HPC MICROWIRE/PLUS Master Slave Handshaking Protocol AN 0474
User Manual: AN-0474
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INTRODUCTION This applications note describes how to use National Semiconductor’s MICROWIRE/PLUS to communicate between two members of the HPC family of microcontrollers, and will discuss the implications of adding other MICROWIRETM peripherals. MICROWIRE/PLUS (mWIRE) may be effectively used to communicate between chips, such as in Small Area Networks (SANs). Possible applications range from setting up a communications network within an automobile to home security systems. Among the standard MICROWIRE peripherals available are display drivers (LCD, VF, LED), memories (RAM, EEPROM), A/D converters, and frequency generators/timers. Each MICROWIRE peripheral requires its own handshaking protocol, however the HPC’s MICROWIRE is flexible enough to work with any peripheral and allows you to define your own handshaking protocol when having two HPC family members communicate. MICROWIRE MICROWIRE/PLUS is an extension of National Semiconductor’s MICROWIRE communications interface. It allows National Semiconductor Application Note 474 Richard Lazovick June 1988 high speed two way serial communications between a master processor and one or more slave processors or peripherals. MICROWIRE/PLUS uses only three wires plus chip selects, therefore it saves on intricate bus routing and does not waste 8-bit ports. Figure 1 shows the block diagram of a sample application using two HPC family members and an 8-bit A/D peripheral to monitor and control certain environmental conditions within a system. MICROWