
The Regents of the University of California.
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in
the documentation and/or other materials provided with the
distribution.
* Neither the name of the copyright holders nor the names of

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2 avr-libc Module Index

7

contributors may be used to endorse or promote products derived
from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.

2

avr-libc Module Index

2.1

avr-libc Modules

Here is a list of all modules:
: Diagnostics

13

: Character Operations

14

: System Errors

17

: Integer Type conversions

17

: Mathematics

29

: Non-local goto

34

: Standard Integer Types

36

: Standard IO facilities

48

: General utilities

66

: Strings

77

: Bootloader Support Utilities

88

: EEPROM handling

94

: Interrupts

97

: Program Space Utilities

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116

3 avr-libc Data Structure Index

8

: Power Reduction Management

128

: Special function registers

133

Additional notes from 

131

: Power Management and Sleep Modes

136

: avr-libc version macros

138

: Watchdog timer handling

140

: CRC Computations

143

: Convenience functions for busy-wait delay loops

147

: Basic busy-wait delay loops

148

: Parity bit generation

149

: TWI bit mask definitions

150

: Deprecated items

154

: Compatibility with IAR EWB 3.x

157

Demo projects

158

3

Combining C and assembly source files

159

A simple project

162

A more sophisticated project

177

Using the standard IO facilities

184

Example using the two-wire interface (TWI)

192

avr-libc Data Structure Index

3.1

avr-libc Data Structures

Here are the data structures with brief descriptions:
div_t

197

ldiv_t

197

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4 avr-libc File Index

4

9

avr-libc File Index

4.1

avr-libc File List

Here is a list of all documented files with brief descriptions:
assert.h

198

atoi.S

198

atol.S

198

boot.h

199

crc16.h

205

ctype.h

205

delay.h

206

delay_basic.h

206

eeprom.h

207

errno.h

208

fdevopen.c

208

ffs.S

209

ffsl.S

209

ffsll.S

209

interrupt.h

209

inttypes.h

209

math.h

212

memccpy.S

213

memchr.S

213

memchr_P.S

214

memcmp.S

214

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4.1

avr-libc File List

10

memcmp_P.S

214

memcpy.S

214

memcpy_P.S

214

memmem.S

215

memmove.S

215

memrchr.S

215

memrchr_P.S

215

memset.S

215

parity.h

216

pgmspace.h

216

power.h

223

setjmp.h

223

sleep.h

224

stdint.h

225

stdio.h

228

stdlib.h

230

strcasecmp.S

232

strcasecmp_P.S

232

strcasestr.S

232

strcat.S

232

strcat_P.S

233

strchr.S

233

strchr_P.S

233

strchrnul.S

233

strchrnul_P.S

233

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4.1

avr-libc File List

11

strcmp.S

233

strcmp_P.S

233

strcpy.S

233

strcpy_P.S

234

strcspn.S

234

strcspn_P.S

234

string.h

234

strlcat.S

236

strlcat_P.S

236

strlcpy.S

236

strlcpy_P.S

236

strlen.S

236

strlen_P.S

236

strlwr.S

237

strncasecmp.S

237

strncasecmp_P.S

237

strncat.S

237

strncat_P.S

237

strncmp.S

238

strncmp_P.S

238

strncpy.S

238

strncpy_P.S

239

strnlen.S

239

strnlen_P.S

239

strpbrk.S

240

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5 avr-libc Page Index

12

strpbrk_P.S

240

strrchr.S

240

strrchr_P.S

242

strrev.S

242

strsep.S

242

strsep_P.S

242

strspn.S

242

strspn_P.S

242

strstr.S

242

strstr_P.S

242

strtok_r.S

242

strupr.S

242

util/twi.h

242

wdt.h

244

5

avr-libc Page Index

5.1

avr-libc Related Pages

Here is a list of all related documentation pages:
Toolchain Overview

245

Memory Areas and Using malloc()

250

Memory Sections

255

Data in Program Space

259

avr-libc and assembler programs

264

Inline Assembler Cookbook

270

How to Build a Library

282

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6 avr-libc Module Documentation

13

Porting From IAR to AVR GCC

285

Frequently Asked Questions

289

Installing the GNU Tool Chain

312

Using the GNU tools

318

Using the avrdude program

328

Release Numbering and Methodology

330

Acknowledgments

333

Todo List

334

Deprecated List

335

6

avr-libc Module Documentation
: Diagnostics

6.1
6.1.1

Detailed Description

#include 

This header file defines a debugging aid.
As there is no standard error output stream available for many applications using this
library, the generation of a printable error message is not enabled by default. These
messages will only be generated if the application defines the macro
__ASSERT_USE_STDERR

before including the  header file. By default, only abort() will be called
to halt the application.
Defines
• #define assert(expression)
6.1.2
6.1.2.1

Define Documentation
#define assert(expression)

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6.2 : Character Operations

14

Parameters:
expression Expression to test for.
The assert() macro tests the given expression and if it is false, the calling process is
terminated. A diagnostic message is written to stderr and the function abort() is called,
effectively terminating the program.
If expression is true, the assert() macro does nothing.
The assert() macro may be removed at compile time by defining NDEBUG as a macro
(e.g., by using the compiler option -DNDEBUG).

: Character Operations

6.2
6.2.1

Detailed Description

These functions perform various operations on characters.
#include 

Character classification routines
These functions perform character classification. They return true or false status depending whether the character passed to the function falls into the function’s classification (i.e. isdigit() returns true if its argument is any value ’0’ though ’9’, inclusive.)
•
•
•
•
•
•
•
•
•
•
•
•
•

int isalnum (int __c) __ATTR_CONST__
int isalpha (int __c) __ATTR_CONST__
int isascii (int __c) __ATTR_CONST__
int isblank (int __c) __ATTR_CONST__
int iscntrl (int __c) __ATTR_CONST__
int isdigit (int __c) __ATTR_CONST__
int isgraph (int __c) __ATTR_CONST__
int islower (int __c) __ATTR_CONST__
int isprint (int __c) __ATTR_CONST__
int ispunct (int __c) __ATTR_CONST__
int isspace (int __c) __ATTR_CONST__
int isupper (int __c) __ATTR_CONST__
int isxdigit (int __c) __ATTR_CONST__

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6.2 : Character Operations

15

Character convertion routines
If c is not an unsigned char value, or EOF, the behaviour of these functions is undefined.
• int toascii (int __c) __ATTR_CONST__
• int tolower (int __c) __ATTR_CONST__
• int toupper (int __c) __ATTR_CONST__
6.2.2
6.2.2.1

Function Documentation
int isalnum (int __c)

Checks for an alphanumeric character.
isdigit(c)).
6.2.2.2

int isalpha (int __c)

Checks for an alphabetic character.
islower(c)).
6.2.2.3

It is equivalent to (isalpha(c) ||

It is equivalent to (isupper(c) ||

int isascii (int __c)

Checks whether c is a 7-bit unsigned char value that fits into the ASCII character set.
6.2.2.4

int isblank (int __c)

Checks for a blank character, that is, a space or a tab.
6.2.2.5

int iscntrl (int __c)

Checks for a control character.
6.2.2.6

int isdigit (int __c)

Checks for a digit (0 through 9).
6.2.2.7

int isgraph (int __c)

Checks for any printable character except space.
6.2.2.8

int islower (int __c)

Checks for a lower-case character.

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6.2 : Character Operations

6.2.2.9

16

int isprint (int __c)

Checks for any printable character including space.
6.2.2.10

int ispunct (int __c)

Checks for any printable character which is not a space or an alphanumeric character.
6.2.2.11

int isspace (int __c)

Checks for white-space characters. For the avr-libc library, these are: space, formfeed (’\f’), newline (’\n’), carriage return (’\r’), horizontal tab (’\t’), and vertical tab
(’\v’).
6.2.2.12

int isupper (int __c)

Checks for an uppercase letter.
6.2.2.13

int isxdigit (int __c)

Checks for a hexadecimal digits, i.e. one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F.

6.2.2.14

int toascii (int __c)

Converts c to a 7-bit unsigned char value that fits into the ASCII character set, by
clearing the high-order bits.
Warning:
Many people will be unhappy if you use this function. This function will convert
accented letters into random characters.

6.2.2.15

int tolower (int __c)

Converts the letter c to lower case, if possible.
6.2.2.16

int toupper (int __c)

Converts the letter c to upper case, if possible.

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6.3 : System Errors

17

: System Errors

6.3
6.3.1

Detailed Description

#include 

Some functions in the library set the global variable errno when an error occurs. The
file, , provides symbolic names for various error codes.
Warning:
The errno global variable is not safe to use in a threaded or multi-task system. A
race condition can occur if a task is interrupted between the call which sets error
and when the task examines errno. If another task changes errno during this
time, the result will be incorrect for the interrupted task.
Defines
• #define EDOM 33
• #define ERANGE 34
6.3.2

Define Documentation

6.3.2.1

#define EDOM 33

Domain error.
6.3.2.2

#define ERANGE 34

Range error.

6.4
6.4.1

: Integer Type conversions
Detailed Description

#include 

This header file includes the exact-width integer definitions from , and
extends them with additional facilities provided by the implementation.
Currently, the extensions include two additional integer types that could hold a "far"
pointer (i.e. a code pointer that can address more than 64 KB), as well as standard
names for all printf and scanf formatting options that are supported by the :
Standard IO facilities. As the library does not support the full range of conversion

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6.4 : Integer Type conversions

18

specifiers from ISO 9899:1999, only those conversions that are actually implemented
will be listed here.
The idea behind these conversion macros is that, for each of the types defined by
, a macro will be supplied that portably allows formatting an object of that
type in printf() or scanf() operations. Example:
#include 
uint8_t smallval;
int32_t longval;
...
printf("The hexadecimal value of smallval is " PRIx8
", the decimal value of longval is " PRId32 ".\n",
smallval, longval);

macros for printf and scanf format specifiers
For C++, these are only included if __STDC_LIMIT_MACROS is defined before including .
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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•
•

#define PRId8 "d"
#define PRIdLEAST8 "d"
#define PRIdFAST8 "d"
#define PRIi8 "i"
#define PRIiLEAST8 "i"
#define PRIiFAST8 "i"
#define PRId16 "d"
#define PRIdLEAST16 "d"
#define PRIdFAST16 "d"
#define PRIi16 "i"
#define PRIiLEAST16 "i"
#define PRIiFAST16 "i"
#define PRId32 "ld"
#define PRIdLEAST32 "ld"
#define PRIdFAST32 "ld"
#define PRIi32 "li"
#define PRIiLEAST32 "li"
#define PRIiFAST32 "li"
#define PRIdPTR PRId16
#define PRIiPTR PRIi16
#define PRIo8 "o"
#define PRIoLEAST8 "o"
#define PRIoFAST8 "o"
#define PRIu8 "u"

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6.4 : Integer Type conversions

•
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•

#define PRIuLEAST8 "u"
#define PRIuFAST8 "u"
#define PRIx8 "x"
#define PRIxLEAST8 "x"
#define PRIxFAST8 "x"
#define PRIX8 "X"
#define PRIXLEAST8 "X"
#define PRIXFAST8 "X"
#define PRIo16 "o"
#define PRIoLEAST16 "o"
#define PRIoFAST16 "o"
#define PRIu16 "u"
#define PRIuLEAST16 "u"
#define PRIuFAST16 "u"
#define PRIx16 "x"
#define PRIxLEAST16 "x"
#define PRIxFAST16 "x"
#define PRIX16 "X"
#define PRIXLEAST16 "X"
#define PRIXFAST16 "X"
#define PRIo32 "lo"
#define PRIoLEAST32 "lo"
#define PRIoFAST32 "lo"
#define PRIu32 "lu"
#define PRIuLEAST32 "lu"
#define PRIuFAST32 "lu"
#define PRIx32 "lx"
#define PRIxLEAST32 "lx"
#define PRIxFAST32 "lx"
#define PRIX32 "lX"
#define PRIXLEAST32 "lX"
#define PRIXFAST32 "lX"
#define PRIoPTR PRIo16
#define PRIuPTR PRIu16
#define PRIxPTR PRIx16
#define PRIXPTR PRIX16
#define SCNd16 "d"
#define SCNdLEAST16 "d"
#define SCNdFAST16 "d"
#define SCNi16 "i"
#define SCNiLEAST16 "i"
#define SCNiFAST16 "i"
#define SCNd32 "ld"

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19

6.4 : Integer Type conversions

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#define SCNdLEAST32 "ld"
#define SCNdFAST32 "ld"
#define SCNi32 "li"
#define SCNiLEAST32 "li"
#define SCNiFAST32 "li"
#define SCNdPTR SCNd16
#define SCNiPTR SCNi16
#define SCNo16 "o"
#define SCNoLEAST16 "o"
#define SCNoFAST16 "o"
#define SCNu16 "u"
#define SCNuLEAST16 "u"
#define SCNuFAST16 "u"
#define SCNx16 "x"
#define SCNxLEAST16 "x"
#define SCNxFAST16 "x"
#define SCNo32 "lo"
#define SCNoLEAST32 "lo"
#define SCNoFAST32 "lo"
#define SCNu32 "lu"
#define SCNuLEAST32 "lu"
#define SCNuFAST32 "lu"
#define SCNx32 "lx"
#define SCNxLEAST32 "lx"
#define SCNxFAST32 "lx"
#define SCNoPTR SCNo16
#define SCNuPTR SCNu16
#define SCNxPTR SCNx16

Far pointers for memory access >64K
• typedef int32_t int_farptr_t
• typedef uint32_t uint_farptr_t
6.4.2
6.4.2.1

Define Documentation
#define PRId16 "d"

decimal printf format for int16_t
6.4.2.2

#define PRId32 "ld"

decimal printf format for int32_t
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20

6.4 : Integer Type conversions

6.4.2.3

#define PRId8 "d"

decimal printf format for int8_t
6.4.2.4

#define PRIdFAST16 "d"

decimal printf format for int_fast16_t
6.4.2.5

#define PRIdFAST32 "ld"

decimal printf format for int_fast32_t
6.4.2.6

#define PRIdFAST8 "d"

decimal printf format for int_fast8_t
6.4.2.7

#define PRIdLEAST16 "d"

decimal printf format for int_least16_t
6.4.2.8

#define PRIdLEAST32 "ld"

decimal printf format for int_least32_t
6.4.2.9

#define PRIdLEAST8 "d"

decimal printf format for int_least8_t
6.4.2.10

#define PRIdPTR PRId16

decimal printf format for intptr_t
6.4.2.11

#define PRIi16 "i"

integer printf format for int16_t
6.4.2.12

#define PRIi32 "li"

integer printf format for int32_t
6.4.2.13

#define PRIi8 "i"

integer printf format for int8_t

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21

6.4 : Integer Type conversions

6.4.2.14

#define PRIiFAST16 "i"

integer printf format for int_fast16_t
6.4.2.15

#define PRIiFAST32 "li"

integer printf format for int_fast32_t
6.4.2.16

#define PRIiFAST8 "i"

integer printf format for int_fast8_t
6.4.2.17

#define PRIiLEAST16 "i"

integer printf format for int_least16_t
6.4.2.18

#define PRIiLEAST32 "li"

integer printf format for int_least32_t
6.4.2.19

#define PRIiLEAST8 "i"

integer printf format for int_least8_t
6.4.2.20

#define PRIiPTR PRIi16

integer printf format for intptr_t
6.4.2.21

#define PRIo16 "o"

octal printf format for uint16_t
6.4.2.22

#define PRIo32 "lo"

octal printf format for uint32_t
6.4.2.23

#define PRIo8 "o"

octal printf format for uint8_t
6.4.2.24

#define PRIoFAST16 "o"

octal printf format for uint_fast16_t

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22

6.4 : Integer Type conversions

6.4.2.25

#define PRIoFAST32 "lo"

octal printf format for uint_fast32_t
6.4.2.26

#define PRIoFAST8 "o"

octal printf format for uint_fast8_t
6.4.2.27

#define PRIoLEAST16 "o"

octal printf format for uint_least16_t
6.4.2.28

#define PRIoLEAST32 "lo"

octal printf format for uint_least32_t
6.4.2.29

#define PRIoLEAST8 "o"

octal printf format for uint_least8_t
6.4.2.30

#define PRIoPTR PRIo16

octal printf format for uintptr_t
6.4.2.31

#define PRIu16 "u"

decimal printf format for uint16_t
6.4.2.32

#define PRIu32 "lu"

decimal printf format for uint32_t
6.4.2.33

#define PRIu8 "u"

decimal printf format for uint8_t
6.4.2.34

#define PRIuFAST16 "u"

decimal printf format for uint_fast16_t
6.4.2.35

#define PRIuFAST32 "lu"

decimal printf format for uint_fast32_t

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23

6.4 : Integer Type conversions

6.4.2.36

#define PRIuFAST8 "u"

decimal printf format for uint_fast8_t
6.4.2.37

#define PRIuLEAST16 "u"

decimal printf format for uint_least16_t
6.4.2.38

#define PRIuLEAST32 "lu"

decimal printf format for uint_least32_t
6.4.2.39

#define PRIuLEAST8 "u"

decimal printf format for uint_least8_t
6.4.2.40

#define PRIuPTR PRIu16

decimal printf format for uintptr_t
6.4.2.41

#define PRIX16 "X"

uppercase hexadecimal printf format for uint16_t
6.4.2.42

#define PRIx16 "x"

hexadecimal printf format for uint16_t
6.4.2.43

#define PRIX32 "lX"

uppercase hexadecimal printf format for uint32_t
6.4.2.44

#define PRIx32 "lx"

hexadecimal printf format for uint32_t
6.4.2.45

#define PRIX8 "X"

uppercase hexadecimal printf format for uint8_t
6.4.2.46

#define PRIx8 "x"

hexadecimal printf format for uint8_t

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24

6.4 : Integer Type conversions

6.4.2.47

#define PRIXFAST16 "X"

uppercase hexadecimal printf format for uint_fast16_t
6.4.2.48

#define PRIxFAST16 "x"

hexadecimal printf format for uint_fast16_t
6.4.2.49

#define PRIXFAST32 "lX"

uppercase hexadecimal printf format for uint_fast32_t
6.4.2.50

#define PRIxFAST32 "lx"

hexadecimal printf format for uint_fast32_t
6.4.2.51

#define PRIXFAST8 "X"

uppercase hexadecimal printf format for uint_fast8_t
6.4.2.52

#define PRIxFAST8 "x"

hexadecimal printf format for uint_fast8_t
6.4.2.53

#define PRIXLEAST16 "X"

uppercase hexadecimal printf format for uint_least16_t
6.4.2.54

#define PRIxLEAST16 "x"

hexadecimal printf format for uint_least16_t
6.4.2.55

#define PRIXLEAST32 "lX"

uppercase hexadecimal printf format for uint_least32_t
6.4.2.56

#define PRIxLEAST32 "lx"

hexadecimal printf format for uint_least32_t
6.4.2.57

#define PRIXLEAST8 "X"

uppercase hexadecimal printf format for uint_least8_t

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25

6.4 : Integer Type conversions

6.4.2.58

#define PRIxLEAST8 "x"

hexadecimal printf format for uint_least8_t
6.4.2.59

#define PRIXPTR PRIX16

uppercase hexadecimal printf format for uintptr_t
6.4.2.60

#define PRIxPTR PRIx16

hexadecimal printf format for uintptr_t
6.4.2.61

#define SCNd16 "d"

decimal scanf format for int16_t
6.4.2.62

#define SCNd32 "ld"

decimal scanf format for int32_t
6.4.2.63

#define SCNdFAST16 "d"

decimal scanf format for int_fast16_t
6.4.2.64

#define SCNdFAST32 "ld"

decimal scanf format for int_fast32_t
6.4.2.65

#define SCNdLEAST16 "d"

decimal scanf format for int_least16_t
6.4.2.66

#define SCNdLEAST32 "ld"

decimal scanf format for int_least32_t
6.4.2.67

#define SCNdPTR SCNd16

decimal scanf format for intptr_t
6.4.2.68

#define SCNi16 "i"

generic-integer scanf format for int16_t

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26

6.4 : Integer Type conversions

6.4.2.69

#define SCNi32 "li"

generic-integer scanf format for int32_t
6.4.2.70

#define SCNiFAST16 "i"

generic-integer scanf format for int_fast16_t
6.4.2.71

#define SCNiFAST32 "li"

generic-integer scanf format for int_fast32_t
6.4.2.72

#define SCNiLEAST16 "i"

generic-integer scanf format for int_least16_t
6.4.2.73

#define SCNiLEAST32 "li"

generic-integer scanf format for int_least32_t
6.4.2.74

#define SCNiPTR SCNi16

generic-integer scanf format for intptr_t
6.4.2.75

#define SCNo16 "o"

octal scanf format for uint16_t
6.4.2.76

#define SCNo32 "lo"

octal scanf format for uint32_t
6.4.2.77

#define SCNoFAST16 "o"

octal scanf format for uint_fast16_t
6.4.2.78

#define SCNoFAST32 "lo"

octal scanf format for uint_fast32_t
6.4.2.79

#define SCNoLEAST16 "o"

octal scanf format for uint_least16_t

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27

6.4 : Integer Type conversions

6.4.2.80

#define SCNoLEAST32 "lo"

octal scanf format for uint_least32_t
6.4.2.81

#define SCNoPTR SCNo16

octal scanf format for uintptr_t
6.4.2.82

#define SCNu16 "u"

decimal scanf format for uint16_t
6.4.2.83

#define SCNu32 "lu"

decimal scanf format for uint32_t
6.4.2.84

#define SCNuFAST16 "u"

decimal scanf format for uint_fast16_t
6.4.2.85

#define SCNuFAST32 "lu"

decimal scanf format for uint_fast32_t
6.4.2.86

#define SCNuLEAST16 "u"

decimal scanf format for uint_least16_t
6.4.2.87

#define SCNuLEAST32 "lu"

decimal scanf format for uint_least32_t
6.4.2.88

#define SCNuPTR SCNu16

decimal scanf format for uintptr_t
6.4.2.89

#define SCNx16 "x"

hexadecimal scanf format for uint16_t
6.4.2.90

#define SCNx32 "lx"

hexadecimal scanf format for uint32_t

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28

6.5 : Mathematics

6.4.2.91

29

#define SCNxFAST16 "x"

hexadecimal scanf format for uint_fast16_t
6.4.2.92

#define SCNxFAST32 "lx"

hexadecimal scanf format for uint_fast32_t
6.4.2.93

#define SCNxLEAST16 "x"

hexadecimal scanf format for uint_least16_t
6.4.2.94

#define SCNxLEAST32 "lx"

hexadecimal scanf format for uint_least32_t
6.4.2.95

#define SCNxPTR SCNx16

hexadecimal scanf format for uintptr_t
6.4.3

Typedef Documentation

6.4.3.1

typedef int32_t int_farptr_t

signed integer type that can hold a pointer > 64 KB
6.4.3.2

typedef uint32_t uint_farptr_t

unsigned integer type that can hold a pointer > 64 KB

: Mathematics

6.5
6.5.1

Detailed Description

#include 

This header file declares basic mathematics constants and functions.
Note:
In order to access the functions delcared herein, it is usually also required to additionally link against the library libm.a. See also the related FAQ entry.

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6.5 : Mathematics

Defines
• #define M_PI 3.141592653589793238462643
• #define M_SQRT2 1.4142135623730950488016887
Functions
•
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•
•
•
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•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

double cos (double __x) __ATTR_CONST__
double fabs (double __x) __ATTR_CONST__
double fmod (double __x, double __y) __ATTR_CONST__
double modf (double __value, double ∗__iptr)
double sin (double __x) __ATTR_CONST__
double sqrt (double __x) __ATTR_CONST__
double tan (double __x) __ATTR_CONST__
double floor (double __x) __ATTR_CONST__
double ceil (double __x) __ATTR_CONST__
double frexp (double __value, int ∗__exp)
double ldexp (double __x, int __exp) __ATTR_CONST__
double exp (double __x) __ATTR_CONST__
double cosh (double __x) __ATTR_CONST__
double sinh (double __x) __ATTR_CONST__
double tanh (double __x) __ATTR_CONST__
double acos (double __x) __ATTR_CONST__
double asin (double __x) __ATTR_CONST__
double atan (double __x) __ATTR_CONST__
double atan2 (double __y, double __x) __ATTR_CONST__
double log (double __x) __ATTR_CONST__
double log10 (double __x) __ATTR_CONST__
double pow (double __x, double __y) __ATTR_CONST__
int isnan (double __x) __ATTR_CONST__
int isinf (double __x) __ATTR_CONST__
double square (double __x) __ATTR_CONST__

6.5.2
6.5.2.1

Define Documentation
#define M_PI 3.141592653589793238462643

The constant pi.
6.5.2.2

#define M_SQRT2 1.4142135623730950488016887

The square root of 2.

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30

6.5 : Mathematics

6.5.3
6.5.3.1

31

Function Documentation
double acos (double __x)

The acos() function computes the principal value of the arc cosine of x. The returned
value is in the range [0, pi] radians. A domain error occurs for arguments not in the
range [-1, +1].
6.5.3.2

double asin (double __x)

The asin() function computes the principal value of the arc sine of x. The returned
value is in the range [0, pi] radians. A domain error occurs for arguments not in the
range [-1, +1].
6.5.3.3

double atan (double __x)

The atan() function computes the principal value of the arc tangent of x. The returned
value is in the range [0, pi] radians. A domain error occurs for arguments not in the
range [-1, +1].
6.5.3.4

double atan2 (double __y, double __x)

The atan2() function computes the principal value of the arc tangent of y / x, using
the signs of both arguments to determine the quadrant of the return value. The returned
value is in the range [-pi, +pi] radians. If both x and y are zero, the global variable
errno is set to EDOM.
6.5.3.5

double ceil (double __x)

The ceil() function returns the smallest integral value greater than or equal to x, expressed as a floating-point number.
6.5.3.6

double cos (double __x)

The cos() function returns the cosine of x, measured in radians.
6.5.3.7

double cosh (double __x)

The cosh() function returns the hyperbolic cosine of x.
6.5.3.8

double exp (double __x)

The exp() function returns the exponential value of x.

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6.5 : Mathematics

6.5.3.9

32

double fabs (double __x)

The fabs() function computes the absolute value of a floating-point number x.
6.5.3.10

double floor (double __x)

The floor() function returns the largest integral value less than or equal to x, expressed
as a floating-point number.
6.5.3.11

double fmod (double __x, double __y)

The function fmod() returns the floating-point remainder of x / y.
6.5.3.12

double frexp (double __value, int ∗ __exp)

The frexp() function breaks a floating-point number into a normalized fraction and an
integral power of 2. It stores the integer in the int object pointed to by exp.
The frexp() function returns the value x, such that x is a double with magnitude in the
interval [1/2, 1) or zero, and value equals x times 2 raised to the power ∗exp. If
value is zero, both parts of the result are zero.
6.5.3.13

int isinf (double __x)

The function isinf() returns 1 if the argument x is either positive or negative infinity,
otherwise 0.
6.5.3.14

int isnan (double __x)

The function isnan() returns 1 if the argument x represents a "not-a-number" (NaN)
object, otherwise 0.
6.5.3.15

double ldexp (double __x, int __exp)

The ldexp() function multiplies a floating-point number by an integral power of 2.
The ldexp() function returns the value of x times 2 raised to the power exp.
If the resultant value would cause an overflow, the global variable errno is set to
ERANGE, and the value NaN is returned.
6.5.3.16

double log (double __x)

The log() function returns the natural logarithm of argument x.
If the argument is less than or equal 0, a domain error will occur.

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6.5 : Mathematics

6.5.3.17

33

double log10 (double __x)

The log10() function returns the logarithm of argument x to base 10.
If the argument is less than or equal 0, a domain error will occur.
6.5.3.18

double modf (double __value, double ∗ __iptr)

The modf() function breaks the argument value into integral and fractional parts,
each of which has the same sign as the argument. It stores the integral part as a double
in the object pointed to by iptr.
The modf() function returns the signed fractional part of value.
6.5.3.19

double pow (double __x, double __y)

The function pow() returns the value of x to the exponent y.
6.5.3.20

double sin (double __x)

The sin() function returns the sine of x, measured in radians.
6.5.3.21

double sinh (double __x)

The sinh() function returns the hyperbolic sine of x.
6.5.3.22

double sqrt (double __x)

The sqrt() function returns the non-negative square root of x.
6.5.3.23

double square (double __x)

The function square() returns x ∗ x.
Note:
This function does not belong to the C standard definition.

6.5.3.24

double tan (double __x)

The tan() function returns the tangent of x, measured in radians.
6.5.3.25

double tanh (double __x)

The tanh() function returns the hyperbolic tangent of x.

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6.6 : Non-local goto

34

: Non-local goto

6.6
6.6.1

Detailed Description

While the C language has the dreaded goto statement, it can only be used to jump to
a label in the same (local) function. In order to jump directly to another (non-local)
function, the C library provides the setjmp() and longjmp() functions. setjmp() and
longjmp() are useful for dealing with errors and interrupts encountered in a low-level
subroutine of a program.
Note:
setjmp() and longjmp() make programs hard to understand and maintain. If possible, an alternative should be used.
longjmp() can destroy changes made to global register variables (see How to permanently bind a variable to a register?).
For a very detailed discussion of setjmp()/longjmp(), see Chapter 7 of Advanced Programming in the UNIX Environment, by W. Richard Stevens.
Example:
#include 
jmp_buf env;
int main (void)
{
if (setjmp (env))
{
... handle error ...
}
while (1)
{
... main processing loop which calls foo() some where ...
}
}
...
void foo (void)
{
... blah, blah, blah ...
if (err)
{
longjmp (env, 1);
}
}

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6.6 : Non-local goto

35

Functions
• int setjmp (jmp_buf __jmpb)
• void longjmp (jmp_buf __jmpb, int __ret) __ATTR_NORETURN__
6.6.2

Function Documentation

6.6.2.1

void longjmp (jmp_buf __jmpb, int __ret)

Non-local jump to a saved stack context.
#include 

longjmp() restores the environment saved by the last call of setjmp() with the corresponding __jmpb argument. After longjmp() is completed, program execution continues as if the corresponding call of setjmp() had just returned the value __ret.
Note:
longjmp() cannot cause 0 to be returned. If longjmp() is invoked with a second
argument of 0, 1 will be returned instead.
Parameters:
__jmpb Information saved by a previous call to setjmp().
__ret Value to return to the caller of setjmp().
Returns:
This function never returns.

6.6.2.2

int setjmp (jmp_buf __jmpb)

Save stack context for non-local goto.
#include 

setjmp() saves the stack context/environment in __jmpb for later use by longjmp(). The
stack context will be invalidated if the function which called setjmp() returns.
Parameters:
__jmpb Variable of type jmp_buf which holds the stack information such that
the environment can be restored.
Returns:
setjmp() returns 0 if returning directly, and non-zero when returning from
longjmp() using the saved context.

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6.7 : Standard Integer Types

36

: Standard Integer Types

6.7
6.7.1

Detailed Description

#include 

Use [u]intN_t if you need exactly N bits.
Since these typedefs are mandated by the C99 standard, they are preferred over rolling
your own typedefs.
Limits of specified-width integer types
C++ implementations should define these macros only when __STDC_LIMIT_MACROS is defined before  is included
•
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#define INT8_MAX 0x7f
#define INT8_MIN (-INT8_MAX - 1)
#define UINT8_MAX (__CONCAT(INT8_MAX, U) ∗ 2U + 1U)
#define INT16_MAX 0x7fff
#define INT16_MIN (-INT16_MAX - 1)
#define UINT16_MAX (__CONCAT(INT16_MAX, U) ∗ 2U + 1U)
#define INT32_MAX 0x7fffffffL
#define INT32_MIN (-INT32_MAX - 1L)
#define UINT32_MAX (__CONCAT(INT32_MAX, U) ∗ 2UL + 1UL)
#define INT64_MAX 0x7fffffffffffffffLL
#define INT64_MIN (-INT64_MAX - 1LL)
#define UINT64_MAX (__CONCAT(INT64_MAX, U) ∗ 2ULL + 1ULL)

Limits of minimum-width integer types
•
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•

#define INT_LEAST8_MAX INT8_MAX
#define INT_LEAST8_MIN INT8_MIN
#define UINT_LEAST8_MAX UINT8_MAX
#define INT_LEAST16_MAX INT16_MAX
#define INT_LEAST16_MIN INT16_MIN
#define UINT_LEAST16_MAX UINT16_MAX
#define INT_LEAST32_MAX INT32_MAX
#define INT_LEAST32_MIN INT32_MIN
#define UINT_LEAST32_MAX UINT32_MAX
#define INT_LEAST64_MAX INT64_MAX
#define INT_LEAST64_MIN INT64_MIN
#define UINT_LEAST64_MAX UINT64_MAX

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6.7 : Standard Integer Types

37

Limits of fastest minimum-width integer types
•
•
•
•
•
•
•
•
•
•
•
•

#define INT_FAST8_MAX INT8_MAX
#define INT_FAST8_MIN INT8_MIN
#define UINT_FAST8_MAX UINT8_MAX
#define INT_FAST16_MAX INT16_MAX
#define INT_FAST16_MIN INT16_MIN
#define UINT_FAST16_MAX UINT16_MAX
#define INT_FAST32_MAX INT32_MAX
#define INT_FAST32_MIN INT32_MIN
#define UINT_FAST32_MAX UINT32_MAX
#define INT_FAST64_MAX INT64_MAX
#define INT_FAST64_MIN INT64_MIN
#define UINT_FAST64_MAX UINT64_MAX

Limits of integer types capable of holding object pointers
• #define INTPTR_MAX INT16_MAX
• #define INTPTR_MIN INT16_MIN
• #define UINTPTR_MAX UINT16_MAX
Limits of greatest-width integer types
• #define INTMAX_MAX INT64_MAX
• #define INTMAX_MIN INT64_MIN
• #define UINTMAX_MAX UINT64_MAX
Limits of other integer types
C++ implementations should define these macros only when __STDC_LIMIT_MACROS is defined before  is included
•
•
•
•
•

#define PTRDIFF_MAX INT16_MAX
#define PTRDIFF_MIN INT16_MIN
#define SIG_ATOMIC_MAX INT8_MAX
#define SIG_ATOMIC_MIN INT8_MIN
#define SIZE_MAX (__CONCAT(INT16_MAX, U))

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6.7 : Standard Integer Types

38

Macros for integer constants
C++ implementations should define these macros only when __STDC_CONSTANT_MACROS is defined before  is included.
These definitions are valid for integer constants without suffix and for macros defined
as integer constant without suffix
•
•
•
•
•
•
•
•
•
•

#define INT8_C(value) ((int8_t) value)
#define UINT8_C(value) ((uint8_t) __CONCAT(value, U))
#define INT16_C(value) value
#define UINT16_C(value) __CONCAT(value, U)
#define INT32_C(value) __CONCAT(value, L)
#define UINT32_C(value) __CONCAT(value, UL)
#define INT64_C(value) __CONCAT(value, LL)
#define UINT64_C(value) __CONCAT(value, ULL)
#define INTMAX_C(value) __CONCAT(value, LL)
#define UINTMAX_C(value) __CONCAT(value, ULL)

Exact-width integer types
Integer types having exactly the specified width
•
•
•
•
•
•
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•

typedef signed char int8_t
typedef unsigned char uint8_t
typedef signed int int16_t
typedef unsigned int uint16_t
typedef signed long int int32_t
typedef unsigned long int uint32_t
typedef signed long long int int64_t
typedef unsigned long long int uint64_t

Integer types capable of holding object pointers
These allow you to declare variables of the same size as a pointer.
• typedef int16_t intptr_t
• typedef uint16_t uintptr_t

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6.7 : Standard Integer Types

39

Minimum-width integer types
Integer types having at least the specified width
•
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typedef int8_t int_least8_t
typedef uint8_t uint_least8_t
typedef int16_t int_least16_t
typedef uint16_t uint_least16_t
typedef int32_t int_least32_t
typedef uint32_t uint_least32_t
typedef int64_t int_least64_t
typedef uint64_t uint_least64_t

Fastest minimum-width integer types
Integer types being usually fastest having at least the specified width
•
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typedef int8_t int_fast8_t
typedef uint8_t uint_fast8_t
typedef int16_t int_fast16_t
typedef uint16_t uint_fast16_t
typedef int32_t int_fast32_t
typedef uint32_t uint_fast32_t
typedef int64_t int_fast64_t
typedef uint64_t uint_fast64_t

Greatest-width integer types
Types designating integer data capable of representing any value of any integer type in
the corresponding signed or unsigned category
• typedef int64_t intmax_t
• typedef uint64_t uintmax_t
6.7.2
6.7.2.1

Define Documentation
#define INT16_C(value) value

define a constant of type int16_t
6.7.2.2

#define INT16_MAX 0x7fff

largest positive value an int16_t can hold.
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6.7 : Standard Integer Types

6.7.2.3

#define INT16_MIN (-INT16_MAX - 1)

smallest negative value an int16_t can hold.
6.7.2.4

#define INT32_C(value) __CONCAT(value, L)

define a constant of type int32_t
6.7.2.5

#define INT32_MAX 0x7fffffffL

largest positive value an int32_t can hold.
6.7.2.6

#define INT32_MIN (-INT32_MAX - 1L)

smallest negative value an int32_t can hold.
6.7.2.7

#define INT64_C(value) __CONCAT(value, LL)

define a constant of type int64_t
6.7.2.8

#define INT64_MAX 0x7fffffffffffffffLL

largest positive value an int64_t can hold.
6.7.2.9

#define INT64_MIN (-INT64_MAX - 1LL)

smallest negative value an int64_t can hold.
6.7.2.10

#define INT8_C(value) ((int8_t) value)

define a constant of type int8_t
6.7.2.11

#define INT8_MAX 0x7f

largest positive value an int8_t can hold.
6.7.2.12

#define INT8_MIN (-INT8_MAX - 1)

smallest negative value an int8_t can hold.
6.7.2.13

#define INT_FAST16_MAX INT16_MAX

largest positive value an int_fast16_t can hold.

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40

6.7 : Standard Integer Types

6.7.2.14

#define INT_FAST16_MIN INT16_MIN

smallest negative value an int_fast16_t can hold.
6.7.2.15

#define INT_FAST32_MAX INT32_MAX

largest positive value an int_fast32_t can hold.
6.7.2.16

#define INT_FAST32_MIN INT32_MIN

smallest negative value an int_fast32_t can hold.
6.7.2.17

#define INT_FAST64_MAX INT64_MAX

largest positive value an int_fast64_t can hold.
6.7.2.18

#define INT_FAST64_MIN INT64_MIN

smallest negative value an int_fast64_t can hold.
6.7.2.19

#define INT_FAST8_MAX INT8_MAX

largest positive value an int_fast8_t can hold.
6.7.2.20

#define INT_FAST8_MIN INT8_MIN

smallest negative value an int_fast8_t can hold.
6.7.2.21

#define INT_LEAST16_MAX INT16_MAX

largest positive value an int_least16_t can hold.
6.7.2.22

#define INT_LEAST16_MIN INT16_MIN

smallest negative value an int_least16_t can hold.
6.7.2.23

#define INT_LEAST32_MAX INT32_MAX

largest positive value an int_least32_t can hold.
6.7.2.24

#define INT_LEAST32_MIN INT32_MIN

smallest negative value an int_least32_t can hold.

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41

6.7 : Standard Integer Types

6.7.2.25

#define INT_LEAST64_MAX INT64_MAX

largest positive value an int_least64_t can hold.
6.7.2.26

#define INT_LEAST64_MIN INT64_MIN

smallest negative value an int_least64_t can hold.
6.7.2.27

#define INT_LEAST8_MAX INT8_MAX

largest positive value an int_least8_t can hold.
6.7.2.28

#define INT_LEAST8_MIN INT8_MIN

smallest negative value an int_least8_t can hold.
6.7.2.29

#define INTMAX_C(value) __CONCAT(value, LL)

define a constant of type intmax_t
6.7.2.30

#define INTMAX_MAX INT64_MAX

largest positive value an intmax_t can hold.
6.7.2.31

#define INTMAX_MIN INT64_MIN

smallest negative value an intmax_t can hold.
6.7.2.32

#define INTPTR_MAX INT16_MAX

largest positive value an intptr_t can hold.
6.7.2.33

#define INTPTR_MIN INT16_MIN

smallest negative value an intptr_t can hold.
6.7.2.34

#define PTRDIFF_MAX INT16_MAX

largest positive value a ptrdiff_t can hold.
6.7.2.35

#define PTRDIFF_MIN INT16_MIN

smallest negative value a ptrdiff_t can hold.

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6.7 : Standard Integer Types

6.7.2.36

43

#define SIG_ATOMIC_MAX INT8_MAX

largest positive value a sig_atomic_t can hold.
6.7.2.37

#define SIG_ATOMIC_MIN INT8_MIN

smallest negative value a sig_atomic_t can hold.
6.7.2.38

#define SIZE_MAX (__CONCAT(INT16_MAX, U))

largest value a size_t can hold.
6.7.2.39

#define UINT16_C(value) __CONCAT(value, U)

define a constant of type uint16_t
6.7.2.40

#define UINT16_MAX (__CONCAT(INT16_MAX, U) ∗ 2U + 1U)

largest value an uint16_t can hold.
6.7.2.41

#define UINT32_C(value) __CONCAT(value, UL)

define a constant of type uint32_t
6.7.2.42

#define UINT32_MAX (__CONCAT(INT32_MAX, U) ∗ 2UL + 1UL)

largest value an uint32_t can hold.
6.7.2.43

#define UINT64_C(value) __CONCAT(value, ULL)

define a constant of type uint64_t
6.7.2.44

#define UINT64_MAX (__CONCAT(INT64_MAX, U) ∗ 2ULL + 1ULL)

largest value an uint64_t can hold.
6.7.2.45

#define UINT8_C(value) ((uint8_t) __CONCAT(value, U))

define a constant of type uint8_t
6.7.2.46

#define UINT8_MAX (__CONCAT(INT8_MAX, U) ∗ 2U + 1U)

largest value an uint8_t can hold.

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6.7 : Standard Integer Types

6.7.2.47

#define UINT_FAST16_MAX UINT16_MAX

largest value an uint_fast16_t can hold.
6.7.2.48

#define UINT_FAST32_MAX UINT32_MAX

largest value an uint_fast32_t can hold.
6.7.2.49

#define UINT_FAST64_MAX UINT64_MAX

largest value an uint_fast64_t can hold.
6.7.2.50

#define UINT_FAST8_MAX UINT8_MAX

largest value an uint_fast8_t can hold.
6.7.2.51

#define UINT_LEAST16_MAX UINT16_MAX

largest value an uint_least16_t can hold.
6.7.2.52

#define UINT_LEAST32_MAX UINT32_MAX

largest value an uint_least32_t can hold.
6.7.2.53

#define UINT_LEAST64_MAX UINT64_MAX

largest value an uint_least64_t can hold.
6.7.2.54

#define UINT_LEAST8_MAX UINT8_MAX

largest value an uint_least8_t can hold.
6.7.2.55

#define UINTMAX_C(value) __CONCAT(value, ULL)

define a constant of type uintmax_t
6.7.2.56

#define UINTMAX_MAX UINT64_MAX

largest value an uintmax_t can hold.
6.7.2.57

#define UINTPTR_MAX UINT16_MAX

largest value an uintptr_t can hold.

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6.7 : Standard Integer Types

6.7.3
6.7.3.1

Typedef Documentation
typedef signed int int16_t

16-bit signed type.
6.7.3.2

typedef signed long int int32_t

32-bit signed type.
6.7.3.3

typedef signed long long int int64_t

64-bit signed type.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.4

typedef signed char int8_t

8-bit signed type.
6.7.3.5

typedef int16_t int_fast16_t

fastest signed int with at least 16 bits.
6.7.3.6

typedef int32_t int_fast32_t

fastest signed int with at least 32 bits.
6.7.3.7

typedef int64_t int_fast64_t

fastest signed int with at least 64 bits.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.8

typedef int8_t int_fast8_t

fastest signed int with at least 8 bits.
6.7.3.9

typedef int16_t int_least16_t

signed int with at least 16 bits.
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6.7 : Standard Integer Types

6.7.3.10

typedef int32_t int_least32_t

signed int with at least 32 bits.
6.7.3.11

typedef int64_t int_least64_t

signed int with at least 64 bits.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.12

typedef int8_t int_least8_t

signed int with at least 8 bits.
6.7.3.13

typedef int64_t intmax_t

largest signed int available.
6.7.3.14

typedef int16_t intptr_t

Signed pointer compatible type.
6.7.3.15

typedef unsigned int uint16_t

16-bit unsigned type.
6.7.3.16

typedef unsigned long int uint32_t

32-bit unsigned type.
6.7.3.17

typedef unsigned long long int uint64_t

64-bit unsigned type.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.18

typedef unsigned char uint8_t

8-bit unsigned type.

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6.7 : Standard Integer Types

6.7.3.19

typedef uint16_t uint_fast16_t

fastest unsigned int with at least 16 bits.
6.7.3.20

typedef uint32_t uint_fast32_t

fastest unsigned int with at least 32 bits.
6.7.3.21

typedef uint64_t uint_fast64_t

fastest unsigned int with at least 64 bits.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.22

typedef uint8_t uint_fast8_t

fastest unsigned int with at least 8 bits.
6.7.3.23

typedef uint16_t uint_least16_t

unsigned int with at least 16 bits.
6.7.3.24

typedef uint32_t uint_least32_t

unsigned int with at least 32 bits.
6.7.3.25

typedef uint64_t uint_least64_t

unsigned int with at least 64 bits.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.7.3.26

typedef uint8_t uint_least8_t

unsigned int with at least 8 bits.
6.7.3.27

typedef uint64_t uintmax_t

largest unsigned int available.

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47

6.8 : Standard IO facilities

6.7.3.28

48

typedef uint16_t uintptr_t

Unsigned pointer compatible type.

6.8
6.8.1

: Standard IO facilities
Detailed Description

#include 

Introduction to the Standard IO facilities This file declares the standard IO facilities that are implemented in avr-libc. Due to the nature of the underlying hardware,
only a limited subset of standard IO is implemented. There is no actual file implementation available, so only device IO can be performed. Since there’s no operating system,
the application needs to provide enough details about their devices in order to make
them usable by the standard IO facilities.
Due to space constraints, some functionality has not been implemented at all (like some
of the printf conversions that have been left out). Nevertheless, potential users of
this implementation should be warned: the printf and scanf families of functions,
although usually associated with presumably simple things like the famous "Hello,
world!" program, are actually fairly complex which causes their inclusion to eat up
a fair amount of code space. Also, they are not fast due to the nature of interpreting
the format string at run-time. Whenever possible, resorting to the (sometimes nonstandard) predetermined conversion facilities that are offered by avr-libc will usually
cost much less in terms of speed and code size.
Tunable options for code size vs. feature set In order to allow programmers a code
size vs. functionality tradeoff, the function vfprintf() which is the heart of the printf
family can be selected in different flavours using linker options. See the documentation
of vfprintf() for a detailed description. The same applies to vfscanf() and the scanf
family of functions.
Outline of the chosen API The standard streams stdin, stdout, and stderr are
provided, but contrary to the C standard, since avr-libc has no knowledge about applicable devices, these streams are not already pre-initialized at application startup. Also,
since there is no notion of "file" whatsoever to avr-libc, there is no function fopen()
that could be used to associate a stream to some device. (See note 1.) Instead, the
function fdevopen() is provided to associate a stream to a device, where the device
needs to provide a function to send a character, to receive a character, or both. There
is no differentiation between "text" and "binary" streams inside avr-libc. Character \n
is sent literally down to the device’s put() function. If the device requires a carriage
return (\r) character to be sent before the linefeed, its put() routine must implement
this (see note 2).

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6.8 : Standard IO facilities

49

As an alternative method to fdevopen(), the macro fdev_setup_stream() might be used
to setup a user-supplied FILE structure.
It should be noted that the automatic conversion of a newline character into a carriage
return - newline sequence breaks binary transfers. If binary transfers are desired, no
automatic conversion should be performed, but instead any string that aims to issue a
CR-LF sequence must use "\r\n" explicitly.
For convenience, the first call to fdevopen() that opens a stream for reading
will cause the resulting stream to be aliased to stdin. Likewise, the first call to
fdevopen() that opens a stream for writing will cause the resulting stream to be
aliased to both, stdout, and stderr. Thus, if the open was done with both, read
and write intent, all three standard streams will be identical. Note that these aliases are
indistinguishable from each other, thus calling fclose() on such a stream will also
effectively close all of its aliases (note 3).
It is possible to tie additional user data to a stream, using fdev_set_udata(). The backend put and get functions can then extract this user data using fdev_get_udata(), and act
appropriately. For example, a single put function could be used to talk to two different
UARTs that way, or the put and get functions could keep internal state between calls
there.
Format strings in flash ROM All the printf and scanf family functions come
in two flavours: the standard name, where the format string is expected to be in SRAM,
as well as a version with the suffix "_P" where the format string is expected to reside
in the flash ROM. The macro PSTR (explained in : Program Space
Utilities) becomes very handy for declaring these format strings.
Running stdio without malloc() By default, fdevopen() requires malloc(). As this is
often not desired in the limited environment of a microcontroller, an alternative option
is provided to run completely without malloc().
The macro fdev_setup_stream() is provided to prepare a user-supplied FILE buffer for
operation with stdio.
Example
#include 
static int uart_putchar(char c, FILE *stream);
static FILE mystdout = FDEV_SETUP_STREAM(uart_putchar, NULL,
_FDEV_SETUP_WRITE);
static int
uart_putchar(char c, FILE *stream)
{

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6.8 : Standard IO facilities

50

if (c == ’\n’)
uart_putchar(’\r’, stream);
loop_until_bit_is_set(UCSRA, UDRE);
UDR = c;
return 0;
}
int
main(void)
{
init_uart();
stdout = &mystdout;
printf("Hello, world!\n");
return 0;
}

This example uses the initializer form FDEV_SETUP_STREAM() rather than the
function-like fdev_setup_stream(), so all data initialization happens during C start-up.
If streams initialized that way are no longer needed, they can be destroyed by first
calling the macro fdev_close(), and then destroying the object itself. No call to fclose()
should be issued for these streams. While calling fclose() itself is harmless, it will cause
an undefined reference to free() and thus cause the linker to link the malloc module into
the application.
Notes
Note 1:
It might have been possible to implement a device abstraction that is compatible
with fopen() but since this would have required to parse a string, and to take all
the information needed either out of this string, or out of an additional table that
would need to be provided by the application, this approach was not taken.
Note 2:
This basically follows the Unix approach: if a device such as a terminal needs
special handling, it is in the domain of the terminal device driver to provide this
functionality. Thus, a simple function suitable as put() for fdevopen() that
talks to a UART interface might look like this:
int
uart_putchar(char c, FILE *stream)
{
if (c == ’\n’)
uart_putchar(’\r’);
loop_until_bit_is_set(UCSRA, UDRE);
UDR = c;
return 0;
}

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6.8 : Standard IO facilities

51

Note 3:
This implementation has been chosen because the cost of maintaining an alias
is considerably smaller than the cost of maintaining full copies of each stream.
Yet, providing an implementation that offers the complete set of standard
streams was deemed to be useful. Not only that writing printf() instead of
fprintf(mystream, ...) saves typing work, but since avr-gcc needs to resort to pass all arguments of variadic functions on the stack (as opposed to passing
them in registers for functions that take a fixed number of parameters), the ability
to pass one parameter less by implying stdin will also save some execution time.
Defines
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•
•

#define FILE struct __file
#define stdin (__iob[0])
#define stdout (__iob[1])
#define stderr (__iob[2])
#define EOF (-1)
#define fdev_set_udata(stream, u) do { (stream) → udata = u; } while(0)
#define fdev_get_udata(stream) ((stream) → udata)
#define fdev_setup_stream(stream, put, get, rwflag)
#define _FDEV_SETUP_READ __SRD
#define _FDEV_SETUP_WRITE __SWR
#define _FDEV_SETUP_RW (__SRD|__SWR)
#define _FDEV_ERR (-1)
#define _FDEV_EOF (-2)
#define FDEV_SETUP_STREAM(put, get, rwflag)
#define fdev_close()
#define putc(__c, __stream) fputc(__c, __stream)
#define putchar(__c) fputc(__c, stdout)
#define getc(__stream) fgetc(__stream)
#define getchar() fgetc(stdin)

Functions
•
•
•
•
•
•
•
•

int fclose (FILE ∗__stream)
int vfprintf (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int vfprintf_P (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int fputc (int __c, FILE ∗__stream)
int printf (const char ∗__fmt,...)
int printf_P (const char ∗__fmt,...)
int vprintf (const char ∗__fmt, va_list __ap)
int sprintf (char ∗__s, const char ∗__fmt,...)

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6.8 : Standard IO facilities

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•
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•

52

int sprintf_P (char ∗__s, const char ∗__fmt,...)
int snprintf (char ∗__s, size_t __n, const char ∗__fmt,...)
int snprintf_P (char ∗__s, size_t __n, const char ∗__fmt,...)
int vsprintf (char ∗__s, const char ∗__fmt, va_list ap)
int vsprintf_P (char ∗__s, const char ∗__fmt, va_list ap)
int vsnprintf (char ∗__s, size_t __n, const char ∗__fmt, va_list ap)
int vsnprintf_P (char ∗__s, size_t __n, const char ∗__fmt, va_list ap)
int fprintf (FILE ∗__stream, const char ∗__fmt,...)
int fprintf_P (FILE ∗__stream, const char ∗__fmt,...)
int fputs (const char ∗__str, FILE ∗__stream)
int fputs_P (const char ∗__str, FILE ∗__stream)
int puts (const char ∗__str)
int puts_P (const char ∗__str)
size_t fwrite (const void ∗__ptr, size_t __size, size_t __nmemb, FILE ∗__stream)
int fgetc (FILE ∗__stream)
int ungetc (int __c, FILE ∗__stream)
char ∗ fgets (char ∗__str, int __size, FILE ∗__stream)
char ∗ gets (char ∗__str)
size_t fread (void ∗__ptr, size_t __size, size_t __nmemb, FILE ∗__stream)
void clearerr (FILE ∗__stream)
int feof (FILE ∗__stream)
int ferror (FILE ∗__stream)
int vfscanf (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int vfscanf_P (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int fscanf (FILE ∗__stream, const char ∗__fmt,...)
int fscanf_P (FILE ∗__stream, const char ∗__fmt,...)
int scanf (const char ∗__fmt,...)
int scanf_P (const char ∗__fmt,...)
int vscanf (const char ∗__fmt, va_list __ap)
int sscanf (const char ∗__buf, const char ∗__fmt,...)
int sscanf_P (const char ∗__buf, const char ∗__fmt,...)
int fflush (FILE ∗stream)
FILE ∗ fdevopen (int(∗put)(char, FILE ∗), int(∗get)(FILE ∗))

6.8.2
6.8.2.1

Define Documentation
#define _FDEV_EOF (-2)

Return code for an end-of-file condition during device read.
To be used in the get function of fdevopen().

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6.8.2.2

53

#define _FDEV_ERR (-1)

Return code for an error condition during device read.
To be used in the get function of fdevopen().
6.8.2.3

#define _FDEV_SETUP_READ __SRD

fdev_setup_stream() with read intent
6.8.2.4

#define _FDEV_SETUP_RW (__SRD|__SWR)

fdev_setup_stream() with read/write intent
6.8.2.5

#define _FDEV_SETUP_WRITE __SWR

fdev_setup_stream() with write intent
6.8.2.6

#define EOF (-1)

EOF declares the value that is returned by various standard IO functions in case of an
error. Since the AVR platform (currently) doesn’t contain an abstraction for actual files,
its origin as "end of file" is somewhat meaningless here.
6.8.2.7

#define fdev_close()

This macro frees up any library resources that might be associated with stream. It
should be called if stream is no longer needed, right before the application is going
to destroy the stream object itself.
(Currently, this macro evaluates to nothing, but this might change in future versions of
the library.)
6.8.2.8

#define fdev_get_udata(stream) ((stream) → udata)

This macro retrieves a pointer to user defined data from a FILE stream object.
6.8.2.9

#define fdev_set_udata(stream, u) do { (stream) → udata = u; } while(0)

This macro inserts a pointer to user defined data into a FILE stream object.
The user data can be useful for tracking state in the put and get functions supplied to
the fdevopen() function.

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6.8.2.10

54

#define FDEV_SETUP_STREAM(put, get, rwflag)

Initializer for a user-supplied stdio stream.
This macro acts similar to fdev_setup_stream(), but it is to be used as the initializer of
a variable of type FILE.
The remaining arguments are to be used as explained in fdev_setup_stream().
6.8.2.11

#define fdev_setup_stream(stream, put, get, rwflag)

Setup a user-supplied buffer as an stdio stream.
This macro takes a user-supplied buffer stream, and sets it up as a stream that is valid
for stdio operations, similar to one that has been obtained dynamically from fdevopen().
The buffer to setup must be of type FILE.
The arguments put and get are identical to those that need to be passed to fdevopen().
The rwflag argument can take one of the values _FDEV_SETUP_READ, _FDEV_SETUP_WRITE, or _FDEV_SETUP_RW, for read, write, or read/write intent, respectively.
Note:
No assignments to the standard streams will be performed by fdev_setup_stream().
If standard streams are to be used, these need to be assigned by the user. See also
under Running stdio without malloc().

6.8.2.12

#define FILE struct __file

FILE is the opaque structure that is passed around between the various standard IO
functions.
6.8.2.13

#define getc(__stream) fgetc(__stream)

The macro getc used to be a "fast" macro implementation with a functionality identical to fgetc(). For space constraints, in avr-libc, it is just an alias for fgetc.

6.8.2.14

#define getchar(void) fgetc(stdin)

The macro getchar reads a character from stdin. Return values and error handling
is identical to fgetc().
6.8.2.15

#define putc(__c, __stream) fputc(__c, __stream)

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55

The macro putc used to be a "fast" macro implementation with a functionality identical to fputc(). For space constraints, in avr-libc, it is just an alias for fputc.

6.8.2.16

#define putchar(__c) fputc(__c, stdout)

The macro putchar sends character c to stdout.
6.8.2.17

#define stderr (__iob[2])

Stream destined for error output. Unless specifically assigned, identical to stdout.
If stderr should point to another stream, the result of another fdevopen() must
be explicitly assigned to it without closing the previous stderr (since this would also
close stdout).
6.8.2.18

#define stdin (__iob[0])

Stream that will be used as an input stream by the simplified functions that don’t take
a stream argument.
The first stream opened with read intent using fdevopen() will be assigned to
stdin.
6.8.2.19

#define stdout (__iob[1])

Stream that will be used as an output stream by the simplified functions that don’t take
a stream argument.
The first stream opened with write intent using fdevopen() will be assigned to both,
stdin, and stderr.
6.8.3
6.8.3.1

Function Documentation
void clearerr (FILE ∗ __stream)

Clear the error and end-of-file flags of stream.
6.8.3.2

int fclose (FILE ∗ __stream)

This function closes stream, and disallows and further IO to and from it.
When using fdevopen() to setup the stream, a call to fclose() is needed in order to free
the internal resources allocated.
If the stream has been set up using fdev_setup_stream() or FDEV_SETUP_STREAM(), use fdev_close() instead.

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6.8 : Standard IO facilities

56

It currently always returns 0 (for success).
6.8.3.3

FILE∗ fdevopen (int(∗)(char, FILE ∗) put, int(∗)(FILE ∗) get)

This function is a replacement for fopen().
It opens a stream for a device where the actual device implementation needs to be
provided by the application. If successful, a pointer to the structure for the opened
stream is returned. Reasons for a possible failure currently include that neither the
put nor the get argument have been provided, thus attempting to open a stream with
no IO intent at all, or that insufficient dynamic memory is available to establish a new
stream.
If the put function pointer is provided, the stream is opened with write intent. The
function passed as put shall take two arguments, the first a character to write to the
device, and the second a pointer to FILE, and shall return 0 if the output was successful,
and a nonzero value if the character could not be sent to the device.
If the get function pointer is provided, the stream is opened with read intent. The
function passed as get shall take a pointer to FILE as its single argument, and return
one character from the device, passed as an int type. If an error occurs when trying
to read from the device, it shall return _FDEV_ERR. If an end-of-file condition was
reached while reading from the device, _FDEV_EOF shall be returned.
If both functions are provided, the stream is opened with read and write intent.
The first stream opened with read intent is assigned to stdin, and the first one opened
with write intent is assigned to both, stdout and stderr.
fdevopen() uses calloc() (und thus malloc()) in order to allocate the storage for the new
stream.
Note:
If the macro __STDIO_FDEVOPEN_COMPAT_12 is declared before including
, a function prototype for fdevopen() will be chosen that is backwards
compatible with avr-libc version 1.2 and before. This is solely intented for providing a simple migration path without the need to immediately change all source
code. Do not use for new code.

6.8.3.4

int feof (FILE ∗ __stream)

Test the end-of-file flag of stream. This flag can only be cleared by a call to clearerr().

6.8.3.5

int ferror (FILE ∗ __stream)

Test the error flag of stream. This flag can only be cleared by a call to clearerr().

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57

int fflush (FILE ∗ stream)

Flush stream.
This is a null operation provided for source-code compatibility only, as the standard IO
implementation currently does not perform any buffering.
6.8.3.7

int fgetc (FILE ∗ __stream)

The function fgetc reads a character from stream. It returns the character, or EOF
in case end-of-file was encountered or an error occurred. The routines feof() or ferror()
must be used to distinguish between both situations.
6.8.3.8

char∗ fgets (char ∗ __str, int __size, FILE ∗ __stream)

Read at most size - 1 bytes from stream, until a newline character was encountered, and store the characters in the buffer pointed to by str. Unless an error was
encountered while reading, the string will then be terminated with a NUL character.
If an error was encountered, the function returns NULL and sets the error flag of
stream, which can be tested using ferror(). Otherwise, a pointer to the string will
be returned.
6.8.3.9

int fprintf (FILE ∗ __stream, const char ∗ __fmt, ...)

The function fprintf performs formatted output to stream. See vfprintf()
for details.
6.8.3.10

int fprintf_P (FILE ∗ __stream, const char ∗ __fmt, ...)

Variant of fprintf() that uses a fmt string that resides in program memory.
6.8.3.11

int fputc (int __c, FILE ∗ __stream)

The function fputc sends the character c (though given as type int) to stream. It
returns the character, or EOF in case an error occurred.
6.8.3.12

int fputs (const char ∗ __str, FILE ∗ __stream)

Write the string pointed to by str to stream stream.
Returns 0 on success and EOF on error.
6.8.3.13

int fputs_P (const char ∗ __str, FILE ∗ __stream)

Variant of fputs() where str resides in program memory.

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6.8 : Standard IO facilities

6.8.3.14
stream)

58

size_t fread (void ∗ __ptr, size_t __size, size_t __nmemb, FILE ∗ __-

Read nmemb objects, size bytes each, from stream, to the buffer pointed to by
ptr.
Returns the number of objects successfully read, i. e. nmemb unless an input error
occured or end-of-file was encountered. feof() and ferror() must be used to distinguish
between these two conditions.
6.8.3.15

int fscanf (FILE ∗ __stream, const char ∗ __fmt, ...)

The function fscanf performs formatted input, reading the input data from stream.
See vfscanf() for details.
6.8.3.16

int fscanf_P (FILE ∗ __stream, const char ∗ __fmt, ...)

Variant of fscanf() using a fmt string in program memory.
6.8.3.17 size_t fwrite (const void ∗ __ptr, size_t __size, size_t __nmemb, FILE ∗
__stream)
Write nmemb objects, size bytes each, to stream. The first byte of the first object
is referenced by ptr.
Returns the number of objects successfully written, i. e. nmemb unless an output error
occured.
6.8.3.18

char∗ gets (char ∗ __str)

Similar to fgets() except that it will operate on stream stdin, and the trailing newline
(if any) will not be stored in the string. It is the caller’s responsibility to provide enough
storage to hold the characters read.
6.8.3.19

int printf (const char ∗ __fmt, ...)

The function printf performs formatted output to stream stderr.
vfprintf() for details.
6.8.3.20

int printf_P (const char ∗ __fmt, ...)

Variant of printf() that uses a fmt string that resides in program memory.
6.8.3.21

int puts (const char ∗ __str)

Write the string pointed to by str, and a trailing newline character, to stdout.
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See

6.8 : Standard IO facilities

6.8.3.22

59

int puts_P (const char ∗ __str)

Variant of puts() where str resides in program memory.
6.8.3.23

int scanf (const char ∗ __fmt, ...)

The function scanf performs formatted input from stream stdin.
See vfscanf() for details.
6.8.3.24

int scanf_P (const char ∗ __fmt, ...)

Variant of scanf() where fmt resides in program memory.
6.8.3.25

int snprintf (char ∗ __s, size_t __n, const char ∗ __fmt, ...)

Like sprintf(), but instead of assuming s to be of infinite size, no more than n
characters (including the trailing NUL character) will be converted to s.
Returns the number of characters that would have been written to s if there were
enough space.
6.8.3.26

int snprintf_P (char ∗ __s, size_t __n, const char ∗ __fmt, ...)

Variant of snprintf() that uses a fmt string that resides in program memory.
6.8.3.27

int sprintf (char ∗ __s, const char ∗ __fmt, ...)

Variant of printf() that sends the formatted characters to string s.
6.8.3.28

int sprintf_P (char ∗ __s, const char ∗ __fmt, ...)

Variant of sprintf() that uses a fmt string that resides in program memory.
6.8.3.29

int sscanf (const char ∗ __buf, const char ∗ __fmt, ...)

The function sscanf performs formatted input, reading the input data from the buffer
pointed to by buf.
See vfscanf() for details.
6.8.3.30

int sscanf_P (const char ∗ __buf, const char ∗ __fmt, ...)

Variant of sscanf() using a fmt string in program memory.

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6.8.3.31

60

int ungetc (int __c, FILE ∗ __stream)

The ungetc() function pushes the character c (converted to an unsigned char) back onto
the input stream pointed to by stream. The pushed-back character will be returned
by a subsequent read on the stream.
Currently, only a single character can be pushed back onto the stream.
The ungetc() function returns the character pushed back after the conversion, or EOF if
the operation fails. If the value of the argument c character equals EOF, the operation
will fail and the stream will remain unchanged.
6.8.3.32

int vfprintf (FILE ∗ __stream, const char ∗ __fmt, va_list __ap)

vfprintf is the central facility of the printf family of functions. It outputs values
to stream under control of a format string passed in fmt. The actual values to print
are passed as a variable argument list ap.
vfprintf returns the number of characters written to stream, or EOF in case of
an error. Currently, this will only happen if stream has not been opened with write
intent.
The format string is composed of zero or more directives: ordinary characters (not
%), which are copied unchanged to the output stream; and conversion specifications,
each of which results in fetching zero or more subsequent arguments. Each conversion
specification is introduced by the % character. The arguments must properly correspond
(after type promotion) with the conversion specifier. After the %, the following appear
in sequence:
• Zero or more of the following flags:
– # The value should be converted to an "alternate form". For c, d, i, s, and
u conversions, this option has no effect. For o conversions, the precision of
the number is increased to force the first character of the output string to
a zero (except if a zero value is printed with an explicit precision of zero).
For x and X conversions, a non-zero result has the string ‘0x’ (or ‘0X’ for
X conversions) prepended to it.
– 0 (zero) Zero padding. For all conversions, the converted value is padded
on the left with zeros rather than blanks. If a precision is given with a
numeric conversion (d, i, o, u, i, x, and X), the 0 flag is ignored.
– - A negative field width flag; the converted value is to be left adjusted on
the field boundary. The converted value is padded on the right with blanks,
rather than on the left with blanks or zeros. A - overrides a 0 if both are
given.
– ’ ’ (space) A blank should be left before a positive number produced by a
signed conversion (d, or i).
– + A sign must always be placed before a number produced by a signed
conversion. A + overrides a space if both are used.
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• An optional decimal digit string specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with
spaces on the left (or right, if the left-adjust ment flag has been given) to fill out
the field width.
• An optional precision, in the form of a period . followed by an optional digit
string. If the digit string is omitted, the precision is taken as zero. This gives the
minimum number of digits to appear for d, i, o, u, x, and X conversions, or the
maximum number of characters to be printed from a string for s conversions.
• An optional l length modifier, that specifies that the argument for the d, i, o, u,
x, or X conversion is a "long int" rather than int.
• A character that specifies the type of conversion to be applied.
The conversion specifiers and their meanings are:
• diouxX The int (or appropriate variant) argument is converted to signed decimal
(d and i), unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal
(x and X) notation. The letters "abcdef" are used for x conversions; the letters
"ABCDEF" are used for X conversions. The precision, if any, gives the minimum
number of digits that must appear; if the converted value requires fewer digits, it
is padded on the left with zeros.
• p The void ∗ argument is taken as an unsigned integer, and converted similarly
as a %#x command would do.
• c The int argument is converted to an "unsigned char", and the resulting
character is written.
• s The "char ∗" argument is expected to be a pointer to an array of character
type (pointer to a string). Characters from the array are written up to (but not
including) a terminating NUL character; if a precision is specified, no more than
the number specified are written. If a precision is given, no null character need
be present; if the precision is not specified, or is greater than the size of the array,
the array must contain a terminating NUL character.
• % A % is written. No argument is converted. The complete conversion specification is "%%".
• eE The double argument is rounded and converted in the format
"[-]d.dddeśdd" where there is one digit before the decimal-point character and the number of digits after it is equal to the precision; if the precision
is missing, it is taken as 6; if the precision is zero, no decimal-point character
appears. An E conversion uses the letter ’E’ (rather than ’e’) to introduce
the exponent. The exponent always contains two digits; if the value is zero, the
exponent is 00.

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• fF The double argument is rounded and converted to decimal notation in the
format "[-]ddd.ddd", where the number of digits after the decimal-point
character is equal to the precision specification. If the precision is missing, it is
taken as 6; if the precision is explicitly zero, no decimal-point character appears.
If a decimal point appears, at least one digit appears before it.
• gG The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of significant digits. If the precision
is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is
used if the exponent from its conversion is less than -4 or greater than or equal to
the precision. Trailing zeros are removed from the fractional part of the result; a
decimal point appears only if it is followed by at least one digit.
• S Similar to the s format, except the pointer is expected to point to a programmemory (ROM) string instead of a RAM string.
In no case does a non-existent or small field width cause truncation of a numeric field;
if the result of a conversion is wider than the field width, the field is expanded to contain
the conversion result.
Since the full implementation of all the mentioned features becomes fairly large, three
different flavours of vfprintf() can be selected using linker options. The default vfprintf() implements all the mentioned functionality except floating point conversions.
A minimized version of vfprintf() is available that only implements the very basic integer and string conversion facilities, but none of the additional options that can be
specified using conversion flags (these flags are parsed correctly from the format specification, but then simply ignored). This version can be requested using the following
compiler options:
-Wl,-u,vfprintf -lprintf_min

If the full functionality including the floating point conversions is required, the following options should be used:
-Wl,-u,vfprintf -lprintf_flt -lm

Limitations:
• The specified width and precision can be at most 127.
• For floating-point conversions, trailing digits will be lost if a number close to
DBL_MAX is converted with a precision > 0.

6.8.3.33

int vfprintf_P (FILE ∗ __stream, const char ∗ __fmt, va_list __ap)

Variant of vfprintf() that uses a fmt string that resides in program memory.

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63

int vfscanf (FILE ∗ __stream, const char ∗ __fmt, va_list __ap)

Formatted input. This function is the heart of the scanf family of functions.
Characters are read from stream and processed in a way described by fmt. Conversion results will be assigned to the parameters passed via ap.
The format string fmt is scanned for conversion specifications. Anything that doesn’t
comprise a conversion specification is taken as text that is matched literally against
the input. White space in the format string will match any white space in the data
(including none), all other characters match only itself. Processing is aborted as soon as
the data and format string no longer match, or there is an error or end-of-file condition
on stream.
Most conversions skip leading white space before starting the actual conversion.
Conversions are introduced with the character %. Possible options can follow the %:
• a ∗ indicating that the conversion should be performed but the conversion result
is to be discarded; no parameters will be processed from ap,
• the character h indicating that the argument is a pointer to short int (rather
than int),
• the character l indicating that the argument is a pointer to long int (rather
than int, for integer type conversions), or a pointer to double (for floating
point conversions).
In addition, a maximal field width may be specified as a nonzero positive decimal
integer, which will restrict the conversion to at most this many characters from the
input stream. This field width is limited to at most 127 characters which is also the
default value (except for the c conversion that defaults to 1).
The following conversion flags are supported:
• % Matches a literal % character. This is not a conversion.
• d Matches an optionally signed decimal integer; the next pointer must be a
pointer to int.
• i Matches an optionally signed integer; the next pointer must be a pointer to
int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it
begins with 0, and in base 10 otherwise. Only characters that correspond to the
base are used.
• o Matches an octal integer; the next pointer must be a pointer to unsigned
int.
• u Matches an optionally signed decimal integer; the next pointer must be a
pointer to unsigned int.

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6.8 : Standard IO facilities

64

• x Matches an optionally signed hexadecimal integer; the next pointer must be a
pointer to unsigned int.
• f Matches an optionally signed floating-point number; the next pointer must be
a pointer to float.
• e, g, E, G Equivalent to f.
• s Matches a sequence of non-white-space characters; the next pointer must be a
pointer to char, and the array must be large enough to accept all the sequence
and the terminating NUL character. The input string stops at white space or at the
maximum field width, whichever occurs first.
• c Matches a sequence of width count characters (default 1); the next pointer must
be a pointer to char, and there must be enough room for all the characters (no
terminating NUL is added). The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.
• [ Matches a nonempty sequence of characters from the specified set of accepted
characters; the next pointer must be a pointer to char, and there must be enough
room for all the characters in the string, plus a terminating NUL character. The
usual skip of leading white space is suppressed. The string is to be made up
of characters in (or not in) a particular set; the set is defined by the characters
between the open bracket [ character and a close bracket ] character. The set
excludes those characters if the first character after the open bracket is a circumflex ∧ . To include a close bracket in the set, make it the first character after the
open bracket or the circumflex; any other position will end the set. The hyphen
character - is also special; when placed between two other characters, it adds all
intervening characters to the set. To include a hyphen, make it the last character
before the final close bracket. For instance, [∧ ]0-9-] means the set of everything except close bracket, zero through nine, and hyphen. The string ends with
the appearance of a character not in the (or, with a circumflex, in) set or when
the field width runs out.
• p Matches a pointer value (as printed by p in printf()); the next pointer must be
a pointer to void.
• n Nothing is expected; instead, the number of characters consumed thus far from
the input is stored through the next pointer, which must be a pointer to int. This
is not a conversion, although it can be suppressed with the ∗ flag.
These functions return the number of input items assigned, which can be fewer than
provided for, or even zero, in the event of a matching failure. Zero indicates that, while
there was input available, no conversions were assigned; typically this is due to an
invalid input character, such as an alphabetic character for a d conversion. The value
EOF is returned if an input failure occurs before any conversion such as an end-of-file
occurs. If an error or end-of-file occurs after conversion has begun, the number of
conversions which were successfully completed is returned.
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By default, all the conversions described above are available except the floating-point
conversions, and the %[ conversion. These conversions will be available in the extended version provided by the library libscanf_flt.a. Note that these conversions require a 40-byte conversion buffer, so the extended version requires more stack
space than the basic version irrespective of whether the actual call in progress actually uses this buffer or not. To link a program against the extended version, use the
following compiler flags in the link stage:
-Wl,-u,vfscanf -lscanf_flt -lm

A third version is available for environments that are tight on space. This version is
provided in the library libscanf_min.a, and can be requested using the following
options in the link stage:
-Wl,-u,vfscanf -lscanf_min -lm

In addition to the restrictions of the standard version, this version implements no field
width specification, no conversion assignment suppression flag (∗), no n specification,
and no general format character matching at all. All characters in fmt that do not
comprise a conversion specification will simply be ignored, including white space (that
is normally used to consume any amount of white space in the input stream). However,
the usual skip of initial white space in the formats that support it is implemented.
6.8.3.35

int vfscanf_P (FILE ∗ __stream, const char ∗ __fmt, va_list __ap)

Variant of vfscanf() using a fmt string in program memory.
6.8.3.36

int vprintf (const char ∗ __fmt, va_list __ap)

The function vprintf performs formatted output to stream stdout, taking a variable argument list as in vfprintf().
See vfprintf() for details.
6.8.3.37

int vscanf (const char ∗ __fmt, va_list __ap)

The function vscanf performs formatted input from stream stdin, taking a variable
argument list as in vfscanf().
See vfscanf() for details.
6.8.3.38

int vsnprintf (char ∗ __s, size_t __n, const char ∗ __fmt, va_list ap)

Like vsprintf(), but instead of assuming s to be of infinite size, no more than n
characters (including the trailing NUL character) will be converted to s.
Returns the number of characters that would have been written to s if there were
enough space.
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66

int vsnprintf_P (char ∗ __s, size_t __n, const char ∗ __fmt, va_list ap)

Variant of vsnprintf() that uses a fmt string that resides in program memory.
6.8.3.40

int vsprintf (char ∗ __s, const char ∗ __fmt, va_list ap)

Like sprintf() but takes a variable argument list for the arguments.
6.8.3.41

int vsprintf_P (char ∗ __s, const char ∗ __fmt, va_list ap)

Variant of vsprintf() that uses a fmt string that resides in program memory.

: General utilities

6.9
6.9.1

Detailed Description

#include 

This file declares some basic C macros and functions as defined by the ISO standard,
plus some AVR-specific extensions.
Data Structures
• struct div_t
• struct ldiv_t
Non-standard (i.e. non-ISO C) functions.
•
•
•
•
•
•
•
•

#define RANDOM_MAX 0x7FFFFFFF
char ∗ ltoa (long int __val, char ∗__s, int __radix)
char ∗ utoa (unsigned int __val, char ∗__s, int __radix)
char ∗ ultoa (unsigned long int __val, char ∗__s, int __radix)
long random (void)
void srandom (unsigned long __seed)
long random_r (unsigned long ∗ctx)
char ∗ itoa (int __val, char ∗__s, int __radix)

Conversion functions for double arguments.
Note that these functions are not located in the default library, libc.a, but in the
mathematical library, libm.a. So when linking the application, the -lm option needs
to be specified.

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6.9 : General utilities

67

•
•
•
•

#define DTOSTR_ALWAYS_SIGN 0x01
#define DTOSTR_PLUS_SIGN 0x02
#define DTOSTR_UPPERCASE 0x04
char ∗ dtostre (double __val, char ∗__s, unsigned char __prec, unsigned char
__flags)
• char ∗ dtostrf (double __val, char __width, char __prec, char ∗__s)
Defines
• #define RAND_MAX 0x7FFF
Typedefs
• typedef int(∗) __compar_fn_t (const void ∗, const void ∗)
Functions
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

__inline__ void abort (void) __ATTR_NORETURN__
int abs (int __i) __ATTR_CONST__
long labs (long __i) __ATTR_CONST__
void ∗ bsearch (const void ∗__key, const void ∗__base, size_t __nmemb, size_t
__size, int(∗__compar)(const void ∗, const void ∗))
div_t div (int __num, int __denom) __asm__("__divmodhi4") __ATTR_CONST__
ldiv_t ldiv (long __num, long __denom) __asm__("__divmodsi4") __ATTR_CONST__
void qsort (void ∗__base, size_t __nmemb, size_t __size, __compar_fn_t __compar)
long strtol (const char ∗__nptr, char ∗∗__endptr, int __base)
unsigned long strtoul (const char ∗__nptr, char ∗∗__endptr, int __base)
long atol (const char ∗s) __ATTR_PURE__
int atoi (const char ∗s) __ATTR_PURE__
void exit (int __status) __ATTR_NORETURN__
void ∗ malloc (size_t __size) __ATTR_MALLOC__
void free (void ∗__ptr)
void ∗ calloc (size_t __nele, size_t __size) __ATTR_MALLOC__
void ∗ realloc (void ∗__ptr, size_t __size) __ATTR_MALLOC__
double strtod (const char ∗__nptr, char ∗∗__endptr)
double atof (const char ∗__nptr)
int rand (void)
void srand (unsigned int __seed)
int rand_r (unsigned long ∗ctx)

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Variables
• size_t __malloc_margin
• char ∗ __malloc_heap_start
• char ∗ __malloc_heap_end
6.9.2
6.9.2.1

Define Documentation
#define DTOSTR_ALWAYS_SIGN 0x01

Bit value that can be passed in flags to dtostre().
6.9.2.2

#define DTOSTR_PLUS_SIGN 0x02

Bit value that can be passed in flags to dtostre().
6.9.2.3

#define DTOSTR_UPPERCASE 0x04

Bit value that can be passed in flags to dtostre().
6.9.2.4

#define RAND_MAX 0x7FFF

Highest number that can be generated by rand().
6.9.2.5

#define RANDOM_MAX 0x7FFFFFFF

Highest number that can be generated by random().
6.9.3
6.9.3.1

Typedef Documentation
typedef int(∗) __compar_fn_t(const void ∗, const void ∗)

Comparision function type for qsort(), just for convenience.
6.9.4
6.9.4.1

Function Documentation
__inline__ void abort (void)

The abort() function causes abnormal program termination to occur. In the limited
AVR environment, execution is effectively halted by entering an infinite loop.
6.9.4.2

int abs (int __i)

The abs() function computes the absolute value of the integer i.

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69

Note:
The abs() and labs() functions are builtins of gcc.

6.9.4.3

double atof (const char ∗ __nptr)

The atof() function converts the initial portion of the string pointed to by nptr to
double representation.
It is equivalent to calling
strtod(nptr, (char **)NULL);

6.9.4.4

int atoi (const char ∗ s)

Convert a string to an integer.
The atoi() function converts the initial portion of the string pointed to by s to integer
representation. In contrast to
(int)strtol(s, (char **)NULL, 10);

this function does not detect overflow (errno is not changed and the result value is
not predictable), uses smaller memory (flash and stack) and works more quickly.
6.9.4.5

long atol (const char ∗ s)

Convert a string to a long integer.
The atol() function converts the initial portion of the string pointed to by s to long
integer representation. In contrast to
strtol(s, (char **)NULL, 10);

this function does not detect overflow (errno is not changed and the result value is
not predictable), uses smaller memory (flash and stack) and works more quickly.
6.9.4.6 void∗ bsearch (const void ∗ __key, const void ∗ __base, size_t __nmemb,
size_t __size, int(∗)(const void ∗, const void ∗) __compar)
The bsearch() function searches an array of nmemb objects, the initial member of
which is pointed to by base, for a member that matches the object pointed to by
key. The size of each member of the array is specified by size.
The contents of the array should be in ascending sorted order according to the comparison function referenced by compar. The compar routine is expected to have two
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arguments which point to the key object and to an array member, in that order, and
should return an integer less than, equal to, or greater than zero if the key object is
found, respectively, to be less than, to match, or be greater than the array member.
The bsearch() function returns a pointer to a matching member of the array, or a null
pointer if no match is found. If two members compare as equal, which member is
matched is unspecified.
6.9.4.7

void∗ calloc (size_t __nele, size_t __size)

Allocate nele elements of size each. Identical to calling malloc() using nele
∗ size as argument, except the allocated memory will be cleared to zero.
6.9.4.8 div_t div (int __num, int __denom)
The div() function computes the value num/denom and returns the quotient and remainder in a structure named div_t that contains two int members named quot and
rem.
6.9.4.9 char∗ dtostre (double __val, char ∗ __s, unsigned char __prec, unsigned
char __flags)
The dtostre() function converts the double value passed in val into an ASCII representation that will be stored under s. The caller is responsible for providing sufficient
storage in s.
Conversion is done in the format "[-]d.dddeśdd" where there is one digit before
the decimal-point character and the number of digits after it is equal to the precision
prec; if the precision is zero, no decimal-point character appears. If flags has the
DTOSTRE_UPPERCASE bit set, the letter ’E’ (rather than ’e’ ) will be used to
introduce the exponent. The exponent always contains two digits; if the value is zero,
the exponent is "00".
If flags has the DTOSTRE_ALWAYS_SIGN bit set, a space character will be placed
into the leading position for positive numbers.
If flags has the DTOSTRE_PLUS_SIGN bit set, a plus sign will be used instead of
a space character in this case.
The dtostre() function returns the pointer to the converted string s.
6.9.4.10

char∗ dtostrf (double __val, char __width, char __prec, char ∗ __s)

The dtostrf() function converts the double value passed in val into an ASCII representationthat will be stored under s. The caller is responsible for providing sufficient
storage in s.
Conversion is done in the format "[-]d.ddd". The minimum field width of the

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output string (including the ’.’ and the possible sign for negative values) is given in
width, and prec determines the number of digits after the decimal sign.
The dtostrf() function returns the pointer to the converted string s.
6.9.4.11

void exit (int __status)

The exit() function terminates the application. Since there is no environment to return to, status is ignored, and code execution will eventually reach an infinite loop,
thereby effectively halting all code processing.
In a C++ context, global destructors will be called before halting execution.
6.9.4.12

void free (void ∗ __ptr)

The free() function causes the allocated memory referenced by ptr to be made available for future allocations. If ptr is NULL, no action occurs.
6.9.4.13

char∗ itoa (int __val, char ∗ __s, int __radix)

Convert an integer to a string.
The function itoa() converts the integer value from val into an ASCII representation
that will be stored under s. The caller is responsible for providing sufficient storage in
s.
Note:
The minimal size of the buffer s depends on the choice of radix. For example, if
the radix is 2 (binary), you need to supply a buffer with a minimal length of 8 ∗
sizeof (int) + 1 characters, i.e. one character for each bit plus one for the string
terminator. Using a larger radix will require a smaller minimal buffer size.
Warning:
If the buffer is too small, you risk a buffer overflow.
Conversion is done using the radix as base, which may be a number between 2
(binary conversion) and up to 36. If radix is greater than 10, the next digit after
’9’ will be the letter ’a’.
If radix is 10 and val is negative, a minus sign will be prepended.
The itoa() function returns the pointer passed as s.
6.9.4.14

long labs (long __i)

The labs() function computes the absolute value of the long integer i.

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Note:
The abs() and labs() functions are builtins of gcc.

6.9.4.15 ldiv_t ldiv (long __num, long __denom)
The ldiv() function computes the value num/denom and returns the quotient and remainder in a structure named ldiv_t that contains two long integer members named
quot and rem.
6.9.4.16

char∗ ltoa (long int __val, char ∗ __s, int __radix)

Convert a long integer to a string.
The function ltoa() converts the long integer value from val into an ASCII representation that will be stored under s. The caller is responsible for providing sufficient
storage in s.
Note:
The minimal size of the buffer s depends on the choice of radix. For example,
if the radix is 2 (binary), you need to supply a buffer with a minimal length of 8
∗ sizeof (long int) + 1 characters, i.e. one character for each bit plus one for the
string terminator. Using a larger radix will require a smaller minimal buffer size.
Warning:
If the buffer is too small, you risk a buffer overflow.
Conversion is done using the radix as base, which may be a number between 2
(binary conversion) and up to 36. If radix is greater than 10, the next digit after
’9’ will be the letter ’a’.
If radix is 10 and val is negative, a minus sign will be prepended.
The ltoa() function returns the pointer passed as s.
6.9.4.17

void∗ malloc (size_t __size)

The malloc() function allocates size bytes of memory. If malloc() fails, a NULL
pointer is returned.
Note that malloc() does not initialize the returned memory to zero bytes.
See the chapter about malloc() usage for implementation details.
6.9.4.18 void qsort (void ∗ __base, size_t __nmemb, size_t __size, __compar_fn_t
__compar)
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The qsort() function is a modified partition-exchange sort, or quicksort.
The qsort() function sorts an array of nmemb objects, the initial member of which is
pointed to by base. The size of each object is specified by size. The contents of the
array base are sorted in ascending order according to a comparison function pointed to
by compar, which requires two arguments pointing to the objects being compared.
The comparison function must return an integer less than, equal to, or greater than zero
if the first argument is considered to be respectively less than, equal to, or greater than
the second.
6.9.4.19

int rand (void)

The rand() function computes a sequence of pseudo-random integers in the range of 0
to RAND_MAX (as defined by the header file ).
The srand() function sets its argument seed as the seed for a new sequence of pseudorandom numbers to be returned by rand(). These sequences are repeatable by calling
srand() with the same seed value.
If no seed value is provided, the functions are automatically seeded with a value of 1.
In compliance with the C standard, these functions operate on int arguments. Since
the underlying algorithm already uses 32-bit calculations, this causes a loss of precision. See random() for an alternate set of functions that retains full 32-bit precision.

6.9.4.20

int rand_r (unsigned long ∗ ctx)

Variant of rand() that stores the context in the user-supplied variable located at ctx
instead of a static library variable so the function becomes re-entrant.
6.9.4.21

long random (void)

The random() function computes a sequence of pseudo-random integers in the range of
0 to RANDOM_MAX (as defined by the header file ).
The srandom() function sets its argument seed as the seed for a new sequence of
pseudo-random numbers to be returned by rand(). These sequences are repeatable by
calling srandom() with the same seed value.
If no seed value is provided, the functions are automatically seeded with a value of 1.
6.9.4.22

long random_r (unsigned long ∗ ctx)

Variant of random() that stores the context in the user-supplied variable located at ctx
instead of a static library variable so the function becomes re-entrant.

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74

void∗ realloc (void ∗ __ptr, size_t __size)

The realloc() function tries to change the size of the region allocated at ptr to the new
size value. It returns a pointer to the new region. The returned pointer might be the
same as the old pointer, or a pointer to a completely different region.
The contents of the returned region up to either the old or the new size value (whatever
is less) will be identical to the contents of the old region, even in case a new region had
to be allocated.
It is acceptable to pass ptr as NULL, in which case realloc() will behave identical to
malloc().
If the new memory cannot be allocated, realloc() returns NULL, and the region at ptr
will not be changed.
6.9.4.24

void srand (unsigned int __seed)

Pseudo-random number generator seeding; see rand().
6.9.4.25

void srandom (unsigned long __seed)

Pseudo-random number generator seeding; see random().
6.9.4.26

double strtod (const char ∗ __nptr, char ∗∗ __endptr)

The strtod() function converts the initial portion of the string pointed to by nptr to
double representation.
The expected form of the string is an optional plus ( ’+’ ) or minus sign ( ’-’ )
followed by a sequence of digits optionally containing a decimal-point character, optionally followed by an exponent. An exponent consists of an ’E’ or ’e’, followed
by an optional plus or minus sign, followed by a sequence of digits.
Leading white-space characters in the string are skipped.
The strtod() function returns the converted value, if any.
If endptr is not NULL, a pointer to the character after the last character used in the
conversion is stored in the location referenced by endptr.
If no conversion is performed, zero is returned and the value of nptr is stored in the
location referenced by endptr.
If the correct value would cause overflow, plus or minus HUGE_VAL is returned (according to the sign of the value), and ERANGE is stored in errno. If the correct value
would cause underflow, zero is returned and ERANGE is stored in errno.
FIXME: HUGE_VAL needs to be defined somewhere. The bit pattern is 0x7fffffff, but
what number would this be?

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75

long strtol (const char ∗ __nptr, char ∗∗ __endptr, int __base)

The strtol() function converts the string in nptr to a long value. The conversion is
done according to the given base, which must be between 2 and 36 inclusive, or be the
special value 0.
The string may begin with an arbitrary amount of white space (as determined by isspace()) followed by a single optional ’+’ or ’-’ sign. If base is zero or 16, the string
may then include a "0x" prefix, and the number will be read in base 16; otherwise, a
zero base is taken as 10 (decimal) unless the next character is ’0’, in which case it is
taken as 8 (octal).
The remainder of the string is converted to a long value in the obvious manner, stopping
at the first character which is not a valid digit in the given base. (In bases above 10, the
letter ’A’ in either upper or lower case represents 10, ’B’ represents 11, and so forth,
with ’Z’ representing 35.)
If endptr is not NULL, strtol() stores the address of the first invalid character in
∗endptr. If there were no digits at all, however, strtol() stores the original value of
nptr in endptr. (Thus, if ∗nptr is not ’\0’ but ∗∗endptr is ’\0’ on return, the
entire string was valid.)
The strtol() function returns the result of the conversion, unless the value would underflow or overflow. If no conversion could be performed, 0 is returned. If an overflow or
underflow occurs, errno is set to ERANGE and the function return value is clamped
to LONG_MIN or LONG_MAX, respectively.
6.9.4.28

unsigned long strtoul (const char ∗ __nptr, char ∗∗ __endptr, int __base)

The strtoul() function converts the string in nptr to an unsigned long value. The conversion is done according to the given base, which must be between 2 and 36 inclusive,
or be the special value 0.
The string may begin with an arbitrary amount of white space (as determined by isspace()) followed by a single optional ’+’ or ’-’ sign. If base is zero or 16, the string
may then include a "0x" prefix, and the number will be read in base 16; otherwise, a
zero base is taken as 10 (decimal) unless the next character is ’0’, in which case it is
taken as 8 (octal).
The remainder of the string is converted to an unsigned long value in the obvious
manner, stopping at the first character which is not a valid digit in the given base.
(In bases above 10, the letter ’A’ in either upper or lower case represents 10, ’B’
represents 11, and so forth, with ’Z’ representing 35.)
If endptr is not NULL, strtoul() stores the address of the first invalid character in
∗endptr. If there were no digits at all, however, strtoul() stores the original value of
nptr in endptr. (Thus, if ∗nptr is not ’\0’ but ∗∗endptr is ’\0’ on return, the
entire string was valid.)

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The strtoul() function return either the result of the conversion or, if there was a leading minus sign, the negation of the result of the conversion, unless the original (nonnegated) value would overflow; in the latter case, strtoul() returns ULONG_MAX, and
errno is set to ERANGE. If no conversion could be performed, 0 is returned.
6.9.4.29

char∗ ultoa (unsigned long int __val, char ∗ __s, int __radix)

Convert an unsigned long integer to a string.
The function ultoa() converts the unsigned long integer value from val into an ASCII
representation that will be stored under s. The caller is responsible for providing sufficient storage in s.
Note:
The minimal size of the buffer s depends on the choice of radix. For example, if
the radix is 2 (binary), you need to supply a buffer with a minimal length of 8 ∗
sizeof (unsigned long int) + 1 characters, i.e. one character for each bit plus one
for the string terminator. Using a larger radix will require a smaller minimal buffer
size.
Warning:
If the buffer is too small, you risk a buffer overflow.
Conversion is done using the radix as base, which may be a number between 2
(binary conversion) and up to 36. If radix is greater than 10, the next digit after
’9’ will be the letter ’a’.
The ultoa() function returns the pointer passed as s.
6.9.4.30

char∗ utoa (unsigned int __val, char ∗ __s, int __radix)

Convert an unsigned integer to a string.
The function utoa() converts the unsigned integer value from val into an ASCII representation that will be stored under s. The caller is responsible for providing sufficient
storage in s.
Note:
The minimal size of the buffer s depends on the choice of radix. For example, if
the radix is 2 (binary), you need to supply a buffer with a minimal length of 8 ∗
sizeof (unsigned int) + 1 characters, i.e. one character for each bit plus one for the
string terminator. Using a larger radix will require a smaller minimal buffer size.
Warning:
If the buffer is too small, you risk a buffer overflow.
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Conversion is done using the radix as base, which may be a number between 2
(binary conversion) and up to 36. If radix is greater than 10, the next digit after
’9’ will be the letter ’a’.
The utoa() function returns the pointer passed as s.
6.9.5

Variable Documentation

6.9.5.1

char∗ __malloc_heap_end

malloc() tunable.
6.9.5.2

char∗ __malloc_heap_start

malloc() tunable.
6.9.5.3

size_t __malloc_margin

malloc() tunable.

6.10

: Strings

6.10.1

Detailed Description

#include 

The string functions perform string operations on NULL terminated strings.
Note:
If the strings you are working on resident in program space (flash), you will need to
use the string functions described in : Program Space Utilities.
Defines
• #define _FFS(x)
Functions
•
•
•
•
•

int ffs (int) __attribute__((const ))
int ffsl (long) __attribute__((const ))
int ffsll (long long) __attribute__((const ))
void ∗ memccpy (void ∗, const void ∗, int, size_t)
void ∗ memchr (const void ∗, int, size_t) __ATTR_PURE__

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•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

78

int memcmp (const void ∗, const void ∗, size_t) __ATTR_PURE__
void ∗ memcpy (void ∗, const void ∗, size_t)
void ∗ memmem (const void ∗, size_t, const void ∗, size_t) __ATTR_PURE__
void ∗ memmove (void ∗, const void ∗, size_t)
void ∗ memrchr (const void ∗, int, size_t) __ATTR_PURE__
void ∗ memset (void ∗, int, size_t)
int strcasecmp (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strcasestr (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strcat (char ∗, const char ∗)
char ∗ strchr (const char ∗, int) __ATTR_PURE__
char ∗ strchrnul (const char ∗, int) __ATTR_PURE__
int strcmp (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strcpy (char ∗, const char ∗)
size_t strcspn (const char ∗s, const char ∗reject) __ATTR_PURE__
size_t strlcat (char ∗, const char ∗, size_t)
size_t strlcpy (char ∗, const char ∗, size_t)
size_t strlen (const char ∗) __ATTR_PURE__
char ∗ strlwr (char ∗)
int strncasecmp (const char ∗, const char ∗, size_t) __ATTR_PURE__
char ∗ strncat (char ∗, const char ∗, size_t)
int strncmp (const char ∗, const char ∗, size_t) __ATTR_PURE__
char ∗ strncpy (char ∗, const char ∗, size_t)
size_t strnlen (const char ∗, size_t) __ATTR_PURE__
char ∗ strpbrk (const char ∗s, const char ∗accept) __ATTR_PURE__
char ∗ strrchr (const char ∗, int) __ATTR_PURE__
char ∗ strrev (char ∗)
char ∗ strsep (char ∗∗, const char ∗)
size_t strspn (const char ∗s, const char ∗accept) __ATTR_PURE__
char ∗ strstr (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strtok_r (char ∗, const char ∗, char ∗∗)
char ∗ strupr (char ∗)

6.10.2
6.10.2.1

Define Documentation
#define _FFS(x)

This macro finds the first (least significant) bit set in the input value.
This macro is very similar to the function ffs() except that it evaluates its argument at
compile-time, so it should only be applied to compile-time constant expressions where
it will reduce to a constant itself. Application of this macro to expressions that are not
constant at compile-time is not recommended, and might result in a huge amount of
code generated.

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Returns:
The _FFS() macro returns the position of the first (least significant) bit set in the
word val, or 0 if no bits are set. The least significant bit is position 1.
6.10.3

Function Documentation

6.10.3.1

int ffs (int val) const

This function finds the first (least significant) bit set in the input value.
Returns:
The ffs() function returns the position of the first (least significant) bit set in the
word val, or 0 if no bits are set. The least significant bit is position 1.
Note:
For expressions that are constant at compile time, consider using the _FFS macro
instead.

6.10.3.2

int ffsl (long) const

Same as ffs(), for an argument of type long.
6.10.3.3

int ffsll (long long) const

Same as ffs(), for an argument of type long long.
6.10.3.4

void ∗ memccpy (void ∗ dest, const void ∗ src, int val, size_t len)

Copy memory area.
The memccpy() function copies no more than len bytes from memory area src to memory area dest, stopping when the character val is found.
Returns:
The memccpy() function returns a pointer to the next character in dest after val, or
NULL if val was not found in the first len characters of src.

6.10.3.5

void ∗ memchr (const void ∗ src, int val, size_t len)

Scan memory for a character.
The memchr() function scans the first len bytes of the memory area pointed to by src
for the character val. The first byte to match val (interpreted as an unsigned character)
stops the operation.
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Returns:
The memchr() function returns a pointer to the matching byte or NULL if the
character does not occur in the given memory area.

6.10.3.6

int memcmp (const void ∗ s1, const void ∗ s2, size_t len)

Compare memory areas.
The memcmp() function compares the first len bytes of the memory areas s1 and s2.
The comparision is performed using unsigned char operations.
Returns:
The memcmp() function returns an integer less than, equal to, or greater than zero
if the first len bytes of s1 is found, respectively, to be less than, to match, or be
greater than the first len bytes of s2.
Note:
Be sure to store the result in a 16 bit variable since you may get incorrect results if
you use an unsigned char or char due to truncation.
Warning:
This function is not -mint8 compatible, although if you only care about testing for
equality, this function should be safe to use.

6.10.3.7

void ∗ memcpy (void ∗ dest, const void ∗ src, size_t len)

Copy a memory area.
The memcpy() function copies len bytes from memory area src to memory area dest.
The memory areas may not overlap. Use memmove() if the memory areas do overlap.
Returns:
The memcpy() function returns a pointer to dest.

6.10.3.8

void ∗ memmem (const void ∗ s1, size_t len1, const void ∗ s2, size_t len2)

The memmem() function finds the start of the first occurrence of the substring s2 of
length len2 in the memory area s1 of length len1.
Returns:
The memmem() function returns a pointer to the beginning of the substring, or
NULL if the substring is not found. If len2 is zero, the function returns s1.

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81

void ∗ memmove (void ∗ dest, const void ∗ src, size_t len)

Copy memory area.
The memmove() function copies len bytes from memory area src to memory area dest.
The memory areas may overlap.
Returns:
The memmove() function returns a pointer to dest.

6.10.3.10

void ∗ memrchr (const void ∗ src, int val, size_t len)

The memrchr() function is like the memchr() function, except that it searches backwards from the end of the len bytes pointed to by src instead of forwards from the
front. (Glibc, GNU extension.)
Returns:
The memrchr() function returns a pointer to the matching byte or NULL if the
character does not occur in the given memory area.

6.10.3.11

void ∗ memset (void ∗ dest, int val, size_t len)

Fill memory with a constant byte.
The memset() function fills the first len bytes of the memory area pointed to by dest
with the constant byte val.
Returns:
The memset() function returns a pointer to the memory area dest.

6.10.3.12

int strcasecmp (const char ∗ s1, const char ∗ s2)

Compare two strings ignoring case.
The strcasecmp() function compares the two strings s1 and s2, ignoring the case of
the characters.
Returns:
The strcasecmp() function returns an integer less than, equal to, or greater than
zero if s1 is found, respectively, to be less than, to match, or be greater than
s2. A consequence of the ordering used by strcasecmp() is that if s1 is an initial
substring of s2, then s1 is considered to be "less than" s2.

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82

char ∗ strcasestr (const char ∗ s1, const char ∗ s2)

The strcasestr() function finds the first occurrence of the substring s2 in the string s1.
This is like strstr(), except that it ignores case of alphabetic symbols in searching for
the substring. (Glibc, GNU extension.)
Returns:
The strcasestr() function returns a pointer to the beginning of the substring, or
NULL if the substring is not found. If s2 points to a string of zero length, the
function returns s1.

6.10.3.14

char ∗ strcat (char ∗ dest, const char ∗ src)

Concatenate two strings.
The strcat() function appends the src string to the dest string overwriting the ’\0’ character at the end of dest, and then adds a terminating ’\0’ character. The strings may not
overlap, and the dest string must have enough space for the result.
Returns:
The strcat() function returns a pointer to the resulting string dest.

6.10.3.15

char ∗ strchr (const char ∗ src, int val)

Locate character in string.
The strchr() function returns a pointer to the first occurrence of the character val in
the string src.
Here "character" means "byte" - these functions do not work with wide or multi-byte
characters.
Returns:
The strchr() function returns a pointer to the matched character or NULL if the
character is not found.

6.10.3.16

char ∗ strchrnul (const char ∗ s, int c)

The strchrnul() function is like strchr() except that if c is not found in s, then it returns
a pointer to the null byte at the end of s, rather than NULL. (Glibc, GNU extension.)
Returns:
The strchrnul() function returns a pointer to the matched character, or a pointer to
the null byte at the end of s (i.e., s+strlen(s)) if the character is not found.

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83

int strcmp (const char ∗ s1, const char ∗ s2)

Compare two strings.
The strcmp() function compares the two strings s1 and s2.
Returns:
The strcmp() function returns an integer less than, equal to, or greater than zero
if s1 is found, respectively, to be less than, to match, or be greater than s2. A
consequence of the ordering used by strcmp() is that if s1 is an initial substring of
s2, then s1 is considered to be "less than" s2.

6.10.3.18

char ∗ strcpy (char ∗ dest, const char ∗ src)

Copy a string.
The strcpy() function copies the string pointed to by src (including the terminating
’\0’ character) to the array pointed to by dest. The strings may not overlap, and the
destination string dest must be large enough to receive the copy.
Returns:
The strcpy() function returns a pointer to the destination string dest.
Note:
If the destination string of a strcpy() is not large enough (that is, if the programmer
was stupid/lazy, and failed to check the size before copying) then anything might
happen. Overflowing fixed length strings is a favourite cracker technique.

6.10.3.19

size_t strcspn (const char ∗ s, const char ∗ reject)

The strcspn() function calculates the length of the initial segment of s which consists
entirely of characters not in reject.
Returns:
The strcspn() function returns the number of characters in the initial segment of s
which are not in the string reject. The terminating zero is not considered as a
part of string.

6.10.3.20

size_t strlcat (char ∗ dst, const char ∗ src, size_t siz)

Concatenate two strings.
Appends src to string dst of size siz (unlike strncat(), siz is the full size of dst, not space
left). At most siz-1 characters will be copied. Always NULL terminates (unless siz <=
strlen(dst)).
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Returns:
The strlcat() function returns strlen(src) + MIN(siz, strlen(initial dst)). If retval >=
siz, truncation occurred.

6.10.3.21

size_t strlcpy (char ∗ dst, const char ∗ src, size_t siz)

Copy a string.
Copy src to string dst of size siz. At most siz-1 characters will be copied. Always
NULL terminates (unless siz == 0).
Returns:
The strlcpy() function returns strlen(src). If retval >= siz, truncation occurred.

6.10.3.22

size_t strlen (const char ∗ src)

Calculate the length of a string.
The strlen() function calculates the length of the string src, not including the terminating ’\0’ character.
Returns:
The strlen() function returns the number of characters in src.

6.10.3.23

char ∗ strlwr (char ∗ s)

Convert a string to lower case.
The strlwr() function will convert a string to lower case. Only the upper case alphabetic
characters [A .. Z] are converted. Non-alphabetic characters will not be changed.
Returns:
The strlwr() function returns a pointer to the converted string.

6.10.3.24

int strncasecmp (const char ∗ s1, const char ∗ s2, size_t len)

Compare two strings ignoring case.
The strncasecmp() function is similar to strcasecmp(), except it only compares the first
len characters of s1.

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85

Returns:
The strncasecmp() function returns an integer less than, equal to, or greater than
zero if s1 (or the first len bytes thereof) is found, respectively, to be less than, to
match, or be greater than s2. A consequence of the ordering used by strncasecmp()
is that if s1 is an initial substring of s2, then s1 is considered to be "less than"
s2.

6.10.3.25

char ∗ strncat (char ∗ dest, const char ∗ src, size_t len)

Concatenate two strings.
The strncat() function is similar to strcat(), except that only the first n characters of src
are appended to dest.
Returns:
The strncat() function returns a pointer to the resulting string dest.

6.10.3.26

int strncmp (const char ∗ s1, const char ∗ s2, size_t len)

Compare two strings.
The strncmp() function is similar to strcmp(), except it only compares the first (at most)
n characters of s1 and s2.
Returns:
The strncmp() function returns an integer less than, equal to, or greater than zero
if s1 (or the first n bytes thereof) is found, respectively, to be less than, to match,
or be greater than s2.

6.10.3.27

char ∗ strncpy (char ∗ dest, const char ∗ src, size_t len)

Copy a string.
The strncpy() function is similar to strcpy(), except that not more than n bytes of src
are copied. Thus, if there is no null byte among the first n bytes of src, the result will
not be null-terminated.
In the case where the length of src is less than that of n, the remainder of dest will be
padded with nulls.
Returns:
The strncpy() function returns a pointer to the destination string dest.

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6.10 : Strings

6.10.3.28

86

size_t strnlen (const char ∗ src, size_t len)

Determine the length of a fixed-size string.
The strnlen function returns the number of characters in the string pointed to by src, not
including the terminating ’\0’ character, but at most len. In doing this, strnlen looks
only at the first len characters at src and never beyond src+len.
Returns:
The strnlen function returns strlen(src), if that is less than len, or len if there is no
’\0’ character among the first len characters pointed to by src.

6.10.3.29

char ∗ strpbrk (const char ∗ s, const char ∗ accept)

The strpbrk() function locates the first occurrence in the string s of any of the characters
in the string accept.
Returns:
The strpbrk() function returns a pointer to the character in s that matches one of
the characters in accept, or NULL if no such character is found. The terminating
zero is not considered as a part of string: if one or both args are empty, the result
will NULL.

6.10.3.30

char ∗ strrchr (const char ∗ src, int val)

Locate character in string.
The strrchr() function returns a pointer to the last occurrence of the character val in the
string src.
Here "character" means "byte" - these functions do not work with wide or multi-byte
characters.
Returns:
The strrchr() function returns a pointer to the matched character or NULL if the
character is not found.

6.10.3.31

char ∗ strrev (char ∗ s)

Reverse a string.
The strrev() function reverses the order of the string.
Returns:
The strrev() function returns a pointer to the beginning of the reversed string.

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6.10.3.32

87

char ∗ strsep (char ∗∗ sp, const char ∗ delim)

Parse a string into tokens.
The strsep() function locates, in the string referenced by ∗sp, the first occurrence of
any character in the string delim (or the terminating ’\0’ character) and replaces it
with a ’\0’. The location of the next character after the delimiter character (or NULL,
if the end of the string was reached) is stored in ∗sp. An “empty” field, i.e. one
caused by two adjacent delimiter characters, can be detected by comparing the location
referenced by the pointer returned in ∗sp to ’\0’.
Returns:
The strsep() function returns a pointer to the original value of ∗sp. If ∗sp is
initially NULL, strsep() returns NULL.

6.10.3.33

size_t strspn (const char ∗ s, const char ∗ accept)

The strspn() function calculates the length of the initial segment of s which consists
entirely of characters in accept.
Returns:
The strspn() function returns the number of characters in the initial segment of
s which consist only of characters from accept. The terminating zero is not
considered as a part of string.

6.10.3.34

char ∗ strstr (const char ∗ s1, const char ∗ s2)

Locate a substring.
The strstr() function finds the first occurrence of the substring s2 in the string s1. The
terminating ’\0’ characters are not compared.
Returns:
The strstr() function returns a pointer to the beginning of the substring, or NULL
if the substring is not found. If s2 points to a string of zero length, the function
returns s1.

6.10.3.35

char ∗ strtok_r (char ∗ string, const char ∗ delim, char ∗∗ last)

Parses the string s into tokens.
strtok_r parses the string s into tokens. The first call to strtok_r should have string as
its first argument. Subsequent calls should have the first argument set to NULL. If a
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88

token ends with a delimiter, this delimiting character is overwritten with a ’\0’ and a
pointer to the next character is saved for the next call to strtok_r. The delimiter string
delim may be different for each call. last is a user allocated char∗ pointer. It must be
the same while parsing the same string. strtok_r is a reentrant version of strtok().
Returns:
The strtok_r() function returns a pointer to the next token or NULL when no more
tokens are found.

6.10.3.36

char ∗ strupr (char ∗ s)

Convert a string to upper case.
The strupr() function will convert a string to upper case. Only the lower case alphabetic
characters [a .. z] are converted. Non-alphabetic characters will not be changed.
Returns:
The strupr() function returns a pointer to the converted string. The pointer is the
same as that passed in since the operation is perform in place.

6.11

: Bootloader Support Utilities

6.11.1

Detailed Description

#include 
#include 

The macros in this module provide a C language interface to the bootloader support
functionality of certain AVR processors. These macros are designed to work with all
sizes of flash memory.
Global interrupts are not automatically disabled for these macros. It is left up to the
programmer to do this. See the code example below. Also see the processor datasheet
for caveats on having global interrupts enabled during writing of the Flash.
Note:
Not all AVR processors provide bootloader support. See your processor datasheet
to see if it provides bootloader support.
Todo
From email with Marek: On smaller devices (all except ATmega64/128), __SPM_REG is in the I/O space, accessible with the shorter "in" and "out" instructions since the boot loader has a limited size, this could be an important optimization.
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89

API Usage Example
The following code shows typical usage of the boot API.
#include 
#include 
#include 
void boot_program_page (uint32_t page, uint8_t *buf)
{
uint16_t i;
uint8_t sreg;
// Disable interrupts.
sreg = SREG;
cli();
eeprom_busy_wait ();
boot_page_erase (page);
boot_spm_busy_wait ();

// Wait until the memory is erased.

for (i=0; i: Bootloader Support Utilities

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

90

#define boot_spm_busy() (__SPM_REG & (uint8_t)_BV(SPMEN))
#define boot_spm_busy_wait() do{}while(boot_spm_busy())
#define GET_LOW_FUSE_BITS (0x0000)
#define GET_LOCK_BITS (0x0001)
#define GET_EXTENDED_FUSE_BITS (0x0002)
#define GET_HIGH_FUSE_BITS (0x0003)
#define boot_lock_fuse_bits_get(address)
#define boot_page_fill(address, data) __boot_page_fill_normal(address, data)
#define boot_page_erase(address) __boot_page_erase_normal(address)
#define boot_page_write(address) __boot_page_write_normal(address)
#define boot_rww_enable() __boot_rww_enable()
#define boot_lock_bits_set(lock_bits) __boot_lock_bits_set(lock_bits)
#define boot_page_fill_safe(address, data)
#define boot_page_erase_safe(address)
#define boot_page_write_safe(address)
#define boot_rww_enable_safe()
#define boot_lock_bits_set_safe(lock_bits)

6.11.2

Define Documentation

6.11.2.1 #define
BV(SPMIE))

boot_is_spm_interrupt()

(__SPM_REG

&

(uint8_t)_-

Check if the SPM interrupt is enabled.
6.11.2.2

#define boot_lock_bits_set(lock_bits) __boot_lock_bits_set(lock_bits)

Set the bootloader lock bits.
Parameters:
lock_bits A mask of which Boot Loader Lock Bits to set.
Note:
In this context, a ’set bit’ will be written to a zero value. Note also that only BLBxx
bits can be programmed by this command.
For example, to disallow the SPM instruction from writing to the Boot Loader memory
section of flash, you would use this macro as such:
boot_lock_bits_set (_BV (BLB11));

Note:
Like any lock bits, the Boot Loader Lock Bits, once set, cannot be cleared again
except by a chip erase which will in turn also erase the boot loader itself.

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91

#define boot_lock_bits_set_safe(lock_bits)

Value:
do { \
boot_spm_busy_wait();
eeprom_busy_wait();
boot_lock_bits_set (lock_bits);
} while (0)

\
\
\

Same as boot_lock_bits_set() except waits for eeprom and spm operations to complete
before setting the lock bits.
6.11.2.4

#define boot_lock_fuse_bits_get(address)

Value:
(__extension__({
uint8_t __result;
__asm__ __volatile__
(
"ldi r30, %3\n\t"
"ldi r31, 0\n\t"
"sts %1, %2\n\t"
"lpm %0, Z\n\t"
: "=r" (__result)
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_LOCK_BITS_SET),
"M" (address)
: "r0", "r30", "r31"
);
__result;
}))

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

Read the lock or fuse bits at address.
Parameter address can be any of GET_LOW_FUSE_BITS, GET_LOCK_BITS,
GET_EXTENDED_FUSE_BITS, or GET_HIGH_FUSE_BITS.
Note:
The lock and fuse bits returned are the physical values, i.e. a bit returned as 0
means the corresponding fuse or lock bit is programmed.

6.11.2.5

#define boot_page_erase(address) __boot_page_erase_normal(address)

Erase the flash page that contains address.
Note:
address is a byte address in flash, not a word address.

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92

#define boot_page_erase_safe(address)

Value:
do { \
boot_spm_busy_wait();
eeprom_busy_wait();
boot_page_erase (address);
} while (0)

\
\
\

Same as boot_page_erase() except it waits for eeprom and spm operations to complete
before erasing the page.
6.11.2.7
data)

#define boot_page_fill(address, data) __boot_page_fill_normal(address,

Fill the bootloader temporary page buffer for flash address with data word.
Note:
The address is a byte address. The data is a word. The AVR writes data to the
buffer a word at a time, but addresses the buffer per byte! So, increment your
address by 2 between calls, and send 2 data bytes in a word format! The LSB of
the data is written to the lower address; the MSB of the data is written to the higher
address.

6.11.2.8

#define boot_page_fill_safe(address, data)

Value:
do { \
boot_spm_busy_wait();
eeprom_busy_wait();
boot_page_fill(address, data);
} while (0)

\
\
\

Same as boot_page_fill() except it waits for eeprom and spm operations to complete
before filling the page.
6.11.2.9

#define boot_page_write(address) __boot_page_write_normal(address)

Write the bootloader temporary page buffer to flash page that contains address.
Note:
address is a byte address in flash, not a word address.

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6.11.2.10

93

#define boot_page_write_safe(address)

Value:
do { \
boot_spm_busy_wait();
eeprom_busy_wait();
boot_page_write (address);
} while (0)

\
\
\

Same as boot_page_write() except it waits for eeprom and spm operations to complete
before writing the page.
6.11.2.11 #define
COMMON_ASB))

boot_rww_busy()

(__SPM_REG

&

(uint8_t)_BV(__-

Check if the RWW section is busy.
6.11.2.12

#define boot_rww_enable() __boot_rww_enable()

Enable the Read-While-Write memory section.
6.11.2.13

#define boot_rww_enable_safe()

Value:
do { \
boot_spm_busy_wait();
eeprom_busy_wait();
boot_rww_enable();
} while (0)

\
\
\

Same as boot_rww_enable() except waits for eeprom and spm operations to complete
before enabling the RWW mameory.
6.11.2.14

#define boot_spm_busy() (__SPM_REG & (uint8_t)_BV(SPMEN))

Check if the SPM instruction is busy.
6.11.2.15

#define boot_spm_busy_wait() do{}while(boot_spm_busy())

Wait while the SPM instruction is busy.
6.11.2.16 #define boot_spm_interrupt_disable() (__SPM_REG &= (uint8_t)∼_BV(SPMIE))
Disable the SPM interrupt.
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94

6.11.2.17 #define boot_spm_interrupt_enable() (__SPM_REG |= (uint8_t)_BV(SPMIE))
Enable the SPM interrupt.
6.11.2.18 #define BOOTLOADER_SECTION __attribute__ ((section (".bootloader")))
Used to declare a function or variable to be placed into a new section called .bootloader. This section and its contents can then be relocated to any address (such as the
bootloader NRWW area) at link-time.
6.11.2.19

#define GET_EXTENDED_FUSE_BITS (0x0002)

address to read the extended fuse bits, using boot_lock_fuse_bits_get
6.11.2.20

#define GET_HIGH_FUSE_BITS (0x0003)

address to read the high fuse bits, using boot_lock_fuse_bits_get
6.11.2.21

#define GET_LOCK_BITS (0x0001)

address to read the lock bits, using boot_lock_fuse_bits_get
6.11.2.22

#define GET_LOW_FUSE_BITS (0x0000)

address to read the low fuse bits, using boot_lock_fuse_bits_get

6.12

: EEPROM handling

6.12.1

Detailed Description

#include 

This header file declares the interface to some simple library routines suitable for handling the data EEPROM contained in the AVR microcontrollers. The implementation
uses a simple polled mode interface. Applications that require interrupt-controlled
EEPROM access to ensure that no time will be wasted in spinloops will have to deploy
their own implementation.
Note:
All of the read/write functions first make sure the EEPROM is ready to be accessed. Since this may cause long delays if a write operation is still pending, timecritical applications should first poll the EEPROM e. g. using eeprom_is_ready()
before attempting any actual I/O.
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95

This header file declares inline functions that call the assembler subroutines directly. This prevents that the compiler generates push/pops for the call-clobbered
registers. This way also a specific calling convention could be used for the eeprom routines e.g. by passing values in __tmp_reg__, eeprom addresses in X and
memory addresses in Z registers. Method is optimized for code size.
Presently supported are two locations of the EEPROM register set:
0x1F,0x20,0x21 and 0x1C,0x1D,0x1E (see __EEPROM_REG_LOCATIONS__).
As these functions modify IO registers, they are known to be non-reentrant. If any
of these functions are used from both, standard and interrupt context, the applications must ensure proper protection (e.g. by disabling interrupts before accessing
them).
avr-libc declarations
•
•
•
•
•
•

#define EEMEM __attribute__((section(".eeprom")))
#define eeprom_is_ready()
#define eeprom_busy_wait() do {} while (!eeprom_is_ready())
uint8_t eeprom_read_byte (const uint8_t ∗addr)
uint16_t eeprom_read_word (const uint16_t ∗addr)
void eeprom_read_block (void ∗pointer_ram, const void ∗pointer_eeprom,
size_t n)
• void eeprom_write_byte (uint8_t ∗addr, uint8_t value)
• void eeprom_write_word (uint16_t ∗addr, uint16_t value)
• void eeprom_write_block (const void ∗pointer_ram, void ∗pointer_eeprom,
size_t n)
IAR C compatibility defines
• #define _EEPUT(addr, val) eeprom_write_byte ((uint8_t ∗)(addr), (uint8_t)(val))
• #define _EEGET(var, addr) (var) = eeprom_read_byte ((uint8_t ∗)(addr))
Defines
• #define __EEPROM_REG_LOCATIONS__ 1C1D1E
6.12.2
6.12.2.1

Define Documentation
#define __EEPROM_REG_LOCATIONS__ 1C1D1E

In order to be able to work without a requiring a multilib approach for dealing with
controllers having the EEPROM registers at different positions in memory space, the
eeprom functions evaluate __EEPROM_REG_LOCATIONS__: It is assumed to be

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96

defined by the device io header and contains 6 uppercase hex digits encoding the addresses of EECR,EEDR and EEAR. First two letters: EECR address. Second two
letters: EEDR address. Last two letters: EEAR address. The default 1C1D1E corresponds to the register location that is valid for most controllers. The value of this define
symbol is used for appending it to the base name of the assembler functions.
6.12.2.2 #define _EEGET(var, addr) (var) = eeprom_read_byte ((uint8_t
∗)(addr))
Read a byte from EEPROM. Compatibility define for IAR C.
6.12.2.3 #define _EEPUT(addr, val) eeprom_write_byte ((uint8_t ∗)(addr),
(uint8_t)(val))
Write a byte to EEPROM. Compatibility define for IAR C.
6.12.2.4

#define EEMEM __attribute__((section(".eeprom")))

Attribute expression causing a variable to be allocated within the .eeprom section.
6.12.2.5

#define eeprom_busy_wait() do {} while (!eeprom_is_ready())

Loops until the eeprom is no longer busy.
Returns:
Nothing.

6.12.2.6

#define eeprom_is_ready()

Returns:
1 if EEPROM is ready for a new read/write operation, 0 if not.
6.12.3

Function Documentation

6.12.3.1 void eeprom_read_block (void ∗ pointer_ram, const void ∗ pointer_eeprom, size_t n)
Read a block of n bytes from EEPROM address pointer_eeprom to pointer_ram. For constant n <= 256 bytes a library function is used. For block sizes unknown
at compile time or block sizes > 256 an inline loop is expanded.

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97

6.12.3.2 uint8_t eeprom_read_byte (const uint8_t ∗ addr)
Read one byte from EEPROM address addr.
6.12.3.3 uint16_t eeprom_read_word (const uint16_t ∗ addr)
Read one 16-bit word (little endian) from EEPROM address addr.
6.12.3.4 void eeprom_write_block (const void ∗ pointer_ram, void ∗ pointer_eeprom, size_t n)
Write a block of n bytes to EEPROM address pointer_eeprom from pointer_ram.
6.12.3.5

void eeprom_write_byte (uint8_t ∗ addr, uint8_t value)

Write a byte value to EEPROM address addr.
6.12.3.6

void eeprom_write_word (uint16_t ∗ addr, uint16_t value)

Write a word value to EEPROM address addr.

6.13

: Interrupts

6.13.1

Detailed Description

Note:
This discussion of interrupts was originally taken from Rich Neswold’s document.
See Acknowledgments.
Introduction to avr-libc’s interrupt handling It’s nearly impossible to find compilers that agree on how to handle interrupt code. Since the C language tries to stay away
from machine dependent details, each compiler writer is forced to design their method
of support.
In the AVR-GCC environment, the vector table is predefined to point to interrupt routines with predetermined names. By using the appropriate name, your routine will be
called when the corresponding interrupt occurs. The device library provides a set of
default interrupt routines, which will get used if you don’t define your own.
Patching into the vector table is only one part of the problem. The compiler uses, by
convention, a set of registers when it’s normally executing compiler-generated code.
It’s important that these registers, as well as the status register, get saved and restored.
The extra code needed to do this is enabled by tagging the interrupt function with __attribute__((signal)).

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98

These details seem to make interrupt routines a little messy, but all these details are
handled by the Interrupt API. An interrupt routine is defined with ISR(). This macro
register and mark the routine as an interrupt handler for the specified peripheral. The
following is an example definition of a handler for the ADC interrupt.
#include 
ISR(ADC_vect)
{
// user code here
}

Refer to the chapter explaining assembler programming for an explanation about interrupt routines written solely in assembler language.
Catch-all interrupt vector If an unexpected interrupt occurs (interrupt is enabled
and no handler is installed, which usually indicates a bug), then the default action is to
reset the device by jumping to the reset vector. You can override this by supplying a
function named __vector_default which should be defined with ISR() as such.
#include 
ISR(__vector_default)
{
// user code here
}

Nested interrupts The AVR hardware clears the global interrupt flag in SREG before entering an interrupt vector. Thus, normally interrupts will remain disabled inside
the handler until the handler exits, where the RETI instruction (that is emitted by the
compiler as part of the normal function epilogue for an interrupt handler) will eventually re-enable further interrupts. For that reason, interrupt handlers normally do not
nest. For most interrupt handlers, this is the desired behaviour, for some it is even
required in order to prevent infinitely recursive interrupts (like UART interrupts, or
level-triggered external interrupts). In rare circumstances though it might be desired to
re-enable the global interrupt flag as early as possible in the interrupt handler, in order
to not defer any other interrupt more than absolutely needed. This could be done using
an sei() instruction right at the beginning of the interrupt handler, but this still leaves
few instructions inside the compiler-generated function prologue to run with global interrupts disabled. The compiler can be instructed to insert an SEI instruction right at
the beginning of an interrupt handler by declaring the handler the following way:
void XXX_vect(void) __attribute__((interrupt));
void XXX_vect(void) {
...
}

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99

where XXX_vect is the name of a valid interrupt vector for the MCU type in question,
as explained below.
Choosing the vector: Interrupt vector names The interrupt is chosen by supplying
one of the symbols in following table.
There are currently two different styles present for naming the vectors. One form uses
names starting with SIG_, followed by a relatively verbose but arbitrarily chosen name
describing the interrupt vector. This has been the only available style in avr-libc up to
version 1.2.x.
Starting with avr-libc version 1.4.0, a second style of interrupt vector names has been
added, where a short phrase for the vector description is followed by _vect. The
short phrase matches the vector name as described in the datasheet of the respective
device (and in Atmel’s XML files), with spaces replaced by an underscore and other
non-alphanumeric characters dropped. Using the suffix _vect is intented to improve
portability to other C compilers available for the AVR that use a similar naming convention.
The historical naming style might become deprecated in a future release, so it is not
recommended for new projects.
Note:
The ISR() macro cannot really spell-check the argument passed to them. Thus, by
misspelling one of the names below in a call to ISR(), a function will be created
that, while possibly being usable as an interrupt function, is not actually wired into
the interrupt vector table. The compiler will generate a warning if it detects a suspiciously looking name of a ISR() function (i.e. one that after macro replacement
does not start with "__vector_").
Vector name

Old
name

vector

Description

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Applicable for device

6.13 : Interrupts

Vector name

100

Description

Applicable for device

ADC_vect

Old
vector
name
SIG_ADC

ADC Conversion
Complete

ANALOG_COMP_0_vect
ANALOG_COMP_1_vect
ANALOG_COMP_2_vect
ANALOG_COMP_vect

SIG_COMPARATOR0
SIG_COMPARATOR1
SIG_COMPARATOR2
SIG_COMPARATOR

Analog
parator 0
Analog
parator 1
Analog
parator 2
Analog
parator

Com-

AT90S2333, AT90S4433, AT90S4434,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega163, ATmega165,
ATmega165P, ATmega169, ATmega169P,
ATmega32, ATmega323, ATmega325,
ATmega3250, ATmega329, ATmega329P,
ATmega3290, ATmega3290P, ATmega64,
ATmega645, ATmega6450, ATmega649,
ATmega6490, ATmega8, ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny13, ATtiny15, ATtiny26, ATtiny24,
ATtiny44, ATtiny84, ATtiny45, ATtiny25,
ATtiny85, ATtiny261, ATtiny461, ATtiny861, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646
AT90PWM3, AT90PWM2, AT90PWM1

Com-

AT90PWM3, AT90PWM2, AT90PWM1

Com-

AT90PWM3, AT90PWM2, AT90PWM1

Com-

ANA_COMP_vect

SIG_Analog
COMPARATOR parator

CANIT_vect

SIG_CAN_INTERRUPT1

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega165,
ATmega165P, ATmega169, ATmega169P,
ATmega325, ATmega3250, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega64, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATmega168,
ATmega48,
ATmega88,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P,
ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90S1200, AT90S2313, AT90S2333,
AT90S4414, AT90S4433, AT90S4434,
AT90S8515,
AT90S8535,
ATmega16,
ATmega161, ATmega162, ATmega163,
ATmega32, ATmega323, ATmega8, ATmega8515,
ATmega8535,
ATtiny11,
ATtiny12, ATtiny13, ATtiny15, ATtiny2313,
ATtiny26, ATtiny28, ATtiny24, ATtiny44,
ATtiny84, ATtiny45, ATtiny25, ATtiny85,
ATtiny261, ATtiny461, ATtiny861
AT90CAN128, AT90CAN32, AT90CAN64

Com-

CAN
Transfer
Complete
or
Error

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.13 : Interrupts

Vector name

Old
vector
name
SIG_EEPROM_READY,
SIG_EE_READY
SIG_EEPROM_READY

Description

EE_READY_vect

SIG_EEPROM_READY

EEPROM Ready

EXT_INT0_vect
INT0_vect

SIG_INTERRUPT0
SIG_INTERRUPT0

External Interrupt
Request 0
External Interrupt
0

EEPROM_READY_vect

EE_RDY_vect

101

Applicable for device
ATtiny2313

EEPROM Ready

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

AT90S2333, AT90S4433, AT90S4434,
AT90S8535,
ATmega16,
ATmega161,
ATmega162, ATmega163, ATmega32,
ATmega323, ATmega8, ATmega8515, ATmega8535, ATtiny12, ATtiny13, ATtiny15,
ATtiny26, ATtiny24, ATtiny44, ATtiny84,
ATtiny45, ATtiny25, ATtiny85, ATtiny261,
ATtiny461, ATtiny861
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega165,
ATmega165P, ATmega169, ATmega169P,
ATmega325, ATmega3250, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega406, ATmega64, ATmega645,
ATmega6450, ATmega649, ATmega6490,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATtiny24, ATtiny44, ATtiny84
AT90S1200, AT90S2313, AT90S2323,
AT90S2333, AT90S2343, AT90S4414,
AT90S4433, AT90S4434, AT90S8515,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega161, ATmega162,
ATmega163, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega32,
ATmega323, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega406, ATmega64,
ATmega645, ATmega6450, ATmega649,
ATmega6490, ATmega8, ATmega8515,
ATmega8535, ATmega168, ATmega48,
ATmega88, ATmega640, ATmega1280,
ATmega1281, ATmega2560, ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, ATtiny11, ATtiny12, ATtiny13, ATtiny15, ATtiny22, ATtiny2313,
ATtiny26, ATtiny28, ATtiny45, ATtiny25,
ATtiny85, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

6.13 : Interrupts

Vector name

102

Old
vector
name
SIG_INTERRUPT1

Description

Applicable for device

External Interrupt
Request 1

INT2_vect

SIG_INTERRUPT2

External Interrupt
Request 2

INT3_vect

SIG_INTERRUPT3

External Interrupt
Request 3

INT4_vect

SIG_INTERRUPT4

External Interrupt
Request 4

INT5_vect

SIG_INTERRUPT5

External Interrupt
Request 5

AT90S2313, AT90S2333, AT90S4414,
AT90S4433, AT90S4434, AT90S8515,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega161, ATmega162,
ATmega163, ATmega32, ATmega323,
ATmega406, ATmega64, ATmega8, ATmega8515, ATmega8535, ATmega168,
ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281, ATmega2560,
ATmega2561, ATmega324P, ATmega164P,
ATmega644P, ATmega644, ATtiny2313,
ATtiny28, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega16,
ATmega161, ATmega162, ATmega32,
ATmega323, ATmega406, ATmega64,
ATmega8515,
ATmega8535,
ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega406,
ATmega64, ATmega640, ATmega1280,
ATmega1281,
ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103,
ATmega128,
ATmega64,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103,
ATmega128,
ATmega64,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

INT1_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.13 : Interrupts

Vector name

103

Old
vector
name
SIG_INTERRUPT6

Description

Applicable for device

External Interrupt
Request 6

INT7_vect

SIG_INTERRUPT7

External Interrupt
Request 7

IO_PINS_vect

SIG_PIN,
SIG_PIN_CHANGE
SIG_LCD

External Interrupt
Request 0

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103,
ATmega128,
ATmega64,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103,
ATmega128,
ATmega64,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATtiny11, ATtiny12, ATtiny15, ATtiny26

SIG_PIN
SIG_CAN_OVERFLOW1
SIG_PIN_CHANGE0

Low-level Input
on Port B
CAN
Timer
Overrun
Pin Change Interrupt Request 0

PCINT1_vect

SIG_PIN_CHANGE1

Pin Change Interrupt Request 1

PCINT2_vect

SIG_PIN_CHANGE2

Pin Change Interrupt Request 2

INT6_vect

LCD_vect

LOWLEVEL_IO_PINS_vect
OVRIT_vect
PCINT0_vect

LCD Start
Frame

of

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

ATmega169, ATmega169P, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega649, ATmega6490
ATtiny28
AT90CAN128, AT90CAN32, AT90CAN64
ATmega162, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega325,
ATmega3250, ATmega329, ATmega329P,
ATmega3290, ATmega3290P, ATmega406,
ATmega645, ATmega6450, ATmega649,
ATmega6490, ATmega168, ATmega48,
ATmega88, ATmega640, ATmega1280,
ATmega1281, ATmega2560, ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, ATtiny13, ATtiny24, ATtiny44, ATtiny84, ATtiny45, ATtiny25,
ATtiny85, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATmega162, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega325,
ATmega3250, ATmega329, ATmega329P,
ATmega3290, ATmega3290P, ATmega406,
ATmega645, ATmega6450, ATmega649,
ATmega6490, ATmega168, ATmega48,
ATmega88, ATmega640, ATmega1280,
ATmega1281, ATmega2560, ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, ATtiny24, ATtiny44, ATtiny84,
AT90USB162, AT90USB82
ATmega3250, ATmega3290, ATmega3290P,
ATmega6450, ATmega6490, ATmega168,
ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, ATmega324P, ATmega164P, ATmega644P, ATmega644

6.13 : Interrupts

Vector name
PCINT3_vect

PCINT_vect

Old
vector
name
SIG_PIN_CHANGE3

PSC0_CAPT_vect
PSC0_EC_vect
PSC1_CAPT_vect
PSC1_EC_vect
PSC2_CAPT_vect
PSC2_EC_vect
SPI_STC_vect

SIG_PIN_CHANGE,
SIG_PCINT
SIG_PSC0_CAPTURE
SIG_PSC0_END_CYCLE
SIG_PSC1_CAPTURE
SIG_PSC1_END_CYCLE
SIG_PSC2_CAPTURE
SIG_PSC2_END_CYCLE
SIG_SPI

SPM_RDY_vect

SIG_SPM_READY

104

Description

Applicable for device

Pin Change Interrupt Request 3

ATmega3250, ATmega3290, ATmega3290P,
ATmega6450, ATmega6490, ATmega324P,
ATmega164P, ATmega644P, ATmega644
ATtiny2313, ATtiny261, ATtiny461, ATtiny861

PSC0
Capture
Event
PSC0 End Cycle

AT90PWM3, AT90PWM2, AT90PWM1

PSC1
Capture
Event
PSC1 End Cycle

AT90PWM3, AT90PWM2, AT90PWM1

PSC2
Capture
Event
PSC2 End Cycle

AT90PWM3, AT90PWM2, AT90PWM1

Serial Transfer
Complete

AT90S2333, AT90S4414, AT90S4433,
AT90S4434, AT90S8515, AT90S8535,
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega16,
ATmega161, ATmega162, ATmega163,
ATmega165, ATmega165P, ATmega169,
ATmega169P, ATmega32, ATmega323,
ATmega325, ATmega3250, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega64, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATmega8,
ATmega8515,
ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATmega16, ATmega162, ATmega32, ATmega323, ATmega8, ATmega8515, ATmega8535

Store
Program
Memory Ready

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

AT90PWM3, AT90PWM2, AT90PWM1

AT90PWM3, AT90PWM2, AT90PWM1

AT90PWM3, AT90PWM2, AT90PWM1

6.13 : Interrupts

Vector name
SPM_READY_vect

TIM0_COMPA_vect
TIM0_COMPB_vect
TIM0_OVF_vect
TIM1_CAPT_vect
TIM1_COMPA_vect
TIM1_COMPB_vect
TIM1_OVF_vect
TIMER0_CAPT_vect
TIMER0_COMPA_vect

TIMER0_COMPB_vect

105

Old
vector
name
SIG_SPM_READY

Description

Applicable for device

Store
Program
Memory Read

SIG_OUTPUT_COMPARE0A
SIG_OUTPUT_COMPARE0B
SIG_OVERFLOW0
SIG_INPUT_CAPTURE1
SIG_OUTPUT_COMPARE1A
SIG_OUTPUT_COMPARE1B
SIG_OVERFLOW1
SIG_INPUT_CAPTURE0
SIG_OUTPUT_COMPARE0A

Timer/Counter
Compare Match
A
Timer/Counter
Compare Match
B
Timer/Counter0
Overflow
Timer/Counter1
Capture Event
Timer/Counter1
Compare Match
A
Timer/Counter1
Compare Match
B
Timer/Counter1
Overflow
ADC Conversion
Complete
TimerCounter0
Compare Match
A

AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega325,
ATmega3250, ATmega329, ATmega329P,
ATmega3290,
ATmega3290P,
ATmega406, ATmega64, ATmega645, ATmega6450, ATmega649, ATmega6490,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATtiny13, ATtiny24, ATtiny44, ATtiny84,
ATtiny45, ATtiny25, ATtiny85

SIG_OUTPUT_COMPARE0B,
SIG_OUTPUT_COMPARE0_B

Timer Counter 0
Compare Match
B

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

ATtiny13, ATtiny24, ATtiny44, ATtiny84,
ATtiny45, ATtiny25, ATtiny85
ATtiny13, ATtiny24, ATtiny44, ATtiny84,
ATtiny45, ATtiny25, ATtiny85
ATtiny24, ATtiny44, ATtiny84
ATtiny24, ATtiny44, ATtiny84, ATtiny45,
ATtiny25, ATtiny85
ATtiny24, ATtiny44, ATtiny84, ATtiny45,
ATtiny25, ATtiny85
ATtiny24, ATtiny44, ATtiny84, ATtiny45,
ATtiny25, ATtiny85
ATtiny261, ATtiny461, ATtiny861
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny2313, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90PWM3, AT90PWM2, AT90PWM1,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny2313, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

6.13 : Interrupts

Vector name

106

Old
vector
name
SIG_OUTPUT_COMPARE0A,
SIG_OUTPUT_COMPARE0_A
SIG_OUTPUT_COMPARE0

Description

Applicable for device

Timer/Counter0
Compare Match
A

AT90PWM3, AT90PWM2, AT90PWM1

Timer/Counter0
Compare Match

TIMER0_OVF0_vect
TIMER0_OVF_vect

SIG_OVERFLOW0
SIG_OVERFLOW0

Timer/Counter0
Overflow
Timer/Counter0
Overflow

TIMER1_CAPT1_vect

SIG_INPUT_CAPTURE1

Timer/Counter1
Capture Event

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega16, ATmega161, ATmega162, ATmega165, ATmega165P, ATmega169, ATmega169P, ATmega32, ATmega323, ATmega325, ATmega3250, ATmega329, ATmega329P, ATmega3290, ATmega3290P, ATmega64, ATmega645, ATmega6450, ATmega649, ATmega6490, ATmega8515, ATmega8535
AT90S2313, AT90S2323, AT90S2343, ATtiny22, ATtiny26
AT90S1200, AT90S2333, AT90S4414,
AT90S4433, AT90S4434, AT90S8515,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega161, ATmega162,
ATmega163, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega32,
ATmega323, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega64, ATmega645,
ATmega6450, ATmega649, ATmega6490,
ATmega8, ATmega8515, ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny11, ATtiny12, ATtiny15, ATtiny2313,
ATtiny28, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90S2313

TIMER0_COMP_A_vect

TIMER0_COMP_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.13 : Interrupts

Vector name
TIMER1_CAPT_vect

TIMER1_CMPA_vect
TIMER1_CMPB_vect
TIMER1_COMP1_vect
TIMER1_COMPA_vect

107

Old
vector
name
SIG_INPUT_CAPTURE1

Description

Applicable for device

Timer/Counter
Capture Event

SIG_OUTPUT_COMPARE1A
SIG_OUTPUT_COMPARE1B
SIG_OUTPUT_COMPARE1A
SIG_OUTPUT_COMPARE1A

Timer/Counter1
Compare Match
1A
Timer/Counter1
Compare Match
1B
Timer/Counter1
Compare Match

AT90S2333, AT90S4414, AT90S4433,
AT90S4434, AT90S8515, AT90S8535,
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega16,
ATmega161, ATmega162, ATmega163,
ATmega165, ATmega165P, ATmega169,
ATmega169P, ATmega32, ATmega323,
ATmega325, ATmega3250, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega64, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATmega8,
ATmega8515, ATmega8535, ATmega168,
ATmega48,
ATmega88,
ATmega640,
ATmega1280,
ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny2313, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATtiny26

Timer/Counter1
Compare Match
A

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

ATtiny26

AT90S2313

AT90S4414, AT90S4434, AT90S8515,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega161, ATmega162,
ATmega163, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega32,
ATmega323, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega64, ATmega645,
ATmega6450, ATmega649, ATmega6490,
ATmega8, ATmega8515, ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny2313, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

6.13 : Interrupts

Vector name

108

Old
vector
name
SIG_OUTPUT_COMPARE1B

Description

Applicable for device

Timer/Counter1
Compare MatchB

TIMER1_COMPC_vect

SIG_OUTPUT_COMPARE1C

Timer/Counter1
Compare Match
C

TIMER1_COMPD_vect

SIG_OUTPUT_COMPARE0D
SIG_OUTPUT_COMPARE1A
SIG_OVERFLOW1
SIG_OVERFLOW1

Timer/Counter1
Compare Match
D
Timer/Counter1
Compare Match

AT90S4414, AT90S4434, AT90S8515,
AT90S8535, AT90PWM3, AT90PWM2,
AT90PWM1, AT90CAN128, AT90CAN32,
AT90CAN64, ATmega103, ATmega128,
ATmega16, ATmega161, ATmega162,
ATmega163, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega32,
ATmega323, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega64, ATmega645,
ATmega6450, ATmega649, ATmega6490,
ATmega8, ATmega8515, ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P, ATmega644P, ATmega644,
ATtiny2313, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega64, ATmega640,
ATmega1280, ATmega1281, ATmega2560,
ATmega2561, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATtiny261, ATtiny461, ATtiny861

TIMER1_COMPB_vect

TIMER1_COMP_vect
TIMER1_OVF1_vect
TIMER1_OVF_vect

Timer/Counter1
Overflow
Timer/Counter1
Overflow

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

AT90S2333, AT90S4433, ATtiny15

AT90S2313, ATtiny26
AT90S2333, AT90S4414, AT90S4433,
AT90S4434, AT90S8515, AT90S8535,
AT90PWM3, AT90PWM2, AT90PWM1,
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega103, ATmega128, ATmega16,
ATmega161, ATmega162, ATmega163,
ATmega165, ATmega165P, ATmega169,
ATmega169P, ATmega32, ATmega323,
ATmega325, ATmega3250, ATmega329,
ATmega329P, ATmega3290, ATmega3290P,
ATmega64, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATmega8,
ATmega8515, ATmega8535, ATmega168,
ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281, ATmega2560,
ATmega2561, ATmega324P, ATmega164P,
ATmega644P, ATmega644,
ATtiny15,
ATtiny2313, ATtiny261, ATtiny461, ATtiny861,
AT90USB162,
AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

6.13 : Interrupts

Vector name

109

Old
vector
name
SIG_OUTPUT_COMPARE2A

Description

Applicable for device

Timer/Counter2
Compare Match
A

TIMER2_COMPB_vect

SIG_OUTPUT_COMPARE2B

Timer/Counter2
Compare Match
A

TIMER2_COMP_vect

SIG_OUTPUT_COMPARE2

Timer/Counter2
Compare Match

TIMER2_OVF_vect

SIG_OVERFLOW2

Timer/Counter2
Overflow

TIMER3_CAPT_vect

SIG_INPUT_CAPTURE3

Timer/Counter3
Capture Event

TIMER3_COMPA_vect

SIG_OUTPUT_COMPARE3A

Timer/Counter3
Compare Match
A

ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P, ATmega644, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P, ATmega644, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646
AT90S4434, AT90S8535, AT90CAN128,
AT90CAN32, AT90CAN64, ATmega103,
ATmega128, ATmega16, ATmega161, ATmega162, ATmega163, ATmega165, ATmega165P, ATmega169, ATmega169P, ATmega32, ATmega323, ATmega325, ATmega3250, ATmega329, ATmega329P, ATmega3290, ATmega3290P, ATmega64, ATmega645, ATmega6450, ATmega649, ATmega6490, ATmega8, ATmega8535
AT90S4434, AT90S8535, AT90CAN128,
AT90CAN32, AT90CAN64, ATmega103,
ATmega128, ATmega16, ATmega161,
ATmega162, ATmega163, ATmega165,
ATmega165P, ATmega169, ATmega169P,
ATmega32, ATmega323, ATmega325, ATmega3250, ATmega329, ATmega329P,
ATmega3290,
ATmega3290P,
ATmega64,
ATmega645,
ATmega6450,
ATmega649, ATmega6490, ATmega8,
ATmega8535, ATmega168, ATmega48,
ATmega88, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, ATmega324P, ATmega164P, ATmega644P, ATmega644, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega64, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, AT90USB1287,
AT90USB1286,
AT90USB647,
AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega64, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, AT90USB1287,
AT90USB1286,
AT90USB647,
AT90USB646

TIMER2_COMPA_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.13 : Interrupts

Vector name

110

Old
vector
name
SIG_OUTPUT_COMPARE3B

Description

Applicable for device

Timer/Counter3
Compare Match
B

TIMER3_COMPC_vect

SIG_OUTPUT_COMPARE3C

Timer/Counter3
Compare Match
C

TIMER3_OVF_vect

SIG_OVERFLOW3

Timer/Counter3
Overflow

TIMER4_CAPT_vect
TIMER4_COMPA_vect

SIG_INPUT_CAPTURE4
SIG_OUTPUT_COMPARE4A
SIG_OUTPUT_COMPARE4B
SIG_OUTPUT_COMPARE4C
SIG_OVERFLOW4
SIG_INPUT_CAPTURE5
SIG_OUTPUT_COMPARE5A
SIG_OUTPUT_COMPARE5B
SIG_OUTPUT_COMPARE5C
SIG_OVERFLOW5
SIG_2WIRE_SERIAL

Timer/Counter4
Capture Event
Timer/Counter4
Compare Match
A
Timer/Counter4
Compare Match
B
Timer/Counter4
Compare Match
C
Timer/Counter4
Overflow
Timer/Counter5
Capture Event
Timer/Counter5
Compare Match
A
Timer/Counter5
Compare Match
B
Timer/Counter5
Compare Match
C
Timer/Counter5
Overflow
2-wire Serial Interface

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega64, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, AT90USB1287,
AT90USB1286,
AT90USB647,
AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega64, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega64, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, AT90USB1287,
AT90USB1286,
AT90USB647,
AT90USB646
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

TIMER3_COMPB_vect

TIMER4_COMPB_vect
TIMER4_COMPC_vect
TIMER4_OVF_vect
TIMER5_CAPT_vect
TIMER5_COMPA_vect
TIMER5_COMPB_vect
TIMER5_COMPC_vect
TIMER5_OVF_vect
TWI_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega16, ATmega163,
ATmega32, ATmega323, ATmega406,
ATmega64,
ATmega8,
ATmega8535,
ATmega168, ATmega48, ATmega88, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P, ATmega644, AT90USB1287, AT90USB1286,
AT90USB647, AT90USB646

6.13 : Interrupts

Vector name
TXDONE_vect

Old
vector
name
SIG_TXDONE

111

Description

Applicable for device

Transmission
Done, Bit Timer
Flag 2 Interrupt
Transmit Buffer
Empty, Bit Itmer
Flag 0 Interrupt
UART0,
Rx
Complete

AT86RF401

UART0,
Complete

Tx

ATmega161

UART0
Data
Register Empty

ATmega161

UART1,
Complete

Rx

ATmega161

UART1,
Complete

Tx

ATmega161

UART1
Data
Register Empty

ATmega161

UART, Rx Complete

AT90S2313, AT90S2333,
AT90S4433, AT90S4434,
AT90S8535, ATmega103,
ATmega8515
AT90S2313, AT90S2333,
AT90S4433, AT90S4434,
AT90S8535, ATmega103,
ATmega8515
AT90S2313, AT90S2333,
AT90S4433, AT90S4434,
AT90S8535, ATmega103,
ATmega8515
ATmega162

TXEMPTY_vect

SIG_TXBE

UART0_RX_vect

UART_RX_vect

SIG_UART0_RECV
SIG_UART0_TRANS
SIG_UART0_DATA
SIG_UART1_RECV
SIG_UART1_TRANS
SIG_UART1_DATA
SIG_UART_RECV

UART_TX_vect

SIG_UART_TRANS

UART, Tx Complete

UART_UDRE_vect

SIG_UART_DATA

UART Data Register Empty

USART0_RXC_vect

SIG_USART0_RECV
SIG_UART0_RECV

USART0,
Complete

Rx

USART0,
Complete

Rx

SIG_USART0_TRANS

USART0,
Complete

Tx

UART0_TX_vect
UART0_UDRE_vect
UART1_RX_vect
UART1_TX_vect
UART1_UDRE_vect

USART0_RX_vect

USART0_TXC_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

AT86RF401

ATmega161

AT90S4414,
AT90S8515,
ATmega163,
AT90S4414,
AT90S8515,
ATmega163,
AT90S4414,
AT90S8515,
ATmega163,

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega325,
ATmega329, ATmega329P, ATmega64,
ATmega645, ATmega649, ATmega640,
ATmega1280, ATmega1281, ATmega2560,
ATmega2561, ATmega324P, ATmega164P,
ATmega644P, ATmega644
ATmega162

6.13 : Interrupts

Vector name

112

Old
vector
name
SIG_UART0_TRANS

Description

USART0_UDRE_vect

SIG_UART0_DATA

USART0
Data
Register Empty

USART1_RXC_vect

SIG_USART1_RECV
SIG_UART1_RECV

USART1,
Complete

Rx

USART1,
Complete

Rx

SIG_USART1_TRANS
SIG_UART1_TRANS

USART1,
Complete

Tx

USART1,
Complete

Tx

USART1_UDRE_vect

SIG_UART1_DATA

USART1, Data
Register Empty

USART2_RX_vect

SIG_USART2_RECV
SIG_USART2_TRANS

USART2,
Complete

Rx

USART2,
Complete

Tx

USART0_TX_vect

USART1_RX_vect

USART1_TXC_vect
USART1_TX_vect

USART2_TX_vect

USART0,
Complete

Applicable for device
Tx

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega165, ATmega165P,
ATmega169, ATmega169P, ATmega325,
ATmega3250, ATmega329, ATmega329P,
ATmega3290, ATmega3290P, ATmega64,
ATmega645, ATmega6450, ATmega649,
ATmega6490, ATmega640, ATmega1280,
ATmega1281, ATmega2560, ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega165, ATmega165P, ATmega169, ATmega169P, ATmega325, ATmega329, ATmega329P, ATmega64, ATmega645, ATmega649, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, ATmega324P, ATmega164P, ATmega644P, ATmega644
ATmega162

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega64, ATmega640,
ATmega1280,
ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P,
ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATmega162

AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega64, ATmega640,
ATmega1280,
ATmega1281,
ATmega2560, ATmega2561, ATmega324P,
ATmega164P,
ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
AT90CAN128, AT90CAN32, AT90CAN64,
ATmega128, ATmega162, ATmega64, ATmega640, ATmega1280, ATmega1281,
ATmega2560,
ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561
ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

6.13 : Interrupts

Vector name

113

Old
vector
name
SIG_USART2_DATA
SIG_USART3_RECV
SIG_USART3_TRANS
SIG_USART3_DATA
SIG_USART_RECV, SIG_UART_RECV
SIG_USART_RECV, SIG_UART_RECV

Description

Applicable for device

USART2
Data
register Empty

ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

USART3,
Complete

Rx

ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

USART3,
Complete

Tx

ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

USART3
Data
register Empty

ATmega640, ATmega1280, ATmega1281,
ATmega2560, ATmega2561

USART,
Complete

Rx

ATmega16, ATmega32, ATmega323, ATmega8

USART,
Complete

Rx

SIG_USART_TRANS,
SIG_UART_TRANS
SIG_USART_TRANS,
SIG_UART_TRANS
SIG_USART_DATA, SIG_UART_DATA

USART,
Complete

Tx

AT90PWM3, AT90PWM2, AT90PWM1,
ATmega3250, ATmega3290, ATmega3290P,
ATmega6450, ATmega6490, ATmega8535,
ATmega168, ATmega48, ATmega88, ATtiny2313
ATmega16, ATmega32, ATmega323, ATmega8

USART,
Complete

Tx

AT90PWM3, AT90PWM2, AT90PWM1,
ATmega8535, ATmega168, ATmega48, ATmega88, ATtiny2313

USART
Data
Register Empty

USI_OVERFLOW_vect

SIG_USI_OVERFLOW

USI Overflow

USI_OVF_vect

SIG_USI_OVERFLOW

USI Overflow

USI_START_vect

SIG_USI_START

USI Start Condition

USI_STRT_vect

SIG_USI_START

USI Start

AT90PWM3, AT90PWM2, AT90PWM1,
ATmega16, ATmega32, ATmega323, ATmega3250, ATmega3290, ATmega3290P,
ATmega6450, ATmega6490, ATmega8, ATmega8535, ATmega168, ATmega48, ATmega88, ATtiny2313
ATmega165, ATmega165P, ATmega169,
ATmega169P, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATtiny2313
ATtiny26, ATtiny24, ATtiny44, ATtiny84,
ATtiny45, ATtiny25, ATtiny85, ATtiny261,
ATtiny461, ATtiny861
ATmega165, ATmega165P, ATmega169,
ATmega169P, ATmega325, ATmega3250,
ATmega329, ATmega329P, ATmega3290,
ATmega3290P, ATmega645, ATmega6450,
ATmega649, ATmega6490, ATtiny2313,
ATtiny45, ATtiny25, ATtiny85, ATtiny261,
ATtiny461, ATtiny861
ATtiny26

USART2_UDRE_vect
USART3_RX_vect
USART3_TX_vect
USART3_UDRE_vect
USART_RXC_vect

USART_RX_vect

USART_TXC_vect

USART_TX_vect

USART_UDRE_vect

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.13 : Interrupts

Vector name
USI_STR_vect
WATCHDOG_vect
WDT_OVERFLOW_vect

WDT_vect

Old
vector
name
SIG_USI_START
SIG_WATCHDOG_TIMEOUT
SIG_WATCHDOG_TIMEOUT,
SIG_WDT_OVERFLOW
SIG_WDT,
SIG_WATCHDOG_TIMEOUT

114

Description

Applicable for device

USI START

ATtiny24, ATtiny44, ATtiny84

Watchdog Timeout

ATtiny24, ATtiny44, ATtiny84

Watchdog Timer
Overflow

ATtiny2313

Watchdog Timeout Interrupt

AT90PWM3, AT90PWM2, AT90PWM1,
ATmega406, ATmega168, ATmega48,
ATmega88, ATmega640, ATmega1280,
ATmega1281, ATmega2560, ATmega2561,
ATmega324P, ATmega164P, ATmega644P,
ATmega644, ATtiny13, ATtiny45, ATtiny25, ATtiny85, ATtiny261, ATtiny461,
ATtiny861, AT90USB162, AT90USB82,
AT90USB1287,
AT90USB1286,
AT90USB647, AT90USB646

Global manipulation of the interrupt flag
The global interrupt flag is maintained in the I bit of the status register (SREG).
• #define sei()
• #define cli()
Macros for writing interrupt handler functions
•
•
•
•

#define ISR(vector)
#define SIGNAL(vector)
#define EMPTY_INTERRUPT(vector)
#define ISR_ALIAS(vector, target_vector)

6.13.2
6.13.2.1

Define Documentation
#define cli()

#include 

Disables all interrupts by clearing the global interrupt mask. This function actually
compiles into a single line of assembly, so there is no function call overhead.

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6.13 : Interrupts

6.13.2.2

115

#define EMPTY_INTERRUPT(vector)

#include 

Defines an empty interrupt handler function. This will not generate any prolog or
epilog code and will only return from the ISR. Do not define a function body as this
will define it for you. Example:
EMPTY_INTERRUPT(ADC_vect);

6.13.2.3

#define ISR(vector)

#include 

Introduces an interrupt handler function (interrupt service routine) that runs with global
interrupts initially disabled.
vector must be one of the interrupt vector names that are valid for the particular
MCU type.
6.13.2.4

#define ISR_ALIAS(vector, target_vector)

#include 

Defines vector to point to the same interrupt vector as target_vector. That
way, a single interrupt vector implementation can be used to serve several interrupt
sources.
Do not define a function body.
Note:
This requires a recent version of AVR-GCC to work (GCC 4.2 or patched GCC
4.1.x).
Example:
ISR(INT0_vect)
{
PORTB = 42;
}
ISR_ALIAS(INT1_vect, INT0_vect);

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.14 : Program Space Utilities

6.13.2.5

116

#define sei()

#include 

Enables interrupts by setting the global interrupt mask. This function actually compiles
into a single line of assembly, so there is no function call overhead.
6.13.2.6

#define SIGNAL(vector)

#include 

Introduces an interrupt handler function that runs with global interrupts initially disabled.
This is the same as the ISR macro.
Deprecated
Do not use SIGNAL() in new code. Use ISR() instead.

6.14

: Program Space Utilities

6.14.1

Detailed Description

#include 
#include 

The functions in this module provide interfaces for a program to access data stored in
program space (flash memory) of the device. In order to use these functions, the target
device must support either the LPM or ELPM instructions.
Note:
These functions are an attempt to provide some compatibility with header files
that come with IAR C, to make porting applications between different compilers
easier. This is not 100% compatibility though (GCC does not have full support for
multiple address spaces yet).
If you are working with strings which are completely based in ram, use the standard string functions described in : Strings.
If possible, put your constant tables in the lower 64 KB and use pgm_read_byte_near() or pgm_read_word_near() instead of pgm_read_byte_far() or pgm_read_word_far() since it is more efficient that way, and you can still use the upper 64K
for executable code. All functions that are suffixed with a _P require their arguments to be in the lower 64 KB of the flash ROM, as they do not use ELPM
instructions. This is normally not a big concern as the linker setup arranges any
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6.14 : Program Space Utilities

117

program space constants declared using the macros from this header file so they
are placed right after the interrupt vectors, and in front of any executable code.
However, it can become a problem if there are too many of these constants, or for
bootloaders on devices with more than 64 KB of ROM. All these functions will not
work in that situation.
Defines
•
•
•
•
•
•
•
•
•
•
•
•
•

#define PROGMEM __ATTR_PROGMEM__
#define PSTR(s) ((const PROGMEM char ∗)(s))
#define pgm_read_byte_near(address_short) __LPM((uint16_t)(address_short))
#define
pgm_read_word_near(address_short)
__LPM_word((uint16_t)(address_short))
#define
pgm_read_dword_near(address_short)
__LPM_dword((uint16_t)(address_short))
#define pgm_read_byte_far(address_long) __ELPM((uint32_t)(address_long))
#define
pgm_read_word_far(address_long)
__ELPM_word((uint32_t)(address_long))
#define
pgm_read_dword_far(address_long)
__ELPM_dword((uint32_t)(address_long))
#define pgm_read_byte(address_short) pgm_read_byte_near(address_short)
#define pgm_read_word(address_short) pgm_read_word_near(address_short)
#define pgm_read_dword(address_short) pgm_read_dword_near(address_short)
#define PGM_P const prog_char ∗
#define PGM_VOID_P const prog_void ∗

Typedefs
•
•
•
•
•
•
•
•
•
•
•

typedef void PROGMEM prog_void
typedef char PROGMEM prog_char
typedef unsigned char PROGMEM prog_uchar
typedef int8_t PROGMEM prog_int8_t
typedef uint8_t PROGMEM prog_uint8_t
typedef int16_t PROGMEM prog_int16_t
typedef uint16_t PROGMEM prog_uint16_t
typedef int32_t PROGMEM prog_int32_t
typedef uint32_t PROGMEM prog_uint32_t
typedef int64_t PROGMEM prog_int64_t
typedef uint64_t PROGMEM prog_uint64_t

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6.14 : Program Space Utilities

118

Functions
• PGM_VOID_P memchr_P (PGM_VOID_P s, int val, size_t len) __ATTR_CONST__
• int memcmp_P (const void ∗, PGM_VOID_P, size_t) __ATTR_PURE__
• void ∗ memcpy_P (void ∗, PGM_VOID_P, size_t)
• PGM_VOID_P memrchr_P (PGM_VOID_P s, int val, size_t len) __ATTR_CONST__
• int strcasecmp_P (const char ∗, PGM_P) __ATTR_PURE__
• char ∗ strcat_P (char ∗, PGM_P)
• PGM_P strchr_P (PGM_P s, int val) __ATTR_CONST__
• PGM_P strchrnul_P (PGM_P s, int val) __ATTR_CONST__
• int strcmp_P (const char ∗, PGM_P) __ATTR_PURE__
• char ∗ strcpy_P (char ∗, PGM_P)
• size_t strcspn_P (const char ∗s, PGM_P reject) __ATTR_PURE__
• size_t strlcat_P (char ∗, PGM_P, size_t)
• size_t strlcpy_P (char ∗, PGM_P, size_t)
• size_t strlen_P (PGM_P) __ATTR_CONST__
• int strncasecmp_P (const char ∗, PGM_P, size_t) __ATTR_PURE__
• char ∗ strncat_P (char ∗, PGM_P, size_t)
• int strncmp_P (const char ∗, PGM_P, size_t) __ATTR_PURE__
• char ∗ strncpy_P (char ∗, PGM_P, size_t)
• size_t strnlen_P (PGM_P, size_t) __ATTR_CONST__
• char ∗ strpbrk_P (const char ∗s, PGM_P accept) __ATTR_PURE__
• PGM_P strrchr_P (PGM_P s, int val) __ATTR_CONST__
• char ∗ strsep_P (char ∗∗sp, PGM_P delim)
• size_t strspn_P (const char ∗s, PGM_P accept) __ATTR_PURE__
• char ∗ strstr_P (const char ∗, PGM_P) __ATTR_PURE__
• void ∗ memmem_P (const void ∗, size_t, PGM_VOID_P, size_t) __ATTR_PURE__
• char ∗ strcasestr_P (const char ∗, PGM_P) __ATTR_PURE__
6.14.2
6.14.2.1

Define Documentation
#define PGM_P const prog_char ∗

Used to declare a variable that is a pointer to a string in program space.
6.14.2.2
short)

#define pgm_read_byte(address_short) pgm_read_byte_near(address_-

Read a byte from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

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6.14 : Program Space Utilities

6.14.2.3 #define
t)(address_long))

119

pgm_read_byte_far(address_long)

__ELPM((uint32_-

Read a byte from the program space with a 32-bit (far) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.4 #define
t)(address_short))

pgm_read_byte_near(address_short)

__LPM((uint16_-

Read a byte from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.5 #define
near(address_short)

pgm_read_dword(address_short)

pgm_read_dword_-

Read a double word from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.6 #define
pgm_read_dword_far(address_long)
dword((uint32_t)(address_long))

__ELPM_-

Read a double word from the program space with a 32-bit (far) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.7 #define
pgm_read_dword_near(address_short)
dword((uint16_t)(address_short))
Read a double word from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

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__LPM_-

6.14 : Program Space Utilities

6.14.2.8 #define
near(address_short)

pgm_read_word(address_short)

120

pgm_read_word_-

Read a word from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.9 #define pgm_read_word_far(address_long) __ELPM_word((uint32_t)(address_long))
Read a word from the program space with a 32-bit (far) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.10 #define pgm_read_word_near(address_short) __LPM_word((uint16_t)(address_short))
Read a word from the program space with a 16-bit (near) address.
Note:
The address is a byte address. The address is in the program space.

6.14.2.11

#define PGM_VOID_P const prog_void ∗

Used to declare a generic pointer to an object in program space.
6.14.2.12

#define PROGMEM __ATTR_PROGMEM__

Attribute to use in order to declare an object being located in flash ROM.
6.14.2.13

#define PSTR(s) ((const PROGMEM char ∗)(s))

Used to declare a static pointer to a string in program space.
6.14.3

Typedef Documentation

6.14.3.1 prog_char
Type of a "char" object located in flash ROM.

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6.14 : Program Space Utilities

6.14.3.2 prog_int16_t
Type of an "int16_t" object located in flash ROM.
6.14.3.3 prog_int32_t
Type of an "int32_t" object located in flash ROM.
6.14.3.4 prog_int64_t
Type of an "int64_t" object located in flash ROM.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.14.3.5 prog_int8_t
Type of an "int8_t" object located in flash ROM.
6.14.3.6 prog_uchar
Type of an "unsigned char" object located in flash ROM.
6.14.3.7 prog_uint16_t
Type of an "uint16_t" object located in flash ROM.
6.14.3.8 prog_uint32_t
Type of an "uint32_t" object located in flash ROM.
6.14.3.9 prog_uint64_t
Type of an "uint64_t" object located in flash ROM.
Note:
This type is not available when the compiler option -mint8 is in effect.

6.14.3.10 prog_uint8_t
Type of an "uint8_t" object located in flash ROM.

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121

6.14 : Program Space Utilities

122

6.14.3.11 prog_void
Type of a "void" object located in flash ROM. Does not make much sense by itself, but
can be used to declare a "void ∗" object in flash ROM.
6.14.4

Function Documentation

6.14.4.1

PGM_VOID_P memchr_P (PGM_VOID_P s, int val, size_t len)

Scan flash memory for a character.
The memchr_P() function scans the first len bytes of the flash memory area pointed
to by s for the character val. The first byte to match val (interpreted as an unsigned
character) stops the operation.
Returns:
The memchr_P() function returns a pointer to the matching byte or NULL if the
character does not occur in the given memory area.

6.14.4.2

int memcmp_P (const void ∗ s1, PGM_VOID_P s2, size_t len)

Compare memory areas.
The memcmp_P() function compares the first len bytes of the memory areas s1 and
flash s2. The comparision is performed using unsigned char operations.
Returns:
The memcmp_P() function returns an integer less than, equal to, or greater than
zero if the first len bytes of s1 is found, respectively, to be less than, to match, or
be greater than the first len bytes of s2.

6.14.4.3

void ∗ memcpy_P (void ∗ dest, PGM_VOID_P src, size_t n)

The memcpy_P() function is similar to memcpy(), except the src string resides in program space.
Returns:
The memcpy_P() function returns a pointer to dest.

6.14.4.4 void ∗ memmem_P (const void ∗ s1, size_t len1, PGM_VOID_P s2,
size_t len2)
The memmem_P() function is similar to memmem() except that s2 is pointer to a
string in program space.
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6.14 : Program Space Utilities

6.14.4.5

123

PGM_VOID_P memrchr_P (PGM_VOID_P src, int val, size_t len)

The memrchr_P() function is like the memchr_P() function, except that it searches
backwards from the end of the len bytes pointed to by src instead of forwards from
the front. (Glibc, GNU extension.)
Returns:
The memrchr_P() function returns a pointer to the matching byte or NULL if the
character does not occur in the given memory area.

6.14.4.6

int strcasecmp_P (const char ∗ s1, PGM_P s2)

Compare two strings ignoring case.
The strcasecmp_P() function compares the two strings s1 and s2, ignoring the case of
the characters.
Parameters:
s1 A pointer to a string in the devices SRAM.
s2 A pointer to a string in the devices Flash.
Returns:
The strcasecmp_P() function returns an integer less than, equal to, or greater than
zero if s1 is found, respectively, to be less than, to match, or be greater than s2.
A consequence of the ordering used by strcasecmp_P() is that if s1 is an initial
substring of s2, then s1 is considered to be "less than" s2.

6.14.4.7

char ∗ strcasestr_P (const char ∗ s1, PGM_P s2)

This funtion is similar to strcasestr() except that s2 is pointer to a string in program
space.
6.14.4.8

char ∗ strcat_P (char ∗ dest, PGM_P src)

The strcat_P() function is similar to strcat() except that the src string must be located
in program space (flash).
Returns:
The strcat() function returns a pointer to the resulting string dest.

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6.14 : Program Space Utilities

6.14.4.9

124

PGM_P strchr_P (PGM_P s, int val)

Locate character in program space string.
The strchr_P() function locates the first occurrence of val (converted to a char) in the
string pointed to by s in program space. The terminating null character is considered
to be part of the string.
The strchr_P() function is similar to strchr() except that s is pointer to a string in
program space.
Returns:
The strchr_P() function returns a pointer to the matched character or NULL if the
character is not found.

6.14.4.10

PGM_P strchrnul_P (PGM_P s, int c)

The strchrnul_P() function is like strchr_P() except that if c is not found in s, then
it returns a pointer to the null byte at the end of s, rather than NULL. (Glibc, GNU
extension.)
Returns:
The strchrnul_P() function returns a pointer to the matched character, or a pointer
to the null byte at the end of s (i.e., s+strlen(s)) if the character is not found.

6.14.4.11

int strcmp_P (const char ∗ s1, PGM_P s2)

The strcmp_P() function is similar to strcmp() except that s2 is pointer to a string in
program space.
Returns:
The strcmp_P() function returns an integer less than, equal to, or greater than zero
if s1 is found, respectively, to be less than, to match, or be greater than s2. A
consequence of the ordering used by strcmp_P() is that if s1 is an initial substring
of s2, then s1 is considered to be "less than" s2.

6.14.4.12

char ∗ strcpy_P (char ∗ dest, PGM_P src)

The strcpy_P() function is similar to strcpy() except that src is a pointer to a string in
program space.
Returns:
The strcpy_P() function returns a pointer to the destination string dest.

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6.14 : Program Space Utilities

6.14.4.13

125

size_t strcspn_P (const char ∗ s, PGM_P reject)

The strcspn_P() function calculates the length of the initial segment of s which consists
entirely of characters not in reject. This function is similar to strcspn() except that
reject is a pointer to a string in program space.
Returns:
The strcspn_P() function returns the number of characters in the initial segment of
s which are not in the string reject. The terminating zero is not considered as a
part of string.

6.14.4.14

size_t strlcat_P (char ∗ dst, PGM_P, size_t siz)

Concatenate two strings.
The strlcat_P() function is similar to strlcat(), except that the src string must be located
in program space (flash).
Appends src to string dst of size siz (unlike strncat(), siz is the full size of dst, not space
left). At most siz-1 characters will be copied. Always NULL terminates (unless siz <=
strlen(dst)).
Returns:
The strlcat_P() function returns strlen(src) + MIN(siz, strlen(initial dst)). If retval
>= siz, truncation occurred.

6.14.4.15

size_t strlcpy_P (char ∗ dst, PGM_P, size_t siz)

Copy a string from progmem to RAM.
Copy src to string dst of size siz. At most siz-1 characters will be copied. Always
NULL terminates (unless siz == 0).
Returns:
The strlcpy_P() function returns strlen(src). If retval >= siz, truncation occurred.

6.14.4.16

size_t strlen_P (PGM_P src)

The strlen_P() function is similar to strlen(), except that src is a pointer to a string in
program space.
Returns:
The strlen() function returns the number of characters in src.

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6.14 : Program Space Utilities

6.14.4.17

126

int strncasecmp_P (const char ∗ s1, PGM_P s2, size_t n)

Compare two strings ignoring case.
The strncasecmp_P() function is similar to strcasecmp_P(), except it only compares the
first n characters of s1.
Parameters:
s1 A pointer to a string in the devices SRAM.
s2 A pointer to a string in the devices Flash.
n The maximum number of bytes to compare.
Returns:
The strncasecmp_P() function returns an integer less than, equal to, or greater
than zero if s1 (or the first n bytes thereof) is found, respectively, to be less
than, to match, or be greater than s2. A consequence of the ordering used by
strncasecmp_P() is that if s1 is an initial substring of s2, then s1 is considered to
be "less than" s2.

6.14.4.18

char ∗ strncat_P (char ∗ dest, PGM_P src, size_t len)

Concatenate two strings.
The strncat_P() function is similar to strncat(), except that the src string must be located
in program space (flash).
Returns:
The strncat_P() function returns a pointer to the resulting string dest.

6.14.4.19

int strncmp_P (const char ∗ s1, PGM_P s2, size_t n)

The strncmp_P() function is similar to strcmp_P() except it only compares the first (at
most) n characters of s1 and s2.
Returns:
The strncmp_P() function returns an integer less than, equal to, or greater than zero
if s1 (or the first n bytes thereof) is found, respectively, to be less than, to match,
or be greater than s2.

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6.14 : Program Space Utilities

6.14.4.20

127

char ∗ strncpy_P (char ∗ dest, PGM_P src, size_t n)

The strncpy_P() function is similar to strcpy_P() except that not more than n bytes of
src are copied. Thus, if there is no null byte among the first n bytes of src, the result
will not be null-terminated.
In the case where the length of src is less than that of n, the remainder of dest will be
padded with nulls.
Returns:
The strncpy_P() function returns a pointer to the destination string dest.

6.14.4.21

size_t strnlen_P (PGM_P src, size_t len)

Determine the length of a fixed-size string.
The strnlen_P() function is similar to strnlen(), except that src is a pointer to a string
in program space.
Returns:
The strnlen_P function returns strlen_P(src), if that is less than len, or len if
there is no ’\0’ character among the first len characters pointed to by src.

6.14.4.22

char ∗ strpbrk_P (const char ∗ s, PGM_P accept)

The strpbrk_P() function locates the first occurrence in the string s of any of the characters in the flash string accept. This function is similar to strpbrk() except that
accept is a pointer to a string in program space.
Returns:
The strpbrk_P() function returns a pointer to the character in s that matches one of
the characters in accept, or NULL if no such character is found. The terminating
zero is not considered as a part of string: if one or both args are empty, the result
will NULL.

6.14.4.23

PGM_P strrchr_P (PGM_P s, int val)

Locate character in string.
The strrchr_P() function returns a pointer to the last occurrence of the character val
in the flash string s.
Returns:
The strrchr_P() function returns a pointer to the matched character or NULL if the
character is not found.

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6.15 : Power Reduction Management

6.14.4.24

128

char ∗ strsep_P (char ∗∗ sp, PGM_P delim)

Parse a string into tokens.
The strsep_P() function locates, in the string referenced by ∗sp, the first occurrence of
any character in the string delim (or the terminating ’\0’ character) and replaces it
with a ’\0’. The location of the next character after the delimiter character (or NULL,
if the end of the string was reached) is stored in ∗sp. An “empty” field, i.e. one
caused by two adjacent delimiter characters, can be detected by comparing the location
referenced by the pointer returned in ∗sp to ’\0’. This function is similar to strsep()
except that delim is a pointer to a string in program space.
Returns:
The strsep_P() function returns a pointer to the original value of ∗sp. If ∗sp is
initially NULL, strsep_P() returns NULL.

6.14.4.25

size_t strspn_P (const char ∗ s, PGM_P accept)

The strspn_P() function calculates the length of the initial segment of s which consists entirely of characters in accept. This function is similar to strspn() except that
accept is a pointer to a string in program space.
Returns:
The strspn_P() function returns the number of characters in the initial segment of
s which consist only of characters from accept. The terminating zero is not
considered as a part of string.

6.14.4.26

char ∗ strstr_P (const char ∗ s1, PGM_P s2)

Locate a substring.
The strstr_P() function finds the first occurrence of the substring s2 in the string s1.
The terminating ’\0’ characters are not compared. The strstr_P() function is similar to
strstr() except that s2 is pointer to a string in program space.
Returns:
The strstr_P() function returns a pointer to the beginning of the substring, or NULL
if the substring is not found. If s2 points to a string of zero length, the function
returns s1.

6.15

: Power Reduction Management

#include 

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6.15 : Power Reduction Management

129

Many AVRs contain a Power Reduction Register (PRR) or Registers (PRRx) that allow
you to reduce power consumption by disabling or enabling various on-board peripherals as needed.
There are many macros in this header file that provide an easy interface to enable or
disable on-board peripherals to reduce power. See the table below.
Note:
Not all AVR devices have a Power Reduction Register (for example the ATmega128). On those devices without a Power Reduction Register, these macros
are not available.
Not all AVR devices contain the same peripherals (for example, the LCD interface), or they will be named differently (for example, USART and USART0).
Please consult your device’s datasheet, or the header file, to find out which macros
are applicable to your device.

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6.15 : Power Reduction Management

130

Power Macro

Description

Applicable for device

power_adc_enable()

Enable the Analog to Digital
Converter module.

ATmega640, ATmega1280,
ATmega1281, ATmega2560,
ATmega2561, AT90USB646,
AT90USB647, AT90USB1286,
AT90USB1287, AT90PWM1,
AT90PWM2, AT90PWM3,
ATmega165, ATmega165P,
ATmega325, ATmega3250,
ATmega645, ATmega6450,
ATmega169, ATmega169P,
ATmega329, ATmega3290,
ATmega649, ATmega6490,
ATmega164P, ATmega324P,
ATmega644, ATmega48,
ATmega88, ATmega168,
ATtiny24, ATtiny44, ATtiny84,
ATtiny25, ATtiny45, ATtiny85,
ATtiny261, ATtiny461,
ATtiny861

power_adc_disable()

Disable the Analog to Digital
Converter module.

ATmega640, ATmega1280,
ATmega1281, ATmega2560,
ATmega2561, AT90USB646,
AT90USB647, AT90USB1286,
AT90USB1287, AT90PWM1,
AT90PWM2, AT90PWM3,
ATmega165, ATmega165P,
ATmega325, ATmega3250,
ATmega645, ATmega6450,
ATmega169, ATmega169P,
ATmega329, ATmega3290,
ATmega649, ATmega6490,
ATmega164P, ATmega324P,
ATmega644, ATmega48,
ATmega88, ATmega168,
ATtiny24, ATtiny44, ATtiny84,
ATtiny25, ATtiny45, ATtiny85,
ATtiny261, ATtiny461,
ATtiny861

power_lcd_enable()

Enable the LCD module.

ATmega169, ATmega169P,
ATmega329, ATmega3290,
ATmega649, ATmega6490

power_lcd_disable().

Disable the LCD module.

ATmega169, ATmega169P,
ATmega329, ATmega3290,
ATmega649, ATmega6490

power_psc0_enable()

Enable the Power Stage
Controller 0 module.

AT90PWM1, AT90PWM2,
AT90PWM3

power_psc0_disable()

Disable the Power Stage
Controller 0 module.

AT90PWM1, AT90PWM2,
AT90PWM3

power_psc1_enable()

Enable the Power Stage
Controller 1 module.

AT90PWM1, AT90PWM2,
AT90PWM3

power_psc1_disable()

Disable the Power Stage

AT90PWM1, AT90PWM2,
AT90PWM3

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power_psc2_enable()

Enable the Power Stage
Controller 2 module.

AT90PWM1, AT90PWM2,
AT90PWM3

power_psc2_disable()

Disable the Power Stage
Controller 2 module.

AT90PWM1, AT90PWM2,
AT90PWM3

power_spi_enable()

Enable the Serial Peripheral
Interface module.

ATmega640, ATmega1280,
ATmega1281, ATmega2560,

Additional notes from 

6.16

131

Some of the newer AVRs contain a System Clock Prescale Register (CLKPR) that
allows you to decrease the system clock frequency and the power consumption when
the need for processing power is low. Below are two macros and an enumerated type
that can be used to interface to the Clock Prescale Register.
Note:
Not all AVR devices have a Clock Prescale Register. On those devices without a
Clock Prescale Register, these macros are not available.
typedef enum
{
clock_div_1 = 0,
clock_div_2 = 1,
clock_div_4 = 2,
clock_div_8 = 3,
clock_div_16 = 4,
clock_div_32 = 5,
clock_div_64 = 6,
clock_div_128 = 7,
clock_div_256 = 8
} clock_div_t;

Clock prescaler setting enumerations.
clock_prescale_set(x)

Set the clock prescaler register select bits, selecting a system clock division setting.
They type of x is clock_div_t.
clock_prescale_get()

Gets and returns the clock prescaler register setting. The return type is clock_div_t.

6.16

Additional notes from 

The  file is included by all of the  files,
which use macros defined here to make the special function register definitions look
like C variables or simple constants, depending on the _SFR_ASM_COMPAT define.
Some examples from  to show how to define such macros:
#define
#define
#define
#define
#define

PORTA
EEAR
UDR0
TCNT3
CANIDT

_SFR_IO8(0x02)
_SFR_IO16(0x21)
_SFR_MEM8(0xC6)
_SFR_MEM16(0x94)
_SFR_MEM32(0xF0)

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6.16

Additional notes from 

132

If _SFR_ASM_COMPAT is not defined, C programs can use names like PORTA directly
in C expressions (also on the left side of assignment operators) and GCC will do the
right thing (use short I/O instructions if possible). The __SFR_OFFSET definition is
not used in any way in this case.
Define _SFR_ASM_COMPAT as 1 to make these names work as simple constants (addresses of the I/O registers). This is necessary when included in preprocessed assembler (∗.S) source files, so it is done automatically if __ASSEMBLER__ is defined. By
default, all addresses are defined as if they were memory addresses (used in lds/sts
instructions). To use these addresses in in/out instructions, you must subtract 0x20
from them.
For more backwards compatibility, insert the following at the start of your old assembler source file:
#define __SFR_OFFSET 0

This automatically subtracts 0x20 from I/O space addresses, but it’s a hack, so it is
recommended to change your source: wrap such addresses in macros defined here, as
shown below. After this is done, the __SFR_OFFSET definition is no longer necessary
and can be removed.
Real example - this code could be used in a boot loader that is portable between devices
with SPMCR at different addresses.
: #define SPMCR _SFR_IO8(0x37)
: #define SPMCR _SFR_MEM8(0x68)
#if _SFR_IO_REG_P(SPMCR)
out
_SFR_IO_ADDR(SPMCR), r24
#else
sts
_SFR_MEM_ADDR(SPMCR), r24
#endif

You can use the in/out/cbi/sbi/sbic/sbis instructions, without the _SFR_IO_REG_P test, if you know that the register is in the I/O space (as with SREG, for
example). If it isn’t, the assembler will complain (I/O address out of range 0...0x3f),
so this should be fairly safe.
If you do not define __SFR_OFFSET (so it will be 0x20 by default), all special register
addresses are defined as memory addresses (so SREG is 0x5f), and (if code size and
speed are not important, and you don’t like the ugly #if above) you can always use
lds/sts to access them. But, this will not work if __SFR_OFFSET != 0x20, so use a
different macro (defined only if __SFR_OFFSET == 0x20) for safety:
sts

_SFR_ADDR(SPMCR), r24

In C programs, all 3 combinations of _SFR_ASM_COMPAT and __SFR_OFFSET are
supported - the _SFR_ADDR(SPMCR) macro can be used to get the address of the
SPMCR register (0x57 or 0x68 depending on device).
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6.17 : Special function registers

6.17

: Special function registers

6.17.1

Detailed Description

133

When working with microcontrollers, many of the tasks usually consist of controlling
the peripherals that are connected to the device, respectively programming the subsystems that are contained in the controller (which by itself communicate with the circuitry
connected to the controller).
The AVR series of microcontrollers offers two different paradigms to perform this task.
There’s a separate IO address space available (as it is known from some high-level
CISC CPUs) that can be addressed with specific IO instructions that are applicable to
some or all of the IO address space (in, out, sbi etc.). The entire IO address space
is also made available as memory-mapped IO, i. e. it can be accessed using all the
MCU instructions that are applicable to normal data memory. The IO register space is
mapped into the data memory address space with an offset of 0x20 since the bottom
of this space is reserved for direct access to the MCU registers. (Actual SRAM is
available only behind the IO register area, starting at either address 0x60, or 0x100
depending on the device.)
AVR Libc supports both these paradigms. While by default, the implementation uses
memory-mapped IO access, this is hidden from the programmer. So the programmer
can access IO registers either with a special function like outb():
#include 
outb(PORTA, 0x33);

or they can assign a value directly to the symbolic address:
PORTA = 0x33;

The compiler’s choice of which method to use when actually accessing the IO port is
completely independent of the way the programmer chooses to write the code. So even
if the programmer uses the memory-mapped paradigm and writes
PORTA |= 0x40;

the compiler can optimize this into the use of an sbi instruction (of course, provided
the target address is within the allowable range for this instruction, and the right-hand
side of the expression is a constant value known at compile-time).
The advantage of using the memory-mapped paradigm in C programs is that it makes
the programs more portable to other C compilers for the AVR platform. Some people
might also feel that this is more readable. For example, the following two statements
would be equivalent:

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134

outb(DDRD, inb(DDRD) & ~LCDBITS);
DDRD &= ~LCDBITS;

The generated code is identical for both. Without optimization, the compiler strictly
generates code following the memory-mapped paradigm, while with optimization
turned on, code is generated using the (faster and smaller) in/out MCU instructions.
Note that special care must be taken when accessing some of the 16-bit timer IO registers where access from both the main program and within an interrupt context can
happen. See Why do some 16-bit timer registers sometimes get trashed?.
Porting programs that use sbi/cbi

As described above, access to the AVR single bit set and clear instructions are provided
via the standard C bit manipulation commands. The sbi and cbi commands are no
longer directly supported. sbi (sfr,bit) can be replaced by sfr |= _BV(bit) .
ie: sbi(PORTB, PB1); is now PORTB |= _BV(PB1);
This actually is more flexible than having sbi directly, as the optimizer will use a hardware sbi if appropriate, or a read/or/write if not. You do not need to keep track of which
registers sbi/cbi will operate on.
Likewise, cbi (sfr,bit) is now sfr &= ∼(_BV(bit));
Modules
• Additional notes from 
Bit manipulation
• #define _BV(bit) (1 << (bit))
IO register bit manipulation
•
•
•
•

#define bit_is_set(sfr, bit) (_SFR_BYTE(sfr) & _BV(bit))
#define bit_is_clear(sfr, bit) (!(_SFR_BYTE(sfr) & _BV(bit)))
#define loop_until_bit_is_set(sfr, bit) do { } while (bit_is_clear(sfr, bit))
#define loop_until_bit_is_clear(sfr, bit) do { } while (bit_is_set(sfr, bit))

6.17.2
6.17.2.1

Define Documentation
#define _BV(bit) (1 << (bit))

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6.17 : Special function registers

135

#include 

Converts a bit number into a byte value.
Note:
The bit shift is performed by the compiler which then inserts the result into the
code. Thus, there is no run-time overhead when using _BV().

6.17.2.2

#define bit_is_clear(sfr, bit) (!(_SFR_BYTE(sfr) & _BV(bit)))

#include 

Test whether bit bit in IO register sfr is clear. This will return non-zero if the bit is
clear, and a 0 if the bit is set.
6.17.2.3

#define bit_is_set(sfr, bit) (_SFR_BYTE(sfr) & _BV(bit))

#include 

Test whether bit bit in IO register sfr is set. This will return a 0 if the bit is clear,
and non-zero if the bit is set.
6.17.2.4

#define loop_until_bit_is_clear(sfr, bit) do { } while (bit_is_set(sfr, bit))

#include 

Wait until bit bit in IO register sfr is clear.
6.17.2.5

#define loop_until_bit_is_set(sfr, bit) do { } while (bit_is_clear(sfr, bit))

#include 

Wait until bit bit in IO register sfr is set.

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6.18 : Power Management and Sleep Modes

6.18

: Power Management and Sleep Modes

6.18.1

Detailed Description

136

#include 

Use of the SLEEP instruction can allow an application to reduce its power comsumption considerably. AVR devices can be put into different sleep modes. Refer to the
datasheet for the details relating to the device you are using.
There are several macros provided in this header file to actually put the device into
sleep mode. The simplest way is to optionally set the desired sleep mode using set_sleep_mode() (it usually defaults to idle mode where the CPU is put to sleep but
all peripheral clocks are still running), and then call sleep_mode(). Unless it is the
purpose to lock the CPU hard (until a hardware reset), interrupts need to be enabled at
this point. This macro automatically takes care to enable the sleep mode in the CPU
before going to sleep, and disable it again afterwards.
As this combined macro might cause race conditions in some situations, the individual
steps of manipulating the sleep enable (SE) bit, and actually issuing the SLEEP instruction are provided in the macros sleep_enable(), sleep_disable(), and
sleep_cpu(). This also allows for test-and-sleep scenarios that take care of not
missing the interrupt that will awake the device from sleep.
Example:
#include 
#include 
...
cli();
if (some_condition) {
sleep_enable();
sei();
sleep_cpu();
sleep_disable();
}
sei();

This sequence ensures an atomic test of some_condition with interrupts being disabled. If the condition is met, sleep mode will be prepared, and the SLEEP instruction
will be scheduled immediately after an SEI instruction. As the intruction right after
the SEI is guaranteed to be executed before an interrupt could trigger, it is sure the
device will really be put to sleep.
Sleep Modes
Note:
Some of these modes are not available on all devices. See the datasheet for target
device for the available sleep modes.
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6.18 : Power Management and Sleep Modes

•
•
•
•
•
•

137

#define SLEEP_MODE_IDLE 0
#define SLEEP_MODE_ADC _BV(SM0)
#define SLEEP_MODE_PWR_DOWN _BV(SM1)
#define SLEEP_MODE_PWR_SAVE (_BV(SM0) | _BV(SM1))
#define SLEEP_MODE_STANDBY (_BV(SM1) | _BV(SM2))
#define SLEEP_MODE_EXT_STANDBY (_BV(SM0) | _BV(SM1) | _BV(SM2))

Sleep Functions
•
•
•
•
•

void set_sleep_mode (uint8_t mode)
void sleep_mode (void)
void sleep_enable (void)
void sleep_disable (void)
void sleep_cpu (void)

6.18.2
6.18.2.1

Define Documentation
#define SLEEP_MODE_ADC _BV(SM0)

ADC Noise Reduction Mode.
6.18.2.2 #define SLEEP_MODE_EXT_STANDBY (_BV(SM0) | _BV(SM1) | _BV(SM2))
Extended Standby Mode.
6.18.2.3

#define SLEEP_MODE_IDLE 0

Idle mode.
6.18.2.4

#define SLEEP_MODE_PWR_DOWN _BV(SM1)

Power Down Mode.
6.18.2.5

#define SLEEP_MODE_PWR_SAVE (_BV(SM0) | _BV(SM1))

Power Save Mode.
6.18.2.6

#define SLEEP_MODE_STANDBY (_BV(SM1) | _BV(SM2))

Standby Mode.

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6.19 : avr-libc version macros

6.18.3
6.18.3.1

138

Function Documentation
void set_sleep_mode (uint8_t mode)

Select a sleep mode.
6.18.3.2

void sleep_cpu (void)

Put the device into sleep mode. The SE bit must be set beforehand, and it is recommended to clear it afterwards.
6.18.3.3

void sleep_disable (void)

Clear the SE (sleep enable) bit.
6.18.3.4

void sleep_enable (void)

Set the SE (sleep enable) bit.
6.18.3.5

void sleep_mode (void)

Put the device in sleep mode. How the device is brought out of sleep mode depends on
the specific mode selected with the set_sleep_mode() function. See the data sheet for
your device for more details.

6.19

: avr-libc version macros

6.19.1

Detailed Description

#include 

This header file defines macros that contain version numbers and strings describing the
current version of avr-libc.
The version number itself basically consists of three pieces that are separated by a
dot: the major number, the minor number, and the revision number. For development
versions (which use an odd minor number), the string representation additionally gets
the date code (YYYYMMDD) appended.
This file will also be included by . That way, portable tests can be
implemented using  that can be used in code that wants to remain
backwards-compatible to library versions prior to the date when the library version
API had been added, as referenced but undefined C preprocessor macros automatically
evaluate to 0.

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6.19 : avr-libc version macros

139

Defines
• #define __AVR_LIBC_VERSION_STRING__ "@AVR_LIBC_VERSION@"
• #define
__AVR_LIBC_VERSION__
@AVR_LIBC_VERSION_NUMERIC@UL
• #define __AVR_LIBC_DATE_STRING__ "@AVR_LIBC_RELDATE@"
• #define __AVR_LIBC_DATE_ @AVR_LIBC_RELDATE@UL
• #define __AVR_LIBC_MAJOR__ @AVR_LIBC_MAJOR@
• #define __AVR_LIBC_MINOR__ @AVR_LIBC_MINOR@
• #define __AVR_LIBC_REVISION__ @AVR_LIBC_REVISION@
6.19.2
6.19.2.1

Define Documentation
#define __AVR_LIBC_DATE_ @AVR_LIBC_RELDATE@UL

Numerical representation of the release date.
6.19.2.2 #define
RELDATE@"

__AVR_LIBC_DATE_STRING__

"@AVR_LIBC_-

String literal representation of the release date.
6.19.2.3

#define __AVR_LIBC_MAJOR__ @AVR_LIBC_MAJOR@

Library major version number.
6.19.2.4

#define __AVR_LIBC_MINOR__ @AVR_LIBC_MINOR@

Library minor version number.
6.19.2.5

#define __AVR_LIBC_REVISION__ @AVR_LIBC_REVISION@

Library revision number.
6.19.2.6 #define
NUMERIC@UL

__AVR_LIBC_VERSION__

@AVR_LIBC_VERSION_-

Numerical representation of the current library version.
In the numerical representation, the major number is multiplied by 10000, the minor
number by 100, and all three parts are then added. It is intented to provide a monotonically increasing numerical value that can easily be used in numerical checks.

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6.20 : Watchdog timer handling

6.19.2.7 #define
VERSION@"

__AVR_LIBC_VERSION_STRING__

140

"@AVR_LIBC_-

String literal representation of the current library version.

6.20

: Watchdog timer handling

6.20.1

Detailed Description

#include 

This header file declares the interface to some inline macros handling the watchdog
timer present in many AVR devices. In order to prevent the watchdog timer configuration from being accidentally altered by a crashing application, a special timed sequence
is required in order to change it. The macros within this header file handle the required
sequence automatically before changing any value. Interrupts will be disabled during
the manipulation.
Note:
Depending on the fuse configuration of the particular device, further restrictions
might apply, in particular it might be disallowed to turn off the watchdog timer.
Note that for newer devices (ATmega88 and newer, effectively any AVR that has the option to also generate interrupts), the watchdog timer remains active even after a system
reset (except a power-on condition), using the fastest prescaler value (approximately
15 ms). It is therefore required to turn off the watchdog early during program startup,
the datasheet recommends a sequence like the following:
#include 
#include 
uint8_t mcusr_mirror;
void get_mcusr(void) \
__attribute__((naked)) \
__attribute__((section(".init3")));
void get_mcusr(void)
{
mcusr_mirror = MCUSR;
MCUSR = 0;
wdt_disable();
}

Saving the value of MCUSR in mcusr_mirror is only needed if the application
later wants to examine the reset source, but clearing in particular the watchdog reset
flag before disabling the watchdog is required, according to the datasheet.

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6.20 : Watchdog timer handling

141

Defines
•
•
•
•
•
•
•
•
•
•
•
•
•

#define wdt_reset() __asm__ __volatile__ ("wdr")
#define wdt_disable()
#define wdt_enable(timeout) _wdt_write(timeout)
#define WDTO_15MS 0
#define WDTO_30MS 1
#define WDTO_60MS 2
#define WDTO_120MS 3
#define WDTO_250MS 4
#define WDTO_500MS 5
#define WDTO_1S 6
#define WDTO_2S 7
#define WDTO_4S 8
#define WDTO_8S 9

6.20.2
6.20.2.1

Define Documentation
#define wdt_disable()

Value:
__asm__ __volatile__ ( \
"in __tmp_reg__, __SREG__" "\n\t" \
"cli" "\n\t" \
"out %0, %1" "\n\t" \
"out %0, __zero_reg__" "\n\t" \
"out __SREG__,__tmp_reg__" "\n\t" \
: /* no outputs */ \
: "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)), \
"r" ((uint8_t)(_BV(_WD_CHANGE_BIT) | _BV(WDE))) \
: "r0" \
)

Disable the watchdog timer, if possible. This attempts to turn off the Enable bit in the
watchdog control register. See the datasheet for details.
6.20.2.2

#define wdt_enable(timeout) _wdt_write(timeout)

Enable the watchdog timer, configuring it for expiry after timeout (which is a combination of the WDP0 through WDP2 bits to write into the WDTCR register; For those
devices that have a WDTCSR register, it uses the combination of the WDP0 through
WDP3 bits).
See also the symbolic constants WDTO_15MS et al.

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6.20 : Watchdog timer handling

6.20.2.3

142

#define wdt_reset() __asm__ __volatile__ ("wdr")

Reset the watchdog timer. When the watchdog timer is enabled, a call to this instruction
is required before the timer expires, otherwise a watchdog-initiated device reset will
occur.
6.20.2.4

#define WDTO_120MS 3

See WDT0_15MS
6.20.2.5

#define WDTO_15MS 0

Symbolic constants for the watchdog timeout. Since the watchdog timer is based on
a free-running RC oscillator, the times are approximate only and apply to a supply
voltage of 5 V. At lower supply voltages, the times will increase. For older devices,
the times will be as large as three times when operating at Vcc = 3 V, while the newer
devices (e. g. ATmega128, ATmega8) only experience a negligible change.
Possible timeout values are: 15 ms, 30 ms, 60 ms, 120 ms, 250 ms, 500 ms, 1 s, 2 s.
(Some devices also allow for 4 s and 8 s.) Symbolic constants are formed by the prefix
WDTO_, followed by the time.
Example that would select a watchdog timer expiry of approximately 500 ms:
wdt_enable(WDTO_500MS);

6.20.2.6

#define WDTO_1S 6

See WDT0_15MS
6.20.2.7

#define WDTO_250MS 4

See WDT0_15MS
6.20.2.8

#define WDTO_2S 7

See WDT0_15MS
6.20.2.9

#define WDTO_30MS 1

See WDT0_15MS
6.20.2.10

#define WDTO_4S 8

See WDT0_15MS Note: This is only available on the ATtiny2313, ATtiny24, ATtiny44, ATtiny84, ATtiny25, ATtiny45, ATtiny85, ATtiny261, ATtiny461, ATtiny861,
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6.21 : CRC Computations

143

ATmega48, ATmega88, ATmega168, ATmega164P, ATmega324P, ATmega644P, ATmega644, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, ATmega8HVA, ATmega16HVA, ATmega406, AT90PWM1, AT90PWM2, AT90PWM3,
AT90USB82, AT90USB162, AT90USB646, AT90USB647, AT90USB1286,
AT90USB1287.
6.20.2.11

#define WDTO_500MS 5

See WDT0_15MS
6.20.2.12

#define WDTO_60MS 2

WDT0_15MS
6.20.2.13

#define WDTO_8S 9

See WDT0_15MS Note: This is only available on the ATtiny2313, ATtiny24, ATtiny44, ATtiny84, ATtiny25, ATtiny45, ATtiny85, ATtiny261, ATtiny461, ATtiny861,
ATmega48, ATmega88, ATmega168, ATmega164P, ATmega324P, ATmega644P, ATmega644, ATmega640, ATmega1280, ATmega1281, ATmega2560, ATmega2561, ATmega8HVA, ATmega16HVA, ATmega406, AT90PWM1, AT90PWM2, AT90PWM3,
AT90USB82, AT90USB162, AT90USB646, AT90USB647, AT90USB1286,
AT90USB1287.

6.21

: CRC Computations

6.21.1

Detailed Description

#include 

This header file provides a optimized inline functions for calculating cyclic redundancy
checks (CRC) using common polynomials.
References:

See the Dallas Semiconductor app note 27 for 8051 assembler example and general
CRC optimization suggestions. The table on the last page of the app note is the key to
understanding these implementations.

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6.21 : CRC Computations

144

Jack Crenshaw’s "Implementing CRCs" article in the January 1992 isue of Embedded
Systems Programming. This may be difficult to find, but it explains CRC’s in very clear
and concise terms. Well worth the effort to obtain a copy.
A typical application would look like:
// Dallas iButton test vector.
uint8_t serno[] = { 0x02, 0x1c, 0xb8, 0x01, 0, 0, 0, 0xa2 };
int
checkcrc(void)
{
uint8_t crc = 0, i;
for (i = 0; i < sizeof serno / sizeof serno[0]; i++)
crc = _crc_ibutton_update(crc, serno[i]);
return crc; // must be 0
}

Functions
•
•
•
•

static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t __data)
static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc, uint8_t __data)
static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t __data)
static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t __data)

6.21.2

Function Documentation

6.21.2.1 static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t __data) [static]
Optimized CRC-16 calculation.
Polynomial: x∧ 16 + x∧ 15 + x∧ 2 + 1 (0xa001)
Initial value: 0xffff
This CRC is normally used in disk-drive controllers.
The following is the equivalent functionality written in C.
uint16_t
crc16_update(uint16_t crc, uint8_t a)
{
int i;
crc ^= a;

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6.21 : CRC Computations

for (i = 0;
{
if (crc
crc
else
crc
}

145

i < 8; ++i)
& 1)
= (crc >> 1) ^ 0xA001;
= (crc >> 1);

return crc;
}

6.21.2.2 static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t __data) [static]
Optimized CRC-CCITT calculation.
Polynomial: x∧ 16 + x∧ 12 + x∧ 5 + 1 (0x8408)
Initial value: 0xffff
This is the CRC used by PPP and IrDA.
See RFC1171 (PPP protocol) and IrDA IrLAP 1.1
Note:
Although the CCITT polynomial is the same as that used by the Xmodem protocol,
they are quite different. The difference is in how the bits are shifted through the
alorgithm. Xmodem shifts the MSB of the CRC and the input first, while CCITT
shifts the LSB of the CRC and the input first.
The following is the equivalent functionality written in C.
uint16_t
crc_ccitt_update (uint16_t crc, uint8_t data)
{
data ^= lo8 (crc);
data ^= data << 4;
return ((((uint16_t)data << 8) | hi8 (crc)) ^ (uint8_t)(data >> 4)
^ ((uint16_t)data << 3));
}

6.21.2.3 static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t __data) [static]
Optimized Dallas (now Maxim) iButton 8-bit CRC calculation.
Polynomial: x∧ 8 + x∧ 5 + x∧ 4 + 1 (0x8C)
Initial value: 0x0
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6.21 : CRC Computations

146

See http://www.maxim-ic.com/appnotes.cfm/appnote_number/27
The following is the equivalent functionality written in C.
uint8_t
_crc_ibutton_update(uint8_t crc, uint8_t data)
{
uint8_t i;
crc = crc ^
for (i = 0;
{
if (crc
crc
else
crc
}

data;
i < 8; i++)
& 0x01)
= (crc >> 1) ^ 0x8C;
>>= 1;

return crc;
}

6.21.2.4 static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc,
uint8_t __data) [static]
Optimized CRC-XMODEM calculation.
Polynomial: x∧ 16 + x∧ 12 + x∧ 5 + 1 (0x1021)
Initial value: 0x0
This is the CRC used by the Xmodem-CRC protocol.
The following is the equivalent functionality written in C.
uint16_t
crc_xmodem_update (uint16_t crc, uint8_t data)
{
int i;
crc = crc ^ ((uint16_t)data << 8);
for (i=0; i<8; i++)
{
if (crc & 0x8000)
crc = (crc << 1) ^ 0x1021;
else
crc <<= 1;
}
return crc;
}

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6.22 : Convenience functions for busy-wait delay loops

147

6.22

: Convenience functions for busy-wait delay
loops

6.22.1

Detailed Description

#define F_CPU 1000000UL // 1 MHz
//#define F_CPU 14.7456E6
#include 

Note:
As an alternative method, it is possible to pass the F_CPU macro down to the compiler from the Makefile. Obviously, in that case, no #define statement should
be used.
The functions in this header file are wrappers around the basic busy-wait functions from
. They are meant as convenience functions where actual time
values can be specified rather than a number of cycles to wait for. The idea behind is
that compile-time constant expressions will be eliminated by compiler optimization so
floating-point expressions can be used to calculate the number of delay cycles needed
based on the CPU frequency passed by the macro F_CPU.
Note:
In order for these functions to work as intended, compiler optimizations must be
enabled, and the delay time must be an expression that is a known constant at
compile-time. If these requirements are not met, the resulting delay will be much
longer (and basically unpredictable), and applications that otherwise do not use
floating-point calculations will experience severe code bloat by the floating-point
library routines linked into the application.
The functions available allow the specification of microsecond, and millisecond delays
directly, using the application-supplied macro F_CPU as the CPU clock frequency (in
Hertz).
Functions
• void _delay_us (double __us)
• void _delay_ms (double __ms)
6.22.2
6.22.2.1

Function Documentation
void _delay_ms (double __ms)

Perform a delay of __ms milliseconds, using _delay_loop_2().

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6.23 : Basic busy-wait delay loops

148

The macro F_CPU is supposed to be defined to a constant defining the CPU clock
frequency (in Hertz).
The maximal possible delay is 262.14 ms / F_CPU in MHz.
6.22.2.2

void _delay_us (double __us)

Perform a delay of __us microseconds, using _delay_loop_1().
The macro F_CPU is supposed to be defined to a constant defining the CPU clock
frequency (in Hertz).
The maximal possible delay is 768 us / F_CPU in MHz.

6.23

: Basic busy-wait delay loops

6.23.1

Detailed Description

#include 

The functions in this header file implement simple delay loops that perform a busywaiting. They are typically used to facilitate short delays in the program execution.
They are implemented as count-down loops with a well-known CPU cycle count per
loop iteration. As such, no other processing can occur simultaneously. It should be
kept in mind that the functions described here do not disable interrupts.
In general, for long delays, the use of hardware timers is much preferrable, as they
free the CPU, and allow for concurrent processing of other events while the timer is
running. However, in particular for very short delays, the overhead of setting up a
hardware timer is too much compared to the overall delay time.
Two inline functions are provided for the actual delay algorithms.
Functions
• void _delay_loop_1 (uint8_t __count)
• void _delay_loop_2 (uint16_t __count)
6.23.2
6.23.2.1

Function Documentation
void _delay_loop_1 (uint8_t __count)

Delay loop using an 8-bit counter __count, so up to 256 iterations are possible. (The
value 256 would have to be passed as 0.) The loop executes three CPU cycles per
iteration, not including the overhead the compiler needs to setup the counter register.
Thus, at a CPU speed of 1 MHz, delays of up to 768 microseconds can be achieved.
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6.24 : Parity bit generation

6.23.2.2

149

void _delay_loop_2 (uint16_t __count)

Delay loop using a 16-bit counter __count, so up to 65536 iterations are possible.
(The value 65536 would have to be passed as 0.) The loop executes four CPU cycles
per iteration, not including the overhead the compiler requires to setup the counter
register pair.
Thus, at a CPU speed of 1 MHz, delays of up to about 262.1 milliseconds can be
achieved.

6.24

: Parity bit generation

6.24.1

Detailed Description

#include 

This header file contains optimized assembler code to calculate the parity bit for a byte.
Defines
• #define parity_even_bit(val)
6.24.2

Define Documentation

6.24.2.1

#define parity_even_bit(val)

Value:
(__extension__({
unsigned char __t;
__asm__ (
"mov __tmp_reg__,%0" "\n\t"
"swap %0" "\n\t"
"eor %0,__tmp_reg__" "\n\t"
"mov __tmp_reg__,%0" "\n\t"
"lsr %0" "\n\t"
"lsr %0" "\n\t"
"eor %0,__tmp_reg__"
: "=r" (__t)
: "0" ((unsigned char)(val))
: "r0"
);
(((__t + 1) >> 1) & 1);
}))

Returns:
1 if val has an odd number of bits set.

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\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

6.25 : TWI bit mask definitions

6.25

: TWI bit mask definitions

6.25.1

Detailed Description

#include 

This header file contains bit mask definitions for use with the AVR TWI interface.
TWSR values
Mnemonics:
TW_MT_xxx - master transmitter
TW_MR_xxx - master receiver
TW_ST_xxx - slave transmitter
TW_SR_xxx - slave receiver
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

#define TW_START 0x08
#define TW_REP_START 0x10
#define TW_MT_SLA_ACK 0x18
#define TW_MT_SLA_NACK 0x20
#define TW_MT_DATA_ACK 0x28
#define TW_MT_DATA_NACK 0x30
#define TW_MT_ARB_LOST 0x38
#define TW_MR_ARB_LOST 0x38
#define TW_MR_SLA_ACK 0x40
#define TW_MR_SLA_NACK 0x48
#define TW_MR_DATA_ACK 0x50
#define TW_MR_DATA_NACK 0x58
#define TW_ST_SLA_ACK 0xA8
#define TW_ST_ARB_LOST_SLA_ACK 0xB0
#define TW_ST_DATA_ACK 0xB8
#define TW_ST_DATA_NACK 0xC0
#define TW_ST_LAST_DATA 0xC8
#define TW_SR_SLA_ACK 0x60
#define TW_SR_ARB_LOST_SLA_ACK 0x68
#define TW_SR_GCALL_ACK 0x70
#define TW_SR_ARB_LOST_GCALL_ACK 0x78
#define TW_SR_DATA_ACK 0x80
#define TW_SR_DATA_NACK 0x88
#define TW_SR_GCALL_DATA_ACK 0x90
#define TW_SR_GCALL_DATA_NACK 0x98

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150

6.25 : TWI bit mask definitions

•
•
•
•
•

#define TW_SR_STOP 0xA0
#define TW_NO_INFO 0xF8
#define TW_BUS_ERROR 0x00
#define TW_STATUS_MASK
#define TW_STATUS (TWSR & TW_STATUS_MASK)

R/∼W bit in SLA+R/W address field.
• #define TW_READ 1
• #define TW_WRITE 0
6.25.2
6.25.2.1

Define Documentation
#define TW_BUS_ERROR 0x00

illegal start or stop condition
6.25.2.2

#define TW_MR_ARB_LOST 0x38

arbitration lost in SLA+R or NACK
6.25.2.3

#define TW_MR_DATA_ACK 0x50

data received, ACK returned
6.25.2.4

#define TW_MR_DATA_NACK 0x58

data received, NACK returned
6.25.2.5

#define TW_MR_SLA_ACK 0x40

SLA+R transmitted, ACK received
6.25.2.6

#define TW_MR_SLA_NACK 0x48

SLA+R transmitted, NACK received
6.25.2.7

#define TW_MT_ARB_LOST 0x38

arbitration lost in SLA+W or data
6.25.2.8

#define TW_MT_DATA_ACK 0x28

data transmitted, ACK received
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151

6.25 : TWI bit mask definitions

6.25.2.9

#define TW_MT_DATA_NACK 0x30

data transmitted, NACK received
6.25.2.10

#define TW_MT_SLA_ACK 0x18

SLA+W transmitted, ACK received
6.25.2.11

#define TW_MT_SLA_NACK 0x20

SLA+W transmitted, NACK received
6.25.2.12

#define TW_NO_INFO 0xF8

no state information available
6.25.2.13

#define TW_READ 1

SLA+R address
6.25.2.14

#define TW_REP_START 0x10

repeated start condition transmitted
6.25.2.15

#define TW_SR_ARB_LOST_GCALL_ACK 0x78

arbitration lost in SLA+RW, general call received, ACK returned
6.25.2.16

#define TW_SR_ARB_LOST_SLA_ACK 0x68

arbitration lost in SLA+RW, SLA+W received, ACK returned
6.25.2.17

#define TW_SR_DATA_ACK 0x80

data received, ACK returned
6.25.2.18

#define TW_SR_DATA_NACK 0x88

data received, NACK returned
6.25.2.19

#define TW_SR_GCALL_ACK 0x70

general call received, ACK returned

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152

6.25 : TWI bit mask definitions

6.25.2.20

#define TW_SR_GCALL_DATA_ACK 0x90

general call data received, ACK returned
6.25.2.21

#define TW_SR_GCALL_DATA_NACK 0x98

general call data received, NACK returned
6.25.2.22

#define TW_SR_SLA_ACK 0x60

SLA+W received, ACK returned
6.25.2.23

#define TW_SR_STOP 0xA0

stop or repeated start condition received while selected
6.25.2.24

#define TW_ST_ARB_LOST_SLA_ACK 0xB0

arbitration lost in SLA+RW, SLA+R received, ACK returned
6.25.2.25

#define TW_ST_DATA_ACK 0xB8

data transmitted, ACK received
6.25.2.26

#define TW_ST_DATA_NACK 0xC0

data transmitted, NACK received
6.25.2.27

#define TW_ST_LAST_DATA 0xC8

last data byte transmitted, ACK received
6.25.2.28

#define TW_ST_SLA_ACK 0xA8

SLA+R received, ACK returned
6.25.2.29

#define TW_START 0x08

start condition transmitted
6.25.2.30

#define TW_STATUS (TWSR & TW_STATUS_MASK)

TWSR, masked by TW_STATUS_MASK

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153

6.26 : Deprecated items

6.25.2.31

154

#define TW_STATUS_MASK

Value:
(_BV(TWS7)|_BV(TWS6)|_BV(TWS5)|_BV(TWS4)|\
_BV(TWS3))

The lower 3 bits of TWSR are reserved on the ATmega163. The 2 LSB carry the
prescaler bits on the newer ATmegas.
6.25.2.32

#define TW_WRITE 0

SLA+W address

6.26

: Deprecated items

6.26.1

Detailed Description

This header file contains several items that used to be available in previous versions of
this library, but have eventually been deprecated over time.
#include 

These items are supplied within that header file for backward compatibility reasons
only, so old source code that has been written for previous library versions could easily
be maintained until its end-of-life. Use of any of these items in new code is strongly
discouraged.
Allowing specific system-wide interrupts
In addition to globally enabling interrupts, each device’s particular interrupt needs to
be enabled separately if interrupts for this device are desired. While some devices
maintain their interrupt enable bit inside the device’s register set, external and timer
interrupts have system-wide configuration registers.
Example:
// Enable timer 1 overflow interrupts.
timer_enable_int(_BV(TOIE1));
// Do some work...
// Disable all timer interrupts.
timer_enable_int(0);

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155

Note:
Be careful when you use these functions. If you already have a different interrupt
enabled, you could inadvertantly disable it by enabling another intterupt.
•
•
•
•

#define enable_external_int(mask) (__EICR = mask)
#define INTERRUPT(signame)
#define __INTR_ATTRS used
static __inline__ void timer_enable_int (unsigned char ints)

Obsolete IO macros
Back in a time when AVR-GCC and avr-libc could not handle IO port access in the direct assignment form as they are handled now, all IO port access had to be done through
specific macros that eventually resulted in inline assembly instructions performing the
desired action.
These macros became obsolete, as reading and writing IO ports can be done by simply
using the IO port name in an expression, and all bit manipulation (including those on
IO ports) can be done using generic C bit manipulation operators.
The macros in this group simulate the historical behaviour. While they are supposed to
be applied to IO ports, the emulation actually uses standard C methods, so they could
be applied to arbitrary memory locations as well.
•
•
•
•
•
•

#define inp(port) (port)
#define outp(val, port) (port) = (val)
#define inb(port) (port)
#define outb(port, val) (port) = (val)
#define sbi(port, bit) (port) |= (1 << (bit))
#define cbi(port, bit) (port) &= ∼(1 << (bit))

6.26.2
6.26.2.1

Define Documentation
#define cbi(port, bit) (port) &= ∼(1 << (bit))

Deprecated

Clear bit in IO port port.

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6.26 : Deprecated items

6.26.2.2

156

#define enable_external_int(mask) (__EICR = mask)

Deprecated

This macro gives access to the GIMSK register (or EIMSK register if using an AVR
Mega device or GICR register for others). Although this macro is essentially the same
as assigning to the register, it does adapt slightly to the type of device being used. This
macro is unavailable if none of the registers listed above are defined.
6.26.2.3

#define inb(port) (port)

Deprecated

Read a value from an IO port port.
6.26.2.4

#define inp(port) (port)

Deprecated

Read a value from an IO port port.
6.26.2.5

#define INTERRUPT(signame)

Value:
void signame (void) __attribute__ ((interrupt,__INTR_ATTRS));
void signame (void)

\

Deprecated

Introduces an interrupt handler function that runs with global interrupts initially enabled. This allows interrupt handlers to be interrupted.
As this macro has been used by too many unsuspecting people in the past, it has been
deprecated, and will be removed in a future version of the library. Users who want to
legitimately re-enable interrupts in their interrupt handlers as quickly as possible are
encouraged to explicitly declare their handlers as described above.
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6.27 : Compatibility with IAR EWB 3.x

6.26.2.6

157

#define outb(port, val) (port) = (val)

Deprecated

Write val to IO port port.
6.26.2.7

#define outp(val, port) (port) = (val)

Deprecated

Write val to IO port port.
6.26.2.8

#define sbi(port, bit) (port) |= (1 << (bit))

Deprecated

Set bit in IO port port.
6.26.3

Function Documentation

6.26.3.1 static
[static]

__inline__

void

timer_enable_int

(unsigned

char

ints)

Deprecated

This function modifies the timsk register. The value you pass via ints is device
specific.

6.27

: Compatibility with IAR EWB 3.x

#include 

This is an attempt to provide some compatibility with header files that come with IAR
C, to make porting applications between different compilers easier. No 100% compatibility though.
Note:
For actual documentation, please see the IAR manual.

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6.28

Demo projects

6.28

Demo projects

6.28.1

Detailed Description

158

Various small demo projects are provided to illustrate several aspects of using the opensource utilities for the AVR controller series. It should be kept in mind that these demos serve mainly educational purposes, and are normally not directly suitable for use
in any production environment. Usually, they have been kept as simple as sufficient to
demonstrate one particular feature.
The simple project is somewhat like the "Hello world!" application for a microcontroller, about the most simple project that can be done. It is explained in good detail,
to allow the reader to understand the basic concepts behind using the tools on an AVR
microcontroller.
The more sophisticated demo project builds on top of that simple project, and adds
some controls to it. It touches a number of avr-libc’s basic concepts on its way.
A comprehensive example on using the standard IO facilities intends to explain that
complex topic, using a practical microcontroller peripheral setup with one RS-232 connection, and an HD44780-compatible industry-standard LCD display.
The Example using the two-wire interface (TWI) project explains the use of the twowire hardware interface (also known as "I2C") that is present on many AVR controllers.
Finally, the Combining C and assembly source files demo shows how C and assembly language source files can collaborate within one project. While the overall project
is managed by a C program part for easy maintenance, time-critical parts are written
directly in manually optimized assembly language for shortest execution times possible. Naturally, this kind of project is very closely tied to the hardware design, thus it is
custom-tailored to a particular controller type and peripheral setup. As an alternative to
the assembly-language solution, this project also offers a C-only implementation (deploying the exact same peripheral setup) based on a more sophisticated (and thus more
expensive) but pin-compatible controller.
While the simple demo is meant to run on about any AVR setup possible where a
LED could be connected to the OCR1[A] output, the large and stdio demos are mainly
targeted to the Atmel STK500 starter kit, and the TWI example requires a controller
where some 24Cxx two-wire EEPPROM can be connected to. For the STK500 demos,
the default CPU (either an AT90S8515 or an ATmega8515) should be removed from
its socket, and the ATmega16 that ships with the kit should be inserted into socket
SCKT3100A3. The ATmega16 offers an on-board ADC that is used in the large demo,
and all AVRs with an ADC feature a different pinout than the industry-standard compatible devices.
In order to fully utilize the large demo, a female 10-pin header with cable, connecting
to a 10 kOhm potentiometer will be useful.
For the stdio demo, an industry-standard HD44780-compatible LCD display of at least
16x1 characters will be needed. Among other things, the LCD4Linux project page
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159

describes many things around these displays, including common pinouts.
Modules
•
•
•
•
•

6.29

Combining C and assembly source files
A simple project
A more sophisticated project
Using the standard IO facilities
Example using the two-wire interface (TWI)

Combining C and assembly source files

For time- or space-critical applications, it can often be desirable to combine C code
(for easy maintenance) and assembly code (for maximal speed or minimal code size)
together. This demo provides an example of how to do that.
The objective of the demo is to decode radio-controlled model PWM signals, and control an output PWM based on the current input signal’s value. The incoming PWM
pulses follow a standard encoding scheme where a pulse width of 920 microseconds
denotes one end of the scale (represented as 0 % pulse width on output), and 2120
microseconds mark the other end (100 % output PWM). Normally, multiple channels
would be encoded that way in subsequent pulses, followed by a larger gap, so the entire frame will repeat each 14 through 20 ms, but this is ignored for the purpose of the
demo, so only a single input PWM channel is assumed.
The basic challenge is to use the cheapest controller available for the task, an ATtiny13
that has only a single timer channel. As this timer channel is required to run the outgoing PWM signal generation, the incoming PWM decoding had to be adjusted to the
constraints set by the outgoing PWM.
As PWM generation toggles the counting direction of timer 0 between up and down
after each 256 timer cycles, the current time cannot be deduced by reading TCNT0
only, but the current counting direction of the timer needs to be considered as well.
This requires servicing interrupts whenever the timer hits TOP (255) and BOTTOM (0)
to learn about each change of the counting direction. For PWM generation, it is usually
desired to run it at the highest possible speed so filtering the PWM frequency from the
modulated output signal is made easy. Thus, the PWM timer runs at full CPU speed.
This causes the overflow and compare match interrupts to be triggered each 256 CPU
clocks, so they must run with the minimal number of processor cycles possible in order
to not impose a too high CPU load by these interrupt service routines. This is the main
reason to implement the entire interrupt handling in fine-tuned assembly code rather
than in C.
In order to verify parts of the algorithm, and the underlying hardware, the demo has
been set up in a way so the pin-compatible but more expensive ATtiny45 (or its siblings

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160

ATtiny25 and ATtiny85) could be used as well. In that case, no separate assembly code
is required, as two timer channels are avaible.
6.29.1

Hardware setup

The incoming PWM pulse train is fed into PB4. It will generate a pin change interrupt
there on eache edge of the incoming signal.
The outgoing PWM is generated through OC0B of timer channel 0 (PB1). For demonstration purposes, a LED should be connected to that pin (like, one of the LEDs of an
STK500).
The controllers run on their internal calibrated RC oscillators, 1.2 MHz on the ATtiny13, and 1.0 MHz on the ATtiny45.
6.29.2

A code walkthrough

6.29.2.1 asmdemo.c After the usual include files, two variables are defined. The
first one, pwm_incoming is used to communicate the most recent pulse width detected by the incoming PWM decoder up to the main loop.
The second variable actually only constitutes of a single bit, intbits.pwm_received. This bit will be set whenever the incoming PWM decoder has updated
pwm_incoming.
Both variables are marked volatile to ensure their readers will always pick up an updated value, as both variables will be set by interrupt service routines.
The function ioinit() initializes the microcontroller peripheral devices. In particular, it starts timer 0 to generate the outgoing PWM signal on OC0B. Setting OCR0A
to 255 (which is the TOP value of timer 0) is used to generate a timer 0 overflow A
interrupt on the ATtiny13. This interrupt is used to inform the incoming PWM decoder
that the counting direction of channel 0 is just changing from up to down. Likewise, an
overflow interrupt will be generated whenever the countdown reached BOTTOM (value
0), where the counter will again alter its counting direction to upwards. This information is needed in order to know whether the current counter value of TCNT0 is to be
evaluated from bottom or top.
Further, ioinit() activates the pin-change interrupt PCINT0 on any edge of PB4.
Finally, PB1 (OC0B) will be activated as an output pin, and global interrupts are being
enabled.
In the ATtiny45 setup, the C code contains an ISR for PCINT0. At each pin-change
interrupt, it will first be analyzed whether the interrupt was caused by a rising or a
falling edge. In case of the rising edge, timer 1 will be started with a prescaler of 16
after clearing the current timer value. Then, at the falling edge, the current timer value
will be recorded (and timer 1 stopped), the pin-change interrupt will be suspended, and
the upper layer will be notified that the incoming PWM measurement data is available.

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Function main() first initializes the hardware by calling ioinit(), and then waits
until some incoming PWM value is available. If it is, the output PWM will be adjusted
by computing the relative value of the incoming PWM. Finally, the pin-change interrupt
is re-enabled, and the CPU is put to sleep.
6.29.2.2 project.h In order for the interrupt service routines to be as fast as possible, some of the CPU registers are set aside completely for use by these routines, so the
compiler would not use them for C code. This is arranged for in project.h.
The file is divided into one section that will be used by the assembly source code, and
another one to be used by C code. The assembly part is distinguished by the preprocessing macro __ASSEMBLER__ (which will be automatically set by the compiler
front-end when preprocessing an assembly-language file), and it contains just macros
that give symbolic names to a number of CPU registers. The preprocessor will then
replace the symbolic names by their right-hand side definitions before calling the assembler.
In C code, the compiler needs to see variable declarations for these objects. This is
done by using declarations that bind a variable permanently to a CPU register (see
How to permanently bind a variable to a register?). Even in case the C code never
has a need to access these variables, declaring the register binding that way causes the
compiler to not use these registers in C code at all.
The flags variable needs to be in the range of r16 through r31 as it is the target of a
load immediate (or SER) instruction that is not applicable to the entire register file.
6.29.2.3 isrs.S This file is a preprocessed assembly source file. The C preprocessor
will be run by the compiler front-end first, resolving all #include, #define etc.
directives. The resulting program text will then be passed on to the assembler.
As the C preprocessor strips all C-style comments, preprocessed assembly source files
can have both, C-style (/∗ ... ∗/, // ...) as well as assembly-style (; ...)
comments.
At the top, the IO register definition file avr/io.h and the project declaration file
project.h are included. The remainder of the file is conditionally assembled only if
the target MCU type is an ATtiny13, so it will be completely ignored for the ATtiny45
option.
Next are the two interrupt service routines for timer 0 compare A match (timer 0 hits
TOP, as OCR0A is set to 255) and timer 0 overflow (timer 0 hits BOTTOM). As discussed above, these are kept as short as possible. They only save SREG (as the flags
will be modified by the INC instruction), increment the counter_hi variable which
forms the high part of the current time counter (the low part is formed by querying
TCNT0 directly), and clear or set the variable flags, respectively, in order to note
the current counting direction. The RETI instruction terminates these interrupt service
routines. Total cycle count is 8 CPU cycles, so together with the 4 CPU cycles needed

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for interrupt setup, and the 2 cycles for the RJMP from the interrupt vector to the handler, these routines will require 14 out of each 256 CPU cycles, or about 5 % of the
overall CPU time.
The pin-change interrupt PCINT0 will be handled in the final part of this file. The
basic algorithm is to quickly evaluate the current system time by fetching the current
timer value of TCNT0, and combining it with the overflow part in counter_hi. If
the counter is currently counting down rather than up, the value fetched from TCNT0
must be negated. Finally, if this pin-change interrupt was triggered by a rising edge,
the time computed will be recorded as the start time only. Then, at the falling edge,
this start time will be subracted from the current time to compute the actual pulse width
seen (left in pwm_incoming), and the upper layers are informed of the new value by
setting bit 0 in the intbits flags. At the same time, this pin-change interrupt will be
disabled so no new measurement can be performed until the upper layer had a chance
to process the current value.
6.29.3

The source code

The source code is installed under
$prefix/share/doc/avr-libc/examples/asmdemo/,
where $prefix is a configuration option. For Unix systems, it is usually set to either
/usr or /usr/local.

6.30

A simple project

At this point, you should have the GNU tools configured, built, and installed on your
system. In this chapter, we present a simple example of using the GNU tools in an AVR
project. After reading this chapter, you should have a better feel as to how the tools are
used and how a Makefile can be configured.
6.30.1

The Project

This project will use the pulse-width modulator (PWM) to ramp an LED on and off every
two seconds. An AT90S2313 processor will be used as the controller. The circuit for
this demonstration is shown in the schematic diagram. If you have a development kit,
you should be able to use it, rather than build the circuit, for this project.
Note:
Meanwhile, the AT90S2313 became obsolete. Either use its successor, the (pincompatible) ATtiny2313 for the project, or perhaps the ATmega8 or one of its
successors (ATmega48/88/168) which have become quite popular since the original demo project had been established. For all these more modern devices, it is no

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longer necessary to use an external crystal for clocking as they ship with the internal 1 MHz oscillator enabled, so C1, C2, and Q1 can be omitted. Normally, for
this experiment, the external circuitry on /RESET (R1, C3) can be omitted as well,
leaving only the AVR, the LED, the bypass capacitor C4, and perhaps R2. For the
ATmega8/48/88/168, use PB1 (pin 15 at the DIP-28 package) to connect the LED
to. Additionally, this demo has been ported to many different other AVRs. The location of the respective OC pin varies between different AVRs, and it is mandated
by the AVR hardware.
VCC
IC1
4mhz

C2

Q1

C3

20K

.01uf

R1

C4

.1uf

18pf

GND
GND

19
18
17
16
15
14
13
12

(ICP)PD6
(T1)PD5
(T0)PD4
(INT1)PD3
(INT0)PD2
(TXD)PD1
(RXD)PD0
AT90S2313P

11
9
8
7
6
3
2

RESET

4

XTAL2

5

XTAL1

C1
18pf

(SCK)PB7
(MISO)PB6
(MOSI)PB5
PB4
(OCI)PB3
PB2
(AIN1)PB1
(AIN0)PB0

1

20 VCC
10 GND

R2*

LED5MM
D1

See note [8]
GND

Figure 1: Schematic of circuit for demo project

The source code is given in demo.c. For the sake of this example, create a file called
demo.c containing this source code. Some of the more important parts of the code
are:
Note [1]:
As the AVR microcontroller series has been developed during the past years,
new features have been added over time. Even though the basic concepts of
the timer/counter1 are still the same as they used to be back in early 2001 when
this simple demo was written initially, the names of registers and bits have been
changed slightly to reflect the new features. Also, the port and pin mapping of
the output compare match 1A (or 1 for older devices) pin which is used to control
the LED varies between different AVRs. The file iocompat.h tries to abstract
between all this differences using some preprocessor #ifdef statements, so the
actual program itself can operate on a common set of symbolic names. The macros
defined by that file are:
• OCR the name of the OCR register used to control the PWM (usually either
OCR1 or OCR1A)

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• DDROC the name of the DDR (data direction register) for the OC output
• OC1 the pin number of the OC1[A] output within its port
• TIMER1_TOP the TOP value of the timer used for the PWM (1023 for 10-bit
PWMs, 255 for devices that can only handle an 8-bit PWM)
• TIMER1_PWM_INIT the initialization bits to be set into control register 1A in
order to setup 10-bit (or 8-bit) phase and frequency correct PWM mode
• TIMER1_CLOCKSOURCE the clock bits to set in the respective control register to start the PWM timer; usually the timer runs at full CPU clock for 10-bit
PWMs, while it runs on a prescaled clock for 8-bit PWMs
Note [2]:
ISR() is a macro that marks the function as an interrupt routine. In this case, the
function will get called when timer 1 overflows. Setting up interrupts is explained
in greater detail in : Interrupts.
Note [3]:
The PWM is being used in 10-bit mode, so we need a 16-bit variable to remember
the current value.
Note [4]:
This section determines the new value of the PWM.
Note [5]:
Here’s where the newly computed value is loaded into the PWM register. Since
we are in an interrupt routine, it is safe to use a 16-bit assignment to the register.
Outside of an interrupt, the assignment should only be performed with interrupts
disabled if there’s a chance that an interrupt routine could also access this register
(or another register that uses TEMP), see the appropriate FAQ entry.
Note [6]:
This routine gets called after a reset. It initializes the PWM and enables interrupts.
Note [7]:
The main loop of the program does nothing – all the work is done by the interrupt
routine! The sleep_mode() puts the processor on sleep until the next interrupt,
to conserve power. Of course, that probably won’t be noticable as we are still
driving a LED, it is merely mentioned here to demonstrate the basic principle.

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Note [8]:
Early AVR devices saturate their outputs at rather low currents when sourcing current, so the LED can be connected directly, the resulting current through the LED
will be about 15 mA. For modern parts (at least for the ATmega 128), however
Atmel has drastically increased the IO source capability, so when operating at 5
V Vcc, R2 is needed. Its value should be about 150 Ohms. When operating the
circuit at 3 V, it can still be omitted though.
6.30.2

The Source Code

/*
* ---------------------------------------------------------------------------* "THE BEER-WARE LICENSE" (Revision 42):
*  wrote this file. As long as you retain this notice you
* can do whatever you want with this stuff. If we meet some day, and you think
Joerg Wunsch
* this stuff is worth it, you can buy me a beer in return.
* ---------------------------------------------------------------------------*
* Simple AVR demonstration. Controls a LED that can be directly
* connected from OC1/OC1A to GND. The brightness of the LED is
* controlled with the PWM. After each period of the PWM, the PWM
* value is either incremented or decremented, that’s all.
*
* $Id: demo.c,v 1.6.2.3 2006/01/05 21:33:08 joerg_wunsch Exp $
*/
#include
#include
#include
#include






#include "iocompat.h"

/* Note [1] */

enum { UP, DOWN };
ISR (TIMER1_OVF_vect)
{
static uint16_t pwm;
static uint8_t direction;

/* Note [2] */
/* Note [3] */

switch (direction)
/* Note [4] */
{
case UP:
if (++pwm == TIMER1_TOP)
direction = DOWN;
break;
case DOWN:
if (--pwm == 0)
direction = UP;
break;
}
OCR = pwm;

/* Note [5] */

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}
void
ioinit (void)
/* Note [6] */
{
/* Timer 1 is 10-bit PWM (8-bit PWM on some ATtinys). */
TCCR1A = TIMER1_PWM_INIT;
/*
* Start timer 1.
*
* NB: TCCR1A and TCCR1B could actually be the same register, so
* take care to not clobber it.
*/
TCCR1B |= TIMER1_CLOCKSOURCE;
/*
* Run any device-dependent timer 1 setup hook if present.
*/
#if defined(TIMER1_SETUP_HOOK)
TIMER1_SETUP_HOOK();
#endif
/* Set PWM value to 0. */
OCR = 0;
/* Enable OC1 as output. */
DDROC = _BV (OC1);
/* Enable timer 1 overflow interrupt. */
TIMSK = _BV (TOIE1);
sei ();
}
int
main (void)
{
ioinit ();
/* loop forever, the interrupts are doing the rest */
for (;;)
sleep_mode();

/* Note [7] */

return (0);
}

6.30.3

Compiling and Linking

This first thing that needs to be done is compile the source. When compiling, the
compiler needs to know the processor type so the -mmcu option is specified. The
-Os option will tell the compiler to optimize the code for efficient space usage (at the
possible expense of code execution speed). The -g is used to embed debug info. The
debug info is useful for disassemblies and doesn’t end up in the .hex files, so I usually
specify it. Finally, the -c tells the compiler to compile and stop – don’t link. This

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demo is small enough that we could compile and link in one step. However, real-world
projects will have several modules and will typically need to break up the building of
the project into several compiles and one link.
$ avr-gcc -g -Os -mmcu=atmega8 -c demo.c

The compilation will create a demo.o file. Next we link it into a binary called
demo.elf.
$ avr-gcc -g -mmcu=atmega8 -o demo.elf demo.o

It is important to specify the MCU type when linking. The compiler uses the -mmcu
option to choose start-up files and run-time libraries that get linked together. If this
option isn’t specified, the compiler defaults to the 8515 processor environment, which
is most certainly what you didn’t want.
6.30.4

Examining the Object File

Now we have a binary file. Can we do anything useful with it (besides put it into the
processor?) The GNU Binutils suite is made up of many useful tools for manipulating
object files that get generated. One tool is avr-objdump, which takes information
from the object file and displays it in many useful ways. Typing the command by itself
will cause it to list out its options.
For instance, to get a feel of the application’s size, the -h option can be used. The
output of this option shows how much space is used in each of the sections (the .stab
and .stabstr sections hold the debugging information and won’t make it into the ROM
file).
An even more useful option is -S. This option disassembles the binary file and intersperses the source code in the output! This method is much better, in my opinion, than
using the -S with the compiler because this listing includes routines from the libraries
and the vector table contents. Also, all the "fix-ups" have been satisfied. In other words,
the listing generated by this option reflects the actual code that the processor will run.
$ avr-objdump -h -S demo.elf > demo.lst

Here’s the output as saved in the demo.lst file:
demo.elf:
Sections:
Idx Name
0 .text
1 .bss

file format elf32-avr

Size
000000ce
CONTENTS,
00000003

VMA
LMA
File off
00000000 00000000 00000074
ALLOC, LOAD, READONLY, CODE
00800060 00800060 00000142

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Algn
2**1
2**0

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ALLOC
2 .debug_aranges 00000020 00000000 00000000 00000142 2**0
CONTENTS, READONLY, DEBUGGING
3 .debug_pubnames 00000035 00000000 00000000 00000162 2**0
CONTENTS, READONLY, DEBUGGING
4 .debug_info
000000db 00000000 00000000 00000197 2**0
CONTENTS, READONLY, DEBUGGING
5 .debug_abbrev 00000098 00000000 00000000 00000272 2**0
CONTENTS, READONLY, DEBUGGING
6 .debug_line
00000121 00000000 00000000 0000030a 2**0
CONTENTS, READONLY, DEBUGGING
7 .debug_frame 00000040 00000000 00000000 0000042c 2**2
CONTENTS, READONLY, DEBUGGING
8 .debug_str
000000be 00000000 00000000 0000046c 2**0
CONTENTS, READONLY, DEBUGGING
Disassembly of section .text:
00000000
0: 10
2: a0
4: b0
6: ee
8: f0
a: 02

<__ctors_end>:
e0
ldi r17,
e6
ldi r26,
e0
ldi r27,
ec
ldi r30,
e0
ldi r31,
c0
rjmp .+4

0x00
0x60
0x00
0xCE
0x00

;
;
;
;
;

0
96
0
206
0
; 0x10 <.do_copy_data_start>

0000000c <.do_copy_data_loop>:
c: 05 90
lpm r0, Z+
e: 0d 92
st X+, r0
00000010
10: a0
12: b1
14: d9

<.do_copy_data_start>:
36
cpi r26, 0x60 ; 96
07
cpc r27, r17
f7
brne .-10
; 0xc <.do_copy_data_loop>

00000016
16: 10
18: a0
1a: b0
1c: 01

<__do_clear_bss>:
e0
ldi r17, 0x00 ; 0
e6
ldi r26, 0x60 ; 96
e0
ldi r27, 0x00 ; 0
c0
rjmp .+2
; 0x20 <.do_clear_bss_start>

0000001e <.do_clear_bss_loop>:
1e: 1d 92
st X+, r1
00000020
20: a3
22: b1
24: e1

<.do_clear_bss_start>:
36
cpi r26, 0x63 ; 99
07
cpc r27, r17
f7
brne .-8
; 0x1e <.do_clear_bss_loop>

00000026 <__vector_8>:
enum { UP, DOWN };
ISR (TIMER1_OVF_vect) /* Note [2] */
{
26: 1f 92
push r1
28: 0f 92
push r0
2a: 0f b6
in r0, 0x3f ; 63

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2c: 0f 92
push r0
2e: 11 24
eor r1, r1
30: 2f 93
push r18
32: 3f 93
push r19
34: 8f 93
push r24
static uint16_t pwm; /* Note [3] */
static uint8_t direction;
switch (direction) /* Note [4] */
36: 80 91 60 00 lds r24, 0x0060
3a: 88 23
and r24, r24
3c: 39 f0
breq .+14
; 0x4c
3e: 81 30
cpi r24, 0x01 ; 1
40: b9 f0
breq .+46
; 0x70
42: 20 91 61 00 lds r18, 0x0061
46: 30 91 62 00 lds r19, 0x0062
4a: 21 c0
rjmp .+66
; 0x8e
{
case UP:
if (++pwm == TIMER1_TOP)
4c: 20 91 61 00 lds r18, 0x0061
50: 30 91 62 00 lds r19, 0x0062
54: 2f 5f
subi r18, 0xFF ; 255
56: 3f 4f
sbci r19, 0xFF ; 255
58: 30 93 62 00 sts 0x0062, r19
5c: 20 93 61 00 sts 0x0061, r18
60: 83 e0
ldi r24, 0x03 ; 3
62: 2f 3f
cpi r18, 0xFF ; 255
64: 38 07
cpc r19, r24
66: 99 f4
brne .+38
; 0x8e
direction = DOWN;
68: 81 e0
ldi r24, 0x01 ; 1
6a: 80 93 60 00 sts 0x0060, r24
6e: 0f c0
rjmp .+30
; 0x8e
break;

70:
74:
78:
7a:
7c:
80:
84:
86:
88:
8a:

<__SREG__+0xd>
<__SREG__+0x31>

<__SREG__+0x4f>

<__SREG__+0x4f>

<__SREG__+0x4f>

case DOWN:
if (--pwm == 0)
20 91 61 00 lds r18, 0x0061
30 91 62 00 lds r19, 0x0062
21 50
subi r18, 0x01 ; 1
30 40
sbci r19, 0x00 ; 0
30 93 62 00 sts 0x0062, r19
20 93 61 00 sts 0x0061, r18
21 15
cp r18, r1
31 05
cpc r19, r1
11 f4
brne .+4
; 0x8e <__SREG__+0x4f>
direction = UP;
10 92 60 00 sts 0x0060, r1
break;

}
OCR = pwm; /* Note [5] */
8e: 3b bd
out 0x2b, r19 ; 43
90: 2a bd
out 0x2a, r18 ; 42
92: 8f 91
pop r24
94: 3f 91
pop r19

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98:
9a:
9c:
9e:
a0:

A simple project

2f
0f
0f
0f
1f
18

91
90
be
90
90
95

pop r18
pop r0
out 0x3f, r0 ; 63
pop r0
pop r1
reti

000000a2 :
}
void
ioinit (void) /* Note [6] */
{
a2: 83 e8
ldi r24, 0x83 ; 131
a4: 8f bd
out 0x2f, r24 ; 47
/* Timer 1 is 10-bit PWM (8-bit PWM on some ATtinys). */
TCCR1A = TIMER1_PWM_INIT;
/*
* Start timer 1.
*
* NB: TCCR1A and TCCR1B could actually be the same register, so
* take care to not clobber it.
*/
TCCR1B |= TIMER1_CLOCKSOURCE;
a6: 8e b5
in r24, 0x2e ; 46
a8: 81 60
ori r24, 0x01 ; 1
aa: 8e bd
out 0x2e, r24 ; 46
/*
* Run any device-dependent timer 1 setup hook if present.
*/
#if defined(TIMER1_SETUP_HOOK)
TIMER1_SETUP_HOOK();
#endif
/* Set PWM value to 0. */
OCR = 0;
ac: 1b bc
out 0x2b, r1 ; 43
ae: 1a bc
out 0x2a, r1 ; 42
/* Enable OC1 as output. */
DDROC = _BV (OC1);
b0: 82 e0
ldi r24, 0x02 ; 2
b2: 87 bb
out 0x17, r24 ; 23
/* Enable timer 1 overflow interrupt. */
TIMSK = _BV (TOIE1);
b4: 84 e0
ldi r24, 0x04 ; 4
b6: 89 bf
out 0x39, r24 ; 57
sei ();
b8: 78 94
sei
ba: 08 95
ret
000000bc :
}
int
main (void)

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{
bc: f2 df

rcall .-28

; 0xa2 

ioinit ();
/* loop forever, the interrupts are doing the rest */
for (;;) /* Note [7] */
sleep_mode();
be: 85 b7
in r24, 0x35 ; 53
c0: 80 68
ori r24, 0x80 ; 128
c2: 85 bf
out 0x35, r24 ; 53
c4: 88 95
sleep
c6: 85 b7
in r24, 0x35 ; 53
c8: 8f 77
andi r24, 0x7F ; 127
ca: 85 bf
out 0x35, r24 ; 53
cc: f8 cf
rjmp .-16
; 0xbe 

6.30.5

Linker Map Files

avr-objdump is very useful, but sometimes it’s necessary to see information about
the link that can only be generated by the linker. A map file contains this information.
A map file is useful for monitoring the sizes of your code and data. It also shows where
modules are loaded and which modules were loaded from libraries. It is yet another
view of your application. To get a map file, I usually add -Wl,-Map,demo.map to
my link command. Relink the application using the following command to generate
demo.map (a portion of which is shown below).
$ avr-gcc -g -mmcu=atmega8 -Wl,-Map,demo.map -o demo.elf demo.o

Some points of interest in the demo.map file are:
.rela.plt
*(.rela.plt)
.text
0x00000000
*(.vectors)
*(.vectors)
*(.progmem.gcc*)
*(.progmem*)
0x00000000
0x00000000
*(.trampolines)
.trampolines
0x00000000
*(.trampolines*)
0x00000000
*(.jumptables)
*(.jumptables*)
*(.lowtext)
*(.lowtext*)
0x00000000

0xce

. = ALIGN (0x2)
__trampolines_start = .
0x0 linker stubs

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__trampolines_end = .

__ctors_start = .

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The .text segment (where program instructions are stored) starts at location 0x0.
*(.fini2)
*(.fini2)
*(.fini1)
*(.fini1)
*(.fini0)
*(.fini0)
0x000000ce
.data

0x00800060
0x00800060

*(.data)
.data
0x00800060
.data
0x00800060
.data
0x00800060
*(.data*)
*(.rodata)
*(.rodata*)
*(.gnu.linkonce.d*)
0x00800060
0x00800060
0x00800060
.bss
*(.bss)
.bss
.bss
.bss
*(.bss*)
*(COMMON)

_etext = .
0x0 load address 0x000000ce
PROVIDE (__data_start, .)
0x0 demo.o
0x0 c:/winavr/bin/../lib/gcc/avr/4.1.2/avr4\libgcc.a(_copy_data.o)
0x0 c:/winavr/bin/../lib/gcc/avr/4.1.2/avr4\libgcc.a(_clear_bss.o)

. = ALIGN (0x2)
_edata = .
PROVIDE (__data_end, .)

0x00800060
0x00800060

0x3

0x00800060
0x00800063
0x00800063

0x3 demo.o
0x0 c:/winavr/bin/../lib/gcc/avr/4.1.2/avr4\libgcc.a(_copy_data.o)
0x0 c:/winavr/bin/../lib/gcc/avr/4.1.2/avr4\libgcc.a(_clear_bss.o)

PROVIDE (__bss_start, .)

0x00800063
0x000000ce
0x000000ce
.noinit

0x00800063
0x00800063

PROVIDE (__bss_end, .)
__data_load_start = LOADADDR (.data)
__data_load_end = (__data_load_start + SIZEOF (.data))
0x0
PROVIDE (__noinit_start, .)

*(.noinit*)
0x00800063
0x00800063
0x00800063
.eeprom
*(.eeprom*)

0x00810000

PROVIDE (__noinit_end, .)
_end = .
PROVIDE (__heap_start, .)
0x0

0x00810000

__eeprom_end = .

The last address in the .text segment is location 0x114 ( denoted by _etext ), so the
instructions use up 276 bytes of FLASH.
The .data segment (where initialized static variables are stored) starts at location 0x60,
which is the first address after the register bank on an ATmega8 processor.
The next available address in the .data segment is also location 0x60, so the application
has no initialized data.

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The .bss segment (where uninitialized data is stored) starts at location 0x60.
The next available address in the .bss segment is location 0x63, so the application uses
3 bytes of uninitialized data.
The .eeprom segment (where EEPROM variables are stored) starts at location 0x0.
The next available address in the .eeprom segment is also location 0x0, so there aren’t
any EEPROM variables.
6.30.6

Generating Intel Hex Files

We have a binary of the application, but how do we get it into the processor? Most (if
not all) programmers will not accept a GNU executable as an input file, so we need to
do a little more processing. The next step is to extract portions of the binary and save
the information into .hex files. The GNU utility that does this is called avr-objcopy.
The ROM contents can be pulled from our project’s binary and put into the file
demo.hex using the following command:
$ avr-objcopy -j .text -j .data -O ihex demo.elf demo.hex

The resulting demo.hex file contains:
:1000000010E0A0E6B0E0EEECF0E002C005900D924A
:10001000A036B107D9F710E0A0E6B0E001C01D920C
:10002000A336B107E1F71F920F920FB60F9211247A
:100030002F933F938F9380916000882339F0813014
:10004000B9F0209161003091620021C020916100DF
:10005000309162002F5F3F4F309362002093610028
:1000600083E02F3F380799F481E0809360000FC050
:100070002091610030916200215030403093620045
:10008000209361002115310511F4109260003BBDF1
:100090002ABD8F913F912F910F900FBE0F901F900F
:1000A000189583E88FBD8EB581608EBD1BBC1ABCD0
:1000B00082E087BB84E089BF78940895F2DF85B73A
:0E00C000806885BF889585B78F7785BFF8CF9C
:00000001FF

The -j option indicates that we want the information from the .text and .data segment
extracted. If we specify the EEPROM segment, we can generate a .hex file that can be
used to program the EEPROM:
$ avr-objcopy -j .eeprom --change-section-lma .eeprom=0 -O ihex demo.elf demo_eeprom.hex

There is no demo_eeprom.hex file written, as that file would be empty.
Starting with version 2.17 of the GNU binutils, the avr-objcopy command that used
to generate the empty EEPROM files now aborts because of the empty input section
.eeprom, so these empty files are not generated. It also signals an error to the Makefile
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which will be caught there, and makes it print a message about the empty file not being
generated.
6.30.7

Letting Make Build the Project

Rather than type these commands over and over, they can all be placed in a make file.
To build the demo project using make, save the following in a file called Makefile.
Note:
This Makefile can only be used as input for the GNU version of make.
PRG
OBJ
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET

= demo
= demo.o
= at90s2313
= at90s2333
= at90s4414
= at90s4433
= at90s4434
= at90s8515
= at90s8535
= atmega128
= atmega1280
= atmega1281
= atmega16
= atmega163
= atmega164p
= atmega165
= atmega165p
= atmega168
= atmega169
= atmega169p
= atmega32
= atmega324p
= atmega325
= atmega3250
= atmega329
= atmega3290
= atmega48
= atmega64
= atmega640
= atmega644
= atmega644p
= atmega645
= atmega6450
= atmega649
= atmega6490
= atmega8
= atmega8515
= atmega8535
= atmega88
= attiny2313
= attiny24
= attiny25

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#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
#MCU_TARGET
OPTIMIZE

=
=
=
=
=
=
=
=

attiny26
attiny261
attiny44
attiny45
attiny461
attiny84
attiny85
attiny861
= -O2

DEFS
LIBS

=
=

# You should not have to change anything below here.
CC

= avr-gcc

# Override is only needed by avr-lib build system.
override CFLAGS
override LDFLAGS
OBJCOPY
OBJDUMP

= -g -Wall $(OPTIMIZE) -mmcu=$(MCU_TARGET) $(DEFS)
= -Wl,-Map,$(PRG).map

= avr-objcopy
= avr-objdump

all: $(PRG).elf lst text eeprom
$(PRG).elf: $(OBJ)
$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS)
# dependency:
demo.o: demo.c iocompat.h
clean:
rm -rf *.o $(PRG).elf *.eps *.png *.pdf *.bak
rm -rf *.lst *.map $(EXTRA_CLEAN_FILES)
lst:

$(PRG).lst

%.lst: %.elf
$(OBJDUMP) -h -S $< > $@
# Rules for building the .text rom images
text: hex bin srec
hex: $(PRG).hex
bin: $(PRG).bin
srec: $(PRG).srec
%.hex: %.elf
$(OBJCOPY) -j .text -j .data -O ihex $< $@
%.srec: %.elf
$(OBJCOPY) -j .text -j .data -O srec $< $@
%.bin: %.elf

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$(OBJCOPY) -j .text -j .data -O binary $< $@
# Rules for building the .eeprom rom images
eeprom: ehex ebin esrec
ehex: $(PRG)_eeprom.hex
ebin: $(PRG)_eeprom.bin
esrec: $(PRG)_eeprom.srec
%_eeprom.hex: %.elf
$(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O ihex $< $@ \
|| { echo empty $@ not generated; exit 0; }
%_eeprom.srec: %.elf
$(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O srec $< $@ \
|| { echo empty $@ not generated; exit 0; }
%_eeprom.bin: %.elf
$(OBJCOPY) -j .eeprom --change-section-lma .eeprom=0 -O binary $< $@ \
|| { echo empty $@ not generated; exit 0; }
# Every thing below here is used by avr-libc’s build system and can be ignored
# by the casual user.
FIG2DEV
EXTRA_CLEAN_FILES

= fig2dev
= *.hex *.bin *.srec

dox: eps png pdf
eps: $(PRG).eps
png: $(PRG).png
pdf: $(PRG).pdf
%.eps: %.fig
$(FIG2DEV) -L eps $< $@
%.pdf: %.fig
$(FIG2DEV) -L pdf $< $@
%.png: %.fig
$(FIG2DEV) -L png $< $@

6.30.8

Reference to the source code

The source code is installed under
$prefix/share/doc/avr-libc/examples/demo/,
where $prefix is a configuration option. For Unix systems, it is usually set to either
/usr or /usr/local.

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A more sophisticated project

This project extends the basic idea of the simple project to control a LED with a PWM
output, but adds methods to adjust the LED brightness. It employs a lot of the basic
concepts of avr-libc to achieve that goal.
Understanding this project assumes the simple project has been understood in full, as
well as being acquainted with the basic hardware concepts of an AVR microcontroller.
6.31.1

Hardware setup

The demo is set up in a way so it can be run on the ATmega16 that ships with the
STK500 development kit. The only external part needed is a potentiometer attached to
the ADC. It is connected to a 10-pin ribbon cable for port A, both ends of the potentiometer to pins 9 (GND) and 10 (VCC), and the wiper to pin 1 (port A0). A bypass
capacitor from pin 1 to pin 9 (like 47 nF) is recommendable.

Figure 2: Setup of the STK500

The coloured patch cables are used to provide various interconnections. As there are
only four of them in the STK500, there are two options to connect them for this demo.
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The second option for the yellow-green cable is shown in parenthesis in the table.
Alternatively, the "squid" cable from the JTAG ICE kit can be used if available.
Port
D0

Header
1

Color
brown

Function
RxD

D1

2

grey

TxD

D2

3

black

button
"down"

D3

4

red

button "up"

D4

5

green

button
"ADC"

D5

6

blue

LED

D6

7

(green)

clock out

D7

8

white

GND
VCC

9
10

1-second
flash
unused
unused

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Connect to
RXD of the
RS-232
header
TXD of the
RS-232
header
SW0 (pin 1
switches
header)
SW1 (pin 2
switches
header)
SW2 (pin 3
switches
header)
LED0 (pin 1
LEDs header)
LED1 (pin 2
LEDs header)
LED2 (pin 3
LEDs header)

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Figure 3: Wiring of the STK500

The following picture shows the alternate wiring where LED1 is connected but SW2 is
not:

Figure 4: Wiring option #2 of the STK500

As an alternative, this demo can also be run on the popular ATmega8 controller, or its
successor ATmega88 as well as the ATmega48 and ATmega168 variants of the latter.
These controllers do not have a port named "A", so their ADC inputs are located on
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port C instead, thus the potentiometer needs to be attached to port C. Likewise, the
OC1A output is not on port D pin 5 but on port B pin 1 (PB1). Thus, the above
cabling scheme needs to be changed so that PB1 connects to the LED0 pin. (PD6
remains unconnected.) When using the STK500, use one of the jumper cables for this
connection. All other port D pins should be connected the same way as described for
the ATmega16 above.
When not using an STK500 starter kit, attach the LEDs through some resistor to Vcc
(low-active LEDs), and attach pushbuttons from the respective input pins to GND. The
internal pull-up resistors are enabled for the pushbutton pins, so no external resistors
are needed.
Finally, the demo has been ported to the ATtiny2313 as well. As this AVR does not
offer an ADC, everything related to handling the ADC is disabled in the code for that
MCU type. Also, port D of this controller type only features 6 pins, so the 1-second
flash LED had to be moved from PD6 to PD4. (PD4 is used as the ADC control button
on the other MCU types, but that is not needed here.) OC1A is located at PB3 on this
device.
The MCU_TARGET macro in the Makefile needs to be adjusted appropriately for the
alternative controller types.
The flash ROM and RAM consumption of this demo are way below the resources
of even an ATmega48, and still well within the capabilities of an ATtiny2313. The
major advantage of experimenting with the ATmega16 (in addition that it ships together
with an STK500 anyway) is that it can be debugged online via JTAG. Likewise, the
ATmega48/88/168 and ATtiny2313 devices can be debugged through debugWire, using
the Atmel JTAG ICE mkII or the low-cost AVR Dragon.
Note that in the explanation below, all port/pin names are applicable to the ATmega16
setup.
6.31.2

Functional overview

PD6 will be toggled with each internal clock tick (approx. 10 ms). PD7 will flash once
per second.
PD0 and PD1 are configured as UART IO, and can be used to connect the demo kit to
a PC (9600 Bd, 8N1 frame format). The demo application talks to the serial port, and
it can be controlled from the serial port.
PD2 through PD4 are configured as inputs, and control the application unless control
has been taken over by the serial port. Shorting PD2 to GND will decrease the current
PWM value, shorting PD3 to GND will increase it.
While PD4 is shorted to GND, one ADC conversion for channel 0 (ADC input is on
PA0) will be triggered each internal clock tick, and the resulting value will be used as
the PWM value. So the brightness of the LED follows the analog input value on PC0.
VAREF on the STK500 should be set to the same value as VCC.

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When running in serial control mode, the function of the watchdog timer can be demonstrated by typing an ‘r’. This will make the demo application run in a tight loop without
retriggering the watchdog so after some seconds, the watchdog will reset the MCU.
This situation can be figured out on startup by reading the MCUCSR register.
The current value of the PWM is backed up in an EEPROM cell after about 3 seconds
of idle time after the last change. If that EEPROM cell contains a reasonable (i. e.
non-erased) value at startup, it is taken as the initial value for the PWM. This virtually
preserves the last value across power cycles. By not updating the EEPROM immmediately but only after a timeout, EEPROM wear is reduced considerably compared to
immediately writing the value at each change.
6.31.3

A code walkthrough

This section explains the ideas behind individual parts of the code. The source code
has been divided into numbered parts, and the following subsections explain each of
these parts.
6.31.3.1 Part 1: Macro definitions A number of preprocessor macros are defined
to improve readability and/or portability of the application.
The first macros describe the IO pins our LEDs and pushbuttons are connected to. This
provides some kind of mini-HAL (hardware abstraction layer) so should some of the
connections be changed, they don’t need to be changed inside the code but only on
top. Note that the location of the PWM output itself is mandated by the hardware, so it
cannot be easily changed. As the ATmega48/88/168 controllers belong to a more recent
generation of AVRs, a number of register and bit names have been changed there, so
they are mapped back to their ATmega8/16 equivalents to keep the actual program code
portable.
The name F_CPU is the conventional name to describe the CPU clock frequency of
the controller. This demo project just uses the internal calibrated 1 MHz RC oscillator
that is enabled by default. Note that when using the  functions,
F_CPU needs to be defined before including that file.
The remaining macros have their own comments in the source code. The macro
TMR1_SCALE shows how to use the preprocessor and the compiler’s constant expression computation to calculate the value of timer 1’s post-scaler in a way so it only
depends on F_CPU and the desired software clock frequency. While the formula looks
a bit complicated, using a macro offers the advantage that the application will automatically scale to new target softclock or master CPU frequencies without having to
manually re-calculate hardcoded constants.
6.31.3.2 Part 2: Variable definitions The intflags structure demonstrates a
way to allocate bit variables in memory. Each of the interrupt service routines just sets

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one bit within that structure, and the application’s main loop then monitors the bits in
order to act appropriately.
Like all variables that are used to communicate values between an interrupt service
routine and the main application, it is declared volatile.
The variable ee_pwm is not a variable in the classical C sense that could be used as an
lvalue or within an expression to obtain its value. Instead, the
__attribute__((section(".eeprom")))

marks it as belonging to the EEPROM section. This section is merely used as a placeholder so the compiler can arrange for each individual variable’s location in EEPROM.
The compiler will also keep track of initial values assigned, and usually the Makefile
is arranged to extract these initial values into a separate load file (largedemo_eeprom.∗ in this case) that can be used to initialize the EEPROM.
The actual EEPROM IO must be performed manually.
Similarly, the variable mcucsr is kept in the .noinit section in order to prevent it from
being cleared upon application startup.
6.31.3.3 Part 3: Interrupt service routines The ISR to handle timer 1’s overflow
interrupt arranges for the software clock. While timer 1 runs the PWM, it calls its
overflow handler rather frequently, so the TMR1_SCALE value is used as a postscaler
to reduce the internal software clock frequency further. If the software clock triggers,
it sets the tmr_int bitfield, and defers all further tasks to the main loop.
The ADC ISR just fetches the value from the ADC conversion, disables the ADC
interrupt again, and announces the presence of the new value in the adc_int bitfield.
The interrupt is kept disabled while not needed, because the ADC will also be triggered
by executing the SLEEP instruction in idle mode (which is the default sleep mode).
Another option would be to turn off the ADC completely here, but that increases the
ADC’s startup time (not that it would matter much for this application).
6.31.3.4 Part 4: Auxiliary functions The function handle_mcucsr() uses two
__attribute__ declarators to achieve specific goals. First, it will instruct the compiler to place the generated code into the .init3 section of the output. Thus, it will become part of the application initialization sequence. This is done in order to fetch (and
clear) the reason of the last hardware reset from MCUCSR as early as possible. There
is a short period of time where the next reset could already trigger before the current
reason has been evaluated. This also explains why the variable mcucsr that mirrors
the register’s value needs to be placed into the .noinit section, because otherwise the
default initialization (which happens after .init3) would blank the value again.
As the initialization code is not called using CALL/RET instructions but rather concatenated together, the compiler needs to be instructed to omit the entire function prologue and epilogue. This is performed by the naked attribute. So while syntactically,
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handle_mcucsr() is a function to the compiler, the compiler will just emit the instructions for it without setting up any stack frame, and not even a RET instruction at
the end.
Function ioinit() centralizes all hardware setup. The very last part of that function
demonstrates the use of the EEPROM variable ee_pwm to obtain an EEPROM address
that can in turn be applied as an argument to eeprom_read_word().
The following functions handle UART character and string output. (UART input
is handled by an ISR.) There are two string output functions, printstr() and
printstr_p(). The latter function fetches the string from program memory. Both
functions translate a newline character into a carriage return/newline sequence, so a
simple \n can be used in the source code.
The function set_pwm() propagates the new PWM value to the PWM, performing
range checking. When the value has been changed, the new percentage will be announced on the serial link. The current value is mirrored in the variable pwm so others
can use it in calculations. In order to allow for a simple calculation of a percentage
value without requiring floating-point mathematics, the maximal value of the PWM is
restricted to 1000 rather than 1023, so a simple division by 10 can be used. Due to the
nature of the human eye, the difference in LED brightness between 1000 and 1023 is
not noticable anyway.
6.31.3.5 Part 5: main() At the start of main(), a variable mode is declared to
keep the current mode of operation. An enumeration is used to improve the readability.
By default, the compiler would allocate a variable of type int for an enumeration. The
packed attribute declarator instructs the compiler to use the smallest possible integer
type (which would be an 8-bit type here).
After some initialization actions, the application’s main loop follows. In an embedded
application, this is normally an infinite loop as there is nothing an application could
"exit" into anyway.
At the beginning of the loop, the watchdog timer will be retriggered. If that timer is
not triggered for about 2 seconds, it will issue a hardware reset. Care needs to be taken
that no code path blocks longer than this, or it needs to frequently perform watchdog
resets of its own. An example of such a code path would be the string IO functions: for
an overly large string to print (about 2000 characters at 9600 Bd), they might block for
too long.
The loop itself then acts on the interrupt indication bitfields as appropriate, and will
eventually put the CPU on sleep at its end to conserve power.
The first interrupt bit that is handled is the (software) timer, at a frequency of approximately 100 Hz. The CLOCKOUT pin will be toggled here, so e. g. an oscilloscope
can be used on that pin to measure the accuracy of our software clock. Then, the LED
flasher for LED2 ("We are alive"-LED) is built. It will flash that LED for about 50
ms, and pause it for another 950 ms. Various actions depending on the operation mode
follow. Finally, the 3-second backup timer is implemented that will write the PWM
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value back to EEPROM once it is not changing anymore.
The ADC interrupt will just adjust the PWM value only.
Finally, the UART Rx interrupt will dispatch on the last character received from the
UART.
All the string literals that are used as informational messages within main() are
placed in program memory so no SRAM needs to be allocated for them. This is done
by using the PSTR macro, and passing the string to printstr_p().
6.31.4

The source code

The source code is installed under
$prefix/share/doc/avr-libc/examples/largedemo/largedemo.c,
where $prefix is a configuration option. For Unix systems, it is usually set to either
/usr or /usr/local.

6.32

Using the standard IO facilities

This project illustrates how to use the standard IO facilities (stdio) provided by this
library. It assumes a basic knowledge of how the stdio subsystem is used in standard C
applications, and concentrates on the differences in this library’s implementation that
mainly result from the differences of the microcontroller environment, compared to a
hosted environment of a standard computer.
This demo is meant to supplement the documentation, not to replace it.
6.32.1

Hardware setup

The demo is set up in a way so it can be run on the ATmega16 that ships with the
STK500 development kit. The UART port needs to be connected to the RS-232 "spare"
port by a jumper cable that connects PD0 to RxD and PD1 to TxD. The RS-232 channel
is set up as standard input (stdin) and standard output (stdout), respectively.
In order to have a different device available for a standard error channel (stderr), an
industry-standard LCD display with an HD44780-compatible LCD controller has been
chosen. This display needs to be connected to port A of the STK500 in the following
way:

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6.32

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Port
A0
A1
A2
A3
A4
A5
A6
A7
GND
VCC

Header
1
2
3
4
5
6
7
8
9
10

185

Function
LCD D4
LCD D5
LCD D6
LCD D7
LCD R/∼W
LCD E
LCD RS
unused
GND
Vcc

Figure 5: Wiring of the STK500

The LCD controller is used in 4-bit mode, including polling the "busy" flag so the
R/∼W line from the LCD controller needs to be connected. Note that the LCD controller has yet another supply pin that is used to adjust the LCD’s contrast (V5). Typically, that pin connects to a potentiometer between Vcc and GND. Often, it might
work to just connect that pin to GND, while leaving it unconnected usually yields an
unreadable display.
Port A has been chosen as 7 pins on a single port are needed to connect the LCD, yet all
other ports are already partially in use: port B has the pins for in-system programming
(ISP), port C has the ports for JTAG (can be used for debugging), and port D is used
for the UART connection.

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186

Functional overview

The project consists of the following files:
• stdiodemo.c This is the main example file.
• defines.h Contains some global defines, like the LCD wiring
• hd44780.c Implementation of an HD44780 LCD display driver
• hd44780.h Interface declarations for the HD44780 driver
• lcd.c Implementation of LCD character IO on top of the HD44780 driver
• lcd.h Interface declarations for the LCD driver
• uart.c Implementation of a character IO driver for the internal UART
• uart.h Interface declarations for the UART driver
6.32.3

A code walkthrough

6.32.3.1 stdiodemo.c As usual, include files go first. While conventionally, system
header files (those in angular brackets < ... >) go before application-specific header
files (in double quotes), defines.h comes as the first header file here. The main
reason is that this file defines the value of F_CPU which needs to be known before
including .
The function ioinit() summarizes all hardware initialization tasks. As this function
is declared to be module-internal only (static), the compiler will notice its simplicity, and with a reasonable optimization level in effect, it will inline that function. That
needs to be kept in mind when debugging, because the inlining might cause the debugger to "jump around wildly" at a first glance when single-stepping.
The definitions of uart_str and lcd_str set up two stdio streams. The initialization is done using the FDEV_SETUP_STREAM() initializer template macro, so a
static object can be constructed that can be used for IO purposes. This initializer macro
takes three arguments, two function macros to connect the corresponding output and
input functions, respectively, the third one describes the intent of the stream (read,
write, or both). Those functions that are not required by the specified intent (like the
input function for lcd_str which is specified to only perform output operations) can
be given as NULL.
The stream uart_str corresponds to input and output operations performed over the
RS-232 connection to a terminal (e.g. from/to a PC running a terminal program), while
the lcd_str stream provides a method to display character data on the LCD text
display.
The function delay_1s() suspends program execution for approximately one second. This is done using the _delay_ms() function from 
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which in turn needs the F_CPU macro in order to adjust the cycle counts. As the
_delay_ms() function has a limited range of allowable argument values (depending
on F_CPU), a value of 10 ms has been chosen as the base delay which would be safe
for CPU frequencies of up to about 26 MHz. This function is then called 100 times to
accomodate for the actual one-second delay.
In a practical application, long delays like this one were better be handled by a hardware
timer, so the main CPU would be free for other tasks while waiting, or could be put on
sleep.
At the beginning of main(), after initializing the peripheral devices, the default stdio
streams stdin, stdout, and stderr are set up by using the existing static FILE
stream objects. While this is not mandatory, the availability of stdin and stdout
allows to use the shorthand functions (e.g. printf() instead of fprintf()), and
stderr can mnemonically be referred to when sending out diagnostic messages.
Just for demonstration purposes, stdin and stdout are connected to a stream that
will perform UART IO, while stderr is arranged to output its data to the LCD text
display.
Finally, a main loop follows that accepts simple "commands" entered via the RS-232
connection, and performs a few simple actions based on the commands.
First, a prompt is sent out using printf_P() (which takes a program space string).
The string is read into an internal buffer as one line of input, using fgets(). While it
would be also possible to use gets() (which implicitly reads from stdin), gets()
has no control that the user’s input does not overflow the input buffer provided so it
should never be used at all.
If fgets() fails to read anything, the main loop is left. Of course, normally the main
loop of a microcontroller application is supposed to never finish, but again, for demonstrational purposes, this explains the error handling of stdio. fgets() will return
NULL in case of an input error or end-of-file condition on input. Both these conditions are in the domain of the function that is used to establish the stream, uart_putchar() in this case. In short, this function returns EOF in case of a serial line
"break" condition (extended start condition) has been recognized on the serial line.
Common PC terminal programs allow to assert this condition as some kind of out-ofband signalling on an RS-232 connection.
When leaving the main loop, a goodbye message is sent to standard error output (i.e. to
the LCD), followed by three dots in one-second spacing, followed by a sequence that
will clear the LCD. Finally, main() will be terminated, and the library will add an
infinite loop, so only a CPU reset will be able to restart the application.
There are three "commands" recognized, each determined by the first letter of the line
entered (converted to lower case):
• The ’q’ (quit) command has the same effect of leaving the main loop.
• The ’l’ (LCD) command takes its second argument, and sends it to the LCD.

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• The ’u’ (UART) command takes its second argument, and sends it back to the
UART connection.
Command recognition is done using sscanf() where the first format in the format
string just skips over the command itself (as the assignment suppression modifier ∗ is
given).
6.32.3.2

defines.h This file just contains a few peripheral definitions.

The F_CPU macro defines the CPU clock frequency, to be used in delay loops, as well
as in the UART baud rate calculation.
The macro UART_BAUD defines the RS-232 baud rate. Depending on the actual CPU
frequency, only a limited range of baud rates can be supported.
The remaining macros customize the IO port and pins used for the HD44780 LCD
driver.
6.32.3.3 hd44780.h This file describes the public interface of the low-level LCD
driver that interfaces to the HD44780 LCD controller. Public functions are available to
initialize the controller into 4-bit mode, to wait for the controller’s busy bit to be clear,
and to read or write one byte from or to the controller.
As there are two different forms of controller IO, one to send a command or receive
the controller status (RS signal clear), and one to send or receive data to/from the
controller’s SRAM (RS asserted), macros are provided that build on the mentioned
function primitives.
Finally, macros are provided for all the controller commands to allow them to be used
symbolically. The HD44780 datasheet explains these basic functions of the controller
in more detail.
6.32.3.4 hd44780.c This is the implementation of the low-level HD44780 LCD
controller driver.
On top, a few preprocessor glueing tricks are used to establish symbolic access to
the hardware port pins the LCD controller is attached to, based on the application’s
definitions made in defines.h.
The hd44780_pulse_e() function asserts a short pulse to the controller’s E (enable) pin. Since reading back the data asserted by the LCD controller needs to be
performed while E is active, this function reads and returns the input data if the parameter readback is true. When called with a compile-time constant parameter that is
false, the compiler will completely eliminate the unused readback operation, as well as
the return value as part of its optimizations.
As the controller is used in 4-bit interface mode, all byte IO to/from the controller
needs to be handled as two nibble IOs. The functions hd44780_outnibble() and
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hd44780_innibble() implement this. They do not belong to the public interface,
so they are declared static.
Building upon these, the public functions hd44780_outbyte() and hd44780_inbyte() transfer one byte to/from the controller.
The function hd44780_wait_ready() waits for the controller to become ready,
by continuously polling the controller’s status (which is read by performing a byte read
with the RS signal cleard), and examining the BUSY flag within the status byte. This
function needs to be called before performing any controller IO.
Finally, hd44780_init() initializes the LCD controller into 4-bit mode, based on
the initialization sequence mandated by the datasheet. As the BUSY flag cannot be
examined yet at this point, this is the only part of this code where timed delays are
used. While the controller can perform a power-on reset when certain constraints on
the power supply rise time are met, always calling the software initialization routine
at startup ensures the controller will be in a known state. This function also puts the
interface into 4-bit mode (which would not be done automatically after a power-on
reset).
6.32.3.5 lcd.h This function declares the public interface of the higher-level (character IO) LCD driver.
6.32.3.6 lcd.c The implementation of the higher-level LCD driver. This driver
builds on top of the HD44780 low-level LCD controller driver, and offers a character
IO interface suitable for direct use by the standard IO facilities. Where the low-level
HD44780 driver deals with setting up controller SRAM addresses, writing data to the
controller’s SRAM, and controlling display functions like clearing the display, or moving the cursor, this high-level driver allows to just write a character to the LCD, in the
assumption this will somehow show up on the display.
Control characters can be handled at this level, and used to perform specific actions
on the LCD. Currently, there is only one control character that is being dealt with: a
newline character (\n) is taken as an indication to clear the display and set the cursor
into its initial position upon reception of the next character, so a "new line" of text
can be displayed. Therefore, a received newline character is remembered until more
characters have been sent by the application, and will only then cause the display to be
cleared before continuing. This provides a convenient abstraction where full lines of
text can be sent to the driver, and will remain visible at the LCD until the next line is
to be displayed.
Further control characters could be implemented, e. g. using a set of escape sequences.
That way, it would be possible to implement self-scrolling display lines etc.
The public function lcd_init() first calls the initialization entry point of the lowerlevel HD44780 driver, and then sets up the LCD in a way we’d like to (display cleared,
non-blinking cursor enabled, SRAM addresses are increasing so characters will be

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written left to right).
The public function lcd_putchar() takes arguments that make it suitable for being passed as a put() function pointer to the stdio stream initialization functions and
macros (fdevopen(), FDEV_SETUP_STREAM() etc.). Thus, it takes two arguments, the character to display itself, and a reference to the underlying stream object,
and it is expected to return 0 upon success.
This function remembers the last unprocessed newline character seen in the functionlocal static variable nl_seen. If a newline character is encountered, it will simply set
this variable to a true value, and return to the caller. As soon as the first non-newline
character is to be displayed with nl_seen still true, the LCD controller is told to clear
the display, put the cursor home, and restart at SRAM address 0. All other characters
are sent to the display.
The single static function-internal variable nl_seen works for this purpose. If multiple LCDs should be controlled using the same set of driver functions, that would not
work anymore, as a way is needed to distinguish between the various displays. This is
where the second parameter can be used, the reference to the stream itself: instead of
keeping the state inside a private variable of the function, it can be kept inside a private
object that is attached to the stream itself. A reference to that private object can be attached to the stream (e.g. inside the function lcd_init() that then also needs to be
passed a reference to the stream) using fdev_set_udata(), and can be accessed
inside lcd_putchar() using fdev_get_udata().
6.32.3.7 uart.h Public interface definition for the RS-232 UART driver, much like
in lcd.h except there is now also a character input function available.
As the RS-232 input is line-buffered in this example, the macro RX_BUFSIZE determines the size of that buffer.
6.32.3.8 uart.c This implements an stdio-compatible RS-232 driver using an
AVR’s standard UART (or USART in asynchronous operation mode). Both, character output as well as character input operations are implemented. Character output
takes care of converting the internal newline \n into its external representation carriage
return/line feed (\r\n).
Character input is organized as a line-buffered operation that allows to minimally edit
the current line until it is "sent" to the application when either a carriage return (\r)
or newline (\n) character is received from the terminal. The line editing functions
implemented are:
• \b (back space) or \177 (delete) deletes the previous character
•

∧

•

∧

u (control-U, ASCII NAK) deletes the entire input buffer

w (control-W, ASCII ETB) deletes the previous input word, delimited by white
space

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∧

191

r (control-R, ASCII DC2) sends a \r, then reprints the buffer (refresh)

• \t (tabulator) will be replaced by a single space
The function uart_init() takes care of all hardware initialization that is required to
put the UART into a mode with 8 data bits, no parity, one stop bit (commonly referred
to as 8N1) at the baud rate configured in defines.h. At low CPU clock frequencies, the
U2X bit in the UART is set, reducing the oversampling from 16x to 8x, which allows
for a 9600 Bd rate to be achieved with tolerable error using the default 1 MHz RC
oscillator.
The public function uart_putchar() again has suitable arguments for direct use
by the stdio stream interface. It performs the \n into \r\n translation by recursively
calling itself when it sees a \n character. Just for demonstration purposes, the \a
(audible bell, ASCII BEL) character is implemented by sending a string to stderr,
so it will be displayed on the LCD.
The public function uart_getchar() implements the line editor. If there are characters available in the line buffer (variable rxp is not NULL), the next character will
be returned from the buffer without any UART interaction.
If there are no characters inside the line buffer, the input loop will be entered. Characters will be read from the UART, and processed accordingly. If the UART signalled a
framing error (FE bit set), typically caused by the terminal sending a line break condition (start condition held much longer than one character period), the function will
return an end-of-file condition using _FDEV_EOF. If there was a data overrun condition on input (DOR bit set), an error condition will be returned as _FDEV_ERR.
Line editing characters are handled inside the loop, potentially modifying the buffer
status. If characters are attempted to be entered beyond the size of the line buffer, their
reception is refused, and a \a character is sent to the terminal. If a \r or \n character is
seen, the variable rxp (receive pointer) is set to the beginning of the buffer, the loop is
left, and the first character of the buffer will be returned to the application. (If no other
characters have been entered, this will just be the newline character, and the buffer is
marked as being exhausted immediately again.)
6.32.4

The source code

The source code is installed under
$prefix/share/doc/avr-libc/examples/stdiodemo/,
where $prefix is a configuration option. For Unix systems, it is usually set to either
/usr or /usr/local.

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

6.33

6.33

Example using the two-wire interface (TWI)

192

Example using the two-wire interface (TWI)

Some newer devices of the ATmega series contain builtin support for interfacing the
microcontroller to a two-wire bus, called TWI. This is essentially the same called I2C
by Philips, but that term is avoided in Atmel’s documentation due to patenting issues.
For the original Philips documentation, see
http://www.semiconductors.philips.com/buses/i2c/index.html
6.33.1

Introduction into TWI

The two-wire interface consists of two signal lines named SDA (serial data) and SCL
(serial clock) (plus a ground line, of course). All devices participating in the bus are
connected together, using open-drain driver circuitry, so the wires must be terminated
using appropriate pullup resistors. The pullups must be small enough to recharge
the line capacity in short enough time compared to the desired maximal clock frequency, yet large enough so all drivers will not be overloaded. There are formulas in
the datasheet that help selecting the pullups.
Devices can either act as a master to the bus (i. e., they initiate a transfer), or as a
slave (they only act when being called by a master). The bus is multi-master capable,
and a particular device implementation can act as either master or slave at different
times. Devices are addressed using a 7-bit address (coordinated by Philips) transfered
as the first byte after the so-called start condition. The LSB of that byte is R/∼W, i. e.
it determines whether the request to the slave is to read or write data during the next
cycles. (There is also an option to have devices using 10-bit addresses but that is not
covered by this example.)
6.33.2

The TWI example project

The ATmega TWI hardware supports both, master and slave operation. This example
will only demonstrate how to use an AVR microcontroller as TWI master. The implementation is kept simple in order to concentrate on the steps that are required to talk to
a TWI slave, so all processing is done in polled-mode, waiting for the TWI interface to
indicate that the next processing step is due (by setting the TWINT interrupt bit). If it
is desired to have the entire TWI communication happen in "background", all this can
be implemented in an interrupt-controlled way, where only the start condition needs to
be triggered from outside the interrupt routine.
There is a variety of slave devices available that can be connected to a TWI bus. For the
purpose of this example, an EEPROM device out of the industry-standard 24Cxx series
has been chosen (where xx can be one of 01, 02, 04, 08, or 16) which are available from
various vendors. The choice was almost arbitrary, mainly triggered by the fact that an
EEPROM device is being talked to in both directions, reading and writing the slave
device, so the example will demonstrate the details of both.

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6.33

Example using the two-wire interface (TWI)

193

Usually, there is probably not much need to add more EEPROM to an ATmega system
that way: the smallest possible AVR device that offers hardware TWI support is the
ATmega8 which comes with 512 bytes of EEPROM, which is equivalent to an 24C04
device. The ATmega128 already comes with twice as much EEPROM as the 24C16
would offer. One exception might be to use an externally connected EEPROM device
that is removable; e. g. SDRAM PC memory comes with an integrated TWI EEPROM
that carries the RAM configuration information.
6.33.3

The Source Code

The source code is installed under
$prefix/share/doc/avr-libc/examples/twitest/twitest.c,
where $prefix is a configuration option. For Unix systems, it is usually set to either
/usr or /usr/local.
Note [1]

The header file  contains some macro definitions for symbolic constants used in the TWI status register. These definitions match the names used in the
Atmel datasheet except that all names have been prefixed with TW_.
Note [2]

The clock is used in timer calculations done by the compiler, for the UART baud rate
and the TWI clock rate.
Note [3]

The address assigned for the 24Cxx EEPROM consists of 1010 in the upper four bits.
The following three bits are normally available as slave sub-addresses, allowing to
operate more than one device of the same type on a single bus, where the actual subaddress used for each device is configured by hardware strapping. However, since the
next data packet following the device selection only allows for 8 bits that are used as
an EEPROM address, devices that require more than 8 address bits (24C04 and above)
"steal" subaddress bits and use them for the EEPROM cell address bits 9 to 11 as required. This example simply assumes all subaddress bits are 0 for the smaller devices,
so the E0, E1, and E2 inputs of the 24Cxx must be grounded.

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6.33

Example using the two-wire interface (TWI)

194

Note [4]

For slow clocks, enable the 2 x U[S]ART clock multiplier, to improve the baud rate
error. This will allow a 9600 Bd communication using the standard 1 MHz calibrated
RC oscillator. See also the Baud rate tables in the datasheets.
Note [5]

The datasheet explains why a minimum TWBR value of 10 should be maintained when
running in master mode. Thus, for system clocks below 3.6 MHz, we cannot run the
bus at the intented clock rate of 100 kHz but have to slow down accordingly.
Note [6]

This function is used by the standard output facilities that are utilized in this example
for debugging and demonstration purposes.
Note [7]

In order to shorten the data to be sent over the TWI bus, the 24Cxx EEPROMs support
multiple data bytes transfered within a single request, maintaining an internal address
counter that is updated after each data byte transfered successfully. When reading
data, one request can read the entire device memory if desired (the counter would wrap
around and start back from 0 when reaching the end of the device).
Note [8]

When reading the EEPROM, a first device selection must be made with write intent
(R/∼W bit set to 0 indicating a write operation) in order to transfer the EEPROM address to start reading from. This is called master transmitter mode. Each completion
of a particular step in TWI communication is indicated by an asserted TWINT bit in
TWCR. (An interrupt would be generated if allowed.) After performing any actions
that are needed for the next communication step, the interrupt condition must be manually cleared by setting the TWINT bit. Unlike with many other interrupt sources, this
would even be required when using a true interrupt routine, since as soon as TWINT is
re-asserted, the next bus transaction will start.

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6.33

Example using the two-wire interface (TWI)

195

Note [9]

Since the TWI bus is multi-master capable, there is potential for a bus contention when
one master starts to access the bus. Normally, the TWI bus interface unit will detect this
situation, and will not initiate a start condition while the bus is busy. However, in case
two masters were starting at exactly the same time, the way bus arbitration works, there
is always a chance that one master could lose arbitration of the bus during any transmit
operation. A master that has lost arbitration is required by the protocol to immediately
cease talking on the bus; in particular it must not initiate a stop condition in order to not
corrupt the ongoing transfer from the active master. In this example, upon detecting a
lost arbitration condition, the entire transfer is going to be restarted. This will cause a
new start condition to be initiated, which will normally be delayed until the currently
active master has released the bus.
Note [10]

Next, the device slave is going to be reselected (using a so-called repeated start condition which is meant to guarantee that the bus arbitration will remain at the current
master) using the same slave address (SLA), but this time with read intent (R/∼W bit
set to 1) in order to request the device slave to start transfering data from the slave to
the master in the next packet.
Note [11]

If the EEPROM device is still busy writing one or more cells after a previous write
request, it will simply leave its bus interface drivers at high impedance, and does not
respond to a selection in any way at all. The master selecting the device will see the
high level at SDA after transfering the SLA+R/W packet as a NACK to its selection
request. Thus, the select process is simply started over (effectively causing a repeated
start condition), until the device will eventually respond. This polling procedure is
recommended in the 24Cxx datasheet in order to minimize the busy wait time when
writing. Note that in case a device is broken and never responds to a selection (e. g.
since it is no longer present at all), this will cause an infinite loop. Thus the maximal
number of iterations made until the device is declared to be not responding at all, and
an error is returned, will be limited to MAX_ITER.
Note [12]

This is called master receiver mode: the bus master still supplies the SCL clock, but the
device slave drives the SDA line with the appropriate data. After 8 data bits, the master
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6.33

Example using the two-wire interface (TWI)

196

responds with an ACK bit (SDA driven low) in order to request another data transfer
from the slave, or it can leave the SDA line high (NACK), indicating to the slave that
it is going to stop the transfer now. Assertion of ACK is handled by setting the TWEA
bit in TWCR when starting the current transfer.
Note [13]

The control word sent out in order to initiate the transfer of the next data packet is
initially set up to assert the TWEA bit. During the last loop iteration, TWEA is deasserted so the client will get informed that no further transfer is desired.
Note [14]

Except in the case of lost arbitration, all bus transactions must properly be terminated
by the master initiating a stop condition.
Note [15]

Writing to the EEPROM device is simpler than reading, since only a master transmitter
mode transfer is needed. Note that the first packet after the SLA+W selection is always
considered to be the EEPROM address for the next operation. (This packet is exactly
the same as the one above sent before starting to read the device.) In case a master
transmitter mode transfer is going to send more than one data packet, all following
packets will be considered data bytes to write at the indicated address. The internal
address pointer will be incremented after each write operation.
Note [16]

24Cxx devices can become write-protected by strapping their ∼WC pin to logic high.
(Leaving it unconnected is explicitly allowed, and constitutes logic low level, i. e. no
write protection.) In case of a write protected device, all data transfer attempts will be
NACKed by the device. Note that some devices might not implement this.

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

7 avr-libc Data Structure Documentation

7

avr-libc Data Structure Documentation

7.1

div_t Struct Reference

7.1.1

Detailed Description

Result type for function div().
Data Fields
• int quot
• int rem
7.1.2

Field Documentation

7.1.2.1

int div_t::quot

The Quotient.
7.1.2.2

int div_t::rem

The Remainder.
The documentation for this struct was generated from the following file:
• stdlib.h

7.2

ldiv_t Struct Reference

7.2.1

Detailed Description

Result type for function ldiv().
Data Fields
• long quot
• long rem
7.2.2
7.2.2.1

Field Documentation
long ldiv_t::quot

The Quotient.
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197

8 avr-libc File Documentation

7.2.2.2

long ldiv_t::rem

The Remainder.
The documentation for this struct was generated from the following file:
• stdlib.h

8

avr-libc File Documentation

8.1

assert.h File Reference

8.1.1

Detailed Description

Defines
• #define assert(expression)

8.2

atoi.S File Reference

8.2.1

Detailed Description

Defines
•
•
•
•
•

8.3

#define str_hi r25
#define str_lo r24
#define num_hi r25
#define num_lo r24
#define tmp r18

atol.S File Reference

8.3.1

Detailed Description

Defines
•
•
•
•
•
•
•

#define str_hi r25
#define str_lo r24
#define num_hi_hi r25
#define num_hi_lo r24
#define num_lo_hi r23
#define num_lo_lo r22
#define tmp r17

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198

8.4

boot.h File Reference

8.4

199

boot.h File Reference

8.4.1

Detailed Description

Defines
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

#define _AVR_BOOT_H_ 1
#define BOOTLOADER_SECTION __attribute__ ((section (".bootloader")))
#define __COMMON_ASB RWWSB
#define __COMMON_ASRE RWWSRE
#define BLB12 5
#define BLB11 4
#define BLB02 3
#define BLB01 2
#define boot_spm_interrupt_enable() (__SPM_REG |= (uint8_t)_BV(SPMIE))
#define boot_spm_interrupt_disable() (__SPM_REG &= (uint8_t)∼_BV(SPMIE))
#define boot_is_spm_interrupt() (__SPM_REG & (uint8_t)_BV(SPMIE))
#define boot_rww_busy() (__SPM_REG & (uint8_t)_BV(__COMMON_ASB))
#define boot_spm_busy() (__SPM_REG & (uint8_t)_BV(SPMEN))
#define boot_spm_busy_wait() do{}while(boot_spm_busy())
#define __BOOT_PAGE_ERASE (_BV(SPMEN) | _BV(PGERS))
#define __BOOT_PAGE_WRITE (_BV(SPMEN) | _BV(PGWRT))
#define __BOOT_PAGE_FILL _BV(SPMEN)
#define __BOOT_RWW_ENABLE (_BV(SPMEN) | _BV(__COMMON_ASRE))
#define __BOOT_LOCK_BITS_SET (_BV(SPMEN) | _BV(BLBSET))
#define __boot_page_fill_normal(address, data)
#define __boot_page_fill_alternate(address, data)
#define __boot_page_fill_extended(address, data)
#define __boot_page_erase_normal(address)
#define __boot_page_erase_alternate(address)
#define __boot_page_erase_extended(address)
#define __boot_page_write_normal(address)
#define __boot_page_write_alternate(address)
#define __boot_page_write_extended(address)
#define __boot_rww_enable()
#define __boot_rww_enable_alternate()
#define __boot_lock_bits_set(lock_bits)
#define __boot_lock_bits_set_alternate(lock_bits)
#define GET_LOW_FUSE_BITS (0x0000)
#define GET_LOCK_BITS (0x0001)
#define GET_EXTENDED_FUSE_BITS (0x0002)

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8.4

boot.h File Reference

•
•
•
•
•
•
•
•
•
•
•
•

200

#define GET_HIGH_FUSE_BITS (0x0003)
#define boot_lock_fuse_bits_get(address)
#define boot_page_fill(address, data) __boot_page_fill_normal(address, data)
#define boot_page_erase(address) __boot_page_erase_normal(address)
#define boot_page_write(address) __boot_page_write_normal(address)
#define boot_rww_enable() __boot_rww_enable()
#define boot_lock_bits_set(lock_bits) __boot_lock_bits_set(lock_bits)
#define boot_page_fill_safe(address, data)
#define boot_page_erase_safe(address)
#define boot_page_write_safe(address)
#define boot_rww_enable_safe()
#define boot_lock_bits_set_safe(lock_bits)

8.4.2
8.4.2.1

Define Documentation
#define __boot_lock_bits_set(lock_bits)

Value:
({
uint8_t value = (uint8_t)(~(lock_bits));
__asm__ __volatile__
(
"ldi r30, 1\n\t"
"ldi r31, 0\n\t"
"mov r0, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_LOCK_BITS_SET),
"r" (value)
: "r0", "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.2

#define __boot_lock_bits_set_alternate(lock_bits)

Value:
({
uint8_t value = (uint8_t)(~(lock_bits));
__asm__ __volatile__
(
"ldi r30, 1\n\t"
"ldi r31, 0\n\t"
"mov r0, %2\n\t"
"sts %0, %1\n\t"

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\
\
\
\
\
\
\
\

8.4

boot.h File Reference

201

"spm\n\t"
".word 0xffff\n\t"
"nop\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_LOCK_BITS_SET),
"r" (value)
: "r0", "r30", "r31"

\
\
\
\
\
\
\
\
\

);
})

8.4.2.3

#define __boot_page_erase_alternate(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
".word 0xffff\n\t"
"nop\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_ERASE),
"r" ((uint16_t)address)
: "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.4

#define __boot_page_erase_extended(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %A3\n\t"
"sts %1, %C3\n\t"
"sts %0, %2\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"i" (_SFR_MEM_ADDR(RAMPZ)),
"r" ((uint8_t)__BOOT_PAGE_ERASE),
"r" ((uint32_t)address)
: "r30", "r31"
);
})

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\
\
\
\
\
\
\
\
\
\
\
\
\
\

8.4

boot.h File Reference

8.4.2.5

202

#define __boot_page_erase_normal(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_ERASE),
"r" ((uint16_t)address)
: "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.6

#define __boot_page_fill_alternate(address, data)

Value:
({
__asm__ __volatile__
(
"movw r0, %3\n\t"
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
".word 0xffff\n\t"
"nop\n\t"
"clr r1\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_FILL),
"r" ((uint16_t)address),
"r" ((uint16_t)data)
: "r0", "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.7

#define __boot_page_fill_extended(address, data)

Value:
({
__asm__ __volatile__
(
"movw r0, %4\n\t"
"movw r30, %A3\n\t"
"sts %1, %C3\n\t"

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\
\
\
\
\
\

8.4

boot.h File Reference

"sts %0, %2\n\t"
"spm\n\t"
"clr r1\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"i" (_SFR_MEM_ADDR(RAMPZ)),
"r" ((uint8_t)__BOOT_PAGE_FILL),
"r" ((uint32_t)address),
"r" ((uint16_t)data)
: "r0", "r30", "r31"

203

);

\
\
\
\
\
\
\
\
\
\
\

8.4.2.8

#define __boot_page_fill_normal(address, data)

})

Value:
({
__asm__ __volatile__
(
"movw r0, %3\n\t"
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
"clr r1\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_FILL),
"r" ((uint16_t)address),
"r" ((uint16_t)data)
: "r0", "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.9

#define __boot_page_write_alternate(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
".word 0xffff\n\t"
"nop\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_WRITE),
"r" ((uint16_t)address)
: "r30", "r31"
);
})

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\
\
\
\
\
\
\
\
\
\
\
\
\
\

8.4

boot.h File Reference

8.4.2.10

204

#define __boot_page_write_extended(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %A3\n\t"
"sts %1, %C3\n\t"
"sts %0, %2\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"i" (_SFR_MEM_ADDR(RAMPZ)),
"r" ((uint8_t)__BOOT_PAGE_WRITE),
"r" ((uint32_t)address)
: "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.11

#define __boot_page_write_normal(address)

Value:
({
__asm__ __volatile__
(
"movw r30, %2\n\t"
"sts %0, %1\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_PAGE_WRITE),
"r" ((uint16_t)address)
: "r30", "r31"
);

\
\
\
\
\
\
\
\
\
\
\
\

})

8.4.2.12

#define __boot_rww_enable()

Value:
({
__asm__ __volatile__
(
"sts %0, %1\n\t"
"spm\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_RWW_ENABLE)
);
})

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

\
\
\
\
\
\
\
\
\

8.5

crc16.h File Reference

8.4.2.13

205

#define __boot_rww_enable_alternate()

Value:
({
__asm__ __volatile__
(
"sts %0, %1\n\t"
"spm\n\t"
".word 0xffff\n\t"
"nop\n\t"
:
: "i" (_SFR_MEM_ADDR(__SPM_REG)),
"r" ((uint8_t)__BOOT_RWW_ENABLE)
);

\
\
\
\
\
\
\
\
\
\
\

})

8.5

crc16.h File Reference

8.5.1

Detailed Description

Functions
•
•
•
•

8.6

static __inline__ uint16_t _crc16_update (uint16_t __crc, uint8_t __data)
static __inline__ uint16_t _crc_xmodem_update (uint16_t __crc, uint8_t __data)
static __inline__ uint16_t _crc_ccitt_update (uint16_t __crc, uint8_t __data)
static __inline__ uint8_t _crc_ibutton_update (uint8_t __crc, uint8_t __data)

ctype.h File Reference

8.6.1

Detailed Description

Character classification routines
These functions perform character classification. They return true or false status depending whether the character passed to the function falls into the function’s classification (i.e. isdigit() returns true if its argument is any value ’0’ though ’9’, inclusive.)
•
•
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•
•
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•
•

int isalnum (int __c) __ATTR_CONST__
int isalpha (int __c) __ATTR_CONST__
int isascii (int __c) __ATTR_CONST__
int isblank (int __c) __ATTR_CONST__
int iscntrl (int __c) __ATTR_CONST__
int isdigit (int __c) __ATTR_CONST__
int isgraph (int __c) __ATTR_CONST__
int islower (int __c) __ATTR_CONST__

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.7

delay.h File Reference

•
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•

206

int isprint (int __c) __ATTR_CONST__
int ispunct (int __c) __ATTR_CONST__
int isspace (int __c) __ATTR_CONST__
int isupper (int __c) __ATTR_CONST__
int isxdigit (int __c) __ATTR_CONST__

Character convertion routines
If c is not an unsigned char value, or EOF, the behaviour of these functions is undefined.
• int toascii (int __c) __ATTR_CONST__
• int tolower (int __c) __ATTR_CONST__
• int toupper (int __c) __ATTR_CONST__
Defines
• #define __CTYPE_H_ 1
• #define __ATTR_CONST__ __attribute__((__const__))

8.7

delay.h File Reference

8.7.1

Detailed Description

Defines
• #define _UTIL_DELAY_H_ 1
• #define F_CPU 1000000UL
Functions
• void _delay_us (double __us)
• void _delay_ms (double __ms)

8.8

delay_basic.h File Reference

8.8.1

Detailed Description

Defines
• #define _UTIL_DELAY_BASIC_H_ 1

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.9

eeprom.h File Reference

207

Functions
• void _delay_loop_1 (uint8_t __count)
• void _delay_loop_2 (uint16_t __count)

8.9

eeprom.h File Reference

8.9.1

Detailed Description

avr-libc declarations
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#define EEMEM __attribute__((section(".eeprom")))
#define eeprom_is_ready()
#define eeprom_busy_wait() do {} while (!eeprom_is_ready())
uint8_t eeprom_read_byte (const uint8_t ∗addr)
uint16_t eeprom_read_word (const uint16_t ∗addr)
void eeprom_read_block (void ∗pointer_ram, const void ∗pointer_eeprom,
size_t n)
• void eeprom_write_byte (uint8_t ∗addr, uint8_t value)
• void eeprom_write_word (uint16_t ∗addr, uint16_t value)
• void eeprom_write_block (const void ∗pointer_ram, void ∗pointer_eeprom,
size_t n)
IAR C compatibility defines
• #define _EEPUT(addr, val) eeprom_write_byte ((uint8_t ∗)(addr), (uint8_t)(val))
• #define _EEGET(var, addr) (var) = eeprom_read_byte ((uint8_t ∗)(addr))
Defines
#define _EEPROM_H_ 1
#define __need_size_t
#define XCALL "rcall"
#define __EEPROM_REG_LOCATIONS__ 1C1D1E
#define _STR2(EXP) _STR1(EXP)
#define _STR1(EXP) #EXP
#define
_REG_LOCATION_SUFFIX
_STR2(__EEPROM_REG_LOCATIONS__)
• #define CR_TAB "\n\t"
•
•
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•

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.10

errno.h File Reference

208

Functions
• static uint8_t __attribute__ ((always_inline)) eeprom_read_byte(const uint8_t
∗addr)
Variables
•
•
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•
•
•

static void const void ∗ pointer_eeprom
static void const void size_t size
static void uint8_t value
static void uint16_t value
static void void ∗ pointer_eeprom
static void void size_t size

8.10

errno.h File Reference

8.10.1

Detailed Description

Defines
• #define __ERRNO_H_ 1
• #define EDOM 33
• #define ERANGE 34
Variables
• int errno

8.11

fdevopen.c File Reference

8.11.1

Detailed Description

Functions
• FILE ∗ fdevopen (int(∗put)(char, FILE ∗), int(∗get)(FILE ∗))

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.12

ffs.S File Reference

8.12

ffs.S File Reference

8.12.1

Detailed Description

8.13

ffsl.S File Reference

8.13.1

Detailed Description

8.14

ffsll.S File Reference

8.14.1

Detailed Description

8.15

interrupt.h File Reference

8.15.1

Detailed Description

209

@{
Global manipulation of the interrupt flag
The global interrupt flag is maintained in the I bit of the status register (SREG).
• #define sei()
• #define cli()
Macros for writing interrupt handler functions
•
•
•
•

#define ISR(vector)
#define SIGNAL(vector)
#define EMPTY_INTERRUPT(vector)
#define ISR_ALIAS(vector, target_vector)

8.16

inttypes.h File Reference

8.16.1

Detailed Description

macros for printf and scanf format specifiers
For C++, these are only included if __STDC_LIMIT_MACROS is defined before including .
• #define PRId8 "d"

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8.16

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inttypes.h File Reference

#define PRIdLEAST8 "d"
#define PRIdFAST8 "d"
#define PRIi8 "i"
#define PRIiLEAST8 "i"
#define PRIiFAST8 "i"
#define PRId16 "d"
#define PRIdLEAST16 "d"
#define PRIdFAST16 "d"
#define PRIi16 "i"
#define PRIiLEAST16 "i"
#define PRIiFAST16 "i"
#define PRId32 "ld"
#define PRIdLEAST32 "ld"
#define PRIdFAST32 "ld"
#define PRIi32 "li"
#define PRIiLEAST32 "li"
#define PRIiFAST32 "li"
#define PRIdPTR PRId16
#define PRIiPTR PRIi16
#define PRIo8 "o"
#define PRIoLEAST8 "o"
#define PRIoFAST8 "o"
#define PRIu8 "u"
#define PRIuLEAST8 "u"
#define PRIuFAST8 "u"
#define PRIx8 "x"
#define PRIxLEAST8 "x"
#define PRIxFAST8 "x"
#define PRIX8 "X"
#define PRIXLEAST8 "X"
#define PRIXFAST8 "X"
#define PRIo16 "o"
#define PRIoLEAST16 "o"
#define PRIoFAST16 "o"
#define PRIu16 "u"
#define PRIuLEAST16 "u"
#define PRIuFAST16 "u"
#define PRIx16 "x"
#define PRIxLEAST16 "x"
#define PRIxFAST16 "x"
#define PRIX16 "X"
#define PRIXLEAST16 "X"
#define PRIXFAST16 "X"

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210

8.16

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inttypes.h File Reference

#define PRIo32 "lo"
#define PRIoLEAST32 "lo"
#define PRIoFAST32 "lo"
#define PRIu32 "lu"
#define PRIuLEAST32 "lu"
#define PRIuFAST32 "lu"
#define PRIx32 "lx"
#define PRIxLEAST32 "lx"
#define PRIxFAST32 "lx"
#define PRIX32 "lX"
#define PRIXLEAST32 "lX"
#define PRIXFAST32 "lX"
#define PRIoPTR PRIo16
#define PRIuPTR PRIu16
#define PRIxPTR PRIx16
#define PRIXPTR PRIX16
#define SCNd16 "d"
#define SCNdLEAST16 "d"
#define SCNdFAST16 "d"
#define SCNi16 "i"
#define SCNiLEAST16 "i"
#define SCNiFAST16 "i"
#define SCNd32 "ld"
#define SCNdLEAST32 "ld"
#define SCNdFAST32 "ld"
#define SCNi32 "li"
#define SCNiLEAST32 "li"
#define SCNiFAST32 "li"
#define SCNdPTR SCNd16
#define SCNiPTR SCNi16
#define SCNo16 "o"
#define SCNoLEAST16 "o"
#define SCNoFAST16 "o"
#define SCNu16 "u"
#define SCNuLEAST16 "u"
#define SCNuFAST16 "u"
#define SCNx16 "x"
#define SCNxLEAST16 "x"
#define SCNxFAST16 "x"
#define SCNo32 "lo"
#define SCNoLEAST32 "lo"
#define SCNoFAST32 "lo"
#define SCNu32 "lu"

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211

8.17

•
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math.h File Reference

#define SCNuLEAST32 "lu"
#define SCNuFAST32 "lu"
#define SCNx32 "lx"
#define SCNxLEAST32 "lx"
#define SCNxFAST32 "lx"
#define SCNoPTR SCNo16
#define SCNuPTR SCNu16
#define SCNxPTR SCNx16

Far pointers for memory access >64K
• typedef int32_t int_farptr_t
• typedef uint32_t uint_farptr_t

8.17

math.h File Reference

8.17.1

Detailed Description

Defines
• #define M_PI 3.141592653589793238462643
• #define M_SQRT2 1.4142135623730950488016887
Functions
•
•
•
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•
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•

double cos (double __x) __ATTR_CONST__
double fabs (double __x) __ATTR_CONST__
double fmod (double __x, double __y) __ATTR_CONST__
double modf (double __value, double ∗__iptr)
double sin (double __x) __ATTR_CONST__
double sqrt (double __x) __ATTR_CONST__
double tan (double __x) __ATTR_CONST__
double floor (double __x) __ATTR_CONST__
double ceil (double __x) __ATTR_CONST__
double frexp (double __value, int ∗__exp)
double ldexp (double __x, int __exp) __ATTR_CONST__
double exp (double __x) __ATTR_CONST__
double cosh (double __x) __ATTR_CONST__
double sinh (double __x) __ATTR_CONST__
double tanh (double __x) __ATTR_CONST__
double acos (double __x) __ATTR_CONST__
double asin (double __x) __ATTR_CONST__

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212

8.18

•
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•

memccpy.S File Reference

double atan (double __x) __ATTR_CONST__
double atan2 (double __y, double __x) __ATTR_CONST__
double log (double __x) __ATTR_CONST__
double log10 (double __x) __ATTR_CONST__
double pow (double __x, double __y) __ATTR_CONST__
int isnan (double __x) __ATTR_CONST__
int isinf (double __x) __ATTR_CONST__
double square (double __x) __ATTR_CONST__

8.18

memccpy.S File Reference

8.18.1

Detailed Description

Defines
•
•
•
•
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•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define val_lo r20
#define len_hi r19
#define len_lo r18
#define ret_hi r25
#define ret_lo r24

8.19

memchr.S File Reference

8.19.1

Detailed Description

Defines
•
•
•
•
•
•
•

#define src_hi r25
#define src_lo r24
#define val_lo r22
#define len_hi r21
#define len_lo r20
#define ret_hi r25
#define ret_lo r24

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213

8.20

memchr_P.S File Reference

8.20

memchr_P.S File Reference

8.20.1

Detailed Description

8.21

memcmp.S File Reference

8.21.1

Detailed Description

Defines
•
•
•
•
•
•
•
•

#define s1_hi r25
#define s1_lo r24
#define s2_hi r23
#define s2_lo r22
#define len_hi r21
#define len_lo r20
#define ret_hi r25
#define ret_lo r24

8.22

memcmp_P.S File Reference

8.22.1

Detailed Description

8.23

memcpy.S File Reference

8.23.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

8.24

memcpy_P.S File Reference

8.24.1

Detailed Description

Defines
• #define dest_hi r25
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214

8.25

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memmem.S File Reference

#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

8.25

memmem.S File Reference

8.25.1

Detailed Description

8.26

memmove.S File Reference

8.26.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

8.27

memrchr.S File Reference

8.27.1

Detailed Description

8.28

memrchr_P.S File Reference

8.28.1

Detailed Description

8.29

memset.S File Reference

8.29.1

Detailed Description

Defines
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define val_lo r22
#define len_hi r21
#define len_lo r20

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215

8.30

parity.h File Reference

8.30

parity.h File Reference

8.30.1

Detailed Description

216

Defines
• #define parity_even_bit(val)

8.31

pgmspace.h File Reference

8.31.1

Detailed Description

Defines
•
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#define __PGMSPACE_H_ 1
#define __need_size_t
#define __ATTR_CONST__ __attribute__((__const__))
#define __ATTR_PROGMEM__ __attribute__((__progmem__))
#define __ATTR_PURE__ __attribute__((__pure__))
#define PROGMEM __ATTR_PROGMEM__
#define PSTR(s) ((const PROGMEM char ∗)(s))
#define __LPM_classic__(addr)
#define __LPM_enhanced__(addr)
#define __LPM_word_classic__(addr)
#define __LPM_word_enhanced__(addr)
#define __LPM_dword_classic__(addr)
#define __LPM_dword_enhanced__(addr)
#define __LPM(addr) __LPM_classic__(addr)
#define __LPM_word(addr) __LPM_word_classic__(addr)
#define __LPM_dword(addr) __LPM_dword_classic__(addr)
#define pgm_read_byte_near(address_short) __LPM((uint16_t)(address_short))
#define
pgm_read_word_near(address_short)
__LPM_word((uint16_t)(address_short))
#define
pgm_read_dword_near(address_short)
__LPM_dword((uint16_t)(address_short))
#define __ELPM_classic__(addr)
#define __ELPM_enhanced__(addr)
#define __ELPM_word_classic__(addr)
#define __ELPM_word_enhanced__(addr)
#define __ELPM_dword_classic__(addr)
#define __ELPM_dword_enhanced__(addr)
#define __ELPM(addr) __ELPM_classic__(addr)
#define __ELPM_word(addr) __ELPM_word_classic__(addr)

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8.31

pgmspace.h File Reference

217

• #define __ELPM_dword(addr) __ELPM_dword_classic__(addr)
• #define pgm_read_byte_far(address_long) __ELPM((uint32_t)(address_long))
• #define
pgm_read_word_far(address_long)
__ELPM_word((uint32_t)(address_long))
• #define
pgm_read_dword_far(address_long)
__ELPM_dword((uint32_t)(address_long))
• #define pgm_read_byte(address_short) pgm_read_byte_near(address_short)
• #define pgm_read_word(address_short) pgm_read_word_near(address_short)
• #define pgm_read_dword(address_short) pgm_read_dword_near(address_short)
• #define PGM_P const prog_char ∗
• #define PGM_VOID_P const prog_void ∗
Typedefs
•
•
•
•
•
•
•
•
•
•
•

typedef void PROGMEM prog_void
typedef char PROGMEM prog_char
typedef unsigned char PROGMEM prog_uchar
typedef int8_t PROGMEM prog_int8_t
typedef uint8_t PROGMEM prog_uint8_t
typedef int16_t PROGMEM prog_int16_t
typedef uint16_t PROGMEM prog_uint16_t
typedef int32_t PROGMEM prog_int32_t
typedef uint32_t PROGMEM prog_uint32_t
typedef int64_t PROGMEM prog_int64_t
typedef uint64_t PROGMEM prog_uint64_t

Functions
• PGM_VOID_P memchr_P (PGM_VOID_P s, int val, size_t len) __ATTR_CONST__
• int memcmp_P (const void ∗, PGM_VOID_P, size_t) __ATTR_PURE__
• void ∗ memcpy_P (void ∗, PGM_VOID_P, size_t)
• void ∗ memmem_P (const void ∗, size_t, PGM_VOID_P, size_t) __ATTR_PURE__
• PGM_VOID_P memrchr_P (PGM_VOID_P s, int val, size_t len) __ATTR_CONST__
• char ∗ strcat_P (char ∗, PGM_P)
• PGM_P strchr_P (PGM_P s, int val) __ATTR_CONST__
• PGM_P strchrnul_P (PGM_P s, int val) __ATTR_CONST__
• int strcmp_P (const char ∗, PGM_P) __ATTR_PURE__
• char ∗ strcpy_P (char ∗, PGM_P)
• int strcasecmp_P (const char ∗, PGM_P) __ATTR_PURE__

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8.31

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pgmspace.h File Reference

char ∗ strcasestr_P (const char ∗, PGM_P) __ATTR_PURE__
size_t strcspn_P (const char ∗s, PGM_P reject) __ATTR_PURE__
size_t strlcat_P (char ∗, PGM_P, size_t)
size_t strlcpy_P (char ∗, PGM_P, size_t)
size_t strlen_P (PGM_P) __ATTR_CONST__
size_t strnlen_P (PGM_P, size_t) __ATTR_CONST__
int strncmp_P (const char ∗, PGM_P, size_t) __ATTR_PURE__
int strncasecmp_P (const char ∗, PGM_P, size_t) __ATTR_PURE__
char ∗ strncat_P (char ∗, PGM_P, size_t)
char ∗ strncpy_P (char ∗, PGM_P, size_t)
char ∗ strpbrk_P (const char ∗s, PGM_P accept) __ATTR_PURE__
PGM_P strrchr_P (PGM_P s, int val) __ATTR_CONST__
char ∗ strsep_P (char ∗∗sp, PGM_P delim)
size_t strspn_P (const char ∗s, PGM_P accept) __ATTR_PURE__
char ∗ strstr_P (const char ∗, PGM_P) __ATTR_PURE__

8.31.2
8.31.2.1

Define Documentation
#define __ELPM_classic__(addr)

Value:
(__extension__({
\
uint32_t __addr32 = (uint32_t)(addr); \
uint8_t __result;
\
__asm__
\
(
\
"out %2, %C1" "\n\t"
\
"mov r31, %B1" "\n\t"
\
"mov r30, %A1" "\n\t"
\
"elpm" "\n\t"
\
"mov %0, r0" "\n\t"
\
: "=r" (__result)
\
: "r" (__addr32),
\
"I" (_SFR_IO_ADDR(RAMPZ)) \
: "r0", "r30", "r31"
\
);
\
__result;
\
}))

8.31.2.2

#define __ELPM_dword_enhanced__(addr)

Value:
(__extension__({
\
uint32_t __addr32 = (uint32_t)(addr); \
uint32_t __result;
\

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218

8.31

pgmspace.h File Reference

__asm__
(
"out %2, %C1"
"\n\t"
"movw r30, %1" "\n\t"
"elpm %A0, Z+" "\n\t"
"elpm %B0, Z+" "\n\t"
"elpm %C0, Z+" "\n\t"
"elpm %D0, Z"
"\n\t"
: "=r" (__result)
: "r" (__addr32),
"I" (_SFR_IO_ADDR(RAMPZ))
: "r30", "r31"
);
__result;

219

\
\
\
\
\
\
\
\
\
\
\
\
\
\

}))

8.31.2.3

#define __ELPM_enhanced__(addr)

Value:
(__extension__({
\
uint32_t __addr32 = (uint32_t)(addr); \
uint8_t __result;
\
__asm__
\
(
\
"out %2, %C1" "\n\t"
\
"movw r30, %1" "\n\t"
\
"elpm %0, Z+" "\n\t"
\
: "=r" (__result)
\
: "r" (__addr32),
\
"I" (_SFR_IO_ADDR(RAMPZ)) \
: "r30", "r31"
\
);
\
__result;
\
}))

8.31.2.4

#define __ELPM_word_classic__(addr)

Value:
(__extension__({
uint32_t __addr32 =
uint16_t __result;
__asm__
(
"out %2, %C1"
"mov r31, %B1"
"mov r30, %A1"
"elpm"
"mov %A0, r0"
"in r0, %2"
"adiw r30, 1"

\
(uint32_t)(addr); \
\
\
\
"\n\t"
\
"\n\t"
\
"\n\t"
\
"\n\t"
\
"\n\t"
\
"\n\t"
\
"\n\t"
\

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8.31

pgmspace.h File Reference

"adc r0, __zero_reg__" "\n\t"
"out %2, r0"
"\n\t"
"elpm"
"\n\t"
"mov %B0, r0"
"\n\t"
: "=r" (__result)
: "r" (__addr32),
"I" (_SFR_IO_ADDR(RAMPZ))
: "r0", "r30", "r31"
);
__result;

220

\
\
\
\
\
\
\
\
\
\

}))

8.31.2.5

#define __ELPM_word_enhanced__(addr)

Value:
(__extension__({
\
uint32_t __addr32 = (uint32_t)(addr); \
uint16_t __result;
\
__asm__
\
(
\
"out %2, %C1"
"\n\t"
\
"movw r30, %1" "\n\t"
\
"elpm %A0, Z+" "\n\t"
\
"elpm %B0, Z"
"\n\t"
\
: "=r" (__result)
\
: "r" (__addr32),
\
"I" (_SFR_IO_ADDR(RAMPZ))
\
: "r30", "r31"
\
);
\
__result;
\
}))

8.31.2.6

#define __LPM_classic__(addr)

Value:
(__extension__({
\
uint16_t __addr16 = (uint16_t)(addr); \
uint8_t __result;
\
__asm__
\
(
\
"lpm" "\n\t"
\
"mov %0, r0" "\n\t"
\
: "=r" (__result)
\
: "z" (__addr16)
\
: "r0"
\
);
\
__result;
\
}))

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.31

pgmspace.h File Reference

8.31.2.7

221

#define __LPM_dword_classic__(addr)

Value:
(__extension__({
uint16_t __addr16 = (uint16_t)(addr);
uint32_t __result;
__asm__
(
"lpm"
"\n\t"
"mov %A0, r0"
"\n\t"
"adiw r30, 1"
"\n\t"
"lpm"
"\n\t"
"mov %B0, r0"
"\n\t"
"adiw r30, 1"
"\n\t"
"lpm"
"\n\t"
"mov %C0, r0"
"\n\t"
"adiw r30, 1"
"\n\t"
"lpm"
"\n\t"
"mov %D0, r0"
"\n\t"
: "=r" (__result), "=z" (__addr16)
: "1" (__addr16)
: "r0"
);
__result;
}))

8.31.2.8

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

#define __LPM_dword_enhanced__(addr)

Value:
(__extension__({
uint16_t __addr16 = (uint16_t)(addr);
uint32_t __result;
__asm__
(
"lpm %A0, Z+"
"\n\t"
"lpm %B0, Z+"
"\n\t"
"lpm %C0, Z+"
"\n\t"
"lpm %D0, Z"
"\n\t"
: "=r" (__result), "=z" (__addr16)
: "1" (__addr16)
);
__result;
}))

8.31.2.9

#define __LPM_enhanced__(addr)

Value:
(__extension__({

\

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

\
\
\
\
\
\
\
\
\
\
\
\
\

8.31

pgmspace.h File Reference

222

uint16_t __addr16 = (uint16_t)(addr); \
uint8_t __result;
\
__asm__
\
(
\
"lpm %0, Z" "\n\t"
\
: "=r" (__result)
\
: "z" (__addr16)
\
);
\
__result;
\
}))

8.31.2.10

#define __LPM_word_classic__(addr)

Value:
(__extension__({
uint16_t __addr16 = (uint16_t)(addr);
uint16_t __result;
__asm__
(
"lpm"
"\n\t"
"mov %A0, r0"
"\n\t"
"adiw r30, 1"
"\n\t"
"lpm"
"\n\t"
"mov %B0, r0"
"\n\t"
: "=r" (__result), "=z" (__addr16)
: "1" (__addr16)
: "r0"
);
__result;
}))

8.31.2.11

\
\
\
\
\
\
\
\
\
\
\
\
\
\
\

#define __LPM_word_enhanced__(addr)

Value:
(__extension__({
uint16_t __addr16 = (uint16_t)(addr);
uint16_t __result;
__asm__
(
"lpm %A0, Z+"
"\n\t"
"lpm %B0, Z"
"\n\t"
: "=r" (__result), "=z" (__addr16)
: "1" (__addr16)
);
__result;
}))

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

\
\
\
\
\
\
\
\
\
\
\

8.32

power.h File Reference

8.32

power.h File Reference

8.32.1

Detailed Description

223

Defines
• #define _AVR_POWER_H_ 1
• #define clock_prescale_set(x)
• #define
clock_prescale_get()
(clock_div_t)(CLKPR
&
t)((1< is included
•
•
•
•
•
•
•
•
•
•
•
•

#define INT8_MAX 0x7f
#define INT8_MIN (-INT8_MAX - 1)
#define UINT8_MAX (__CONCAT(INT8_MAX, U) ∗ 2U + 1U)
#define INT16_MAX 0x7fff
#define INT16_MIN (-INT16_MAX - 1)
#define UINT16_MAX (__CONCAT(INT16_MAX, U) ∗ 2U + 1U)
#define INT32_MAX 0x7fffffffL
#define INT32_MIN (-INT32_MAX - 1L)
#define UINT32_MAX (__CONCAT(INT32_MAX, U) ∗ 2UL + 1UL)
#define INT64_MAX 0x7fffffffffffffffLL
#define INT64_MIN (-INT64_MAX - 1LL)
#define UINT64_MAX (__CONCAT(INT64_MAX, U) ∗ 2ULL + 1ULL)

Limits of minimum-width integer types
•
•
•
•
•
•
•
•
•
•
•
•

#define INT_LEAST8_MAX INT8_MAX
#define INT_LEAST8_MIN INT8_MIN
#define UINT_LEAST8_MAX UINT8_MAX
#define INT_LEAST16_MAX INT16_MAX
#define INT_LEAST16_MIN INT16_MIN
#define UINT_LEAST16_MAX UINT16_MAX
#define INT_LEAST32_MAX INT32_MAX
#define INT_LEAST32_MIN INT32_MIN
#define UINT_LEAST32_MAX UINT32_MAX
#define INT_LEAST64_MAX INT64_MAX
#define INT_LEAST64_MIN INT64_MIN
#define UINT_LEAST64_MAX UINT64_MAX

Limits of fastest minimum-width integer types
• #define INT_FAST8_MAX INT8_MAX
• #define INT_FAST8_MIN INT8_MIN
• #define UINT_FAST8_MAX UINT8_MAX

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.35

•
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•
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•

stdint.h File Reference

226

#define INT_FAST16_MAX INT16_MAX
#define INT_FAST16_MIN INT16_MIN
#define UINT_FAST16_MAX UINT16_MAX
#define INT_FAST32_MAX INT32_MAX
#define INT_FAST32_MIN INT32_MIN
#define UINT_FAST32_MAX UINT32_MAX
#define INT_FAST64_MAX INT64_MAX
#define INT_FAST64_MIN INT64_MIN
#define UINT_FAST64_MAX UINT64_MAX

Limits of integer types capable of holding object pointers
• #define INTPTR_MAX INT16_MAX
• #define INTPTR_MIN INT16_MIN
• #define UINTPTR_MAX UINT16_MAX
Limits of greatest-width integer types
• #define INTMAX_MAX INT64_MAX
• #define INTMAX_MIN INT64_MIN
• #define UINTMAX_MAX UINT64_MAX
Limits of other integer types
C++ implementations should define these macros only when __STDC_LIMIT_MACROS is defined before  is included
•
•
•
•
•

#define PTRDIFF_MAX INT16_MAX
#define PTRDIFF_MIN INT16_MIN
#define SIG_ATOMIC_MAX INT8_MAX
#define SIG_ATOMIC_MIN INT8_MIN
#define SIZE_MAX (__CONCAT(INT16_MAX, U))

Macros for integer constants
C++ implementations should define these macros only when __STDC_CONSTANT_MACROS is defined before  is included.
These definitions are valid for integer constants without suffix and for macros defined
as integer constant without suffix
• #define INT8_C(value) ((int8_t) value)

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.35

•
•
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•
•
•
•

stdint.h File Reference

#define UINT8_C(value) ((uint8_t) __CONCAT(value, U))
#define INT16_C(value) value
#define UINT16_C(value) __CONCAT(value, U)
#define INT32_C(value) __CONCAT(value, L)
#define UINT32_C(value) __CONCAT(value, UL)
#define INT64_C(value) __CONCAT(value, LL)
#define UINT64_C(value) __CONCAT(value, ULL)
#define INTMAX_C(value) __CONCAT(value, LL)
#define UINTMAX_C(value) __CONCAT(value, ULL)

Exact-width integer types
Integer types having exactly the specified width
•
•
•
•
•
•
•
•

typedef signed char int8_t
typedef unsigned char uint8_t
typedef signed int int16_t
typedef unsigned int uint16_t
typedef signed long int int32_t
typedef unsigned long int uint32_t
typedef signed long long int int64_t
typedef unsigned long long int uint64_t

Integer types capable of holding object pointers
These allow you to declare variables of the same size as a pointer.
• typedef int16_t intptr_t
• typedef uint16_t uintptr_t
Minimum-width integer types
Integer types having at least the specified width
•
•
•
•
•
•
•
•

typedef int8_t int_least8_t
typedef uint8_t uint_least8_t
typedef int16_t int_least16_t
typedef uint16_t uint_least16_t
typedef int32_t int_least32_t
typedef uint32_t uint_least32_t
typedef int64_t int_least64_t
typedef uint64_t uint_least64_t

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227

8.36

stdio.h File Reference

228

Fastest minimum-width integer types
Integer types being usually fastest having at least the specified width
•
•
•
•
•
•
•
•

typedef int8_t int_fast8_t
typedef uint8_t uint_fast8_t
typedef int16_t int_fast16_t
typedef uint16_t uint_fast16_t
typedef int32_t int_fast32_t
typedef uint32_t uint_fast32_t
typedef int64_t int_fast64_t
typedef uint64_t uint_fast64_t

Greatest-width integer types
Types designating integer data capable of representing any value of any integer type in
the corresponding signed or unsigned category
• typedef int64_t intmax_t
• typedef uint64_t uintmax_t
Defines
• #define __USING_MINT8 0
• #define __CONCATenate(left, right) left ## right
• #define __CONCAT(left, right) __CONCATenate(left, right)

8.36

stdio.h File Reference

8.36.1

Detailed Description

Defines
•
•
•
•
•
•
•
•
•

#define _STDIO_H_ 1
#define __need_NULL
#define __need_size_t
#define FILE struct __file
#define stdin (__iob[0])
#define stdout (__iob[1])
#define stderr (__iob[2])
#define EOF (-1)
#define fdev_set_udata(stream, u) do { (stream) → udata = u; } while(0)

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.36

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•
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•

stdio.h File Reference

229

#define fdev_get_udata(stream) ((stream) → udata)
#define fdev_setup_stream(stream, put, get, rwflag)
#define _FDEV_SETUP_READ __SRD
#define _FDEV_SETUP_WRITE __SWR
#define _FDEV_SETUP_RW (__SRD|__SWR)
#define _FDEV_ERR (-1)
#define _FDEV_EOF (-2)
#define FDEV_SETUP_STREAM(put, get, rwflag)
#define fdev_close()
#define putc(__c, __stream) fputc(__c, __stream)
#define putchar(__c) fputc(__c, stdout)
#define getc(__stream) fgetc(__stream)
#define getchar() fgetc(stdin)
#define SEEK_SET 0
#define SEEK_CUR 1
#define SEEK_END 2

Functions
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

int fclose (FILE ∗__stream)
int vfprintf (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int vfprintf_P (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int fputc (int __c, FILE ∗__stream)
int printf (const char ∗__fmt,...)
int printf_P (const char ∗__fmt,...)
int vprintf (const char ∗__fmt, va_list __ap)
int sprintf (char ∗__s, const char ∗__fmt,...)
int sprintf_P (char ∗__s, const char ∗__fmt,...)
int snprintf (char ∗__s, size_t __n, const char ∗__fmt,...)
int snprintf_P (char ∗__s, size_t __n, const char ∗__fmt,...)
int vsprintf (char ∗__s, const char ∗__fmt, va_list ap)
int vsprintf_P (char ∗__s, const char ∗__fmt, va_list ap)
int vsnprintf (char ∗__s, size_t __n, const char ∗__fmt, va_list ap)
int vsnprintf_P (char ∗__s, size_t __n, const char ∗__fmt, va_list ap)
int fprintf (FILE ∗__stream, const char ∗__fmt,...)
int fprintf_P (FILE ∗__stream, const char ∗__fmt,...)
int fputs (const char ∗__str, FILE ∗__stream)
int fputs_P (const char ∗__str, FILE ∗__stream)
int puts (const char ∗__str)
int puts_P (const char ∗__str)
size_t fwrite (const void ∗__ptr, size_t __size, size_t __nmemb, FILE ∗__stream)

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.37

•
•
•
•
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•
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•
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•
•
•
•
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•
•
•
•

stdlib.h File Reference

int fgetc (FILE ∗__stream)
int ungetc (int __c, FILE ∗__stream)
char ∗ fgets (char ∗__str, int __size, FILE ∗__stream)
char ∗ gets (char ∗__str)
size_t fread (void ∗__ptr, size_t __size, size_t __nmemb, FILE ∗__stream)
void clearerr (FILE ∗__stream)
int feof (FILE ∗__stream)
int ferror (FILE ∗__stream)
int vfscanf (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int vfscanf_P (FILE ∗__stream, const char ∗__fmt, va_list __ap)
int fscanf (FILE ∗__stream, const char ∗__fmt,...)
int fscanf_P (FILE ∗__stream, const char ∗__fmt,...)
int scanf (const char ∗__fmt,...)
int scanf_P (const char ∗__fmt,...)
int vscanf (const char ∗__fmt, va_list __ap)
int sscanf (const char ∗__buf, const char ∗__fmt,...)
int sscanf_P (const char ∗__buf, const char ∗__fmt,...)
int fflush (FILE ∗stream)

8.37

stdlib.h File Reference

8.37.1

Detailed Description

Data Structures
• struct div_t
• struct ldiv_t
Non-standard (i.e. non-ISO C) functions.
•
•
•
•
•
•
•
•

#define RANDOM_MAX 0x7FFFFFFF
char ∗ itoa (int __val, char ∗__s, int __radix)
char ∗ ltoa (long int __val, char ∗__s, int __radix)
char ∗ utoa (unsigned int __val, char ∗__s, int __radix)
char ∗ ultoa (unsigned long int __val, char ∗__s, int __radix)
long random (void)
void srandom (unsigned long __seed)
long random_r (unsigned long ∗ctx)

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230

8.37

stdlib.h File Reference

231

Conversion functions for double arguments.
Note that these functions are not located in the default library, libc.a, but in the
mathematical library, libm.a. So when linking the application, the -lm option needs
to be specified.
•
•
•
•

#define DTOSTR_ALWAYS_SIGN 0x01
#define DTOSTR_PLUS_SIGN 0x02
#define DTOSTR_UPPERCASE 0x04
char ∗ dtostre (double __val, char ∗__s, unsigned char __prec, unsigned char
__flags)
• char ∗ dtostrf (double __val, char __width, char __prec, char ∗__s)
Defines
•
•
•
•
•
•

#define _STDLIB_H_ 1
#define __need_NULL
#define __need_size_t
#define __need_wchar_t
#define __ptr_t void ∗
#define RAND_MAX 0x7FFF

Typedefs
• typedef int(∗) __compar_fn_t (const void ∗, const void ∗)
Functions
•
•
•
•
•
•
•
•
•
•

__inline__ void abort (void) __ATTR_NORETURN__
int abs (int __i) __ATTR_CONST__
long labs (long __i) __ATTR_CONST__
void ∗ bsearch (const void ∗__key, const void ∗__base, size_t __nmemb, size_t
__size, int(∗__compar)(const void ∗, const void ∗))
div_t div (int __num, int __denom) __asm__("__divmodhi4") __ATTR_CONST__
ldiv_t ldiv (long __num, long __denom) __asm__("__divmodsi4") __ATTR_CONST__
void qsort (void ∗__base, size_t __nmemb, size_t __size, __compar_fn_t __compar)
long strtol (const char ∗__nptr, char ∗∗__endptr, int __base)
unsigned long strtoul (const char ∗__nptr, char ∗∗__endptr, int __base)
long atol (const char ∗s) __ATTR_PURE__

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.38

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•
•
•

strcasecmp.S File Reference

int atoi (const char ∗s) __ATTR_PURE__
void exit (int __status) __ATTR_NORETURN__
void ∗ malloc (size_t __size) __ATTR_MALLOC__
void free (void ∗__ptr)
void ∗ calloc (size_t __nele, size_t __size) __ATTR_MALLOC__
void ∗ realloc (void ∗__ptr, size_t __size) __ATTR_MALLOC__
double strtod (const char ∗__nptr, char ∗∗__endptr)
double atof (const char ∗__nptr)
int rand (void)
void srand (unsigned int __seed)
int rand_r (unsigned long ∗ctx)

Variables
• size_t __malloc_margin
• char ∗ __malloc_heap_start
• char ∗ __malloc_heap_end

8.38

strcasecmp.S File Reference

8.38.1

Detailed Description

8.39

strcasecmp_P.S File Reference

8.39.1

Detailed Description

8.40

strcasestr.S File Reference

8.40.1

Detailed Description

8.41

strcat.S File Reference

8.41.1

Detailed Description

Defines
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22

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232

8.42

strcat_P.S File Reference

8.42

strcat_P.S File Reference

8.42.1

Detailed Description

Defines
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22

8.43

strchr.S File Reference

8.43.1

Detailed Description

8.44

strchr_P.S File Reference

8.44.1

Detailed Description

8.45

strchrnul.S File Reference

8.45.1

Detailed Description

8.46

strchrnul_P.S File Reference

8.46.1

Detailed Description

8.47

strcmp.S File Reference

8.47.1

Detailed Description

8.48

strcmp_P.S File Reference

8.48.1

Detailed Description

8.49

strcpy.S File Reference

8.49.1

Detailed Description

Defines
• #define dest_hi r25
• #define dest_lo r24

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233

8.50

strcpy_P.S File Reference

• #define src_hi r23
• #define src_lo r22

8.50

strcpy_P.S File Reference

8.50.1

Detailed Description

Defines
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22

8.51

strcspn.S File Reference

8.51.1

Detailed Description

8.52

strcspn_P.S File Reference

8.52.1

Detailed Description

8.53

string.h File Reference

8.53.1

Detailed Description

Defines
•
•
•
•
•

#define _STRING_H_ 1
#define __need_NULL
#define __need_size_t
#define __ATTR_PURE__ __attribute__((__pure__))
#define _FFS(x)

Functions
•
•
•
•
•
•

int ffs (int) __attribute__((const ))
int ffsl (long) __attribute__((const ))
int ffsll (long long) __attribute__((const ))
void ∗ memccpy (void ∗, const void ∗, int, size_t)
void ∗ memchr (const void ∗, int, size_t) __ATTR_PURE__
int memcmp (const void ∗, const void ∗, size_t) __ATTR_PURE__

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234

8.53

•
•
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•
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•
•
•
•
•
•
•
•
•
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•
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•
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•
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•
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•

string.h File Reference

235

void ∗ memcpy (void ∗, const void ∗, size_t)
void ∗ memmem (const void ∗, size_t, const void ∗, size_t) __ATTR_PURE__
void ∗ memmove (void ∗, const void ∗, size_t)
void ∗ memrchr (const void ∗, int, size_t) __ATTR_PURE__
void ∗ memset (void ∗, int, size_t)
char ∗ strcat (char ∗, const char ∗)
char ∗ strchr (const char ∗, int) __ATTR_PURE__
char ∗ strchrnul (const char ∗, int) __ATTR_PURE__
int strcmp (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strcpy (char ∗, const char ∗)
int strcasecmp (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strcasestr (const char ∗, const char ∗) __ATTR_PURE__
size_t strcspn (const char ∗s, const char ∗reject) __ATTR_PURE__
size_t strlcat (char ∗, const char ∗, size_t)
size_t strlcpy (char ∗, const char ∗, size_t)
size_t strlen (const char ∗) __ATTR_PURE__
char ∗ strlwr (char ∗)
char ∗ strncat (char ∗, const char ∗, size_t)
int strncmp (const char ∗, const char ∗, size_t) __ATTR_PURE__
char ∗ strncpy (char ∗, const char ∗, size_t)
int strncasecmp (const char ∗, const char ∗, size_t) __ATTR_PURE__
size_t strnlen (const char ∗, size_t) __ATTR_PURE__
char ∗ strpbrk (const char ∗s, const char ∗accept) __ATTR_PURE__
char ∗ strrchr (const char ∗, int) __ATTR_PURE__
char ∗ strrev (char ∗)
char ∗ strsep (char ∗∗, const char ∗)
size_t strspn (const char ∗s, const char ∗accept) __ATTR_PURE__
char ∗ strstr (const char ∗, const char ∗) __ATTR_PURE__
char ∗ strtok_r (char ∗, const char ∗, char ∗∗)
char ∗ strupr (char ∗)

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

8.54

strlcat.S File Reference

8.54

strlcat.S File Reference

8.54.1

Detailed Description

8.55

strlcat_P.S File Reference

8.55.1

Detailed Description

8.56

strlcpy.S File Reference

8.56.1

Detailed Description

8.57

strlcpy_P.S File Reference

8.57.1

Detailed Description

8.58

strlen.S File Reference

8.58.1

Detailed Description

Defines
• #define src_hi r25
• #define src_lo r24

8.59

strlen_P.S File Reference

8.59.1

Detailed Description

Defines
• #define src_hi r25
• #define src_lo r24

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236

8.60

strlwr.S File Reference

8.60

strlwr.S File Reference

8.60.1

Detailed Description

8.61

strncasecmp.S File Reference

8.61.1

Detailed Description

8.62

strncasecmp_P.S File Reference

8.62.1

Detailed Description

8.63

strncat.S File Reference

8.63.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

8.64

strncat_P.S File Reference

8.64.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

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237

8.65

strncmp.S File Reference

8.65

strncmp.S File Reference

8.65.1

Detailed Description

Defines
•
•
•
•
•
•
•
•

#define s1_hi r25
#define s1_lo r24
#define s2_hi r23
#define s2_lo r22
#define len_hi r21
#define len_lo r20
#define ret_hi r25
#define ret_lo r24

8.66

strncmp_P.S File Reference

8.66.1

Detailed Description

Defines
•
•
•
•
•
•
•
•

#define s1_hi r25
#define s1_lo r24
#define s2_hi r23
#define s2_lo r22
#define len_hi r21
#define len_lo r20
#define ret_hi r25
#define ret_lo r24

8.67

strncpy.S File Reference

8.67.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

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238

8.68

strncpy_P.S File Reference

8.68

strncpy_P.S File Reference

8.68.1

Detailed Description

Defines
•
•
•
•
•
•

#define dest_hi r25
#define dest_lo r24
#define src_hi r23
#define src_lo r22
#define len_hi r21
#define len_lo r20

8.69

strnlen.S File Reference

8.69.1

Detailed Description

Defines
•
•
•
•

#define src_hi r25
#define src_lo r24
#define len_hi r23
#define len_lo r22

8.70

strnlen_P.S File Reference

8.70.1

Detailed Description

Defines
•
•
•
•

#define src_hi r25
#define src_lo r24
#define len_hi r23
#define len_lo r22

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239

8.71

strpbrk.S File Reference

8.71

strpbrk.S File Reference

8.71.1

Detailed Description

8.72

strpbrk_P.S File Reference

8.72.1

Detailed Description

8.73

strrchr.S File Reference

8.73.1

Detailed Description

Defines
•
•
•
•
•

#define src_hi r25
#define src_lo r24
#define val_lo r22
#define ret_hi r25
#define ret_lo r24

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240

8.73

strrchr.S File Reference

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241

8.74

strrchr_P.S File Reference

8.74

strrchr_P.S File Reference

8.74.1

Detailed Description

8.75

strrev.S File Reference

8.75.1

Detailed Description

8.76

strsep.S File Reference

8.76.1

Detailed Description

8.77

strsep_P.S File Reference

8.77.1

Detailed Description

8.78

strspn.S File Reference

8.78.1

Detailed Description

8.79

strspn_P.S File Reference

8.79.1

Detailed Description

8.80

strstr.S File Reference

8.80.1

Detailed Description

8.81

strstr_P.S File Reference

8.81.1

Detailed Description

8.82

strtok_r.S File Reference

8.82.1

Detailed Description

8.83

strupr.S File Reference

8.83.1

Detailed Description

8.84

twi.h File Reference

8.84.1

Detailed Description

TWSR values
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Mnemonics:

242

8.84

twi.h File Reference

TW_MT_xxx - master transmitter
TW_MR_xxx - master receiver
TW_ST_xxx - slave transmitter
TW_SR_xxx - slave receiver
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

#define TW_START 0x08
#define TW_REP_START 0x10
#define TW_MT_SLA_ACK 0x18
#define TW_MT_SLA_NACK 0x20
#define TW_MT_DATA_ACK 0x28
#define TW_MT_DATA_NACK 0x30
#define TW_MT_ARB_LOST 0x38
#define TW_MR_ARB_LOST 0x38
#define TW_MR_SLA_ACK 0x40
#define TW_MR_SLA_NACK 0x48
#define TW_MR_DATA_ACK 0x50
#define TW_MR_DATA_NACK 0x58
#define TW_ST_SLA_ACK 0xA8
#define TW_ST_ARB_LOST_SLA_ACK 0xB0
#define TW_ST_DATA_ACK 0xB8
#define TW_ST_DATA_NACK 0xC0
#define TW_ST_LAST_DATA 0xC8
#define TW_SR_SLA_ACK 0x60
#define TW_SR_ARB_LOST_SLA_ACK 0x68
#define TW_SR_GCALL_ACK 0x70
#define TW_SR_ARB_LOST_GCALL_ACK 0x78
#define TW_SR_DATA_ACK 0x80
#define TW_SR_DATA_NACK 0x88
#define TW_SR_GCALL_DATA_ACK 0x90
#define TW_SR_GCALL_DATA_NACK 0x98
#define TW_SR_STOP 0xA0
#define TW_NO_INFO 0xF8
#define TW_BUS_ERROR 0x00
#define TW_STATUS_MASK
#define TW_STATUS (TWSR & TW_STATUS_MASK)

R/∼W bit in SLA+R/W address field.
• #define TW_READ 1
• #define TW_WRITE 0

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243

8.85

wdt.h File Reference

Defines
• #define _UTIL_TWI_H_ 1

8.85

wdt.h File Reference

8.85.1

Detailed Description

Defines
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

#define wdt_reset() __asm__ __volatile__ ("wdr")
#define _WD_PS3_MASK 0x00
#define _WD_CONTROL_REG WDTCR
#define _WD_CHANGE_BIT WDCE
#define _wdt_write(value)
#define wdt_disable()
#define wdt_enable(timeout) _wdt_write(timeout)
#define WDTO_15MS 0
#define WDTO_30MS 1
#define WDTO_60MS 2
#define WDTO_120MS 3
#define WDTO_250MS 4
#define WDTO_500MS 5
#define WDTO_1S 6
#define WDTO_2S 7
#define WDTO_4S 8
#define WDTO_8S 9

8.85.2
8.85.2.1

Define Documentation
#define _wdt_write(value)

Value:
__asm__ __volatile__ ( \
"in __tmp_reg__,__SREG__" "\n\t"
\
"cli" "\n\t"
\
"wdr" "\n\t"
\
"out %0,%1" "\n\t" \
"out __SREG__,__tmp_reg__" "\n\t"
\
"out %0,%2" \
: /* no outputs */ \
: "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)), \
"r" (_BV(_WD_CHANGE_BIT) | _BV(WDE)),
\
"r" ((uint8_t) ((value & 0x08 ? _WD_PS3_MASK : 0x00) | \
_BV(WDE) | (value & 0x07)) ) \
: "r0" \
)

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244

9 avr-libc Page Documentation

9
9.1
9.1.1

245

avr-libc Page Documentation
Toolchain Overview
Introduction

Welcome to the open source software development toolset for the Atmel AVR!
There is not a single tool that provides everything needed to develop software for the
AVR. It takes many tools working together. Collectively, the group of tools are called a
toolset, or commonly a toolchain, as the tools are chained together to produce the final
executable application for the AVR microcontroller.
The following sections provide an overview of all of these tools. You may be used
to cross-compilers that provide everything with a GUI front-end, and not know what
goes on "underneath the hood". You may be coming from a desktop or server computer
background and not used to embedded systems. Or you may be just learning about the
most common software development toolchain available on Unix and Linux systems.
Hopefully the following overview will be helpful in putting everything in perspective.
9.1.2

FSF and GNU

According to its website, "the Free Software Foundation (FSF), established in 1985, is
dedicated to promoting computer users’ rights to use, study, copy, modify, and redistribute computer programs. The FSF promotes the development and use of free software, particularly the GNU operating system, used widely in its GNU/Linux variant."
The FSF remains the primary sponsor of the GNU project.
The GNU Project was launched in 1984 to develop a complete Unix-like operating
system which is free software: the GNU system. GNU is a recursive acronym for
ŞGNU’s Not UnixŤ; it is pronounced guh-noo, approximately like canoe.
One of the main projects of the GNU system is the GNU Compiler Collection, or GCC,
and its sister project, GNU Binutils. These two open source projects provide a foundation for a software development toolchain. Note that these projects were designed to
originally run on Unix-like systems.
9.1.3

GCC

GCC stands for GNU Compiler Collection. GCC is highly flexible compiler system. It
has different compiler front-ends for different languages. It has many back-ends that
generate assembly code for many different processors and host operating systems. All
share a common "middle-end", containing the generic parts of the compiler, including
a lot of optimizations.
In GCC, a host system is the system (processor/OS) that the compiler runs on. A

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246

target system is the system that the compiler compiles code for. And, a system is
the system that the compiler is built (from source code) on. If a compiler has the same
system for host and for target, it is known as a native compiler. If a compiler
has different systems for host and target, it is known as a cross-compiler. (And if
all three, build, host, and target systems are different, it is known as a Canadian
cross compiler, but we won’t discuss that here.) When GCC is built to execute on a
host system such as FreeBSD, Linux, or Windows, and it is built to generate code for
the AVR microcontroller target, then it is a cross compiler, and this version of GCC
is commonly known as "AVR GCC". In documentation, or discussion, AVR GCC is
used when referring to GCC targeting specifically the AVR, or something that is AVR
specific about GCC. The term "GCC" is usually used to refer to something generic
about GCC, or about GCC as a whole.
GCC is different from most other compilers. GCC focuses on translating a high-level
language to the target assembly only. AVR GCC has three available compilers for the
AVR: C language, C++, and Ada. The compiler itself does not assemble or link the
final code.
GCC is also known as a "driver" program, in that it knows about, and drives other
programs seamlessly to create the final output. The assembler, and the linker are part
of another open source project called GNU Binutils. GCC knows how to drive the
GNU assembler (gas) to assemble the output of the compiler. GCC knows how to drive
the GNU linker (ld) to link all of the object modules into a final executable.
The two projects, GCC and Binutils, are very much interrelated and many of the same
volunteers work on both open source projects.
When GCC is built for the AVR target, the actual program names are prefixed with
"avr-". So the actual executable name for AVR GCC is: avr-gcc. The name "avr-gcc"
is used in documentation and discussion when referring to the program itself and not
just the whole AVR GCC system.
See the GCC Web Site and GCC User Manual for more information about GCC.
9.1.4

GNU Binutils

The name GNU Binutils stands for "Binary Utilities". It contains the GNU assembler
(gas), and the GNU linker (ld), but also contains many other utilities that work with
binary files that are created as part of the software development toolchain.
Again, when these tools are built for the AVR target, the actual program names are
prefixed with "avr-". For example, the assembler program name, for a native assembler
is "as" (even though in documentation the GNU assembler is commonly referred to as
"gas"). But when built for an AVR target, it becomes "avr-as". Below is a list of the
programs that are included in Binutils:
avr-as
The Assembler.
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9.1

Toolchain Overview

avr-ld
The Linker.
avr-ar
Create, modify, and extract from libraries (archives).
avr-ranlib
Generate index to library (archive) contents.
avr-objcopy
Copy and translate object files to different formats.
avr-objdump
Display information from object files including disassembly.
avr-size
List section sizes and total size.
avr-nm
List symbols from object files.
avr-strings
List printable strings from files.
avr-strip
Discard symbols from files.
avr-readelf
Display the contents of ELF format files.
avr-addr2line
Convert addresses to file and line.
avr-c++filt
Filter to demangle encoded C++ symbols.

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247

9.1

9.1.5

Toolchain Overview

248

avr-libc

GCC and Binutils provides a lot of the tools to develop software, but there is one critical
component that they do not provide: a Standard C Library.
There are different open source projects that provide a Standard C Library depending
upon your system time, whether for a native compiler (GNU Libc), for some other
embedded system (newlib), or for some versions of Linux (uCLibc). The open source
AVR toolchain has its own Standard C Library project: avr-libc.
AVR-Libc provides many of the same functions found in a regular Standard C Library
and many additional library functions that is specific to an AVR. Some of the Standard
C Library functions that are commonly used on a PC environment have limitations or
additional issues that a user needs to be aware of when used on an embedded system.
AVR-Libc also contains the most documentation about the whole AVR toolchain.
9.1.6

Building Software

Even though GCC, Binutils, and avr-libc are the core projects that are used to build
software for the AVR, there is another piece of software that ties it all together: Make.
GNU Make is a program that makes things, and mainly software. Make interprets and
executes a Makefile that is written for a project. A Makefile contains dependency rules,
showing which output files are dependent upon which input files, and instructions on
how to build output files from input files.
Some distributions of the toolchains, and other AVR tools such as MFile, contain a
Makefile template written for the AVR toolchain and AVR applications that you can
copy and modify for your application.
See the GNU Make User Manual for more information.
9.1.7

AVRDUDE

After creating your software, you’ll want to program your device. You can do this by
using the program AVRDUDE which can interface with various hardware devices to
program your processor.
AVRDUDE is a very flexible package. All the information about AVR processors
and various hardware programmers is stored in a text database. This database can be
modified by any user to add new hardware or to add an AVR processor if it is not
already listed.
9.1.8

GDB / Insight / DDD

The GNU Debugger (GDB) is a command-line debugger that can be used with the rest
of the AVR toolchain. Insight is GDB plus a GUI written in Tcl/Tk. Both GDB and
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249

Insight are configured for the AVR and the main executables are prefixed with the target
name: avr-gdb, and avr-insight. There is also a "text mode" GUI for GDB: avr-gdbtui.
DDD (Data Display Debugger) is another popular GUI front end to GDB, available on
Unix and Linux systems.
9.1.9

AVaRICE

AVaRICE is a back-end program to AVR GDB and interfaces to the Atmel JTAG InCircuit Emulator (ICE), to provide emulation capabilities.
9.1.10

SimulAVR

SimulAVR is an AVR simulator used as a back-end with AVR GDB. Unfortunately,
this project is currently unmaintained and could use some help.
9.1.11

Utilities

There are also other optional utilities available that may be useful to add to your toolset.
SRecord is a collection of powerful tools for manipulating EPROM load files. It
reads and writes numerous EPROM file formats, and can perform many different manipulations.
MFile is a simple Makefile generator is meant as an aid to quickly customize a Makefile to use for your AVR application.
9.1.12

Toolchain Distributions (Distros)

All of the various open source projects that comprise the entire toolchain are normally
distributed as source code. It is left up to the user to build the tool application from its
source code. This can be a very daunting task to any potential user of these tools.
Luckily there are people who help out in this area. Volunteers take the time to build the
application from source code on particular host platforms and sometimes packaging
the tools for convenient installation by the end user. These packages contain the binary
executables of the tools, pre-made and ready to use. These packages are known as
"distributions" of the AVR toolchain, or by a more shortened name, "distros".
AVR toolchain distros are available on FreeBSD, Windows, Mac OS X, and certain
flavors of Linux.

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Memory Areas and Using malloc()

9.1.13

250

Open Source

All of these tools, from the original source code in the multitude of projects, to the
various distros, are put together by many, many volunteers. All of these projects could
always use more help from other people who are willing to volunteer some of their time.
There are many different ways to help, for people with varying skill levels, abilities,
and available time.
You can help to answer questions in mailing lists such as the avr-gcc-list, or on forums
at the AVR Freaks website. This helps many people new to the open source AVR tools.
If you think you found a bug in any of the tools, it is always a big help to submit a good
bug report to the proper project. A good bug report always helps other volunteers to
analyze the problem and to get it fixed for future versions of the software.
You can also help to fix bugs in various software projects, or to add desirable new
features.
Volunteers are always welcome! :-)

9.2

Memory Areas and Using malloc()

9.2.1

Introduction

Many of the devices that are possible targets of avr-libc have a minimal amount of
RAM. The smallest parts supported by the C environment come with 128 bytes of
RAM. This needs to be shared between initialized and uninitialized variables (sections
.data and .bss), the dynamic memory allocator, and the stack that is used for calling
subroutines and storing local (automatic) variables.
Also, unlike larger architectures, there is no hardware-supported memory management
which could help in separating the mentioned RAM regions from being overwritten by
each other.
The standard RAM layout is to place .data variables first, from the beginning of the
internal RAM, followed by .bss. The stack is started from the top of internal RAM,
growing downwards. The so-called "heap" available for the dynamic memory allocator
will be placed beyond the end of .bss. Thus, there’s no risk that dynamic memory will
ever collide with the RAM variables (unless there were bugs in the implementation of
the allocator). There is still a risk that the heap and stack could collide if there are large
requirements for either dynamic memory or stack space. The former can even happen
if the allocations aren’t all that large but dynamic memory allocations get fragmented
over time such that new requests don’t quite fit into the "holes" of previously freed
regions. Large stack space requirements can arise in a C function containing large
and/or numerous local variables or when recursively calling function.
Note:
The pictures shown in this document represent typical situations where the RAM
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Memory Areas and Using malloc()

251

on−board RAM

.data
.bss
variables variables

!

heap

external RAM

0xFFFF

0x10FF
0x1100

0x0100

locations refer to an ATmega128. The memory addresses used are not displayed
in a linear scale.

stack

SP

RAMEND

*(__brkval) (<= *SP − *(__malloc_margin))
*(__malloc_heap_start) == __heap_start
__bss_end
__data_end == __bss_start
__data_start

Figure 6: RAM map of a device with internal RAM

On a simple device like a microcontroller it is a challenge to implement a dynamic
memory allocator that is simple enough so the code size requirements will remain low,
yet powerful enough to avoid unnecessary memory fragmentation and to get it all done
with reasonably few CPU cycles. Microcontrollers are often low on space and also run
at much lower speeds than the typical PC these days.
The memory allocator implemented in avr-libc tries to cope with all of these constraints, and offers some tuning options that can be used if there are more resources
available than in the default configuration.
9.2.2

Internal vs. external RAM

Obviously, the constraints are much harder to satisfy in the default configuration where
only internal RAM is available. Extreme care must be taken to avoid a stack-heap
collision, both by making sure functions aren’t nesting too deeply, and don’t require
too much stack space for local variables, as well as by being cautious with allocating
too much dynamic memory.
If external RAM is available, it is strongly recommended to move the heap into the external RAM, regardless of whether or not the variables from the .data and .bss sections
are also going to be located there. The stack should always be kept in internal RAM.
Some devices even require this, and in general, internal RAM can be accessed faster
since no extra wait states are required. When using dynamic memory allocation and
stack and heap are separated in distinct memory areas, this is the safest way to avoid a
stack-heap collision.

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Memory Areas and Using malloc()

9.2.3

252

Tunables for malloc()

There are a number of variables that can be tuned to adapt the behavior of malloc()
to the expected requirements and constraints of the application. Any changes to these
tunables should be made before the very first call to malloc(). Note that some library
functions might also use dynamic memory (notably those from the : Standard IO facilities), so make sure the changes will be done early enough in the startup
sequence.
The variables __malloc_heap_start and __malloc_heap_end can be used
to restrict the malloc() function to a certain memory region. These variables are statically initialized to point to __heap_start and __heap_end, respectively, where
__heap_start is filled in by the linker to point just beyond .bss, and __heap_end
is set to 0 which makes malloc() assume the heap is below the stack.
If the heap is going to be moved to external RAM, __malloc_heap_end must be
adjusted accordingly. This can either be done at run-time, by writing directly to this
variable, or it can be done automatically at link-time, by adjusting the value of the
symbol __heap_end.
The following example shows a linker command to relocate the entire .data and .bss
segments, and the heap to location 0x1100 in external RAM. The heap will extend up
to address 0xffff.
avr-gcc ... -Wl,-Tdata=0x801100,--defsym=__heap_end=0x80ffff ...

Note:

on−board RAM

stack

external RAM

.data
.bss
variables variables

0xFFFF

0x10FF
0x1100

0x0100

See explanation for offset 0x800000. See the chapter about using gcc for the -Wl
options.

heap

SP
RAMEND

*(__malloc_heap_end) == __heap_end
*(__brkval)
*(__malloc_heap_start) == __heap_start
__bss_end
__data_end == __bss_start
__data_start

Figure 7: Internal RAM: stack only, external RAM: variables and heap

If dynamic memory should be placed in external RAM, while keeping the variables in
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253

internal RAM, something like the following could be used. Note that for demonstration
purposes, the assignment of the various regions has not been made adjacent in this
example, so there are "holes" below and above the heap in external RAM that remain
completely unaccessible by regular variables or dynamic memory allocations (shown
in light bisque color in the picture below).
avr-gcc ... -Wl,--defsym=__heap_start=0x802000,--defsym=__heap_end=0x803fff ...

.data

0xFFFF

0x3FFF

on−board RAM

0x2000

0x10FF
0x1100

0x0100

external RAM

.bss

variables variables

stack

heap

SP
RAMEND
__bss_end
__data_end == __bss_start

*(__malloc_heap_end) == __heap_end
*(__brkval)
*(__malloc_heap_start) == __heap_start

__data_start

Figure 8: Internal RAM: variables and stack, external RAM: heap

If __malloc_heap_end is 0, the allocator attempts to detect the bottom of stack
in order to prevent a stack-heap collision when extending the actual size of the heap
to gain more space for dynamic memory. It will not try to go beyond the current
stack limit, decreased by __malloc_margin bytes. Thus, all possible stack frames
of interrupt routines that could interrupt the current function, plus all further nested
function calls must not require more stack space, or they will risk colliding with the
data segment.
The default value of __malloc_margin is set to 32.
9.2.4

Implementation details

Dynamic memory allocation requests will be returned with a two-byte header
prepended that records the size of the allocation. This is later used by free(). The
returned address points just beyond that header. Thus, if the application accidentally
writes before the returned memory region, the internal consistency of the memory allocator is compromised.
The implementation maintains a simple freelist that accounts for memory blocks that
have been returned in previous calls to free(). Note that all of this memory is considered

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254

to be successfully added to the heap already, so no further checks against stack-heap
collisions are done when recycling memory from the freelist.
The freelist itself is not maintained as a separate data structure, but rather by modifying
the contents of the freed memory to contain pointers chaining the pieces together. That
way, no additional memory is reqired to maintain this list except for a variable that
keeps track of the lowest memory segment available for reallocation. Since both, a
chain pointer and the size of the chunk need to be recorded in each chunk, the minimum
chunk size on the freelist is four bytes.
When allocating memory, first the freelist is walked to see if it could satisfy the request.
If there’s a chunk available on the freelist that will fit the request exactly, it will be
taken, disconnected from the freelist, and returned to the caller. If no exact match could
be found, the closest match that would just satisfy the request will be used. The chunk
will normally be split up into one to be returned to the caller, and another (smaller)
one that will remain on the freelist. In case this chunk was only up to two bytes larger
than the request, the request will simply be altered internally to also account for these
additional bytes since no separate freelist entry could be split off in that case.
If nothing could be found on the freelist, heap extension is attempted. This is where
__malloc_margin will be considered if the heap is operating below the stack, or
where __malloc_heap_end will be verified otherwise.
If the remaining memory is insufficient to satisfy the request, NULL will eventually be
returned to the caller.
When calling free(), a new freelist entry will be prepared. An attempt is then made to
aggregate the new entry with possible adjacent entries, yielding a single larger entry
available for further allocations. That way, the potential for heap fragmentation is
hopefully reduced.
A call to realloc() first determines whether the operation is about to grow or shrink the
current allocation. When shrinking, the case is easy: the existing chunk is split, and the
tail of the region that is no longer to be used is passed to the standard free() function for
insertion into the freelist. Checks are first made whether the tail chunk is large enough
to hold a chunk of its own at all, otherwise realloc() will simply do nothing, and return
the original region.
When growing the region, it is first checked whether the existing allocation can be extended in-place. If so, this is done, and the original pointer is returned without copying
any data contents. As a side-effect, this check will also record the size of the largest
chunk on the freelist.
If the region cannot be extended in-place, but the old chunk is at the top of heap, and
the above freelist walk did not reveal a large enough chunk on the freelist to satisfy
the new request, an attempt is made to quickly extend this topmost chunk (and thus
the heap), so no need arises to copy over the existing data. If there’s no more space
available in the heap (same check is done as in malloc()), the entire request will fail.
Otherwise, malloc() will be called with the new request size, the existing data will be

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Memory Sections

255

copied over, and free() will be called on the old region.

9.3

Memory Sections

Remarks:
Need to list all the sections which are available to the avr.
Weak Bindings
FIXME: need to discuss the .weak directive.
The following describes the various sections available.
9.3.1

The .text Section

The .text section contains the actual machine instructions which make up your program.
This section is further subdivided by the .initN and .finiN sections dicussed below.
Note:
The avr-size program (part of binutils), coming from a Unix background,
doesn’t account for the .data initialization space added to the .text section, so in
order to know how much flash the final program will consume, one needs to add
the values for both, .text and .data (but not .bss), while the amount of pre-allocated
SRAM is the sum of .data and .bss.
9.3.2

The .data Section

This section contains static data which was defined in your code. Things like the following would end up in .data:
char err_str[] = "Your program has died a horrible death!";
struct point pt = { 1, 1 };

It is possible to tell the linker the SRAM address of the beginning of the .data section.
This is accomplished by adding -Wl,-Tdata,addr to the avr-gcc command
used to the link your program. Not that addr must be offset by adding 0x800000
the to real SRAM address so that the linker knows that the address is in the SRAM
memory space. Thus, if you want the .data section to start at 0x1100, pass 0x801100
at the address to the linker. [offset explained]
Note:
When using malloc() in the application (which could even happen inside library
calls), additional adjustments are required.

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9.3.3

256

The .bss Section

Uninitialized global or static variables end up in the .bss section.
9.3.4

The .eeprom Section

This is where eeprom variables are stored.
9.3.5

The .noinit Section

This sections is a part of the .bss section. What makes the .noinit section special is that
variables which are defined as such:
int foo __attribute__ ((section (".noinit")));

will not be initialized to zero during startup as would normal .bss data.
Only uninitialized variables can be placed in the .noinit section. Thus, the following
code will cause avr-gcc to issue an error:
int bar __attribute__ ((section (".noinit"))) = 0xaa;

It is possible to tell the linker explicitly where to place the .noinit section by adding
-Wl,-section-start=.noinit=0x802000 to the avr-gcc command line
at the linking stage. For example, suppose you wish to place the .noinit section at
SRAM address 0x2000:
$ avr-gcc ... -Wl,--section-start=.noinit=0x802000 ...

Note:
Because of the Harvard architecture of the AVR devices, you must manually add
0x800000 to the address you pass to the linker as the start of the section. Otherwise, the linker thinks you want to put the .noinit section into the .text section
instead of .data/.bss and will complain.
Alternatively, you can write your own linker script to automate this. [FIXME: need an
example or ref to dox for writing linker scripts.]
9.3.6

The .initN Sections

These sections are used to define the startup code from reset up through the start of
main(). These all are subparts of the .text section.
The purpose of these sections is to allow for more specific placement of code within
your program.
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Note:
Sometimes, it is convenient to think of the .initN and .finiN sections as functions,
but in reality they are just symbolic names which tell the linker where to stick a
chunk of code which is not a function. Notice that the examples for asm and C can
not be called as functions and should not be jumped into.
The .initN sections are executed in order from 0 to 9.
.init0:
Weakly bound to __init(). If user defines __init(), it will be jumped into immediately after a reset.
.init1:
Unused. User definable.
.init2:
In C programs, weakly bound to initialize the stack, and to clear __zero_reg__
(r1).
.init3:
Unused. User definable.
.init4:

For devices with > 64 KB of ROM, .init4 defines the code which takes care of copying
the contents of .data from the flash to SRAM. For all other devices, this code as well
as the code to zero out the .bss section is loaded from libgcc.a.
.init5:
Unused. User definable.
.init6:
Unused for C programs, but used for constructors in C++ programs.
.init7:
Unused. User definable.
.init8:
Unused. User definable.
.init9:
Jumps into main().

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9.3.7

258

The .finiN Sections

These sections are used to define the exit code executed after return from main() or a
call to exit(). These all are subparts of the .text section.
The .finiN sections are executed in descending order from 9 to 0.
.finit9:
Unused. User definable. This is effectively where _exit() starts.
.fini8:
Unused. User definable.
.fini7:
Unused. User definable.
.fini6:
Unused for C programs, but used for destructors in C++ programs.
.fini5:
Unused. User definable.
.fini4:
Unused. User definable.
.fini3:
Unused. User definable.
.fini2:
Unused. User definable.
.fini1:
Unused. User definable.
.fini0:
Goes into an infinite loop after program termination and completion of any _exit()
code (execution of code in the .fini9 -> .fini1 sections).
9.3.8

Using Sections in Assembler Code

Example:
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#include 
.section .init1,"ax",@progbits
ldi
r0, 0xff
out
_SFR_IO_ADDR(PORTB), r0
out
_SFR_IO_ADDR(DDRB), r0

Note:
The ,"ax",@progbits tells the assembler that the section is allocatable ("a"),
executable ("x") and contains data ("@progbits"). For more detailed information
on the .section directive, see the gas user manual.
9.3.9

Using Sections in C Code

Example:
#include 
void my_init_portb (void) __attribute__ ((naked)) \
__attribute__ ((section (".init3")));
void
my_init_portb (void)
{
PORTB = 0xff;
DDRB = 0xff;
}

Note:
Section .init3 is used in this example, as this ensures the inernal __zero_reg__ has already been set up. The code generated by the compiler might blindly rely
on __zero_reg__ being really 0.

9.4
9.4.1

Data in Program Space
Introduction

So you have some constant data and you’re running out of room to store it? Many
AVRs have limited amount of RAM in which to store data, but may have more Flash
space available. The AVR is a Harvard architecture processor, where Flash is used for
the program, RAM is used for data, and they each have separate address spaces. It is
a challenge to get constant data to be stored in the Program Space, and to retrieve that
data to use it in the AVR application.
The problem is exacerbated by the fact that the C Language was not designed for
Harvard architectures, it was designed for Von Neumann architectures where code and
data exist in the same address space. This means that any compiler for a Harvard
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architecture processor, like the AVR, has to use other means to operate with separate
address spaces.
Some compilers use non-standard C language keywords, or they extend the standard
syntax in ways that are non-standard. The AVR toolset takes a different approach.
GCC has a special keyword, __attribute__ that is used to attach different attributes to things such as function declarations, variables, and types. This keyword is
followed by an attribute specification in double parentheses. In AVR GCC, there is a
special attribute called progmem. This attribute is use on data declarations, and tells
the compiler to place the data in the Program Memory (Flash).
AVR-Libc provides a simple macro PROGMEM that is defined as the attribute syntax of GCC with the progmem attribute. This macro was created as a convenience
to the end user, as we will see below. The PROGMEM macro is defined in the
 system header file.
It is difficult to modify GCC to create new extensions to the C language syntax, so
instead, avr-libc has created macros to retrieve the data from the Program Space. These
macros are also found in the  system header file.
9.4.2

A Note On const

Many users bring up the idea of using C’s keyword const as a means of declaring
data to be in Program Space. Doing this would be an abuse of the intended meaning of
the const keyword.
const is used to tell the compiler that the data is to be "read-only". It is used to help
make it easier for the compiler to make certain transformations, or to help the compiler
check for incorrect usage of those variables.
For example, the const keyword is commonly used in many functions as a modifier on
the parameter type. This tells the compiler that the function will only use the parameter
as read-only and will not modify the contents of the parameter variable.
const was intended for uses such as this, not as a means to identify where the data
should be stored. If it were used as a means to define data storage, then it loses its
correct meaning (changes its semantics) in other situations such as in the function parameter example.
9.4.3

Storing and Retrieving Data in the Program Space

Let’s say you have some global data:
unsigned char mydata[11][10] =
{
{0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09},
{0x0A,0x0B,0x0C,0x0D,0x0E,0x0F,0x10,0x11,0x12,0x13},
{0x14,0x15,0x16,0x17,0x18,0x19,0x1A,0x1B,0x1C,0x1D},

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{0x1E,0x1F,0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27},
{0x28,0x29,0x2A,0x2B,0x2C,0x2D,0x2E,0x2F,0x30,0x31},
{0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39,0x3A,0x3B},
{0x3C,0x3D,0x3E,0x3F,0x40,0x41,0x42,0x43,0x44,0x45},
{0x46,0x47,0x48,0x49,0x4A,0x4B,0x4C,0x4D,0x4E,0x4F},
{0x50,0x51,0x52,0x53,0x54,0x55,0x56,0x57,0x58,0x59},
{0x5A,0x5B,0x5C,0x5D,0x5E,0x5F,0x60,0x61,0x62,0x63},
{0x64,0x65,0x66,0x67,0x68,0x69,0x6A,0x6B,0x6C,0x6D}
};

and later in your code you access this data in a function and store a single byte into a
variable like so:
byte = mydata[i][j];

Now you want to store your data in Program Memory. Use the PROGMEM macro found
in  and put it after the declaration of the variable, but before
the initializer, like so:
#include 
.
.
.
unsigned char mydata[11][10] PROGMEM =
{
{0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09},
{0x0A,0x0B,0x0C,0x0D,0x0E,0x0F,0x10,0x11,0x12,0x13},
{0x14,0x15,0x16,0x17,0x18,0x19,0x1A,0x1B,0x1C,0x1D},
{0x1E,0x1F,0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27},
{0x28,0x29,0x2A,0x2B,0x2C,0x2D,0x2E,0x2F,0x30,0x31},
{0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39,0x3A,0x3B},
{0x3C,0x3D,0x3E,0x3F,0x40,0x41,0x42,0x43,0x44,0x45},
{0x46,0x47,0x48,0x49,0x4A,0x4B,0x4C,0x4D,0x4E,0x4F},
{0x50,0x51,0x52,0x53,0x54,0x55,0x56,0x57,0x58,0x59},
{0x5A,0x5B,0x5C,0x5D,0x5E,0x5F,0x60,0x61,0x62,0x63},
{0x64,0x65,0x66,0x67,0x68,0x69,0x6A,0x6B,0x6C,0x6D}
};

That’s it! Now your data is in the Program Space. You can compile, link, and check
the map file to verify that mydata is placed in the correct section.
Now that your data resides in the Program Space, your code to access (read) the data
will no longer work. The code that gets generated will retrieve the data that is located
at the address of the mydata array, plus offsets indexed by the i and j variables.
However, the final address that is calculated where to the retrieve the data points to
the Data Space! Not the Program Space where the data is actually located. It is likely
that you will be retrieving some garbage. The problem is that AVR GCC does not
intrinsically know that the data resides in the Program Space.
The solution is fairly simple. The "rule of thumb" for accessing data stored in the
Program Space is to access the data as you normally would (as if the variable is stored
in Data Space), like so:
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byte = mydata[i][j];

then take the address of the data:
byte = &(mydata[i][j]);

then use the appropriate pgm_read_∗ macro, and the address of your data becomes
the parameter to that macro:
byte = pgm_read_byte(&(mydata[i][j]));

The pgm_read_∗ macros take an address that points to the Program Space, and retrieves the data that is stored at that address. This is why you take the address of the
offset into the array. This address becomes the parameter to the macro so it can generate the correct code to retrieve the data from the Program Space. There are different
pgm_read_∗ macros to read different sizes of data at the address given.
9.4.4

Storing and Retrieving Strings in the Program Space

Now that you can successfully store and retrieve simple data from Program Space you
want to store and retrive strings from Program Space. And specifically you want to
store and array of strings to Program Space. So you start off with your array, like so:
char *string_table[] =
{
"String 1",
"String 2",
"String 3",
"String 4",
"String 5"
};

and then you add your PROGMEM macro to the end of the declaration:
char *string_table[] PROGMEM =
{
"String 1",
"String 2",
"String 3",
"String 4",
"String 5"
};

Right? WRONG!
Unfortunately, with GCC attributes, they affect only the declaration that they are attached to. So in this case, we successfully put the string_table variable, the array
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itself, in the Program Space. This DOES NOT put the actual strings themselves into
Program Space. At this point, the strings are still in the Data Space, which is probably
not what you want.
In order to put the strings in Program Space, you have to have explicit declarations for
each string, and put each string in Program Space:
char
char
char
char
char

string_1[]
string_2[]
string_3[]
string_4[]
string_5[]

PROGMEM
PROGMEM
PROGMEM
PROGMEM
PROGMEM

=
=
=
=
=

"String
"String
"String
"String
"String

1";
2";
3";
4";
5";

Then use the new symbols in your table, like so:
PGM_P string_table[] PROGMEM =
{
string_1,
string_2,
string_3,
string_4,
string_5
};

Now this has the effect of putting string_table in Program Space, where
string_table is an array of pointers to characters (strings), where each pointer
is a pointer to the Program Space, where each string is also stored.
The PGM_P type above is also a macro that defined as a pointer to a character in the
Program Space.
Retrieving the strings are a different matter. You probably don’t want to pull the string
out of Program Space, byte by byte, using the pgm_read_byte() macro. There are
other functions declared in the  header file that work with strings
that are stored in the Program Space.
For example if you want to copy the string from Program Space to a buffer in RAM
(like an automatic variable inside a function, that is allocated on the stack), you can do
this:
void food(void)
{
char buffer[10];
for (unsigned char i = 0; i < 5; i++)
{
strcpy_P(buffer, pgm_read_word(&(string_table[i])));
// Display buffer on LCD.
}
return;
}

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264

Here, the string_table array is stored in Program Space, so we access it normally,
as if were stored in Data Space, then take the address of the location we want to access,
and use the address as a parameter to pgm_read_word. We use the pgm_read_word macro to read the string pointer out of the string_table array. Remember
that a pointer is 16-bits, or word size. This pointer is an address in Program Space
of the string that we want to copy. This pointer is used as a parameter to the function
strcpy_P. The function strcpy_P is just like the regular strcpy function, except
that it copies a string from Program Space (the second parameter) to a buffer in the Data
Space (the first parameter).
There are many string functions available that work with strings located in Program
Space. All of these special string functions have a suffix of _P in the function name,
and are declared in the  header file.
9.4.5

Caveats

The macros and functions used to retrieve data from the Program Space have to generate some extra code in order to actually load the data from the Program Space. This
incurs some extra overhead in terms of code space (extra opcodes) and execution time.
Usually, both the space and time overhead is minimal compared to the space savings
of putting data in Program Space. But you should be aware of this so you can minimize the number of calls within a single function that gets the same piece of data from
Program Space. It is always instructive to look at the resulting disassembly from the
compiler.

9.5

avr-libc and assembler programs

9.5.1

Introduction

There might be several reasons to write code for AVR microcontrollers using plain
assembler source code. Among them are:
• Code for devices that do not have RAM and are thus not supported by the C
compiler.
• Code for very time-critical applications.
• Special tweaks that cannot be done in C.
Usually, all but the first could probably be done easily using the inline assembler facility
of the compiler.
Although avr-libc is primarily targeted to support programming AVR microcontrollers
using the C (and C++) language, there’s limited support for direct assembler usage as
well. The benefits of it are:

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• Use of the C preprocessor and thus the ability to use the same symbolic constants
that are available to C programs, as well as a flexible macro concept that can use
any valid C identifier as a macro (whereas the assembler’s macro concept is
basically targeted to use a macro in place of an assembler instruction).
• Use of the runtime framework like automatically assigning interrupt vectors. For
devices that have RAM, initializing the RAM variables can also be utilized.
9.5.2

Invoking the compiler

For the purpose described in this document, the assembler and linker are usually not
invoked manually, but rather using the C compiler frontend (avr-gcc) that in turn
will call the assembler and linker as required.
This approach has the following advantages:
• There is basically only one program to be called directly, avr-gcc, regardless
of the actual source language used.
• The invokation of the C preprocessor will be automatic, and will include the
appropriate options to locate required include files in the filesystem.
• The invokation of the linker will be automatic, and will include the appropriate options to locate additional libraries as well as the application start-up code
(crtXXX.o) and linker script.
Note that the invokation of the C preprocessor will be automatic when the filename
provided for the assembler file ends in .S (the capital letter "s"). This would even apply
to operating systems that use case-insensitive filesystems since the actual decision is
made based on the case of the filename suffix given on the command-line, not based on
the actual filename from the file system.
Alternatively, the language can
assembler-with-cpp option.
9.5.3

explicitly

be

specified

using

the

-x

Example program

The following annotated example features a simple 100 kHz square wave generator
using an AT90S1200 clocked with a 10.7 MHz crystal. Pin PD6 will be used for the
square wave output.
#include 

; Note [1]

work
tmp

=
=

16
17

; Note [2]

inttmp

=

19

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avr-libc and assembler programs

intsav

=

0

SQUARE

=

PD6

tmconst= 10700000 / 200000
fuzz=
8

266

; Note [3]
; Note [4]:
; 100 kHz => 200000 edges/s
; # clocks in ISR until TCNT0 is set

.section .text
.global main

; Note [5]

main:
rcall

ioinit

rjmp

1b

1:

.global TIMER0_OVF_vect
TIMER0_OVF_vect:
ldi
inttmp, 256 - tmconst + fuzz
out
_SFR_IO_ADDR(TCNT0), inttmp

1:
2:

in

intsav, _SFR_IO_ADDR(SREG)

sbic
rjmp
sbi
rjmp
cbi

_SFR_IO_ADDR(PORTD), SQUARE
1f
_SFR_IO_ADDR(PORTD), SQUARE
2f
_SFR_IO_ADDR(PORTD), SQUARE

out
reti

_SFR_IO_ADDR(SREG), intsav

sbi

_SFR_IO_ADDR(DDRD), SQUARE

ldi
out

work, _BV(TOIE0)
_SFR_IO_ADDR(TIMSK), work

ldi
out

work, _BV(CS00)
; tmr0:
_SFR_IO_ADDR(TCCR0), work

ldi
out

work, 256 - tmconst
_SFR_IO_ADDR(TCNT0), work

; Note [6]
; Note [7]

; Note [8]
; Note [9]

ioinit:

CK/1

sei
ret
.global __vector_default
__vector_default:
reti
.end

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; Note [10]

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267

Note [1]

As in C programs, this includes the central processor-specific file containing the IO port
definitions for the device. Note that not all include files can be included into assembler
sources.
Note [2]

Assignment of registers to symbolic names used locally. Another option would be to
use a C preprocessor macro instead:
#define work 16

Note [3]

Our bit number for the square wave output. Note that the right-hand side consists of a
CPP macro which will be substituted by its value (6 in this case) before actually being
passed to the assembler.
Note [4]

The assembler uses integer operations in the host-defined integer size (32 bits or longer)
when evaluating expressions. This is in contrast to the C compiler that uses the C type
int by default in order to calculate constant integer expressions.
In order to get a 100 kHz output, we need to toggle the PD6 line 200000 times per
second. Since we use timer 0 without any prescaling options in order to get the desired frequency and accuracy, we already run into serious timing considerations: while
accepting and processing the timer overflow interrupt, the timer already continues to
count. When pre-loading the TCCNT0 register, we therefore have to account for the
number of clock cycles required for interrupt acknowledge and for the instructions to
reload TCCNT0 (4 clock cycles for interrupt acknowledge, 2 cycles for the jump from
the interrupt vector, 2 cycles for the 2 instructions that reload TCCNT0). This is what
the constant fuzz is for.
Note [5]

External functions need to be declared to be .global. main is the application entry
point that will be jumped to from the ininitalization routine in crts1200.o.
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Note [6]

The main loop is just a single jump back to itself. Square wave generation itself is
completely handled by the timer 0 overflow interrupt service. A sleep instruction
(using idle mode) could be used as well, but probably would not conserve much energy
anyway since the interrupt service is executed quite frequently.
Note [7]

Interrupt functions can get the usual names that are also available to C programs. The
linker will then put them into the appropriate interrupt vector slots. Note that they must
be declared .global in order to be acceptable for this purpose. This will only work if
 has been included. Note that the assembler or linker have no chance
to check the correct spelling of an interrupt function, so it should be double-checked.
(When analyzing the resulting object file using avr-objdump or avr-nm, a name
like __vector_N should appear, with N being a small integer number.)
Note [8]

As explained in the section about special function registers, the actual IO port address
should be obtained using the macro _SFR_IO_ADDR. (The AT90S1200 does not have
RAM thus the memory-mapped approach to access the IO registers is not available. It
would be slower than using in / out instructions anyway.)
Since the operation to reload TCCNT0 is time-critical, it is even performed before
saving SREG. Obviously, this requires that the instructions involved would not change
any of the flag bits in SREG.
Note [9]

Interrupt routines must not clobber the global CPU state. Thus, it is usually necessary
to save at least the state of the flag bits in SREG. (Note that this serves as an example
here only since actually, all the following instructions would not modify SREG either,
but that’s not commonly the case.)
Also, it must be made sure that registers used inside the interrupt routine do not conflict
with those used outside. In the case of a RAM-less device like the AT90S1200, this can
only be done by agreeing on a set of registers to be used exclusively inside the interrupt
routine; there would not be any other chance to "save" a register anywhere.
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269

If the interrupt routine is to be linked together with C modules, care must be taken
to follow the register usage guidelines imposed by the C compiler. Also, any register
modified inside the interrupt sevice needs to be saved, usually on the stack.
Note [10]

As explained in Interrupts, a global "catch-all" interrupt handler that gets all unassigned
interrupt vectors can be installed using the name __vector_default. This must
be .global, and obviously, should end in a reti instruction. (By default, a jump to
location 0 would be implied instead.)
9.5.4

Pseudo-ops and operators

The available pseudo-ops in the assembler are described in the GNU assembler (gas)
manual. The manual can be found online as part of the current binutils release under
http://sources.redhat.com/binutils/.
As gas comes from a Unix origin, its pseudo-op and overall assembler syntax is slightly
different than the one being used by other assemblers. Numeric constants follow the C
notation (prefix 0x for hexadecimal constants), expressions use a C-like syntax.
Some common pseudo-ops include:
• .byte allocates single byte constants
• .ascii allocates a non-terminated string of characters
• .asciz allocates a \0-terminated string of characters (C string)
• .data switches to the .data section (initialized RAM variables)
• .text switches to the .text section (code and ROM constants)
• .set declares a symbol as a constant expression (identical to .equ)
• .global (or .globl) declares a public symbol that is visible to the linker (e. g.
function entry point, global variable)
• .extern declares a symbol to be externally defined; this is effectively a comment
only, as gas treats all undefined symbols it encounters as globally undefined anyway

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Note that .org is available in gas as well, but is a fairly pointless pseudo-op in an assembler environment that uses relocatable object files, as it is the linker that determines
the final position of some object in ROM or RAM.
Along with the architecture-independent standard operators, there are some AVRspecific operators available which are unfortunately not yet described in the official
documentation. The most notable operators are:
• lo8 Takes the least significant 8 bits of a 16-bit integer
• hi8 Takes the most significant 8 bits of a 16-bit integer
• pm Takes a program-memory (ROM) address, and converts it into a RAM address. This implies a division by 2 as the AVR handles ROM addresses as 16-bit
words (e.g. in an IJMP or ICALL instruction), and can also handle relocatable
symbols on the right-hand side.
Example:
ldi r24, lo8(pm(somefunc))
ldi r25, hi8(pm(somefunc))
call something

This passes the address of function somefunc as the first parameter to function
something.

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AVR-GCC
Inline Assembler Cookbook
About this Document
The GNU C compiler for Atmel AVR RISC processors offers, to embed assembly
language code into C programs. This cool feature may be used for manually optimizing
time critical parts of the software or to use specific processor instruction, which are not
available in the C language.
Because of a lack of documentation, especially for the AVR version of the compiler, it
may take some time to figure out the implementation details by studying the compiler
and assembler source code. There are also a few sample programs available in the net.
Hopefully this document will help to increase their number.
It’s assumed, that you are familiar with writing AVR assembler programs, because this
is not an AVR assembler programming tutorial. It’s not a C language tutorial either.

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Note that this document does not cover file written completely in assembler language,
refer to avr-libc and assembler programs for this.
Copyright (C) 2001-2002 by egnite Software GmbH
Permission is granted to copy and distribute verbatim copies of this manual provided
that the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual provided that
the entire resulting derived work is distributed under the terms of a permission notice
identical to this one.
This document describes version 3.3 of the compiler. There may be some parts, which
hadn’t been completely understood by the author himself and not all samples had been
tested so far. Because the author is German and not familiar with the English language,
there are definitely some typos and syntax errors in the text. As a programmer the
author knows, that a wrong documentation sometimes might be worse than none. Anyway, he decided to offer his little knowledge to the public, in the hope to get enough
response to improve this document. Feel free to contact the author via e-mail. For the
latest release check http://www.ethernut.de/.
Herne, 17th of May 2002 Harald Kipp harald.kipp-at-egnite.de
Note:
As of 26th of July 2002, this document has been merged into the
documentation for avr-libc.
The latest version is now available at
http://savannah.nongnu.org/projects/avr-libc/.
9.6.1

GCC asm Statement

Let’s start with a simple example of reading a value from port D:
asm("in %0, %1" : "=r" (value) : "I" (_SFR_IO_ADDR(PORTD)) );

Each asm statement is devided by colons into (up to) four parts:
1. The assembler instructions, defined as a single string constant:
"in %0, %1"

2. A list of output operands, separated by commas. Our example uses just one:
"=r" (value)

3. A comma separated list of input operands. Again our example uses one operand
only:

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"I" (_SFR_IO_ADDR(PORTD))

4. Clobbered registers, left empty in our example.
You can write assembler instructions in much the same way as you would write assembler programs. However, registers and constants are used in a different way if they refer
to expressions of your C program. The connection between registers and C operands is
specified in the second and third part of the asm instruction, the list of input and output
operands, respectively. The general form is
asm(code : output operand list : input operand list [: clobber list]);

In the code section, operands are referenced by a percent sign followed by a single digit.
0 refers to the first 1 to the second operand and so forth. From the above example:
0 refers to "=r" (value) and
1 refers to "I" (_SFR_IO_ADDR(PORTD)).
This may still look a little odd now, but the syntax of an operand list will be explained
soon. Let us first examine the part of a compiler listing which may have been generated
from our example:
lds r24,value
/* #APP */
in r24, 12
/* #NOAPP */
sts value,r24

The comments have been added by the compiler to inform the assembler that the included code was not generated by the compilation of C statements, but by inline assembler statements. The compiler selected register r24 for storage of the value read
from PORTD. The compiler could have selected any other register, though. It may not
explicitely load or store the value and it may even decide not to include your assembler
code at all. All these decisions are part of the compiler’s optimization strategy. For
example, if you never use the variable value in the remaining part of the C program,
the compiler will most likely remove your code unless you switched off optimization.
To avoid this, you can add the volatile attribute to the asm statement:
asm volatile("in %0, %1" : "=r" (value) : "I" (_SFR_IO_ADDR(PORTD)));

The last part of the asm instruction, the clobber list, is mainly used to tell the compiler
about modifications done by the assembler code. This part may be omitted, all other
parts are required, but may be left empty. If your assembler routine won’t use any
input or output operand, two colons must still follow the assembler code string. A
good example is a simple statement to disable interrupts:
asm volatile("cli"::);

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273

Assembler Code

You can use the same assembler instruction mnemonics as you’d use with any other
AVR assembler. And you can write as many assembler statements into one code string
as you like and your flash memory is able to hold.
Note:
The available assembler directives vary from one assembler to another.
To make it more readable, you should put each statement on a seperate line:
asm volatile("nop\n\t"
"nop\n\t"
"nop\n\t"
"nop\n\t"
::);

The linefeed and tab characters will make the assembler listing generated by the compiler more readable. It may look a bit odd for the first time, but that’s the way the
compiler creates it’s own assembler code.
You may also make use of some special registers.
Symbol
__SREG__
__SP_H__
__SP_L__
__tmp_reg__
__zero_reg__

Register
Status register at address 0x3F
Stack pointer high byte at address 0x3E
Stack pointer low byte at address 0x3D
Register r0, used for temporary storage
Register r1, always zero

Register r0 may be freely used by your assembler code and need not be restored at
the end of your code. It’s a good idea to use __tmp_reg__ and __zero_reg__
instead of r0 or r1, just in case a new compiler version changes the register usage
definitions.
9.6.3

Input and Output Operands

Each input and output operand is described by a constraint string followed by a C
expression in parantheses. AVR-GCC 3.3 knows the following constraint characters:
Note:
The most up-to-date and detailed information on contraints for the avr can be found
in the gcc manual.
The x register is r27:r26, the y register is r29:r28, and the z register is
r31:r30

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Constraint
a
b
d
e
G
I
J
K
L
l
M
N
O
P
q
r
t
w
x
y
z

Used for
Simple upper registers
Base pointer registers
pairs
Upper register
Pointer register pairs
Floating point constant
6-bit positive integer
constant
6-bit negative integer
constant
Integer constant
Integer constant
Lower registers
8-bit integer constant
Integer constant
Integer constant
Integer constant
Stack pointer register
Any register
Temporary register
Special upper register
pairs
Pointer register pair X
Pointer register pair Y
Pointer register pair Z

274

Range
r16 to r23
y, z
r16 to r31
x, y, z
0.0
0 to 63
-63 to 0
2
0
r0 to r15
0 to 255
-1
8, 16, 24
1
SPH:SPL
r0 to r31
r0
r24, r26, r28, r30
x (r27:r26)
y (r29:r28)
z (r31:r30)

These definitions seem not to fit properly to the AVR instruction set. The author’s assumption is, that this part of the compiler has never been really finished in this version,
but that assumption may be wrong. The selection of the proper contraint depends on
the range of the constants or registers, which must be acceptable to the AVR instruction
they are used with. The C compiler doesn’t check any line of your assembler code. But
it is able to check the constraint against your C expression. However, if you specify
the wrong constraints, then the compiler may silently pass wrong code to the assembler. And, of course, the assembler will fail with some cryptic output or internal errors.
For example, if you specify the constraint "r" and you are using this register with an
"ori" instruction in your assembler code, then the compiler may select any register.
This will fail, if the compiler chooses r2 to r15. (It will never choose r0 or r1,
because these are uses for special purposes.) That’s why the correct constraint in that
case is "d". On the other hand, if you use the constraint "M", the compiler will make
sure that you don’t pass anything else but an 8-bit value. Later on we will see how to
pass multibyte expression results to the assembler code.
The following table shows all AVR assembler mnemonics which require operands, and
the related contraints. Because of the improper constraint definitions in version 3.3,
they aren’t strict enough. There is, for example, no constraint, which restricts integer

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constants to the range 0 to 7 for bit set and bit clear operations.
Mnemonic
adc
adiw
andi
bclr
brbc
bset
cbi
com
cpc
cpse
elpm
in
ld
ldi
lpm
lsr
movw
neg
ori
pop
rol
sbc
sbi
sbiw
sbrc
ser
std
sub
swap

Constraints
r,r
w,I
d,M
I
I,label
I
I,I
r
r,r
r,r
t,z
r,I
r,e
d,M
t,z
r
r,r
r
d,M
r
r
r,r
I,I
w,I
r,I
d
b,r
r,r
r

Mnemonic
add
and
asr
bld
brbs
bst
cbr
cp
cpi
dec
eor
inc
ldd
lds
lsl
mov
mul
or
out
push
ror
sbci
sbic
sbr
sbrs
st
sts
subi

Constraints
r,r
r,r
r
r,I
I,label
r,I
d,I
r,r
d,M
r
r,r
r
r,b
r,label
r
r,r
r,r
r,r
I,r
r
r
d,M
I,I
d,M
r,I
e,r
label,r
d,M

Constraint characters may be prepended by a single constraint modifier. Contraints
without a modifier specify read-only operands. Modifiers are:
Modifier
=
+
&

Specifies
Write-only operand, usually used for all
output operands.
Read-write operand (not supported by
inline assembler)
Register should be used for output only

Output operands must be write-only and the C expression result must be an lvalue,
which means that the operands must be valid on the left side of assignments. Note,
that the compiler will not check if the operands are of reasonable type for the kind of
operation used in the assembler instructions.
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Input operands are, you guessed it, read-only. But what if you need the same operand
for input and output? As stated above, read-write operands are not supported in inline
assembler code. But there is another solution. For input operators it is possible to use
a single digit in the constraint string. Using digit n tells the compiler to use the same
register as for the n-th operand, starting with zero. Here is an example:
asm volatile("swap %0" : "=r" (value) : "0" (value));

This statement will swap the nibbles of an 8-bit variable named value. Constraint "0"
tells the compiler, to use the same input register as for the first operand. Note however,
that this doesn’t automatically imply the reverse case. The compiler may choose the
same registers for input and output, even if not told to do so. This is not a problem in
most cases, but may be fatal if the output operator is modified by the assembler code
before the input operator is used. In the situation where your code depends on different
registers used for input and output operands, you must add the & constraint modifier to
your output operand. The following example demonstrates this problem:
asm volatile("in %0,%1"
"\n\t"
"out %1, %2" "\n\t"
: "=&r" (input)
: "I" (_SFR_IO_ADDR(port)), "r" (output)
);

In this example an input value is read from a port and then an output value is written to
the same port. If the compiler would have choosen the same register for input and output, then the output value would have been destroyed on the first assembler instruction.
Fortunately, this example uses the & constraint modifier to instruct the compiler not to
select any register for the output value, which is used for any of the input operands.
Back to swapping. Here is the code to swap high and low byte of a 16-bit value:
asm volatile("mov __tmp_reg__, %A0" "\n\t"
"mov %A0, %B0"
"\n\t"
"mov %B0, __tmp_reg__" "\n\t"
: "=r" (value)
: "0" (value)
);

First you will notice the usage of register __tmp_reg__, which we listed among
other special registers in the Assembler Code section. You can use this register without
saving its contents. Completely new are those letters A and B in %A0 and %B0. In fact
they refer to two different 8-bit registers, both containing a part of value.
Another example to swap bytes of a 32-bit value:
asm volatile("mov
"mov
"mov
"mov

__tmp_reg__, %A0"
%A0, %D0"
%D0, __tmp_reg__"
__tmp_reg__, %B0"

"\n\t"
"\n\t"
"\n\t"
"\n\t"

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"mov %B0, %C0"
"\n\t"
"mov %C0, __tmp_reg__" "\n\t"
: "=r" (value)
: "0" (value)
);

If operands do not fit into a single register, the compiler will automatically assign
enough registers to hold the entire operand. In the assembler code you use %A0 to refer
to the lowest byte of the first operand, %A1 to the lowest byte of the second operand
and so on. The next byte of the first operand will be %B0, the next byte %C0 and so on.
This also implies, that it is often neccessary to cast the type of an input operand to the
desired size.
A final problem may arise while using pointer register pairs. If you define an input
operand
"e" (ptr)

and the compiler selects register Z (r30:r31), then
%A0 refers to r30 and
%B0 refers to r31.
But both versions will fail during the assembly stage of the compiler, if you explicitely
need Z, like in
ld r24,Z

If you write
ld r24, %a0

with a lower case a following the percent sign, then the compiler will create the proper
assembler line.
9.6.4

Clobbers

As stated previously, the last part of the asm statement, the list of clobbers, may be
omitted, including the colon seperator. However, if you are using registers, which
had not been passed as operands, you need to inform the compiler about this. The
following example will do an atomic increment. It increments an 8-bit value pointed
to by a pointer variable in one go, without being interrupted by an interrupt routine
or another thread in a multithreaded environment. Note, that we must use a pointer,
because the incremented value needs to be stored before interrupts are enabled.

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asm volatile(
"cli"
"ld r24, %a0"
"inc r24"
"st %a0, r24"
"sei"
:
: "e" (ptr)
: "r24"
);

278

"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"

The compiler might produce the following code:
cli
ld r24, Z
inc r24
st Z, r24
sei

One easy solution to avoid clobbering register r24 is, to make use of the special temporary register __tmp_reg__ defined by the compiler.
asm volatile(
"cli"
"ld __tmp_reg__, %a0"
"inc __tmp_reg__"
"st %a0, __tmp_reg__"
"sei"
:
: "e" (ptr)
);

"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"

The compiler is prepared to reload this register next time it uses it. Another problem
with the above code is, that it should not be called in code sections, where interrupts
are disabled and should be kept disabled, because it will enable interrupts at the end.
We may store the current status, but then we need another register. Again we can solve
this without clobbering a fixed, but let the compiler select it. This could be done with
the help of a local C variable.
{
uint8_t s;
asm volatile(
"in %0, __SREG__"
"cli"
"ld __tmp_reg__, %a1"
"inc __tmp_reg__"
"st %a1, __tmp_reg__"
"out __SREG__, %0"
: "=&r" (s)
: "e" (ptr)
);

"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"

}

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Now every thing seems correct, but it isn’t really. The assembler code modifies the
variable, that ptr points to. The compiler will not recognize this and may keep its
value in any of the other registers. Not only does the compiler work with the wrong
value, but the assembler code does too. The C program may have modified the value
too, but the compiler didn’t update the memory location for optimization reasons. The
worst thing you can do in this case is:
{
uint8_t s;
asm volatile(
"in %0, __SREG__"
"cli"
"ld __tmp_reg__, %a1"
"inc __tmp_reg__"
"st %a1, __tmp_reg__"
"out __SREG__, %0"
: "=&r" (s)
: "e" (ptr)
: "memory"
);

"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"
"\n\t"

}

The special clobber "memory" informs the compiler that the assembler code may modify any memory location. It forces the compiler to update all variables for which the
contents are currently held in a register before executing the assembler code. And of
course, everything has to be reloaded again after this code.
In most situations, a much better solution would be to declare the pointer destination
itself volatile:
volatile uint8_t *ptr;

This way, the compiler expects the value pointed to by ptr to be changed and will
load it whenever used and store it whenever modified.
Situations in which you need clobbers are very rare. In most cases there will be better
ways. Clobbered registers will force the compiler to store their values before and reload
them after your assembler code. Avoiding clobbers gives the compiler more freedom
while optimizing your code.
9.6.5

Assembler Macros

In order to reuse your assembler language parts, it is useful to define them as macros
and put them into include files. AVR Libc comes with a bunch of them, which could be
found in the directory avr/include. Using such include files may produce compiler
warnings, if they are used in modules, which are compiled in strict ANSI mode. To
avoid that, you can write __asm__ instead of asm and __volatile__ instead of
volatile. These are equivalent aliases.

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Another problem with reused macros arises if you are using labels. In such
cases you may make use of the special pattern =, which is replaced by a unique
number on each asm statement.
The following code had been taken from
avr/include/iomacros.h:
#define loop_until_bit_is_clear(port,bit) \
__asm__ __volatile__ (
\
"L_%=: " "sbic %0, %1" "\n\t"
\
"rjmp L_%="
\
: /* no outputs */
\
: "I" (_SFR_IO_ADDR(port)),
"I" (bit)
)

When used for the first time, L_= may be translated to L_1404, the next usage might
create L_1405 or whatever. In any case, the labels became unique too.
Another option is to use Unix-assembler style numeric labels. They are explained in
How do I trace an assembler file in avr-gdb?. The above example would then look like:
#define loop_until_bit_is_clear(port,bit)
__asm__ __volatile__ (
"1: " "sbic %0, %1" "\n\t"
"rjmp 1b"
: /* no outputs */
: "I" (_SFR_IO_ADDR(port)),
"I" (bit)
)

9.6.6

C Stub Functions

Macro definitions will include the same assembler code whenever they are referenced.
This may not be acceptable for larger routines. In this case you may define a C stub
function, containing nothing other than your assembler code.
void delay(uint8_t ms)
{
uint16_t cnt;
asm volatile (
"\n"
"L_dl1%=:" "\n\t"
"mov %A0, %A2" "\n\t"
"mov %B0, %B2" "\n"
"L_dl2%=:" "\n\t"
"sbiw %A0, 1" "\n\t"
"brne L_dl2%=" "\n\t"
"dec %1" "\n\t"
"brne L_dl1%=" "\n\t"
: "=&w" (cnt)
: "r" (ms), "r" (delay_count)
);
}

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The purpose of this function is to delay the program execution by a specified number
of milliseconds using a counting loop. The global 16 bit variable delay_count must
contain the CPU clock frequency in Hertz divided by 4000 and must have been set
before calling this routine for the first time. As described in the clobber section, the
routine uses a local variable to hold a temporary value.
Another use for a local variable is a return value. The following function returns a 16
bit value read from two successive port addresses.
uint16_t inw(uint8_t port)
{
uint16_t result;
asm volatile (
"in %A0,%1" "\n\t"
"in %B0,(%1) + 1"
: "=r" (result)
: "I" (_SFR_IO_ADDR(port))
);
return result;
}

Note:
inw() is supplied by avr-libc.
9.6.7

C Names Used in Assembler Code

By default AVR-GCC uses the same symbolic names of functions or variables in C and
assembler code. You can specify a different name for the assembler code by using a
special form of the asm statement:
unsigned long value asm("clock") = 3686400;

This statement instructs the compiler to use the symbol name clock rather than value.
This makes sense only for external or static variables, because local variables do not
have symbolic names in the assembler code. However, local variables may be held in
registers.
With AVR-GCC you can specify the use of a specific register:
void Count(void)
{
register unsigned char counter asm("r3");
... some code...
asm volatile("clr r3");
... more code...
}

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The assembler instruction, "clr r3", will clear the variable counter. AVR-GCC will
not completely reserve the specified register. If the optimizer recognizes that the variable will not be referenced any longer, the register may be re-used. But the compiler
is not able to check wether this register usage conflicts with any predefined register. If
you reserve too many registers in this way, the compiler may even run out of registers
during code generation.
In order to change the name of a function, you need a prototype declaration, because
the compiler will not accept the asm keyword in the function definition:
extern long Calc(void) asm ("CALCULATE");

Calling the function Calc() will create assembler instructions to call the function
CALCULATE.
9.6.8

Links

For a more thorough discussion of inline assembly usage, see the gcc user manual. The
latest version of the gcc manual is always available here: http://gcc.gnu.org/onlinedocs/

9.7
9.7.1

How to Build a Library
Introduction

So you keep reusing the same functions that you created over and over? Tired of cut and
paste going from one project to the next? Would you like to reduce your maintenance
overhead? Then you’re ready to create your own library! Code reuse is a very laudable
goal. With some upfront investment, you can save time and energy on future projects
by having ready-to-go libraries. This chapter describes some background information,
design considerations, and practical knowledge that you will need to create and use
your own libraries.
9.7.2

How the Linker Works

The compiler compiles a single high-level language file (C language, for example) into
a single object module file. The linker (ld) can only work with object modules to link
them together. Object modules are the smallest unit that the linker works with.
Typically, on the linker command line, you will specify a set of object modules (that
has been previously compiled) and then a list of libraries, including the Standard C
Library. The linker takes the set of object modules that you specify on the command
line and links them together. Afterwards there will probably be a set of "undefined

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references". A reference is essentially a function call. An undefined reference is a
function call, with no defined function to match the call.
The linker will then go through the libraries, in order, to match the undefined references
with function definitions that are found in the libraries. If it finds the function that
matches the call, the linker will then link in the object module in which the function is
located. This part is important: the linker links in THE ENTIRE OBJECT MODULE in
which the function is located. Remember, the linker knows nothing about the functions
internal to an object module, other than symbol names (such as function names). The
smallest unit the linker works with is object modules.
When there are no more undefined references, the linker has linked everything and is
done and outputs the final application.
9.7.3

How to Design a Library

How the linker behaves is very important in designing a library. Ideally, you want to
design a library where only the functions that are called are the only functions to be
linked into the final application. This helps keep the code size to a minimum. In order
to do this, with the way the linker works, is to only write one function per code module.
This will compile to one function per object module. This is usually a very different
way of doing things than writing an application!
There are always exceptions to the rule. There are generally two cases where you
would want to have more than one function per object module.
The first is when you have very complementary functions that it doesn’t make much
sense to split them up. For example, malloc() and free(). If someone is going to use
malloc(), they will very likely be using free() (or at least should be using free()). In this
case, it makes more sense to aggregate those two functions in the same object module.
The second case is when you want to have an Interrupt Service Routine (ISR) in your
library that you want to link in. The problem in this case is that the linker looks for
unresolved references and tries to resolve them with code in libraries. A reference is
the same as a function call. But with ISRs, there is no function call to initiate the ISR.
The ISR is placed in the Interrupt Vector Table (IVT), hence no call, no reference,
and no linking in of the ISR. In order to do this, you have to trick the linker in a way.
Aggregate the ISR, with another function in the same object module, but have the other
function be something that is required for the user to call in order to use the ISR, like
perhaps an initialization function for the subsystem, or perhaps a function that enables
the ISR in the first place.
9.7.4

Creating a Library

The librarian program is called ar (for "archiver") and is found in the GNU Binutils
project. This program will have been built for the AVR target and will therefore be
named avr-ar.

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The job of the librarian program is simple: aggregate a list of object modules into
a single library (archive) and create an index for the linker to use. The name that
you create for the library filename must follow a specific pattern: lib.a. The
 part is the unique part of the filename that you create. It makes it easier if the
 part relates to what the library is about. This  part must be prefixed
by "lib", and it must have a file extension of .a, for "archive". The reason for the special
form of the filename is for how the library gets used by the toolchain, as we will see
later on.
Note:
The filename is case-sensitive. Use a lowercase "lib" prefix, and a lowercase ".a"
as the file extension.
The command line is fairly simple:
avr-as rcs  

The r command switch tells the program to insert the object modules into the archive
with replacement. The c command line switch tells the program to create the archive.
And the s command line switch tells the program to write an object-file index into the
archive, or update an existing one. This last switch is very important as it helps the
linker to find what it needs to do its job.
Note:
The command line switches are case sensitive! There are uppercase switches that
have completely different actions.
MFile and the WinAVR distribution contain a Makefile Template that includes the
necessary command lines to build a library. You will have to manually modify the
template to switch it over to build a library instead of an application.
See the GNU Binutils manual for more information on the ar program.
9.7.5

Using a Library

To use a library, use the -l switch on your linker command line. The string immediately following the -l is the unique part of the library filename that the linker will link
in. For example, if you use:
-lm

this will expand to the library filename:
libm.a

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which happens to be the math library included in avr-libc.
If you use this on your linker command line:
-lprintf_flt

then the linker will look for a library called:
libprintf_flt.a

This is why naming your library is so important when you create it!
The linker will search libraries in the order that they appear on the command line.
Whichever function is found first that matches the undefined reference, it will be linked
in.
There are also command line switches that tell GCC which directory to look in (-L)
for the libraries that are specified to be linke in with -l.
See the GNU Binutils manual for more information on the GNU linker (ld) program.

9.8
9.8.1

Porting From IAR to AVR GCC
Introduction

C language was designed to be a portable language. There two main types of porting activities: porting an application to a different platform (OS and/or processor),
and porting to a different compiler. Porting to a different compiler can be exacerbated
when the application is an embedded system. For example, the C language Standard,
strangely, does not specify a standard for declaring and defining Interrupt Service Routines (ISRs). Different compilers have different ways of defining registers, some of
which use non-standard language constructs.
This chapter describes some methods and pointers on porting an AVR application built
with the IAR compiler to the GNU toolchain (AVR GCC). Note that this may not be
an exhaustive list.
9.8.2

Registers

IO header files contain identifiers for all the register names and bit names for a particular processor. IAR has individual header files for each processor and they must be
included when registers are being used in the code. For example:
#include 

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Note:
IAR does not always use the same register names or bit names that are used in the
AVR datasheet.
AVR GCC also has individual IO header files for each processor. However, the actual
processor type is specified as a command line flag to the compiler. (Using the -mmcu=
 flag.) This is usually done in the Makefile. This allows you to specify
only a single header file for any processor type:
#include 

Note:
The forward slash in the  file name that is used to separate subdirectories can be used on Windows distributions of the toolchain and is the recommended
method of including this file.
The compiler knows the processor type and through the single header file above, it can
pull in and include the correct individual IO header file. This has the advantage that you
only have to specify one generic header file, and you can easily port your application
to another processor type without having to change every file to include the new IO
header file.
The AVR toolchain tries to adhere to the exact names of the registers and names of
the bits found in the AVR datasheet. There may be some descrepencies between the
register names found in the IAR IO header files and the AVR GCC IO header files.
9.8.3

Interrupt Service Routines (ISRs)

As mentioned above, the C language Standard, strangely, does not specify a standard
way of declaring and defining an ISR. Hence, every compiler seems to have their own
special way of doing so.
IAR declares an ISR like so:
#pragma vector=TIMER0_OVF_vect
__interrupt void MotorPWMBottom()
{
// code
}

In AVR GCC, you declare an ISR like so:
ISR(PCINT1_vect)
{
//code
}

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AVR GCC uses the ISR macro to define an ISR. This macro requries the header file:
#include 

The names of the various interrupt vectors are found in the individual processor IO
header files that you must include with .
Note:
The names of the interrupt vectors in AVR GCC has been changed to match the
names of the vectors in IAR. This significantly helps in porting applications from
IAR to AVR GCC.
9.8.4

Intrinsic Routines

IAR has a number of intrinsic routine such as
__enable_interrupts() __disable_interrupts() __watchdog_reset()
These intrinsic functions compile to specific AVR opcodes (SEI, CLI, WDR).
There are equivalent macros that are used in AVR GCC, however they are not located
in a single include file.
AVR GCC has sei() for __enable_interrupts(), and cli()
for __disable_interrupts().
Both of these macros are located in
.
AVR GCC has the macro wdt_reset() in place of __watchdog_reset().
However, there is a whole Watchdog Timer API available in AVR GCC that can be
found in .
9.8.5

Flash Variables

The C language was not designed for Harvard architecture processors with separate
memory spaces. This means that there are various non-standard ways to define a variable whose data resides in the Program Memory (Flash).
IAR uses a non-standard keyword to declare a variable in Program Memory:
__flash int mydata[] = ....

AVR GCC uses Variable Attributes to achieve the same effect:
int mydata[] __attribute__((progmem))

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Note:
See the GCC User Manual for more information about Variable Attributes.
avr-libc provides a convenience macro for the Variable Attribute:
#include 
.
.
.
int mydata[] PROGMEM = ....

Note:
The PROGMEM macro expands to the Variable Attribute of progmem. This
macro requires that you include . This is the canonical
method for defining a variable in Program Space.
To read back flash data, use the pgm_read_∗() macros defined in
.
All Program Memory handling macros are defined
there.
There is also a way to create a method to define variables in Program Memory that is
common between the two compilers (IAR and AVR GCC). Create a header file that has
these definitions:
#if defined(__ICCAVR__) // IAR C Compiler
#define FLASH_DECLARE(x) __flash x
#endif
#if defined(__GNUC__) // GNU Compiler
#define FLASH_DECLARE(x) x __attribute__((__progmem__))
#endif

This code snippet checks for the IAR compiler or for the GCC compiler and defines a
macro FLASH_DECLARE(x) that will declare a variable in Program Memory using
the appropriate method based on the compiler that is being used. Then you would used
it like so:
FLASH_DECLARE(int mydata[] = ...);

9.8.6

Non-Returning main()

To declare main() to be a non-returning function in IAR, it is done like this:
__C_task void main(void)
{
// code
}

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To do the equivalent in AVR GCC, do this:
void main(void) __attribute__((noreturn));
void main(void)
{
//...
}

Note:
See the GCC User Manual for more information on Function Attributes.
In AVR GCC, a prototype for main() is required so you can declare the function attribute to specify that the main() function is of type "noreturn". Then, define main() as
normal. Note that the return type for main() is now void.
9.8.7

Locking Registers

The IAR compiler allows a user to lock general registers from r15 and down by using
compiler options and this keyword syntax:
__regvar __no_init volatile unsigned int filteredTimeSinceCommutation @14;

This line locks r14 for use only when explicitly referenced in your code thorugh the var
name "filteredTimeSinceCommutation". This means that the compiler cannot dispose
of it at its own will.
To do this in AVR GCC, do this:
register unsigned char counter asm("r3");

Typically, it should be possible to use r2 through r15 that way.
Note:
Do not reserve r0 or r1 as these are used internally by the compiler for a temporary
register and for a zero value.
Locking registers is not recommended in AVR GCC as it removes this register
from the control of the compiler, which may make code generation worse. Use at
your own risk.

9.9
9.9.1

Frequently Asked Questions
FAQ Index

1. My program doesn’t recognize a variable updated within an interrupt routine
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2. I get "undefined reference to..." for functions like "sin()"
3. How to permanently bind a variable to a register?
4. How to modify MCUCR or WDTCR early?
5. What is all this _BV() stuff about?
6. Can I use C++ on the AVR?
7. Shouldn’t I initialize all my variables?
8. Why do some 16-bit timer registers sometimes get trashed?
9. How do I use a #define’d constant in an asm statement?
10. Why does the PC randomly jump around when single-stepping through my program in avr-gdb?
11. How do I trace an assembler file in avr-gdb?
12. How do I pass an IO port as a parameter to a function?
13. What registers are used by the C compiler?
14. How do I put an array of strings completely in ROM?
15. How to use external RAM?
16. Which -O flag to use?
17. How do I relocate code to a fixed address?
18. My UART is generating nonsense! My ATmega128 keeps crashing! Port F is
completely broken!
19. Why do all my "foo...bar" strings eat up the SRAM?
20. Why does the compiler compile an 8-bit operation that uses bitwise operators
into a 16-bit operation in assembly?
21. How to detect RAM memory and variable overlap problems?
22. Is it really impossible to program the ATtinyXX in C?
23. What is this "clock skew detected" messsage?
24. Why are (many) interrupt flags cleared by writing a logical 1?
25. Why have "programmed" fuses the bit value 0?
26. Which AVR-specific assembler operators are available?
27. Why are interrupts re-enabled in the middle of writing the stack pointer?
28. Why are there five different linker scripts?
29. How to add a raw binary image to linker output?

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9.9.2

Frequently Asked Questions

291

My program doesn’t recognize a variable updated within an interrupt routine

When using the optimizer, in a loop like the following one:
uint8_t flag;
...
ISR(SOME_vect) {
flag = 1;
}
...
while (flag == 0) {
...
}

the compiler will typically access flag only once, and optimize further accesses completely away, since its code path analysis shows that nothing inside the loop could
change the value of flag anyway. To tell the compiler that this variable could be
changed outside the scope of its code path analysis (e. g. from within an interrupt
routine), the variable needs to be declared like:
volatile uint8_t flag;

Back to FAQ Index.

9.9.3

I get "undefined reference to..." for functions like "sin()"

In order to access the mathematical functions that are declared in , the
linker needs to be told to also link the mathematical library, libm.a.
Typically, system libraries like libm.a are given to the final C compiler command
line that performs the linking step by adding a flag -lm at the end. (That is, the initial
lib and the filename suffix from the library are written immediately after a -l flag. So
for a libfoo.a library, -lfoo needs to be provided.) This will make the linker
search the library in a path known to the system.
An alternative would be to specify the full path to the libm.a file at the same place
on the command line, i. e. after all the object files (∗.o). However, since this requires knowledge of where the build system will exactly find those library files, this is
deprecated for system libraries.
Back to FAQ Index.

9.9.4

How to permanently bind a variable to a register?

This can be done with
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register unsigned char counter asm("r3");

Typically, it should be possible to use r2 through r15 that way.
See C Names Used in Assembler Code for more details.
Back to FAQ Index.

9.9.5

How to modify MCUCR or WDTCR early?

The method of early initialization (MCUCR, WDTCR or anything else) is different (and
more flexible) in the current version. Basically, write a small assembler file which
looks like this:
;; begin xram.S
#include 
.section .init1,"ax",@progbits
ldi r16,_BV(SRE) | _BV(SRW)
out _SFR_IO_ADDR(MCUCR),r16
;; end xram.S

Assemble it, link the resulting xram.o with other files in your program, and this piece
of code will be inserted in initialization code, which is run right after reset. See the
linker script for comments about the new .initN sections (which one to use, etc.).
The advantage of this method is that you can insert any initialization code you want
(just remember that this is very early startup – no stack and no __zero_reg__ yet),
and no program memory space is wasted if this feature is not used.
There should be no need to modify linker scripts anymore, except for some very special cases. It is best to leave __stack at its default value (end of internal SRAM
– faster, and required on some devices like ATmega161 because of errata), and add
-Wl,-Tdata,0x801100 to start the data section above the stack.
For more information on using sections, see Memory Sections. There is also an example for Using Sections in C Code. Note that in C code, any such function would
preferrably be placed into section .init3 as the code in .init2 ensures the internal register __zero_reg__ is already cleared.
Back to FAQ Index.

9.9.6

What is all this _BV() stuff about?

When performing low-level output work, which is a very central point in microcontroller programming, it is quite common that a particular bit needs to be set or cleared
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in some IO register. While the device documentation provides mnemonic names for
the various bits in the IO registers, and the AVR device-specific IO definitions reflect
these names in definitions for numerical constants, a way is needed to convert a bit
number (usually within a byte register) into a byte value that can be assigned directly
to the register. However, sometimes the direct bit numbers are needed as well (e. g. in
an SBI() instruction), so the definitions cannot usefully be made as byte values in the
first place.
So in order to access a particular bit number as a byte value, use the _BV() macro.
Of course, the implementation of this macro is just the usual bit shift (which is done
by the compiler anyway, thus doesn’t impose any run-time penalty), so the following
applies:
_BV(3) => 1 << 3 => 0x08

However, using the macro often makes the program better readable.
"BV" stands for "bit value", in case someone might ask you. :-)
Example: clock timer 2 with full IO clock (CS2x = 0b001), toggle OC2 output on
compare match (COM2x = 0b01), and clear timer on compare match (CTC2 = 1). Make
OC2 (PD7) an output.
TCCR2 = _BV(COM20)|_BV(CTC2)|_BV(CS20);
DDRD = _BV(PD7);

Back to FAQ Index.

9.9.7

Can I use C++ on the AVR?

Basically yes, C++ is supported (assuming your compiler has been configured and
compiled to support it, of course). Source files ending in .cc, .cpp or .C will automatically cause the compiler frontend to invoke the C++ compiler. Alternatively, the C++
compiler could be explicitly called by the name avr-c++.
However, there’s currently no support for libstdc++, the standard support library
needed for a complete C++ implementation. This imposes a number of restrictions on
the C++ programs that can be compiled. Among them are:
• Obviously, none of the C++ related standard functions, classes, and template
classes are available.
• The operators new and delete are not implemented, attempting to use them
will cause the linker to complain about undefined external references. (This
could perhaps be fixed.)

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• Some of the supplied include files are not C++ safe, i. e. they need to be wrapped
into
extern "C" { . . . }

(This could certainly be fixed, too.)
• Exceptions are not supported. Since exceptions are enabled by default in the
C++ frontend, they explicitly need to be turned off using -fno-exceptions
in the compiler options. Failing this, the linker will complain about an undefined
external reference to __gxx_personality_sj0.
Constructors and destructors are supported though, including global ones.
When programming C++ in space- and runtime-sensitive environments like microcontrollers, extra care should be taken to avoid unwanted side effects of the C++ calling
conventions like implied copy constructors that could be called upon function invocation etc. These things could easily add up into a considerable amount of time and
program memory wasted. Thus, casual inspection of the generated assembler code
(using the -S compiler option) seems to be warranted.
Back to FAQ Index.

9.9.8

Shouldn’t I initialize all my variables?

Global and static variables are guaranteed to be initialized to 0 by the C standard.
avr-gcc does this by placing the appropriate code into section .init4 (see The .initN
Sections). With respect to the standard, this sentence is somewhat simplified (because
the standard allows for machines where the actual bit pattern used differs from all bits
being 0), but for the AVR target, in general, all integer-type variables are set to 0, all
pointers to a NULL pointer, and all floating-point variables to 0.0.
As long as these variables are not initialized (i. e. they don’t have an equal sign and
an initialization expression to the right within the definition of the variable), they go
into the .bss section of the file. This section simply records the size of the variable,
but otherwise doesn’t consume space, neither within the object file nor within flash
memory. (Of course, being a variable, it will consume space in the target’s SRAM.)
In contrast, global and static variables that have an initializer go into the .data section
of the file. This will cause them to consume space in the object file (in order to record
the initializing value), and in the flash ROM of the target device. The latter is needed
since the flash ROM is the only way that the compiler can tell the target device the
value this variable is going to be initialized to.
Now if some programmer "wants to make doubly sure" their variables really get a 0
at program startup, and adds an initializer just containing 0 on the right-hand side,
they waste space. While this waste of space applies to virtually any platform C is
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implemented on, it’s usually not noticeable on larger machines like PCs, while the
waste of flash ROM storage can be very painful on a small microcontroller like the
AVR.
So in general, variables should only be explicitly initialized if the initial value is nonzero.
Note:
Recent versions of GCC are now smart enough to detect this situation, and revert
variables that are explicitly initialized to 0 to the .bss section. Still, other compilers
might not do that optimization, and as the C standard guarantees the initialization,
it is safe to rely on it.
Back to FAQ Index.

9.9.9

Why do some 16-bit timer registers sometimes get trashed?

Some of the timer-related 16-bit IO registers use a temporary register (called TEMP in
the Atmel datasheet) to guarantee an atomic access to the register despite the fact that
two separate 8-bit IO transfers are required to actually move the data. Typically, this
includes access to the current timer/counter value register (TCNTn), the input capture
register (ICRn), and write access to the output compare registers (OCRnM). Refer to
the actual datasheet for each device’s set of registers that involves the TEMP register.
When accessing one of the registers that use TEMP from the main application, and
possibly any other one from within an interrupt routine, care must be taken that no
access from within an interrupt context could clobber the TEMP register data of an
in-progress transaction that has just started elsewhere.
To protect interrupt routines against other interrupt routines, it’s usually best to use the
ISR() macro when declaring the interrupt function, and to ensure that interrupts are still
disabled when accessing those 16-bit timer registers.
Within the main program, access to those registers could be encapsulated in calls to the
cli() and sei() macros. If the status of the global interrupt flag before accessing one of
those registers is uncertain, something like the following example code can be used.
uint16_t
read_timer1(void)
{
uint8_t sreg;
uint16_t val;
sreg = SREG;
cli();
val = TCNT1;
SREG = sreg;
return val;
}

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Back to FAQ Index.

9.9.10

How do I use a #define’d constant in an asm statement?

So you tried this:
asm volatile("sbi 0x18,0x07;");

Which works. When you do the same thing but replace the address of the port by its
macro name, like this:
asm volatile("sbi PORTB,0x07;");

you get a compilation error: "Error:

constant value required".

PORTB is a precompiler definition included in the processor specific file included in
avr/io.h. As you may know, the precompiler will not touch strings and PORTB,
instead of 0x18, gets passed to the assembler. One way to avoid this problem is:
asm volatile("sbi %0, 0x07" : "I" (_SFR_IO_ADDR(PORTB)):);

Note:
For C programs, rather use the standard C bit operators instead, so the above would
be expressed as PORTB |= (1 << 7). The optimizer will take care to transform this into a single SBI instruction, assuming the operands allow for this.
Back to FAQ Index.

9.9.11

Why does the PC randomly jump around when single-stepping through
my program in avr-gdb?

When compiling a program with both optimization (-O) and debug information (-g)
which is fortunately possible in avr-gcc, the code watched in the debugger is optimized code. While it is not guaranteed, very often this code runs with the exact same
optimizations as it would run without the -g switch.
This can have unwanted side effects. Since the compiler is free to reorder code execution as long as the semantics do not change, code is often rearranged in order to
make it possible to use a single branch instruction for conditional operations. Branch
instructions can only cover a short range for the target PC (-63 through +64 words from
the current PC). If a branch instruction cannot be used directly, the compiler needs to
work around it by combining a skip instruction together with a relative jump (rjmp)
instruction, which will need one additional word of ROM.

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Another side effect of optimzation is that variable usage is restricted to the area of code
where it is actually used. So if a variable was placed in a register at the beginning of
some function, this same register can be re-used later on if the compiler notices that the
first variable is no longer used inside that function, even though the variable is still in
lexical scope. When trying to examine the variable in avr-gdb, the displayed result
will then look garbled.
So in order to avoid these side effects, optimization can be turned off while debugging.
However, some of these optimizations might also have the side effect of uncovering
bugs that would otherwise not be obvious, so it must be noted that turning off optimization can easily change the bug pattern. In most cases, you are better off leaving
optimizations enabled while debugging.
Back to FAQ Index.

9.9.12

How do I trace an assembler file in avr-gdb?

When using the -g compiler option, avr-gcc only generates line number and other
debug information for C (and C++) files that pass the compiler. Functions that don’t
have line number information will be completely skipped by a single step command
in gdb. This includes functions linked from a standard library, but by default also
functions defined in an assembler source file, since the -g compiler switch does not
apply to the assembler.
So in order to debug an assembler input file (possibly one that has to be passed through
the C preprocessor), it’s the assembler that needs to be told to include line-number
information into the output file. (Other debug information like data types and variable
allocation cannot be generated, since unlike a compiler, the assembler basically doesn’t
know about this.) This is done using the (GNU) assembler option -gstabs.
Example:
$ avr-as -mmcu=atmega128 --gstabs -o foo.o foo.s

When the assembler is not called directly but through the C compiler frontend
(either implicitly by passing a source file ending in .S, or explicitly using -x
assembler-with-cpp), the compiler frontend needs to be told to pass the
-gstabs option down to the assembler. This is done using -Wa,-gstabs. Please
take care to only pass this option when compiling an assembler input file. Otherwise,
the assembler code that results from the C compilation stage will also get line number
information, which confuses the debugger.
Note:
You can also use -Wa,-gstabs since the compiler will add the extra ’-’ for
you.
Example:
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$ EXTRA_OPTS="-Wall -mmcu=atmega128 -x assembler-with-cpp"
$ avr-gcc -Wa,--gstabs ${EXTRA_OPTS} -c -o foo.o foo.S

Also note that the debugger might get confused when entering a piece of code that has
a non-local label before, since it then takes this label as the name of a new function that
appears to have been entered. Thus, the best practice to avoid this confusion is to only
use non-local labels when declaring a new function, and restrict anything else to local
labels. Local labels consist just of a number only. References to these labels consist
of the number, followed by the letter b for a backward reference, or f for a forward
reference. These local labels may be re-used within the source file, references will pick
the closest label with the same number and given direction.
Example:
myfunc: push
push
push
push
push
...
eor
ldi
ldi
rjmp
1:
ld
...
breq
...
inc
2:
cmp
brlo
1:

pop
pop
pop
pop
pop
ret

r16
r17
r18
YL
YH
r16, r16
; start loop
YL, lo8(sometable)
YH, hi8(sometable)
2f
; jump to loop test at end
r17, Y+
; loop continues here
1f

; return from myfunc prematurely

r16
r16, r18
1b

; jump back to top of loop

YH
YL
r18
r17
r16

Back to FAQ Index.

9.9.13

How do I pass an IO port as a parameter to a function?

Consider this example code:
#include 
#include 
void
set_bits_func_wrong (volatile uint8_t port, uint8_t mask)
{

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port |= mask;
}
void
set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
{
*port |= mask;
}
#define set_bits_macro(port,mask) ((port) |= (mask))
int main (void)
{
set_bits_func_wrong (PORTB, 0xaa);
set_bits_func_correct (&PORTB, 0x55);
set_bits_macro (PORTB, 0xf0);
return (0);
}

The first function will generate object code which is not even close to what is intended.
The major problem arises when the function is called. When the compiler sees this call,
it will actually pass the value of the PORTB register (using an IN instruction), instead
of passing the address of PORTB (e.g. memory mapped io addr of 0x38, io port 0x18
for the mega128). This is seen clearly when looking at the disassembly of the call:
set_bits_func_wrong
10a:
6a ea
10c:
88 b3
10e:
0e 94 65 00

(PORTB,
ldi
in
call

0xaa);
r22, 0xAA
r24, 0x18
0xca

; 170
; 24

So, the function, once called, only sees the value of the port register and knows nothing
about which port it came from. At this point, whatever object code is generated for
the function by the compiler is irrelevant. The interested reader can examine the full
disassembly to see that the function’s body is completely fubar.
The second function shows how to pass (by reference) the memory mapped address of
the io port to the function so that you can read and write to it in the function. Here’s
the object code generated for the function call:
set_bits_func_correct (&PORTB, 0x55);
112:
65 e5
ldi
r22, 0x55
114:
88 e3
ldi
r24, 0x38
116:
90 e0
ldi
r25, 0x00
118:
0e 94 7c 00
call
0xf8

; 85
; 56
; 0

You can clearly see that 0x0038 is correctly passed for the address of the io port.
Looking at the disassembled object code for the body of the function, we can see that
the function is indeed performing the operation we intended:
void

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set_bits_func_correct (volatile uint8_t *port, uint8_t mask)
{
f8:
fc 01
movw
r30, r24
*port |= mask;
fa:
80 81
ld
r24, Z
fc:
86 2b
or
r24, r22
fe:
80 83
st
Z, r24
}
100:
08 95
ret

Notice that we are accessing the io port via the LD and ST instructions.
The port parameter must be volatile to avoid a compiler warning.
Note:
Because of the nature of the IN and OUT assembly instructions, they can not be
used inside the function when passing the port in this way. Readers interested in
the details should consult the Instruction Set data sheet.
Finally we come to the macro version of the operation. In this contrived example, the
macro is the most efficient method with respect to both execution speed and code size:
set_bits_macro (PORTB, 0xf0);
11c:
88 b3
in
r24, 0x18
11e:
80 6f
ori
r24, 0xF0
120:
88 bb
out
0x18, r24

; 24
; 240
; 24

Of course, in a real application, you might be doing a lot more in your function which
uses a passed by reference io port address and thus the use of a function over a macro
could save you some code space, but still at a cost of execution speed.
Care should be taken when such an indirect port access is going to one of the 16-bit
IO registers where the order of write access is critical (like some timer registers). All
versions of avr-gcc up to 3.3 will generate instructions that use the wrong access order
in this situation (since with normal memory operands where the order doesn’t matter,
this sometimes yields shorter code).
See http://mail.nongnu.org/archive/html/avr-libc-dev/2003-01/msg00044.html for a
possible workaround.
avr-gcc versions after 3.3 have been fixed in a way where this optimization will be
disabled if the respective pointer variable is declared to be volatile, so the correct
behaviour for 16-bit IO ports can be forced that way.
Back to FAQ Index.

9.9.14

What registers are used by the C compiler?

• Data types:
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char is 8 bits, int is 16 bits, long is 32 bits, long long is 64 bits, float and
double are 32 bits (this is the only supported floating point format), pointers
are 16 bits (function pointers are word addresses, to allow addressing the whole
128K program memory space on the ATmega devices with > 64 KB of flash
ROM). There is a -mint8 option (see Options for the C compiler avr-gcc) to
make int 8 bits, but that is not supported by avr-libc and violates C standards
(int must be at least 16 bits). It may be removed in a future release.
• Call-used registers (r18-r27, r30-r31):
May be allocated by gcc for local data. You may use them freely in assembler
subroutines. Calling C subroutines can clobber any of them - the caller is responsible for saving and restoring.
• Call-saved registers (r2-r17, r28-r29):
May be allocated by gcc for local data. Calling C subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these
registers, if changed. r29:r28 (Y pointer) is used as a frame pointer (points to
local data on stack) if necessary.
• Fixed registers (r0, r1):
Never allocated by gcc for local data, but often used for fixed purposes:
r0 - temporary register, can be clobbered by any C code (except interrupt handlers
which save it), may be used to remember something for a while within one piece of
assembler code
r1 - assumed to be always zero in any C code, may be used to remember something for
a while within one piece of assembler code, but must then be cleared after use (clr
r1). This includes any use of the [f]mul[s[u]] instructions, which return their
result in r1:r0. Interrupt handlers save and clear r1 on entry, and restore r1 on exit (in
case it was non-zero).
• Function call conventions:
Arguments - allocated left to right, r25 to r8. All arguments are aligned to start in
even-numbered registers (odd-sized arguments, including char, have one free
register above them). This allows making better use of the movw instruction on
the enhanced core.
If too many, those that don’t fit are passed on the stack.
Return values: 8-bit in r24 (not r25!), 16-bit in r25:r24, up to 32 bits in r22-r25, up to
64 bits in r18-r25. 8-bit return values are zero/sign-extended to 16 bits by the caller
(unsigned char is more efficient than signed char - just clr r25). Arguments to functions with variable argument lists (printf etc.) are all passed on stack, and
char is extended to int.
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Warning:
There was no such alignment before 2000-07-01, including the old patches for
gcc-2.95.2. Check your old assembler subroutines, and adjust them accordingly.
Back to FAQ Index.

9.9.15

How do I put an array of strings completely in ROM?

There are times when you may need an array of strings which will never be modified.
In this case, you don’t want to waste ram storing the constant strings. The most obvious
(and incorrect) thing to do is this:
#include 
PGM_P array[2] PROGMEM = {
"Foo",
"Bar"
};
int main (void)
{
char buf[32];
strcpy_P (buf, array[1]);
return 0;
}

The result is not what you want though. What you end up with is the array stored in
ROM, while the individual strings end up in RAM (in the .data section).
To work around this, you need to do something like this:
#include 
const char foo[] PROGMEM = "Foo";
const char bar[] PROGMEM = "Bar";
PGM_P array[2] PROGMEM = {
foo,
bar
};
int main (void)
{
char buf[32];
PGM_P p;
int i;
memcpy_P(&p, &array[i], sizeof(PGM_P));
strcpy_P(buf, p);
return 0;
}

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Looking at the disassembly of the resulting object file we see that array is in flash as
such:
00000026 :
26:
2e 00
28:
2a 00

.word
.word

0x002e
0x002a

; ????
; ????

0000002a :
2a:
42 61 72 00

Bar.

0000002e :
2e:
46 6f 6f 00

Foo.

foo is at addr 0x002e.
bar is at addr 0x002a.
array is at addr 0x0026.
Then in main we see this:
memcpy_P(&p, &array[i], sizeof(PGM_P));
70:
66 0f
add
r22, r22
72:
77 1f
adc
r23, r23
74:
6a 5d
subi
r22, 0xDA
76:
7f 4f
sbci
r23, 0xFF
78:
42 e0
ldi
r20, 0x02
7a:
50 e0
ldi
r21, 0x00
7c:
ce 01
movw
r24, r28
7e:
81 96
adiw
r24, 0x21
80:
08 d0
rcall
.+16

;
;
;
;

218
255
2
0

; 33
; 0x92

This code reads the pointer to the desired string from the ROM table array into a
register pair.
The value of i (in r22:r23) is doubled to accomodate for the word offset required to
access array[], then the address of array (0x26) is added, by subtracting the negated
address (0xffda). The address of variable p is computed by adding its offset within the
stack frame (33) to the Y pointer register, and memcpy_P is called.
strcpy_P(buf, p);
82:
69 a1
84:
7a a1
86:
ce 01
88:
01 96
8a:
0c d0

ldd
ldd
movw
adiw
rcall

r22,
r23,
r24,
r24,
.+24

Y+33
Y+34
r28
0x01

; 0x21
; 0x22
; 1
; 0xa4

This will finally copy the ROM string into the local buffer buf.
Variable p (located at Y+33) is read, and passed together with the address of buf (Y+1)
to strcpy_P. This will copy the string from ROM to buf.

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Note that when using a compile-time constant index, omitting the first step (reading
the pointer from ROM via memcpy_P) usually remains unnoticed, since the compiler
would then optimize the code for accessing array at compile-time.
Back to FAQ Index.

9.9.16

How to use external RAM?

Well, there is no universal answer to this question; it depends on what the external
RAM is going to be used for.
Basically, the bit SRE (SRAM enable) in the MCUCR register needs to be set in order
to enable the external memory interface. Depending on the device to be used, and
the application details, further registers affecting the external memory operation like
XMCRA and XMCRB, and/or further bits in MCUCR might be configured. Refer to the
datasheet for details.
If the external RAM is going to be used to store the variables from the C program
(i. e., the .data and/or .bss segment) in that memory area, it is essential to set up the
external memory interface early during the device initialization so the initialization of
these variable will take place. Refer to How to modify MCUCR or WDTCR early? for
a description how to do this using few lines of assembler code, or to the chapter about
memory sections for an example written in C.
The explanation of malloc() contains a discussion about the use of internal RAM vs.
external RAM in particular with respect to the various possible locations of the heap
(area reserved for malloc()). It also explains the linker command-line options that are
required to move the memory regions away from their respective standard locations in
internal RAM.
Finally, if the application simply wants to use the additional RAM for private data
storage kept outside the domain of the C compiler (e. g. through a char ∗ variable
initialized directly to a particular address), it would be sufficient to defer the initialization of the external RAM interface to the beginning of main(), so no tweaking of the
.init3 section is necessary. The same applies if only the heap is going to be located
there, since the application start-up code does not affect the heap.
It is not recommended to locate the stack in external RAM. In general, accessing external RAM is slower than internal RAM, and errata of some AVR devices even prevent
this configuration from working properly at all.
Back to FAQ Index.

9.9.17

Which -O flag to use?

There’s a common misconception that larger numbers behind the -O option might automatically cause "better" optimization. First, there’s no universal definition for "better",

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with optimization often being a speed vs. code size tradeoff. See the detailed discussion for which option affects which part of the code generation.
A test case was run on an ATmega128 to judge the effect of compiling the library itself
using different optimization levels. The following table lists the results. The test case
consisted of around 2 KB of strings to sort. Test #1 used qsort() using the standard
library strcmp(), test #2 used a function that sorted the strings by their size (thus had
two calls to strlen() per invocation).
When comparing the resulting code size, it should be noted that a floating point version
of fvprintf() was linked into the binary (in order to print out the time elapsed) which
is entirely not affected by the different optimization levels, and added about 2.5 KB to
the code.
Optimization
flags
-O3
-O2
-Os
-Os
-mcall-prologues

Size of .text

Time for test #1

Time for test #2

6898
6666
6618
6474

903 µs
972 µs
955 µs
972 µs

19.7 ms
20.1 ms
20.1 ms
20.1 ms

(The difference between 955 µs and 972 µs was just a single timer-tick, so take this
with a grain of salt.)
So generally, it seems -Os -mcall-prologues is the most universal "best" optimization level. Only applications that need to get the last few percent of speed benefit
from using -O3.
Back to FAQ Index.

9.9.18

How do I relocate code to a fixed address?

First, the code should be put into a new named section. This is done with a section
attribute:
__attribute__ ((section (".bootloader")))

In this example, .bootloader is the name of the new section. This attribute needs to be
placed after the prototype of any function to force the function into the new section.
void boot(void) __attribute__ ((section (".bootloader")));

To relocate the section to a fixed address the linker flag -section-start is used.
This option can be passed to the linker using the -Wl compiler option:
-Wl,--section-start=.bootloader=0x1E000

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The name after section-start is the name of the section to be relocated. The number
after the section name is the beginning address of the named section.
Back to FAQ Index.

9.9.19

My UART is generating nonsense! My ATmega128 keeps crashing! Port
F is completely broken!

Well, certain odd problems arise out of the situation that the AVR devices as shipped
by Atmel often come with a default fuse bit configuration that doesn’t match the user’s
expectations. Here is a list of things to care for:
• All devices that have an internal RC oscillator ship with the fuse enabled that
causes the device to run off this oscillator, instead of an external crystal. This
often remains unnoticed until the first attempt is made to use something critical
in timing, like UART communication.
• The ATmega128 ships with the fuse enabled that turns this device into ATmega103 compatibility mode. This means that some ports are not fully usable,
and in particular that the internal SRAM is located at lower addresses. Since by
default, the stack is located at the top of internal SRAM, a program compiled for
an ATmega128 running on such a device will immediately crash upon the first
function call (or rather, upon the first function return).
• Devices with a JTAG interface have the JTAGEN fuse programmed by default.
This will make the respective port pins that are used for the JTAG interface unavailable for regular IO.
Back to FAQ Index.

9.9.20

Why do all my "foo...bar" strings eat up the SRAM?

By default, all strings are handled as all other initialized variables: they occupy RAM
(even though the compiler might warn you when it detects write attempts to these RAM
locations), and occupy the same amount of flash ROM so they can be initialized to the
actual string by startup code. The compiler can optimize multiple identical strings into
a single one, but obviously only for one compilation unit (i. e., a single C source file).
That way, any string literal will be a valid argument to any C function that expects a
const char ∗ argument.
Of course, this is going to waste a lot of SRAM. In Program Space String Utilities, a
method is described how such constant data can be moved out to flash ROM. However, a constant string located in flash ROM is no longer a valid argument to pass to a
function that expects a const char ∗-type string, since the AVR processor needs
the special instruction LPM to access these strings. Thus, separate functions are needed

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that take this into account. Many of the standard C library functions have equivalents
available where one of the string arguments can be located in flash ROM. Private functions in the applications need to handle this, too. For example, the following can be
used to implement simple debugging messages that will be sent through a UART:
#include 
#include 
#include 
int
uart_putchar(char c)
{
if (c == ’\n’)
uart_putchar(’\r’);
loop_until_bit_is_set(USR, UDRE);
UDR = c;
return 0; /* so it could be used for fdevopen(), too */
}
void
debug_P(const char *addr)
{
char c;
while ((c = pgm_read_byte(addr++)))
uart_putchar(c);
}
int
main(void)
{
ioinit(); /* initialize UART, ... */
debug_P(PSTR("foo was here\n"));
return 0;
}

Note:
By convention, the suffix _P to the function name is used as an indication that
this function is going to accept a "program-space string". Note also the use of the
PSTR() macro.
Back to FAQ Index.

9.9.21

Why does the compiler compile an 8-bit operation that uses bitwise operators into a 16-bit operation in assembly?

Bitwise operations in Standard C will automatically promote their operands to an int,
which is (by default) 16 bits in avr-gcc.
To work around this use typecasts on the operands, including literals, to declare that
the values are to be 8 bit operands.
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This may be especially important when clearing a bit:
var &= ~mask;

/* wrong way! */

The bitwise "not" operator (∼) will also promote the value in mask to an int. To keep
it an 8-bit value, typecast before the "not" operator:
var &= (unsigned char)~mask;

Back to FAQ Index.

9.9.22

How to detect RAM memory and variable overlap problems?

You can simply run avr-nm on your output (ELF) file. Run it with the -n option, and
it will sort the symbols numerically (by default, they are sorted alphabetically).
Look for the symbol _end, that’s the first address in RAM that is not allocated by
a variable. (avr-gcc internally adds 0x800000 to all data/bss variable addresses, so
please ignore this offset.) Then, the run-time initialization code initializes the stack
pointer (by default) to point to the last avaialable address in (internal) SRAM. Thus,
the region between _end and the end of SRAM is what is available for stack. (If your
application uses malloc(), which e. g. also can happen inside printf(), the heap for
dynamic memory is also located there. See Memory Areas and Using malloc().)
The amount of stack required for your application cannot be determined that easily.
For example, if you recursively call a function and forget to break that recursion, the
amount of stack required is infinite. :-) You can look at the generated assembler code
(avr-gcc ... -S), there’s a comment in each generated assembler file that tells
you the frame size for each generated function. That’s the amount of stack required for
this function, you have to add up that for all functions where you know that the calls
could be nested.
Back to FAQ Index.

9.9.23

Is it really impossible to program the ATtinyXX in C?

While some small AVRs are not directly supported by the C compiler since they do not
have a RAM-based stack (and some do not even have RAM at all), it is possible anyway
to use the general-purpose registers as a RAM replacement since they are mapped into
the data memory region.
Bruce D. Lightner wrote an excellent description of how to do this, and offers this
together with a toolkit on his web page:
http://lightner.net/avr/ATtinyAvrGcc.html
Back to FAQ Index.

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9.9.24

309

What is this "clock skew detected" messsage?

It’s a known problem of the MS-DOS FAT file system. Since the FAT file system has
only a granularity of 2 seconds for maintaining a file’s timestamp, and it seems that
some MS-DOS derivative (Win9x) perhaps rounds up the current time to the next second when calculating the timestamp of an updated file in case the current time cannot
be represented in FAT’s terms, this causes a situation where make sees a "file coming
from the future".
Since all make decisions are based on file timestamps, and their dependencies, make
warns about this situation.
Solution: don’t use inferior file systems / operating systems. Neither Unix file systems
nor HPFS (aka NTFS) do experience that problem.
Workaround: after saving the file, wait a second before starting make. Or simply
ignore the warning. If you are paranoid, execute a make clean all to make sure
everything gets rebuilt.
In networked environments where the files are accessed from a file server, this message
can also happen if the file server’s clock differs too much from the network client’s
clock. In this case, the solution is to use a proper time keeping protocol on both systems, like NTP. As a workaround, synchronize the client’s clock frequently with the
server’s clock.
Back to FAQ Index.

9.9.25

Why are (many) interrupt flags cleared by writing a logical 1?

Usually, each interrupt has its own interrupt flag bit in some control register, indicating
the specified interrupt condition has been met by representing a logical 1 in the respective bit position. When working with interrupt handlers, this interrupt flag bit usually
gets cleared automatically in the course of processing the interrupt, sometimes by just
calling the handler at all, sometimes (e. g. for the U[S]ART) by reading a particular
hardware register that will normally happen anyway when processing the interrupt.
From the hardware’s point of view, an interrupt is asserted as long as the respective bit
is set, while global interrupts are enabled. Thus, it is essential to have the bit cleared
before interrupts get re-enabled again (which usually happens when returning from an
interrupt handler).
Only few subsystems require an explicit action to clear the interrupt request when using
interrupt handlers. (The notable exception is the TWI interface, where clearing the
interrupt indicates to proceed with the TWI bus hardware handshake, so it’s never done
automatically.)
However, if no normal interrupt handlers are to be used, or in order to make extra
sure any pending interrupt gets cleared before re-activating global interrupts (e. g.
an external edge-triggered one), it can be necessary to explicitly clear the respective

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310

hardware interrupt bit by software. This is usually done by writing a logical 1 into this
bit position. This seems to be illogical at first, the bit position already carries a logical
1 when reading it, so why does writing a logical 1 to it clear the interrupt bit?
The solution is simple: writing a logical 1 to it requires only a single OUT instruction,
and it is clear that only this single interrupt request bit will be cleared. There is no need
to perform a read-modify-write cycle (like, an SBI instruction), since all bits in these
control registers are interrupt bits, and writing a logical 0 to the remaining bits (as it
is done by the simple OUT instruction) will not alter them, so there is no risk of any
race condition that might accidentally clear another interrupt request bit. So instead of
writing
TIFR |= _BV(TOV0); /* wrong! */

simply use
TIFR = _BV(TOV0);

Back to FAQ Index.

9.9.26

Why have "programmed" fuses the bit value 0?

Basically, fuses are just a bit in a special EEPROM area. For technical reasons, erased
E[E]PROM cells have all bits set to the value 1, so unprogrammed fuses also have a
logical 1. Conversely, programmed fuse cells read out as bit value 0.
Back to FAQ Index.

9.9.27

Which AVR-specific assembler operators are available?

See Pseudo-ops and operators.
Back to FAQ Index.

9.9.28

Why are interrupts re-enabled in the middle of writing the stack pointer?

When setting up space for local variables on the stack, the compiler generates code like
this:
/* prologue: frame size=20 */
push r28
push r29
in r28,__SP_L__
in r29,__SP_H__
sbiw r28,20
in __tmp_reg__,__SREG__

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9.9

Frequently Asked Questions

cli
out
out
out
/* prologue

311

__SP_H__,r29
__SREG__,__tmp_reg__
__SP_L__,r28
end (size=10) */

It reads the current stack pointer value, decrements it by the required amount of bytes,
then disables interrupts, writes back the high part of the stack pointer, writes back
the saved SREG (which will eventually re-enable interrupts if they have been enabled
before), and finally writes the low part of the stack pointer.
At the first glance, there’s a race between restoring SREG, and writing SPL. However,
after enabling interrupts (either explicitly by setting the I flag, or by restoring it as part
of the entire SREG), the AVR hardware executes (at least) the next instruction still with
interrupts disabled, so the write to SPL is guaranteed to be executed with interrupts
disabled still. Thus, the emitted sequence ensures interrupts will be disabled only for
the minimum time required to guarantee the integrity of this operation.
Back to FAQ Index.

9.9.29

Why are there five different linker scripts?

From a comment in the source code:
Which one of the five linker script files is actually used depends on command line
options given to ld.
A .x script file is the default script A .xr script is for linking without relocation (-r flag)
A .xu script is like .xr but ∗do∗ create constructors (-Ur flag) A .xn script is for linking
with -n flag (mix text and data on same page). A .xbn script is for linking with -N flag
(mix text and data on same page).
Back to FAQ Index.

9.9.30

How to add a raw binary image to linker output?

The GNU linker avr-ld cannot handle binary data directly. However, there’s a companion tool called avr-objcopy. This is already known from the output side: it’s
used to extract the contents of the linked ELF file into an Intel Hex load file.
avr-objcopy can create a relocatable object file from arbitrary binary input, like
avr-objcopy -I binary -O elf32-avr foo.bin foo.o

This will create a file named foo.o, with the contents of foo.bin. The contents will
default to section .data, and two symbols will be created named _binary_foo_bin_start_ and _binary_foo_bin_end_. These symbols can be referred to
inside a C source to access these data.
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If the goal is to have those data go to flash ROM (similar to having used the PROGMEM
attribute in C source code), the sections have to be renamed while copying, and it’s also
useful to set the section flags:

avr-objcopy --rename-section .data=.progmem.data,contents,alloc,load,readonly,data -I binary -O elf32

Note that all this could be conveniently wired into a Makefile, so whenever foo.bin
changes, it will trigger the recreation of foo.o, and a subsequent relink of the final
ELF file.
Back to FAQ Index.

9.10

Installing the GNU Tool Chain

Note:
This discussion was taken directly from Rich Neswold’s document. (See Acknowledgments).
This discussion is Unix specific. [FIXME: troth/2002-08-13: we need a volunteer
to add windows specific notes to these instructions.]
This chapter shows how to build and install a complete development environment for
the AVR processors using the GNU toolset.
The default behaviour for most of these tools is to install every thing under the
/usr/local directory. In order to keep the AVR tools separate from the base
system, it is usually better to install everything into /usr/local/avr. If the
/usr/local/avr directory does not exist, you should create it before trying to
install anything. You will need root access to install there. If you don’t have root
access to the system, you can alternatively install in your home directory, for example, in $HOME/local/avr. Where you install is a completely arbitrary decision, but
should be consistent for all the tools.
You specify the installation directory by using the -prefix=dir option with the
configure script. It is important to install all the AVR tools in the same directory
or some of the tools will not work correctly. To ensure consistency and simplify the
discussion, we will use $PREFIX to refer to whatever directory you wish to install in.
You can set this as an environment variable if you wish as such (using a Bourne-like
shell):
$ PREFIX=$HOME/local/avr
$ export PREFIX

Note:
Be sure that you have your PATH environment variable set to search the directory you install everything in before you start installing anything. For example, if
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313

you use -prefix=$PREFIX, you must have $PREFIX/bin in your exported
PATH. As such:
$ PATH=$PATH:$PREFIX/bin
$ export PATH

Warning:
If you have CC set to anything other than avr-gcc in your environment, this will
cause the configure script to fail. It is best to not have CC set at all.
Note:
It is usually the best to use the latest released version of each of the tools.
9.10.1

Required Tools

• GNU Binutils
http://sources.redhat.com/binutils/
Installation
• GCC
http://gcc.gnu.org/
Installation
• AVR Libc
http://savannah.gnu.org/projects/avr-libc/
Installation
9.10.2

Optional Tools

You can develop programs for AVR devices without the following tools. They may or
may not be of use for you.
• uisp
http://savannah.gnu.org/projects/uisp/
Installation
• avrdude
http://savannah.nongnu.org/projects/avrdude/
Installation
Usage Notes
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• GDB
http://sources.redhat.com/gdb/
Installation
• Simulavr
http://savannah.gnu.org/projects/simulavr/
Installation
• AVaRice
http://avarice.sourceforge.net/
Installation
9.10.3

GNU Binutils for the AVR target

The binutils package provides all the low-level utilities needed in building and manipulating object files. Once installed, your environment will have an AVR assembler
(avr-as), linker (avr-ld), and librarian (avr-ar and avr-ranlib). In addition, you get tools which extract data from object files (avr-objcopy), dissassemble object file information (avr-objdump), and strip information from object files
(avr-strip). Before we can build the C compiler, these tools need to be in place.
Download and unpack the source files:
$ bunzip2 -c binutils-.tar.bz2 | tar xf $ cd binutils-

Note:
Replace  with the version of the package you downloaded.
If you obtained a gzip compressed file (.gz), use gunzip instead of bunzip2.
It is usually a good idea to configure and build binutils in a subdirectory so as not
to pollute the source with the compiled files. This is recommended by the binutils
developers.
$ mkdir obj-avr
$ cd obj-avr

The next step is to configure and build the tools. This is done by supplying arguments
to the configure script that enable the AVR-specific options.
$ ../configure --prefix=$PREFIX --target=avr --disable-nls

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If you don’t specify the -prefix option, the tools will get installed in the
/usr/local hierarchy (i.e. the binaries will get installed in /usr/local/bin,
the info pages get installed in /usr/local/info, etc.) Since these tools are changing frequently, It is preferrable to put them in a location that is easily removed.
When configure is run, it generates a lot of messages while it determines what
is available on your operating system. When it finishes, it will have created several
Makefiles that are custom tailored to your platform. At this point, you can build the
project.
$ make

Note:
BSD users should note that the project’s Makefile uses GNU make syntax.
This means FreeBSD users may need to build the tools by using gmake.
If the tools compiled cleanly, you’re ready to install them. If you specified a destination
that isn’t owned by your account, you’ll need root access to install them. To install:
$ make install

You should now have the programs from binutils installed into $PREFIX/bin. Don’t
forget to set your PATH environment variable before going to build avr-gcc.
Note:
The official version of binutils might lack support for recent AVR
devices.
A patch that adds more AVR types can be found at
http://www.freebsd.org/cgi/cvsweb.cgi/ports/devel/avr-binutils/files/patchnewdevices
9.10.4

GCC for the AVR target

Warning:
You must install avr-binutils and make sure your path is set properly before installing avr-gcc.
The steps to build avr-gcc are essentially same as for binutils:
$
$
$
$
$

bunzip2 -c gcc-.tar.bz2 | tar xf cd gcc-
mkdir obj-avr
cd obj-avr
../configure --prefix=$PREFIX --target=avr --enable-languages=c,c++ \
--disable-nls --disable-libssp --with-dwarf2
$ make
$ make install

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To save your self some download time, you can alternatively download only the
gcc-core-.tar.bz2 and gcc-c++-.tar.bz2
parts of the gcc. Also, if you don’t need C++ support, you only need the core part
and should only enable the C language support.
Note:
Early versions of these tools did not support C++.
The stdc++ libs are not included with C++ for AVR due to the size limitations of
the devices.
The official version of GCC might lack support for recent AVR
devices.
A patch that adds more AVR types can be found at
http://www.freebsd.org/cgi/cvsweb.cgi/ports/devel/avr-gcc/files/patch-newdevices
9.10.5

AVR Libc

Warning:
You must install avr-binutils, avr-gcc and make sure your path is set properly
before installing avr-libc.
Note:
If you have obtained the latest avr-libc from cvs, you will have to run the
bootstrap script before using either of the build methods described below.
To build and install avr-libc:
$
$
$
$
$

gunzip -c avr-libc-.tar.gz | tar xf cd avr-libc-
./configure --prefix=$PREFIX --build=‘./config.guess‘ --host=avr
make
make install

9.10.6

Avrdude

Note:
It has been ported to windows (via MinGW or cygwin), Linux and Solaris. Other
Unix systems should be trivial to port to.
avrdude is part of the FreeBSD ports system. To install it, simply do the following:
# cd /usr/ports/devel/avrdude
# make install

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317

Note:
Installation into the default location usually requires root permissions. However,
running the program only requires access permissions to the appropriate ppi(4)
device.
Building and installing on other systems should use the configure system, as such:
$
$
$
$
$
$
$

gunzip -c avrdude-.tar.gz | tar xf cd avrdude-
mkdir obj-avr
cd obj-avr
../configure --prefix=$PREFIX
make
make install

9.10.7

GDB for the AVR target

Gdb also uses the configure system, so to build and install:
$
$
$
$
$
$
$

bunzip2 -c gdb-.tar.bz2 | tar xf cd gdb-
mkdir obj-avr
cd obj-avr
../configure --prefix=$PREFIX --target=avr
make
make install

Note:
If you are planning on using avr-gdb, you will probably want to install either
simulavr or avarice since avr-gdb needs one of these to run as a a remote target
backend.
9.10.8

Simulavr

Simulavr also uses the configure system, so to build and install:
$
$
$
$
$
$
$

gunzip -c simulavr-.tar.gz | tar xf cd simulavr-
mkdir obj-avr
cd obj-avr
../configure --prefix=$PREFIX
make
make install

Note:
You might want to have already installed avr-binutils, avr-gcc and avr-libc if you
want to have the test programs built in the simulavr source.

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Using the GNU tools

9.10.9

318

AVaRice

Note:
These install notes are not applicable to avarice-1.5 or older. You probably don’t
want to use anything that old anyways since there have been many improvements
and bug fixes since the 1.5 release.
AVaRice also uses the configure system, so to build and install:
$
$
$
$
$
$
$

gunzip -c avarice-.tar.gz | tar xf cd avarice-
mkdir obj-avr
cd obj-avr
../configure --prefix=$PREFIX
make
make install

Note:
AVaRice uses the bfd library for accessing various binary file formats. You may
need to tell the configure script where to find the lib and headers for the link to
work. This is usually done by invoking the configure script like this (Replace
 with the path to the bfd.h file on your system. Replace  with the path to libbfd.a on your system.):
$ CPPFLAGS=-I LDFLAGS=-L ../configure --prefix=$PREFIX

9.11

Using the GNU tools

This is a short summary of the AVR-specific aspects of using the GNU tools. Normally,
the generic documentation of these tools is fairly large and maintained in texinfo
files. Command-line options are explained in detail in the manual page.
9.11.1

Options for the C compiler avr-gcc

9.11.1.1 Machine-specific options for the AVR The following machine-specific
options are recognized by the C compiler frontend.
• -mmcu=architecture
Compile code for architecture. Currently known architectures are
Architecture
avr1

Macros
__AVR_ARCH__=1__AVR_ASM_ONLY____AVR_2_BYTE_PC__ [2]

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Architecture
avr2
avr25 [1]
avr3
avr4
avr5
avr6 [2]

319

Macros
__AVR_ARCH__=2__AVR_2_BYTE_PC__ [2]
__AVR_ARCH__=25__AVR_HAVE_MOVW__ [1]__AVR_HAVE_LPMX__ [1]__AVR_2_BYTE_PC
__AVR_ARCH__=3__AVR_MEGA____AVR_2_BYTE_PC__ [2]
__AVR_ARCH__=4__AVR_ENHANCED____AVR_HAVE_MOVW__ [1]__AVR_HAVE_LPMX__
__AVR_ARCH__=5__AVR_MEGA____AVR_ENHANCED____AVR_HAVE_MOVW__ [1]__AVR_
__AVR_ARCH__=6__AVR_MEGA____AVR_ENHANCED____AVR_HAVE_MOVW__ [1]__AVR_

[1] New in GCC 4.2
[2] Unofficial patch for GCC 4.1
By default, code is generated for the avr2 architecture.
Note that when only using -mmcu=architecture but no -mmcu=MCU type, including
the file  cannot work since it cannot decide which device’s definitions
to select.
• -mmcu=MCU type
The following MCU types are currently understood by avr-gcc. The table matches
them against the corresponding avr-gcc architecture name, and shows the preprocessor
symbol declared by the -mmcu option.
Architecture
avr1
avr1
avr1
avr1
avr1
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]

MCU name
at90s1200
attiny11
attiny12
attiny15
attiny28
at90s2313
at90s2323
at90s2333
at90s2343
attiny22
attiny26
at90s4414
at90s4433
at90s4434
at90s8515
at90c8534
at90s8535
at86rf401
attiny13
attiny2313

Macro
__AVR_AT90S1200__
__AVR_ATtiny11__
__AVR_ATtiny12__
__AVR_ATtiny15__
__AVR_ATtiny28__
__AVR_AT90S2313__
__AVR_AT90S2323__
__AVR_AT90S2333__
__AVR_AT90S2343__
__AVR_ATtiny22__
__AVR_ATtiny26__
__AVR_AT90S4414__
__AVR_AT90S4433__
__AVR_AT90S4434__
__AVR_AT90S8515__
__AVR_AT90C8534__
__AVR_AT90S8535__
__AVR_AT86RF401__
__AVR_ATtiny13__
__AVR_ATtiny2313__

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Using the GNU tools

Architecture
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr2/avr25 [1]
avr3
avr3
avr3
avr3
avr3
avr4
avr4
avr4
avr4
avr4
avr4
avr4
avr4
avr4
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5

320

MCU name
attiny24
attiny25
attiny261
attiny44
attiny45
attiny461
attiny84
attiny85
attiny861
atmega103
atmega603
at43usb320
at43usb355
at76c711
atmega48
atmega8
atmega8515
atmega8535
atmega88
atmega8hva
at90pwm1
at90pwm2
at90pwm3
at90can32
at90can64
at90can128
at90usb82
at90usb162
at90usb646
at90usb647
at90usb1286
at90usb1287
atmega128
atmega1280
atmega1281
atmega16
atmega161
atmega162
atmega163
atmega164p
atmega165
atmega165p
atmega168

Macro
__AVR_ATtiny24__
__AVR_ATtiny25__
__AVR_ATtiny261__
__AVR_ATtiny44__
__AVR_ATtiny45__
__AVR_ATtiny461__
__AVR_ATtiny84__
__AVR_ATtiny85__
__AVR_ATtiny861__
__AVR_ATmega103__
__AVR_ATmega603__
__AVR_AT43USB320__
__AVR_AT43USB355__
__AVR_AT76C711__
__AVR_ATmega48__
__AVR_ATmega8__
__AVR_ATmega8515__
__AVR_ATmega8535__
__AVR_ATmega88__
__AVR_ATmega8HVA__
__AVR_AT90PWM1__
__AVR_AT90PWM2__
__AVR_AT90PWM3__
__AVR_AT90CAN32__
__AVR_AT90CAN64__
__AVR_AT90CAN128__
__AVR_AT90USB82__
__AVR_AT90USB162__
__AVR_AT90USB646__
__AVR_AT90USB647__
__AVR_AT90USB1286__
__AVR_AT90USB1287__
__AVR_ATmega128__
__AVR_ATmega1280__
__AVR_ATmega1281__
__AVR_ATmega16__
__AVR_ATmega161__
__AVR_ATmega162__
__AVR_ATmega163__
__AVR_ATmega164P__
__AVR_ATmega165__
__AVR_ATmega165P__
__AVR_ATmega168__

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9.11

Using the GNU tools

Architecture
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr5
avr6
avr6

321

MCU name
atmega169
atmega169p
atmega16hva
atmega32
atmega323
atmega324p
atmega325
atmega325p
atmega3250
atmega3250p
atmega329
atmega329p
atmega3290
atmega3290p
atmega406
atmega64
atmega640
atmega644
atmega644p
atmega645
atmega6450
atmega649
atmega6490
at94k
atmega2560
atmega2561

Macro
__AVR_ATmega169__
__AVR_ATmega169P__
__AVR_ATmega16HVA__
__AVR_ATmega32__
__AVR_ATmega323__
__AVR_ATmega324P__
__AVR_ATmega325__
__AVR_ATmega325P__
__AVR_ATmega3250__
__AVR_ATmega3250P__
__AVR_ATmega329__
__AVR_ATmega329P__
__AVR_ATmega3290__
__AVR_ATmega3290P__
__AVR_ATmega406__
__AVR_ATmega64__
__AVR_ATmega640__
__AVR_ATmega644__
__AVR_ATmega644P__
__AVR_ATmega645__
__AVR_ATmega6450__
__AVR_ATmega649__
__AVR_ATmega6490__
__AVR_AT94K__
__AVR_ATmega2560__
__AVR_ATmega2561__

[1] ’avr25’ architecture is new in GCC 4.2
• -morder1
• -morder2
Change the order of register assignment. The default is
r24, r25, r18, r19, r20, r21, r22, r23, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13,
r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r0, r1
Order 1 uses
r18, r19, r20, r21, r22, r23, r24, r25, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13,
r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r0, r1
Order 2 uses
r25, r24, r23, r22, r21, r20, r19, r18, r30, r31, r26, r27, r28, r29, r17, r16, r15, r14, r13,
r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r1, r0
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• -mint8
Assume int to be an 8-bit integer. Note that this is not really supported by
avr-libc, so it should normally not be used. The default is to use 16-bit integers.
• -mno-interrupts
Generates code that changes the stack pointer without disabling interrupts. Normally,
the state of the status register SREG is saved in a temporary register, interrupts are
disabled while changing the stack pointer, and SREG is restored.
• -mcall-prologues
Use subroutines for function prologue/epilogue. For complex functions that use many
registers (that needs to be saved/restored on function entry/exit), this saves some space
at the cost of a slightly increased execution time.
• -minit-stack=nnnn
Set the initial stack pointer to nnnn. By default, the stack pointer is initialized to the
symbol __stack, which is set to RAMEND by the run-time initialization code.
• -mtiny-stack
Change only the low 8 bits of the stack pointer.
• -mno-tablejump
Do not generate tablejump instructions. By default, jump tables can be used to optimize switch statements. When turned off, sequences of compare statements are
used instead. Jump tables are usually faster to execute on average, but in particular for
switch statements where most of the jumps would go to the default label, they might
waste a bit of flash memory.
• -mshort-calls
Use rjmp/rcall (limited range) on >8K devices. On avr2 and avr4 architectures (less than 8 KB or flash memory), this is always the case. On avr3 and avr5
architectures, calls and jumps to targets outside the current function will by default use
jmp/call instructions that can cover the entire address range, but that require more
flash ROM and execution time.
• -mrtl
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Dump the internal compilation result called "RTL" into comments in the generated
assembler code. Used for debugging avr-gcc.
• -msize
Dump the address, size, and relative cost of each statement into comments in the generated assembler code. Used for debugging avr-gcc.
• -mdeb
Generate lots of debugging information to stderr.
9.11.1.2 Selected general compiler options The following general gcc options
might be of some interest to AVR users.
• -On
Optimization level n. Increasing n is meant to optimize more, an optimization level of
0 means no optimization at all, which is the default if no -O option is present. The
special option -Os is meant to turn on all -O2 optimizations that are not expected to
increase code size.
Note that at -O3, gcc attempts to inline all "simple" functions. For the AVR target,
this will normally constitute a large pessimization due to the code increasement. The
only other optimization turned on with -O3 is -frename-registers, which could
rather be enabled manually instead.
A simple -O option is equivalent to -O1.
Note also that turning off all optimizations will prevent some warnings from being
issued since the generation of those warnings depends on code analysis steps that are
only performed when optimizing (unreachable code, unused variables).
See also the appropriate FAQ entry for issues regarding debugging optimized code.
• -Wa,assembler-options
• -Wl,linker-options
Pass the listed options to the assembler, or linker, respectively.
• -g
Generate debugging information that can be used by avr-gdb.

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• -ffreestanding
Assume a "freestanding" environment as per the C standard. This turns off automatic
builtin functions (though they can still be reached by prepending __builtin_ to
the actual function name). It also makes the compiler not complain when main()
is declared with a void return type which makes some sense in a microcontroller
environment where the application cannot meaningfully provide a return value to its
environment (in most cases, main() won’t even return anyway). However, this also
turns off all optimizations normally done by the compiler which assume that functions
known by a certain name behave as described by the standard. E. g., applying the
function strlen() to a literal string will normally cause the compiler to immediately
replace that call by the actual length of the string, while with -ffreestanding, it
will always call strlen() at run-time.
• -funsigned-char
Make any unqualfied char type an unsigned char. Without this option, they default to
a signed char.
• -funsigned-bitfields
Make any unqualified bitfield type unsigned. By default, they are signed.
• -fshort-enums
Allocate to an enum type only as many bytes as it needs for the declared range of
possible values. Specifically, the enum type will be equivalent to the smallest integer
type which has enough room.
• -fpack-struct
Pack all structure members together without holes.
9.11.2
9.11.2.1

Options for the assembler avr-as
Machine-specific assembler options

• -mmcu=architecture
• -mmcu=MCU name
avr-as understands the same -mmcu= options as avr-gcc. By default, avr2 is assumed,
but this can be altered by using the appropriate .arch pseudo-instruction inside the
assembler source file.
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9.11

Using the GNU tools

325

• -mall-opcodes
Turns off opcode checking for the actual MCU type, and allows any possible AVR
opcode to be assembled.
• -mno-skip-bug
Don’t emit a warning when trying to skip a 2-word instruction with a
CPSE/SBIC/SBIS/SBRC/SBRS instruction. Early AVR devices suffered from a
hardware bug where these instructions could not be properly skipped.
• -mno-wrap
For RJMP/RCALL instructions, don’t allow the target address to wrap around for devices that have more than 8 KB of memory.
• -gstabs
Generate .stabs debugging symbols for assembler source lines. This enables avr-gdb
to trace through assembler source files. This option must not be used when assembling
sources that have been generated by the C compiler; these files already contain the
appropriate line number information from the C source files.
• -a[cdhlmns=file]
Turn on the assembler listing. The sub-options are:
• c omit false conditionals
• d omit debugging directives
• h include high-level source
• l include assembly
• m include macro expansions
• n omit forms processing
• s include symbols
• =file set the name of the listing file
The various sub-options can be combined into a single -a option list; =file must be the
last one in that case.
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326

9.11.2.2 Examples for assembler options passed through the C compiler Remember that assembler options can be passed from the C compiler frontend using -Wa
(see above), so in order to include the C source code into the assembler listing in
file foo.lst, when compiling foo.c, the following compiler command-line can be
used:
$ avr-gcc -c -O foo.c -o foo.o -Wa,-ahls=foo.lst

In order to pass an assembler file through the C preprocessor first, and have the assembler generate line number debugging information for it, the following command can be
used:
$ avr-gcc -c -x assembler-with-cpp -o foo.o foo.S -Wa,--gstabs

Note that on Unix systems that have case-distinguishing file systems, specifying a file
name with the suffix .S (upper-case letter S) will make the compiler automatically
assume -x assembler-with-cpp, while using .s would pass the file directly to
the assembler (no preprocessing done).
9.11.3

Controlling the linker avr-ld

9.11.3.1 Selected linker options While there are no machine-specific options for
avr-ld, a number of the standard options might be of interest to AVR users.
• -lname
Locate the archive library named libname.a, and use it to resolve currently
unresolved symbols from it. The library is searched along a path that consists of builtin pathname entries that have been specified at compile time (e. g.
/usr/local/avr/lib on Unix systems), possibly extended by pathname entries
as specified by -L options (that must precede the -l options on the command-line).
• -Lpath
Additional location to look for archive libraries requested by -l options.
• -defsym symbol=expr
Define a global symbol symbol using expr as the value.
• -M
Print a linker map to stdout.
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327

• -Map mapfile
Print a linker map to mapfile.
• -cref
Output a cross reference table to the map file (in case -Map is also present), or to
stdout.
• -section-start sectionname=org
Start section sectionname at absolute address org.
• -Tbss org
• -Tdata org
• -Ttext org
Start the bss, data, or text section at org, respectively.
• -T scriptfile
Use scriptfile as the linker script, replacing the default linker script.
Default linker scripts are stored in a system-specific location (e.
g.
under
/usr/local/avr/lib/ldscripts on Unix systems), and consist of the AVR
architecture name (avr2 through avr5) with the suffix .x appended. They describe how
the various memory sections will be linked together.
9.11.3.2 Passing linker options from the C compiler By default, all unknown
non-option arguments on the avr-gcc command-line (i. e., all filename arguments that
don’t have a suffix that is handled by avr-gcc) are passed straight to the linker. Thus,
all files ending in .o (object files) and .a (object libraries) are provided to the linker.
System libraries are usually not passed by their explicit filename but rather using the
-l option which uses an abbreviated form of the archive filename (see above). avrlibc ships two system libraries, libc.a, and libm.a. While the standard library
libc.a will always be searched for unresolved references when the linker is started
using the C compiler frontend (i. e., there’s always at least one implied -lc option),
the mathematics library libm.a needs to be explicitly requested using -lm. See also
the entry in the FAQ explaining this.
Conventionally, Makefiles use the make macro LDLIBS to keep track of -l (and
possibly -L) options that should only be appended to the C compiler command-line

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Using the avrdude program

328

when linking the final binary. In contrast, the macro LDFLAGS is used to store other
command-line options to the C compiler that should be passed as options during the
linking stage. The difference is that options are placed early on the command-line,
while libraries are put at the end since they are to be used to resolve global symbols
that are still unresolved at this point.
Specific linker flags can be passed from the C compiler command-line using the -Wl
compiler option, see above. This option requires that there be no spaces in the appended
linker option, while some of the linker options above (like -Map or -defsym) would
require a space. In these situations, the space can be replaced by an equal sign as
well. For example, the following command-line can be used to compile foo.c into an
executable, and also produce a link map that contains a cross-reference list in the file
foo.map:
$ avr-gcc -O -o foo.out -Wl,-Map=foo.map -Wl,--cref foo.c

Alternatively, a comma as a placeholder will be replaced by a space before passing the
option to the linker. So for a device with external SRAM, the following command-line
would cause the linker to place the data segment at address 0x2000 in the SRAM:
$ avr-gcc -mmcu=atmega128 -o foo.out -Wl,-Tdata,0x802000

See the explanation of the data section for why 0x800000 needs to be added to the actual value. Note that unless a -minit-stack option has been given when compiling
the C source file that contains the function main(), the stack will still remain in internal RAM, through the symbol __stack that is provided by the run-time startup code.
This is probably a good idea anyway (since internal RAM access is faster), and even
required for some early devices that had hardware bugs preventing them from using
a stack in external RAM. Note also that the heap for malloc() will still be placed
after all the variables in the data section, so in this situation, no stack/heap collision
can occur.

9.12

Using the avrdude program

Note:
This section was contributed by Brian Dean [ bsd@bsdhome.com ].
The avrdude program was previously called avrprog. The name was changed to
avoid confusion with the avrprog program that Atmel ships with AvrStudio.
avrdude is a program that is used to update or read the flash and EEPROM memories
of Atmel AVR microcontrollers on FreeBSD Unix. It supports the Atmel serial programming protocol using the PC’s parallel port and can upload either a raw binary file
or an Intel Hex format file. It can also be used in an interactive mode to individually
update EEPROM cells, fuse bits, and/or lock bits (if their access is supported by the
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Using the avrdude program

329

Atmel serial programming protocol.) The main flash instruction memory of the AVR
can also be programmed in interactive mode, however this is not very useful because
one can only turn bits off. The only way to turn flash bits on is to erase the entire
memory (using avrdude’s -e option).
avrdude is part of the FreeBSD ports system. To install it, simply do the following:
# cd /usr/ports/devel/avrdude
# make install

Once installed, avrdude can program processors using the contents of the .hex file
specified on the command line. In this example, the file main.hex is burned into the
flash memory:
# avrdude -p 2313 -e -m flash -i main.hex
avrdude: AVR device initialized and ready to accept instructions
avrdude: Device signature = 0x1e9101
avrdude:
avrdude:
avrdude:
avrdude:

erasing chip
done.
reading input file "main.hex"
input file main.hex auto detected as Intel Hex

avrdude: writing flash:
1749 0x00
avrdude: 1750 bytes of flash written
avrdude: verifying flash memory against main.hex:
avrdude: reading on-chip flash data:
1749 0x00
avrdude: verifying ...
avrdude: 1750 bytes of flash verified
avrdude done.

Thank you.

The -p 2313 option lets avrdude know that we are operating on an AT90S2313
chip. This option specifies the device id and is matched up with the device of the same
id in avrdude’s configuration file ( /usr/local/etc/avrdude.conf ). To list
valid parts, specify the -v option. The -e option instructs avrdude to perform a
chip-erase before programming; this is almost always necessary before programming
the flash. The -m flash option indicates that we want to upload data into the flash
memory, while -i main.hex specifies the name of the input file.
The EEPROM is uploaded in the same way, the only difference is that you would use
-m eeprom instead of -m flash.
To use interactive mode, use the -t option:
# avrdude -p 2313 -t
avrdude: AVR device initialized and ready to accept instructions

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Release Numbering and Methodology

330

avrdude: Device signature = 0x1e9101
avrdude>
The ’?’ command displays a list of valid
commands:
avrdude> ?
>>> ?
Valid commands:
dump
read
write
erase
sig
part
send
help
?
quit

:
:
:
:
:
:
:
:
:
:

dump memory : dump   
alias for dump
write memory : write     ... 
perform a chip erase
display device signature bytes
display the current part information
send a raw command : send    
help
help
quit

Use the ’part’ command to display valid memory types for use with the
’dump’ and ’write’ commands.
avrdude>

9.13

Release Numbering and Methodology

9.13.1

Release Version Numbering Scheme

9.13.1.1 Stable Versions A stable release will always have a minor number that is
an even number. This implies that you should be able to upgrade to a new version of
the library with the same major and minor numbers without fear that any of the APIs
have changed. The only changes that should be made to a stable branch are bug fixes
and under some circumstances, additional functionality (e.g. adding support for a new
device).
If major version number has changed, this implies that the required versions of gcc and
binutils have changed. Consult the README file in the toplevel directory of the AVR
Libc source for which versions are required.
9.13.1.2 Development Versions The major version number of a development series is always the same as the last stable release.
The minor version number of a development series is always an odd number and is 1
more than the last stable release.
The patch version number of a development series is always 0 until a new branch is cut
at which point the patch number is changed to 90 to denote the branch is approaching
a release and the date appended to the version to denote that it is still in development.

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331

All versions in development in cvs will also always have the date appended as a fourth
version number. The format of the date will be YYYYMMDD.
So, the development version number will look like this:
1.1.0.20030825

While a pre-release version number on a branch (destined to become either 1.2 or 2.0)
will look like this:
1.1.90.20030828

9.13.2

Releasing AVR Libc

The information in this section is only relevant to AVR Libc developers and can be
ignored by end users.
Note:
In what follows, I assume you know how to use cvs and how to checkout multiple
source trees in a single directory without having them clobber each other. If you
don’t know how to do this, you probably shouldn’t be making releases or cutting
branches.
9.13.2.1 Creating a cvs branch The following steps should be taken to cut a
branch in cvs:
1. Check out a fresh source tree from cvs HEAD.
2. Update the NEWS file with pending release number and commit to cvs HEAD:
Change "Changes since avr-libc-:" to "Changes in avr-libc:".
3. Set the branch-point tag (setting  and  accordingly):
’cvs tag avr-libc-_-branchpoint’
4. Create the branch:
’cvs tag -b avr-libc-_-branch’
5. Update the package version in configure.ac and commit configure.ac to cvs
HEAD:
Change minor number to next odd value.
6. Update the NEWS file and commit to cvs HEAD:
Add "Changes since avr-libc-:"
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332

7. Check out a new tree for the branch:
’cvs co -r avr-libc-_-branch’
8. Update the package version in configure.ac and commit configure.ac to cvs
branch:
Change the patch number to 90 to denote that this now a branch leading up to a
release. Be sure to leave the  part of the version.
9. Bring the build system up to date by running bootstrap and configure.
10. Perform a ’make distcheck’ and make sure it succeeds. This will create the
snapshot source tarball. This should be considered the first release candidate.
11. Upload the snapshot tarball to savannah.
12. Announce the branch and the branch tag to the avr-libc-dev list so other developers can checkout the branch.
Note:
CVS tags do not allow the use of periods (’.’).
9.13.2.2 Making a release
the cvs HEAD.

A stable release will only be done on a branch, not from

The following steps should be taken when making a release:
1. Make sure the source tree you are working from is on the correct branch:
’cvs update -r avr-libc-_-branch’
2. Update the package version in configure.ac and commit it to cvs.
3. Update the gnu tool chain version requirements in the README and commit to
cvs.
4. Update the ChangeLog file to note the release and commit to cvs on the branch:
Add "Released avr-libc-."
5. Update the NEWS file with pending release number and commit to cvs:
Change "Changes since avr-libc-:" to "Changes in avr-libc:".
6. Bring the build system up to date by running bootstrap and configure.
7. Perform a ’make distcheck’ and make sure it succeeds. This will create the
source tarball.
8. Tag the release:
’cvs tag avr-libc-__-release’
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9.14

Acknowledgments

333

9. Upload the tarball to savannah.
10. Update the NEWS file, and commit to cvs:
Add "Changes since avr-libc-__:"
11. Generate the latest documentation and upload to savannah.
12. Announce the release.
The following hypothetical diagram should help clarify version and branch relationships.

HEAD

1.0 Branch

1.2 Branch

cvs tag avr−libc−1_0−branchpoint

set version to 1.1.0.
cvs tag −b avr−libc−1_0−branch

set version to 0.90.90.
set version to 1.0
cvs tag avr−libc−1_0−release

set version to 1.0.0.
set version to 1.0.1
cvs tag avr−libc−1_0_1−release
cvs tag avr−libc−1_2−branchpoint

set version to 1.3.0.

cvs tag −b avr−libc−1_2−branch

set version to 1.1.90.
set version to 1.2
cvs tag avr−libc−1_2−release

cvs tag avr−libc−2.0−branchpoint

set version to 2.1.0.

Figure 9: Release tree

9.14

Acknowledgments

This document tries to tie together the labors of a large group of people. Without
these individuals’ efforts, we wouldn’t have a terrific, free set of tools to develop AVR
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9.15

Todo List

334

projects. We all owe thanks to:
• The GCC Team, which produced a very capable set of development tools for an
amazing number of platforms and processors.
• Denis Chertykov [ denisc@overta.ru ] for making the AVR-specific changes to
the GNU tools.
• Denis Chertykov and Marek Michalkiewicz [ marekm@linux.org.pl ] for developing the standard libraries and startup code for AVR-GCC.
• Uros Platise for developing the AVR programmer tool, uisp.
• Joerg Wunsch [ joerg@FreeBSD.ORG ] for adding all the AVR development
tools to the FreeBSD [ http://www.freebsd.org ] ports tree and for providing the
basics for the demo project.
• Brian Dean [ bsd@bsdhome.com ] for developing avrdude (an alternative to
uisp) and for contributing documentation which describes how to use it. Avrdude was previously called avrprog.
• Eric Weddington [ eric@evcohs.com ] for maintaining the WinAVR package
and thus making the continued improvements to the Opensource AVR toolchain
available to many users.
• Rich Neswold for writing the original avr-tools document (which he graciously
allowed to be merged into this document) and his improvements to the demo
project.
• Theodore A. Roth for having been a long-time maintainer of many of the tools
(AVR-Libc, the AVR port of GDB, AVaRICE, uisp, avrdude).
• All the people who currently maintain the tools, and/or have submitted suggestions, patches and bug reports. (See the AUTHORS files of the various tools.)
• And lastly, all the users who use the software. If nobody used the software, we
would probably not be very motivated to continue to develop it. Keep those bug
reports coming. ;-)

9.15

Todo List

Group avr_boot From email with Marek: On smaller devices (all except ATmega64/128), __SPM_REG is in the I/O space, accessible with the shorter "in"
and "out" instructions - since the boot loader has a limited size, this could be an
important optimization.

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9.16

9.16

Deprecated List

Deprecated List

Global SIGNAL Do not use SIGNAL() in new code. Use ISR() instead.

Global enable_external_int

Global INTERRUPT

Global timer_enable_int

Global inp

Global outp

Global inb

Global outb

Global sbi

Global cbi

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335

Index
$PATH, 312
$PREFIX, 312
–prefix, 312
: Diagnostics, 13
: Bootloader Support Utilities, 88
: EEPROM handling, 94
: Interrupts, 97
: Program Space Utilities, 116
: Power Reduction Management, 128
: Special function registers, 133
: Power Management and
Sleep Modes, 136
:
avr-libc version
macros, 138
: Watchdog timer handling,
140
:
Deprecated
items, 154
: Compatibility with
IAR EWB 3.x, 157
: Character Operations, 14
: System Errors, 16
: Integer Type conversions,
17
: Mathematics, 29
: Non-local goto, 33
: Standard Integer Types, 35
: Standard IO facilities, 47
: General utilities, 66
: Strings, 77
: CRC Computations, 143
: Convenience functions
for busy-wait delay loops, 147
: Basic busy-wait
delay loops, 148
: Parity bit generation, 149
: TWI bit mask definitions,
150

_BV
avr_sfr, 134
_EEGET
avr_eeprom, 96
_EEPUT
avr_eeprom, 96
_FDEV_EOF
avr_stdio, 52
_FDEV_ERR
avr_stdio, 52
_FDEV_SETUP_READ
avr_stdio, 52
_FDEV_SETUP_RW
avr_stdio, 52
_FDEV_SETUP_WRITE
avr_stdio, 52
_FFS
avr_string, 78
__AVR_LIBC_DATE_
avr_version, 139
__AVR_LIBC_DATE_STRING__
avr_version, 139
__AVR_LIBC_MAJOR__
avr_version, 139
__AVR_LIBC_MINOR__
avr_version, 139
__AVR_LIBC_REVISION__
avr_version, 139
__AVR_LIBC_VERSION_STRING__
avr_version, 139
__AVR_LIBC_VERSION__
avr_version, 139
__EEPROM_REG_LOCATIONS__
avr_eeprom, 95
__ELPM_classic__
pgmspace.h, 218
__ELPM_dword_enhanced__
pgmspace.h, 218
__ELPM_enhanced__
pgmspace.h, 219
__ELPM_word_classic__
pgmspace.h, 219

INDEX

__ELPM_word_enhanced__
pgmspace.h, 220
__LPM_classic__
pgmspace.h, 220
__LPM_dword_classic__
pgmspace.h, 220
__LPM_dword_enhanced__
pgmspace.h, 221
__LPM_enhanced__
pgmspace.h, 221
__LPM_word_classic__
pgmspace.h, 222
__LPM_word_enhanced__
pgmspace.h, 222
__boot_lock_bits_set
boot.h, 200
__boot_lock_bits_set_alternate
boot.h, 200
__boot_page_erase_alternate
boot.h, 201
__boot_page_erase_extended
boot.h, 201
__boot_page_erase_normal
boot.h, 201
__boot_page_fill_alternate
boot.h, 202
__boot_page_fill_extended
boot.h, 202
__boot_page_fill_normal
boot.h, 203
__boot_page_write_alternate
boot.h, 203
__boot_page_write_extended
boot.h, 203
__boot_page_write_normal
boot.h, 204
__boot_rww_enable
boot.h, 204
__boot_rww_enable_alternate
boot.h, 204
__compar_fn_t
avr_stdlib, 68
__malloc_heap_end
avr_stdlib, 76
__malloc_heap_start
avr_stdlib, 76

337

__malloc_margin
avr_stdlib, 77
_crc16_update
util_crc, 144
_crc_ccitt_update
util_crc, 145
_crc_ibutton_update
util_crc, 145
_crc_xmodem_update
util_crc, 146
_delay_loop_1
util_delay_basic, 148
_delay_loop_2
util_delay_basic, 148
_delay_ms
util_delay, 147
_delay_us
util_delay, 148
_wdt_write
wdt.h, 244
A more sophisticated project, 177
A simple project, 162
abort
avr_stdlib, 68
abs
avr_stdlib, 68
acos
avr_math, 30
Additional notes from ,
131
asin
avr_math, 30
assert
avr_assert, 13
assert.h, 198
atan
avr_math, 30
atan2
avr_math, 31
atof
avr_stdlib, 68
atoi
avr_stdlib, 69
atoi.S, 198
atol

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INDEX

avr_stdlib, 69
atol.S, 198
avr_assert
assert, 13
avr_boot
boot_is_spm_interrupt, 90
boot_lock_bits_set, 90
boot_lock_bits_set_safe, 90
boot_lock_fuse_bits_get, 91
boot_page_erase, 91
boot_page_erase_safe, 91
boot_page_fill, 92
boot_page_fill_safe, 92
boot_page_write, 92
boot_page_write_safe, 92
boot_rww_busy, 93
boot_rww_enable, 93
boot_rww_enable_safe, 93
boot_spm_busy, 93
boot_spm_busy_wait, 93
boot_spm_interrupt_disable, 93
boot_spm_interrupt_enable, 93
BOOTLOADER_SECTION, 94
GET_EXTENDED_FUSE_BITS,
94
GET_HIGH_FUSE_BITS, 94
GET_LOCK_BITS, 94
GET_LOW_FUSE_BITS, 94
avr_eeprom
_EEGET, 96
_EEPUT, 96
__EEPROM_REG_LOCATIONS__, 95
EEMEM, 96
eeprom_busy_wait, 96
eeprom_is_ready, 96
eeprom_read_block, 96
eeprom_read_byte, 96
eeprom_read_word, 97
eeprom_write_block, 97
eeprom_write_byte, 97
eeprom_write_word, 97
avr_errno
EDOM, 17
ERANGE, 17
avr_interrupts

338

cli, 114
EMPTY_INTERRUPT, 114
ISR, 115
ISR_ALIAS, 115
sei, 115
SIGNAL, 116
avr_inttypes
int_farptr_t, 29
PRId16, 20
PRId32, 20
PRId8, 20
PRIdFAST16, 20
PRIdFAST32, 20
PRIdFAST8, 20
PRIdLEAST16, 21
PRIdLEAST32, 21
PRIdLEAST8, 21
PRIdPTR, 21
PRIi16, 21
PRIi32, 21
PRIi8, 21
PRIiFAST16, 21
PRIiFAST32, 21
PRIiFAST8, 21
PRIiLEAST16, 21
PRIiLEAST32, 22
PRIiLEAST8, 22
PRIiPTR, 22
PRIo16, 22
PRIo32, 22
PRIo8, 22
PRIoFAST16, 22
PRIoFAST32, 22
PRIoFAST8, 22
PRIoLEAST16, 22
PRIoLEAST32, 22
PRIoLEAST8, 23
PRIoPTR, 23
PRIu16, 23
PRIu32, 23
PRIu8, 23
PRIuFAST16, 23
PRIuFAST32, 23
PRIuFAST8, 23
PRIuLEAST16, 23
PRIuLEAST32, 23

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INDEX

PRIuLEAST8, 23
PRIuPTR, 24
PRIX16, 24
PRIx16, 24
PRIX32, 24
PRIx32, 24
PRIX8, 24
PRIx8, 24
PRIXFAST16, 24
PRIxFAST16, 24
PRIXFAST32, 24
PRIxFAST32, 24
PRIXFAST8, 25
PRIxFAST8, 25
PRIXLEAST16, 25
PRIxLEAST16, 25
PRIXLEAST32, 25
PRIxLEAST32, 25
PRIXLEAST8, 25
PRIxLEAST8, 25
PRIXPTR, 25
PRIxPTR, 25
SCNd16, 25
SCNd32, 26
SCNdFAST16, 26
SCNdFAST32, 26
SCNdLEAST16, 26
SCNdLEAST32, 26
SCNdPTR, 26
SCNi16, 26
SCNi32, 26
SCNiFAST16, 26
SCNiFAST32, 26
SCNiLEAST16, 26
SCNiLEAST32, 27
SCNiPTR, 27
SCNo16, 27
SCNo32, 27
SCNoFAST16, 27
SCNoFAST32, 27
SCNoLEAST16, 27
SCNoLEAST32, 27
SCNoPTR, 27
SCNu16, 27
SCNu32, 27
SCNuFAST16, 28

339

SCNuFAST32, 28
SCNuLEAST16, 28
SCNuLEAST32, 28
SCNuPTR, 28
SCNx16, 28
SCNx32, 28
SCNxFAST16, 28
SCNxFAST32, 28
SCNxLEAST16, 28
SCNxLEAST32, 28
SCNxPTR, 29
uint_farptr_t, 29
avr_math
acos, 30
asin, 30
atan, 30
atan2, 31
ceil, 31
cos, 31
cosh, 31
exp, 31
fabs, 31
floor, 31
fmod, 31
frexp, 31
isinf, 32
isnan, 32
ldexp, 32
log, 32
log10, 32
M_PI, 30
M_SQRT2, 30
modf, 32
pow, 32
sin, 33
sinh, 33
sqrt, 33
square, 33
tan, 33
tanh, 33
avr_pgmspace
memchr_P, 122
memcmp_P, 122
memcpy_P, 122
memmem_P, 122
memrchr_P, 122

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

PGM_P, 118
pgm_read_byte, 118
pgm_read_byte_far, 118
pgm_read_byte_near, 119
pgm_read_dword, 119
pgm_read_dword_far, 119
pgm_read_dword_near, 119
pgm_read_word, 119
pgm_read_word_far, 120
pgm_read_word_near, 120
PGM_VOID_P, 120
prog_char, 120
prog_int16_t, 120
prog_int32_t, 121
prog_int64_t, 121
prog_int8_t, 121
prog_uchar, 121
prog_uint16_t, 121
prog_uint32_t, 121
prog_uint64_t, 121
prog_uint8_t, 121
prog_void, 121
PROGMEM, 120
PSTR, 120
strcasecmp_P, 123
strcasestr_P, 123
strcat_P, 123
strchr_P, 123
strchrnul_P, 124
strcmp_P, 124
strcpy_P, 124
strcspn_P, 124
strlcat_P, 125
strlcpy_P, 125
strlen_P, 125
strncasecmp_P, 125
strncat_P, 126
strncmp_P, 126
strncpy_P, 126
strnlen_P, 127
strpbrk_P, 127
strrchr_P, 127
strsep_P, 127
strspn_P, 128
strstr_P, 128
avr_sfr

340

_BV, 134
bit_is_clear, 135
bit_is_set, 135
loop_until_bit_is_clear, 135
loop_until_bit_is_set, 135
avr_sleep
set_sleep_mode, 138
sleep_cpu, 138
sleep_disable, 138
sleep_enable, 138
sleep_mode, 138
SLEEP_MODE_ADC, 137
SLEEP_MODE_EXT_STANDBY,
137
SLEEP_MODE_IDLE, 137
SLEEP_MODE_PWR_DOWN, 137
SLEEP_MODE_PWR_SAVE, 137
SLEEP_MODE_STANDBY, 137
avr_stdint
INT16_C, 39
INT16_MAX, 39
INT16_MIN, 39
int16_t, 44
INT32_C, 39
INT32_MAX, 39
INT32_MIN, 39
int32_t, 44
INT64_C, 40
INT64_MAX, 40
INT64_MIN, 40
int64_t, 44
INT8_C, 40
INT8_MAX, 40
INT8_MIN, 40
int8_t, 45
INT_FAST16_MAX, 40
INT_FAST16_MIN, 40
int_fast16_t, 45
INT_FAST32_MAX, 40
INT_FAST32_MIN, 40
int_fast32_t, 45
INT_FAST64_MAX, 40
INT_FAST64_MIN, 41
int_fast64_t, 45
INT_FAST8_MAX, 41
INT_FAST8_MIN, 41

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

int_fast8_t, 45
INT_LEAST16_MAX, 41
INT_LEAST16_MIN, 41
int_least16_t, 45
INT_LEAST32_MAX, 41
INT_LEAST32_MIN, 41
int_least32_t, 45
INT_LEAST64_MAX, 41
INT_LEAST64_MIN, 41
int_least64_t, 45
INT_LEAST8_MAX, 41
INT_LEAST8_MIN, 41
int_least8_t, 45
INTMAX_C, 42
INTMAX_MAX, 42
INTMAX_MIN, 42
intmax_t, 46
INTPTR_MAX, 42
INTPTR_MIN, 42
intptr_t, 46
PTRDIFF_MAX, 42
PTRDIFF_MIN, 42
SIG_ATOMIC_MAX, 42
SIG_ATOMIC_MIN, 42
SIZE_MAX, 42
UINT16_C, 42
UINT16_MAX, 43
uint16_t, 46
UINT32_C, 43
UINT32_MAX, 43
uint32_t, 46
UINT64_C, 43
UINT64_MAX, 43
uint64_t, 46
UINT8_C, 43
UINT8_MAX, 43
uint8_t, 46
UINT_FAST16_MAX, 43
uint_fast16_t, 46
UINT_FAST32_MAX, 43
uint_fast32_t, 46
UINT_FAST64_MAX, 43
uint_fast64_t, 46
UINT_FAST8_MAX, 43
uint_fast8_t, 47
UINT_LEAST16_MAX, 44

341

uint_least16_t, 47
UINT_LEAST32_MAX, 44
uint_least32_t, 47
UINT_LEAST64_MAX, 44
uint_least64_t, 47
UINT_LEAST8_MAX, 44
uint_least8_t, 47
UINTMAX_C, 44
UINTMAX_MAX, 44
uintmax_t, 47
UINTPTR_MAX, 44
uintptr_t, 47
avr_stdio
_FDEV_EOF, 52
_FDEV_ERR, 52
_FDEV_SETUP_READ, 52
_FDEV_SETUP_RW, 52
_FDEV_SETUP_WRITE, 52
clearerr, 55
EOF, 53
fclose, 55
fdev_close, 53
fdev_get_udata, 53
fdev_set_udata, 53
FDEV_SETUP_STREAM, 53
fdev_setup_stream, 53
fdevopen, 55
feof, 56
ferror, 56
fflush, 56
fgetc, 56
fgets, 56
FILE, 54
fprintf, 57
fprintf_P, 57
fputc, 57
fputs, 57
fputs_P, 57
fread, 57
fscanf, 57
fscanf_P, 57
fwrite, 58
getc, 54
getchar, 54
gets, 58
printf, 58

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

printf_P, 58
putc, 54
putchar, 54
puts, 58
puts_P, 58
scanf, 58
scanf_P, 58
snprintf, 58
snprintf_P, 59
sprintf, 59
sprintf_P, 59
sscanf, 59
sscanf_P, 59
stderr, 54
stdin, 54
stdout, 55
ungetc, 59
vfprintf, 59
vfprintf_P, 62
vfscanf, 62
vfscanf_P, 65
vprintf, 65
vscanf, 65
vsnprintf, 65
vsnprintf_P, 65
vsprintf, 65
vsprintf_P, 65
avr_stdlib
__compar_fn_t, 68
__malloc_heap_end, 76
__malloc_heap_start, 76
__malloc_margin, 77
abort, 68
abs, 68
atof, 68
atoi, 69
atol, 69
bsearch, 69
calloc, 69
div, 70
DTOSTR_ALWAYS_SIGN, 68
DTOSTR_PLUS_SIGN, 68
DTOSTR_UPPERCASE, 68
dtostre, 70
dtostrf, 70
exit, 70

342

free, 71
itoa, 71
labs, 71
ldiv, 71
ltoa, 71
malloc, 72
qsort, 72
rand, 72
RAND_MAX, 68
rand_r, 73
random, 73
RANDOM_MAX, 68
random_r, 73
realloc, 73
srand, 74
srandom, 74
strtod, 74
strtol, 74
strtoul, 75
ultoa, 75
utoa, 76
avr_string
_FFS, 78
ffs, 78
ffsl, 79
ffsll, 79
memccpy, 79
memchr, 79
memcmp, 79
memcpy, 80
memmem, 80
memmove, 80
memrchr, 81
memset, 81
strcasecmp, 81
strcasestr, 81
strcat, 82
strchr, 82
strchrnul, 82
strcmp, 82
strcpy, 83
strcspn, 83
strlcat, 83
strlcpy, 84
strlen, 84
strlwr, 84

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

strncasecmp, 84
strncat, 85
strncmp, 85
strncpy, 85
strnlen, 85
strpbrk, 86
strrchr, 86
strrev, 86
strsep, 86
strspn, 87
strstr, 87
strtok_r, 87
strupr, 88
avr_version
__AVR_LIBC_DATE_, 139
__AVR_LIBC_DATE_STRING__,
139
__AVR_LIBC_MAJOR__, 139
__AVR_LIBC_MINOR__, 139
__AVR_LIBC_REVISION__, 139
__AVR_LIBC_VERSION_STRING__, 139
__AVR_LIBC_VERSION__, 139
avr_watchdog
wdt_disable, 141
wdt_enable, 141
wdt_reset, 141
WDTO_120MS, 142
WDTO_15MS, 142
WDTO_1S, 142
WDTO_250MS, 142
WDTO_2S, 142
WDTO_30MS, 142
WDTO_4S, 142
WDTO_500MS, 143
WDTO_60MS, 143
WDTO_8S, 143
avrdude, usage, 328
avrprog, usage, 328
bit_is_clear
avr_sfr, 135
bit_is_set
avr_sfr, 135
boot.h, 199
__boot_lock_bits_set, 200

343

__boot_lock_bits_set_alternate, 200
__boot_page_erase_alternate, 201
__boot_page_erase_extended, 201
__boot_page_erase_normal, 201
__boot_page_fill_alternate, 202
__boot_page_fill_extended, 202
__boot_page_fill_normal, 203
__boot_page_write_alternate, 203
__boot_page_write_extended, 203
__boot_page_write_normal, 204
__boot_rww_enable, 204
__boot_rww_enable_alternate, 204
boot_is_spm_interrupt
avr_boot, 90
boot_lock_bits_set
avr_boot, 90
boot_lock_bits_set_safe
avr_boot, 90
boot_lock_fuse_bits_get
avr_boot, 91
boot_page_erase
avr_boot, 91
boot_page_erase_safe
avr_boot, 91
boot_page_fill
avr_boot, 92
boot_page_fill_safe
avr_boot, 92
boot_page_write
avr_boot, 92
boot_page_write_safe
avr_boot, 92
boot_rww_busy
avr_boot, 93
boot_rww_enable
avr_boot, 93
boot_rww_enable_safe
avr_boot, 93
boot_spm_busy
avr_boot, 93
boot_spm_busy_wait
avr_boot, 93
boot_spm_interrupt_disable
avr_boot, 93
boot_spm_interrupt_enable
avr_boot, 93

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

BOOTLOADER_SECTION
avr_boot, 94
bsearch
avr_stdlib, 69
calloc
avr_stdlib, 69
cbi
deprecated_items, 155
ceil
avr_math, 31
clearerr
avr_stdio, 55
cli
avr_interrupts, 114
clock_prescale_set
power.h, 223
Combining C and assembly source files,
159
cos
avr_math, 31
cosh
avr_math, 31
crc16.h, 205
ctype
isalnum, 14
isalpha, 14
isascii, 15
isblank, 15
iscntrl, 15
isdigit, 15
isgraph, 15
islower, 15
isprint, 15
ispunct, 15
isspace, 15
isupper, 15
isxdigit, 16
toascii, 16
tolower, 16
toupper, 16
ctype.h, 205
delay.h, 206
delay_basic.h, 206
Demo projects, 158

344

deprecated_items
cbi, 155
enable_external_int, 155
inb, 156
inp, 156
INTERRUPT, 156
outb, 156
outp, 157
sbi, 157
timer_enable_int, 157
disassembling, 167
div
avr_stdlib, 70
div_t, 197
quot, 197
rem, 197
DTOSTR_ALWAYS_SIGN
avr_stdlib, 68
DTOSTR_PLUS_SIGN
avr_stdlib, 68
DTOSTR_UPPERCASE
avr_stdlib, 68
dtostre
avr_stdlib, 70
dtostrf
avr_stdlib, 70
EDOM
avr_errno, 17
EEMEM
avr_eeprom, 96
eeprom.h, 207
eeprom_busy_wait
avr_eeprom, 96
eeprom_is_ready
avr_eeprom, 96
eeprom_read_block
avr_eeprom, 96
eeprom_read_byte
avr_eeprom, 96
eeprom_read_word
avr_eeprom, 97
eeprom_write_block
avr_eeprom, 97
eeprom_write_byte
avr_eeprom, 97

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

eeprom_write_word
avr_eeprom, 97
EMPTY_INTERRUPT
avr_interrupts, 114
enable_external_int
deprecated_items, 155
EOF
avr_stdio, 53
ERANGE
avr_errno, 17
errno.h, 208
Example using the two-wire interface
(TWI), 192
exit
avr_stdlib, 70
exp
avr_math, 31
fabs
avr_math, 31
FAQ, 289
fclose
avr_stdio, 55
fdev_close
avr_stdio, 53
fdev_get_udata
avr_stdio, 53
fdev_set_udata
avr_stdio, 53
FDEV_SETUP_STREAM
avr_stdio, 53
fdev_setup_stream
avr_stdio, 53
fdevopen
avr_stdio, 55
fdevopen.c, 208
feof
avr_stdio, 56
ferror
avr_stdio, 56
fflush
avr_stdio, 56
ffs
avr_string, 78
ffs.S, 209
ffsl

345

avr_string, 79
ffsl.S, 209
ffsll
avr_string, 79
ffsll.S, 209
fgetc
avr_stdio, 56
fgets
avr_stdio, 56
FILE
avr_stdio, 54
floor
avr_math, 31
fmod
avr_math, 31
fprintf
avr_stdio, 57
fprintf_P
avr_stdio, 57
fputc
avr_stdio, 57
fputs
avr_stdio, 57
fputs_P
avr_stdio, 57
fread
avr_stdio, 57
free
avr_stdlib, 71
frexp
avr_math, 31
fscanf
avr_stdio, 57
fscanf_P
avr_stdio, 57
fwrite
avr_stdio, 58
GET_EXTENDED_FUSE_BITS
avr_boot, 94
GET_HIGH_FUSE_BITS
avr_boot, 94
GET_LOCK_BITS
avr_boot, 94
GET_LOW_FUSE_BITS
avr_boot, 94

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

getc
avr_stdio, 54
getchar
avr_stdio, 54
gets
avr_stdio, 58
inb
deprecated_items, 156
inp
deprecated_items, 156
installation, 312
installation, avarice, 318
installation, avr-libc, 316
installation, avrdude, 316
installation, avrprog, 316
installation, binutils, 314
installation, gcc, 315
Installation, gdb, 317
installation, simulavr, 317
INT16_C
avr_stdint, 39
INT16_MAX
avr_stdint, 39
INT16_MIN
avr_stdint, 39
int16_t
avr_stdint, 44
INT32_C
avr_stdint, 39
INT32_MAX
avr_stdint, 39
INT32_MIN
avr_stdint, 39
int32_t
avr_stdint, 44
INT64_C
avr_stdint, 40
INT64_MAX
avr_stdint, 40
INT64_MIN
avr_stdint, 40
int64_t
avr_stdint, 44
INT8_C
avr_stdint, 40

346

INT8_MAX
avr_stdint, 40
INT8_MIN
avr_stdint, 40
int8_t
avr_stdint, 45
int_farptr_t
avr_inttypes, 29
INT_FAST16_MAX
avr_stdint, 40
INT_FAST16_MIN
avr_stdint, 40
int_fast16_t
avr_stdint, 45
INT_FAST32_MAX
avr_stdint, 40
INT_FAST32_MIN
avr_stdint, 40
int_fast32_t
avr_stdint, 45
INT_FAST64_MAX
avr_stdint, 40
INT_FAST64_MIN
avr_stdint, 41
int_fast64_t
avr_stdint, 45
INT_FAST8_MAX
avr_stdint, 41
INT_FAST8_MIN
avr_stdint, 41
int_fast8_t
avr_stdint, 45
INT_LEAST16_MAX
avr_stdint, 41
INT_LEAST16_MIN
avr_stdint, 41
int_least16_t
avr_stdint, 45
INT_LEAST32_MAX
avr_stdint, 41
INT_LEAST32_MIN
avr_stdint, 41
int_least32_t
avr_stdint, 45
INT_LEAST64_MAX
avr_stdint, 41

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

INT_LEAST64_MIN
avr_stdint, 41
int_least64_t
avr_stdint, 45
INT_LEAST8_MAX
avr_stdint, 41
INT_LEAST8_MIN
avr_stdint, 41
int_least8_t
avr_stdint, 45
INTERRUPT
deprecated_items, 156
interrupt.h, 209
INTMAX_C
avr_stdint, 42
INTMAX_MAX
avr_stdint, 42
INTMAX_MIN
avr_stdint, 42
intmax_t
avr_stdint, 46
INTPTR_MAX
avr_stdint, 42
INTPTR_MIN
avr_stdint, 42
intptr_t
avr_stdint, 46
inttypes.h, 209
isalnum
ctype, 14
isalpha
ctype, 14
isascii
ctype, 15
isblank
ctype, 15
iscntrl
ctype, 15
isdigit
ctype, 15
isgraph
ctype, 15
isinf
avr_math, 32
islower
ctype, 15

347

isnan
avr_math, 32
isprint
ctype, 15
ispunct
ctype, 15
ISR
avr_interrupts, 115
ISR_ALIAS
avr_interrupts, 115
isspace
ctype, 15
isupper
ctype, 15
isxdigit
ctype, 16
itoa
avr_stdlib, 71
labs
avr_stdlib, 71
ldexp
avr_math, 32
ldiv
avr_stdlib, 71
ldiv_t, 197
quot, 197
rem, 197
log
avr_math, 32
log10
avr_math, 32
longjmp
setjmp, 34
loop_until_bit_is_clear
avr_sfr, 135
loop_until_bit_is_set
avr_sfr, 135
ltoa
avr_stdlib, 71
M_PI
avr_math, 30
M_SQRT2
avr_math, 30
malloc

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

avr_stdlib, 72
math.h, 212
memccpy
avr_string, 79
memccpy.S, 213
memchr
avr_string, 79
memchr.S, 213
memchr_P
avr_pgmspace, 122
memchr_P.S, 214
memcmp
avr_string, 79
memcmp.S, 214
memcmp_P
avr_pgmspace, 122
memcmp_P.S, 214
memcpy
avr_string, 80
memcpy.S, 214
memcpy_P
avr_pgmspace, 122
memcpy_P.S, 214
memmem
avr_string, 80
memmem.S, 215
memmem_P
avr_pgmspace, 122
memmove
avr_string, 80
memmove.S, 215
memrchr
avr_string, 81
memrchr.S, 215
memrchr_P
avr_pgmspace, 122
memrchr_P.S, 215
memset
avr_string, 81
memset.S, 215
modf
avr_math, 32
outb
deprecated_items, 156
outp

348

deprecated_items, 157
parity.h, 216
parity_even_bit
util_parity, 149
PGM_P
avr_pgmspace, 118
pgm_read_byte
avr_pgmspace, 118
pgm_read_byte_far
avr_pgmspace, 118
pgm_read_byte_near
avr_pgmspace, 119
pgm_read_dword
avr_pgmspace, 119
pgm_read_dword_far
avr_pgmspace, 119
pgm_read_dword_near
avr_pgmspace, 119
pgm_read_word
avr_pgmspace, 119
pgm_read_word_far
avr_pgmspace, 120
pgm_read_word_near
avr_pgmspace, 120
PGM_VOID_P
avr_pgmspace, 120
pgmspace.h, 216
__ELPM_classic__, 218
__ELPM_dword_enhanced__, 218
__ELPM_enhanced__, 219
__ELPM_word_classic__, 219
__ELPM_word_enhanced__, 220
__LPM_classic__, 220
__LPM_dword_classic__, 220
__LPM_dword_enhanced__, 221
__LPM_enhanced__, 221
__LPM_word_classic__, 222
__LPM_word_enhanced__, 222
pow
avr_math, 32
power.h, 223
clock_prescale_set, 223
PRId16
avr_inttypes, 20
PRId32

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

avr_inttypes, 20
PRId8
avr_inttypes, 20
PRIdFAST16
avr_inttypes, 20
PRIdFAST32
avr_inttypes, 20
PRIdFAST8
avr_inttypes, 20
PRIdLEAST16
avr_inttypes, 21
PRIdLEAST32
avr_inttypes, 21
PRIdLEAST8
avr_inttypes, 21
PRIdPTR
avr_inttypes, 21
PRIi16
avr_inttypes, 21
PRIi32
avr_inttypes, 21
PRIi8
avr_inttypes, 21
PRIiFAST16
avr_inttypes, 21
PRIiFAST32
avr_inttypes, 21
PRIiFAST8
avr_inttypes, 21
PRIiLEAST16
avr_inttypes, 21
PRIiLEAST32
avr_inttypes, 22
PRIiLEAST8
avr_inttypes, 22
PRIiPTR
avr_inttypes, 22
printf
avr_stdio, 58
printf_P
avr_stdio, 58
PRIo16
avr_inttypes, 22
PRIo32
avr_inttypes, 22
PRIo8

349

avr_inttypes, 22
PRIoFAST16
avr_inttypes, 22
PRIoFAST32
avr_inttypes, 22
PRIoFAST8
avr_inttypes, 22
PRIoLEAST16
avr_inttypes, 22
PRIoLEAST32
avr_inttypes, 22
PRIoLEAST8
avr_inttypes, 23
PRIoPTR
avr_inttypes, 23
PRIu16
avr_inttypes, 23
PRIu32
avr_inttypes, 23
PRIu8
avr_inttypes, 23
PRIuFAST16
avr_inttypes, 23
PRIuFAST32
avr_inttypes, 23
PRIuFAST8
avr_inttypes, 23
PRIuLEAST16
avr_inttypes, 23
PRIuLEAST32
avr_inttypes, 23
PRIuLEAST8
avr_inttypes, 23
PRIuPTR
avr_inttypes, 24
PRIX16
avr_inttypes, 24
PRIx16
avr_inttypes, 24
PRIX32
avr_inttypes, 24
PRIx32
avr_inttypes, 24
PRIX8
avr_inttypes, 24
PRIx8

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

avr_inttypes, 24
PRIXFAST16
avr_inttypes, 24
PRIxFAST16
avr_inttypes, 24
PRIXFAST32
avr_inttypes, 24
PRIxFAST32
avr_inttypes, 24
PRIXFAST8
avr_inttypes, 25
PRIxFAST8
avr_inttypes, 25
PRIXLEAST16
avr_inttypes, 25
PRIxLEAST16
avr_inttypes, 25
PRIXLEAST32
avr_inttypes, 25
PRIxLEAST32
avr_inttypes, 25
PRIXLEAST8
avr_inttypes, 25
PRIxLEAST8
avr_inttypes, 25
PRIXPTR
avr_inttypes, 25
PRIxPTR
avr_inttypes, 25
prog_char
avr_pgmspace, 120
prog_int16_t
avr_pgmspace, 120
prog_int32_t
avr_pgmspace, 121
prog_int64_t
avr_pgmspace, 121
prog_int8_t
avr_pgmspace, 121
prog_uchar
avr_pgmspace, 121
prog_uint16_t
avr_pgmspace, 121
prog_uint32_t
avr_pgmspace, 121
prog_uint64_t

350

avr_pgmspace, 121
prog_uint8_t
avr_pgmspace, 121
prog_void
avr_pgmspace, 121
PROGMEM
avr_pgmspace, 120
PSTR
avr_pgmspace, 120
PTRDIFF_MAX
avr_stdint, 42
PTRDIFF_MIN
avr_stdint, 42
putc
avr_stdio, 54
putchar
avr_stdio, 54
puts
avr_stdio, 58
puts_P
avr_stdio, 58
qsort
avr_stdlib, 72
quot
div_t, 197
ldiv_t, 197
rand
avr_stdlib, 72
RAND_MAX
avr_stdlib, 68
rand_r
avr_stdlib, 73
random
avr_stdlib, 73
RANDOM_MAX
avr_stdlib, 68
random_r
avr_stdlib, 73
realloc
avr_stdlib, 73
rem
div_t, 197
ldiv_t, 197
sbi

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

deprecated_items, 157
scanf
avr_stdio, 58
scanf_P
avr_stdio, 58
SCNd16
avr_inttypes, 25
SCNd32
avr_inttypes, 26
SCNdFAST16
avr_inttypes, 26
SCNdFAST32
avr_inttypes, 26
SCNdLEAST16
avr_inttypes, 26
SCNdLEAST32
avr_inttypes, 26
SCNdPTR
avr_inttypes, 26
SCNi16
avr_inttypes, 26
SCNi32
avr_inttypes, 26
SCNiFAST16
avr_inttypes, 26
SCNiFAST32
avr_inttypes, 26
SCNiLEAST16
avr_inttypes, 26
SCNiLEAST32
avr_inttypes, 27
SCNiPTR
avr_inttypes, 27
SCNo16
avr_inttypes, 27
SCNo32
avr_inttypes, 27
SCNoFAST16
avr_inttypes, 27
SCNoFAST32
avr_inttypes, 27
SCNoLEAST16
avr_inttypes, 27
SCNoLEAST32
avr_inttypes, 27
SCNoPTR

351

avr_inttypes, 27
SCNu16
avr_inttypes, 27
SCNu32
avr_inttypes, 27
SCNuFAST16
avr_inttypes, 28
SCNuFAST32
avr_inttypes, 28
SCNuLEAST16
avr_inttypes, 28
SCNuLEAST32
avr_inttypes, 28
SCNuPTR
avr_inttypes, 28
SCNx16
avr_inttypes, 28
SCNx32
avr_inttypes, 28
SCNxFAST16
avr_inttypes, 28
SCNxFAST32
avr_inttypes, 28
SCNxLEAST16
avr_inttypes, 28
SCNxLEAST32
avr_inttypes, 28
SCNxPTR
avr_inttypes, 29
sei
avr_interrupts, 115
set_sleep_mode
avr_sleep, 138
setjmp
longjmp, 34
setjmp, 35
setjmp.h, 223
SIG_ATOMIC_MAX
avr_stdint, 42
SIG_ATOMIC_MIN
avr_stdint, 42
SIGNAL
avr_interrupts, 116
sin
avr_math, 33
sinh

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

avr_math, 33
SIZE_MAX
avr_stdint, 42
sleep.h, 224
sleep_cpu
avr_sleep, 138
sleep_disable
avr_sleep, 138
sleep_enable
avr_sleep, 138
sleep_mode
avr_sleep, 138
SLEEP_MODE_ADC
avr_sleep, 137
SLEEP_MODE_EXT_STANDBY
avr_sleep, 137
SLEEP_MODE_IDLE
avr_sleep, 137
SLEEP_MODE_PWR_DOWN
avr_sleep, 137
SLEEP_MODE_PWR_SAVE
avr_sleep, 137
SLEEP_MODE_STANDBY
avr_sleep, 137
snprintf
avr_stdio, 58
snprintf_P
avr_stdio, 59
sprintf
avr_stdio, 59
sprintf_P
avr_stdio, 59
sqrt
avr_math, 33
square
avr_math, 33
srand
avr_stdlib, 74
srandom
avr_stdlib, 74
sscanf
avr_stdio, 59
sscanf_P
avr_stdio, 59
stderr
avr_stdio, 54

352

stdin
avr_stdio, 54
stdint.h, 225
stdio.h, 228
stdlib.h, 230
stdout
avr_stdio, 55
strcasecmp
avr_string, 81
strcasecmp.S, 232
strcasecmp_P
avr_pgmspace, 123
strcasecmp_P.S, 232
strcasestr
avr_string, 81
strcasestr.S, 232
strcasestr_P
avr_pgmspace, 123
strcat
avr_string, 82
strcat.S, 232
strcat_P
avr_pgmspace, 123
strcat_P.S, 233
strchr
avr_string, 82
strchr.S, 233
strchr_P
avr_pgmspace, 123
strchr_P.S, 233
strchrnul
avr_string, 82
strchrnul.S, 233
strchrnul_P
avr_pgmspace, 124
strchrnul_P.S, 233
strcmp
avr_string, 82
strcmp.S, 233
strcmp_P
avr_pgmspace, 124
strcmp_P.S, 233
strcpy
avr_string, 83
strcpy.S, 233
strcpy_P

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

avr_pgmspace, 124
strcpy_P.S, 234
strcspn
avr_string, 83
strcspn.S, 234
strcspn_P
avr_pgmspace, 124
strcspn_P.S, 234
string.h, 234
strlcat
avr_string, 83
strlcat.S, 236
strlcat_P
avr_pgmspace, 125
strlcat_P.S, 236
strlcpy
avr_string, 84
strlcpy.S, 236
strlcpy_P
avr_pgmspace, 125
strlcpy_P.S, 236
strlen
avr_string, 84
strlen.S, 236
strlen_P
avr_pgmspace, 125
strlen_P.S, 236
strlwr
avr_string, 84
strlwr.S, 237
strncasecmp
avr_string, 84
strncasecmp.S, 237
strncasecmp_P
avr_pgmspace, 125
strncasecmp_P.S, 237
strncat
avr_string, 85
strncat.S, 237
strncat_P
avr_pgmspace, 126
strncat_P.S, 237
strncmp
avr_string, 85
strncmp.S, 238
strncmp_P

353

avr_pgmspace, 126
strncmp_P.S, 238
strncpy
avr_string, 85
strncpy.S, 238
strncpy_P
avr_pgmspace, 126
strncpy_P.S, 239
strnlen
avr_string, 85
strnlen.S, 239
strnlen_P
avr_pgmspace, 127
strnlen_P.S, 239
strpbrk
avr_string, 86
strpbrk.S, 240
strpbrk_P
avr_pgmspace, 127
strpbrk_P.S, 240
strrchr
avr_string, 86
strrchr.S, 240
strrchr_P
avr_pgmspace, 127
strrchr_P.S, 242
strrev
avr_string, 86
strrev.S, 242
strsep
avr_string, 86
strsep.S, 242
strsep_P
avr_pgmspace, 127
strsep_P.S, 242
strspn
avr_string, 87
strspn.S, 242
strspn_P
avr_pgmspace, 128
strspn_P.S, 242
strstr
avr_string, 87
strstr.S, 242
strstr_P
avr_pgmspace, 128

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

strstr_P.S, 242
strtod
avr_stdlib, 74
strtok_r
avr_string, 87
strtok_r.S, 242
strtol
avr_stdlib, 74
strtoul
avr_stdlib, 75
strupr
avr_string, 88
strupr.S, 242
supported devices, 2
tan
avr_math, 33
tanh
avr_math, 33
timer_enable_int
deprecated_items, 157
toascii
ctype, 16
tolower
ctype, 16
tools, optional, 313
tools, required, 313
toupper
ctype, 16
TW_BUS_ERROR
util_twi, 151
TW_MR_ARB_LOST
util_twi, 151
TW_MR_DATA_ACK
util_twi, 151
TW_MR_DATA_NACK
util_twi, 151
TW_MR_SLA_ACK
util_twi, 151
TW_MR_SLA_NACK
util_twi, 151
TW_MT_ARB_LOST
util_twi, 151
TW_MT_DATA_ACK
util_twi, 151
TW_MT_DATA_NACK

354

util_twi, 151
TW_MT_SLA_ACK
util_twi, 152
TW_MT_SLA_NACK
util_twi, 152
TW_NO_INFO
util_twi, 152
TW_READ
util_twi, 152
TW_REP_START
util_twi, 152
TW_SR_ARB_LOST_GCALL_ACK
util_twi, 152
TW_SR_ARB_LOST_SLA_ACK
util_twi, 152
TW_SR_DATA_ACK
util_twi, 152
TW_SR_DATA_NACK
util_twi, 152
TW_SR_GCALL_ACK
util_twi, 152
TW_SR_GCALL_DATA_ACK
util_twi, 152
TW_SR_GCALL_DATA_NACK
util_twi, 153
TW_SR_SLA_ACK
util_twi, 153
TW_SR_STOP
util_twi, 153
TW_ST_ARB_LOST_SLA_ACK
util_twi, 153
TW_ST_DATA_ACK
util_twi, 153
TW_ST_DATA_NACK
util_twi, 153
TW_ST_LAST_DATA
util_twi, 153
TW_ST_SLA_ACK
util_twi, 153
TW_START
util_twi, 153
TW_STATUS
util_twi, 153
TW_STATUS_MASK
util_twi, 153
TW_WRITE

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

util_twi, 154
twi.h, 242
UINT16_C
avr_stdint, 42
UINT16_MAX
avr_stdint, 43
uint16_t
avr_stdint, 46
UINT32_C
avr_stdint, 43
UINT32_MAX
avr_stdint, 43
uint32_t
avr_stdint, 46
UINT64_C
avr_stdint, 43
UINT64_MAX
avr_stdint, 43
uint64_t
avr_stdint, 46
UINT8_C
avr_stdint, 43
UINT8_MAX
avr_stdint, 43
uint8_t
avr_stdint, 46
uint_farptr_t
avr_inttypes, 29
UINT_FAST16_MAX
avr_stdint, 43
uint_fast16_t
avr_stdint, 46
UINT_FAST32_MAX
avr_stdint, 43
uint_fast32_t
avr_stdint, 46
UINT_FAST64_MAX
avr_stdint, 43
uint_fast64_t
avr_stdint, 46
UINT_FAST8_MAX
avr_stdint, 43
uint_fast8_t
avr_stdint, 47
UINT_LEAST16_MAX

355

avr_stdint, 44
uint_least16_t
avr_stdint, 47
UINT_LEAST32_MAX
avr_stdint, 44
uint_least32_t
avr_stdint, 47
UINT_LEAST64_MAX
avr_stdint, 44
uint_least64_t
avr_stdint, 47
UINT_LEAST8_MAX
avr_stdint, 44
uint_least8_t
avr_stdint, 47
UINTMAX_C
avr_stdint, 44
UINTMAX_MAX
avr_stdint, 44
uintmax_t
avr_stdint, 47
UINTPTR_MAX
avr_stdint, 44
uintptr_t
avr_stdint, 47
ultoa
avr_stdlib, 75
ungetc
avr_stdio, 59
Using the standard IO facilities, 184
util_crc
_crc16_update, 144
_crc_ccitt_update, 145
_crc_ibutton_update, 145
_crc_xmodem_update, 146
util_delay
_delay_ms, 147
_delay_us, 148
util_delay_basic
_delay_loop_1, 148
_delay_loop_2, 148
util_parity
parity_even_bit, 149
util_twi
TW_BUS_ERROR, 151
TW_MR_ARB_LOST, 151

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

INDEX

TW_MR_DATA_ACK, 151
TW_MR_DATA_NACK, 151
TW_MR_SLA_ACK, 151
TW_MR_SLA_NACK, 151
TW_MT_ARB_LOST, 151
TW_MT_DATA_ACK, 151
TW_MT_DATA_NACK, 151
TW_MT_SLA_ACK, 152
TW_MT_SLA_NACK, 152
TW_NO_INFO, 152
TW_READ, 152
TW_REP_START, 152
TW_SR_ARB_LOST_GCALL_ACK, 152
TW_SR_ARB_LOST_SLA_ACK,
152
TW_SR_DATA_ACK, 152
TW_SR_DATA_NACK, 152
TW_SR_GCALL_ACK, 152
TW_SR_GCALL_DATA_ACK, 152
TW_SR_GCALL_DATA_NACK,
153
TW_SR_SLA_ACK, 153
TW_SR_STOP, 153
TW_ST_ARB_LOST_SLA_ACK,
153
TW_ST_DATA_ACK, 153
TW_ST_DATA_NACK, 153
TW_ST_LAST_DATA, 153
TW_ST_SLA_ACK, 153
TW_START, 153
TW_STATUS, 153
TW_STATUS_MASK, 153
TW_WRITE, 154
utoa
avr_stdlib, 76
vfprintf
avr_stdio, 59
vfprintf_P
avr_stdio, 62
vfscanf
avr_stdio, 62
vfscanf_P
avr_stdio, 65
vprintf

356

avr_stdio, 65
vscanf
avr_stdio, 65
vsnprintf
avr_stdio, 65
vsnprintf_P
avr_stdio, 65
vsprintf
avr_stdio, 65
vsprintf_P
avr_stdio, 65
wdt.h, 244
_wdt_write, 244
wdt_disable
avr_watchdog, 141
wdt_enable
avr_watchdog, 141
wdt_reset
avr_watchdog, 141
WDTO_120MS
avr_watchdog, 142
WDTO_15MS
avr_watchdog, 142
WDTO_1S
avr_watchdog, 142
WDTO_250MS
avr_watchdog, 142
WDTO_2S
avr_watchdog, 142
WDTO_30MS
avr_watchdog, 142
WDTO_4S
avr_watchdog, 142
WDTO_500MS
avr_watchdog, 143
WDTO_60MS
avr_watchdog, 143
WDTO_8S
avr_watchdog, 143

Generated on Tue May 15 14:56:11 2007 for avr-libc by Doxygen

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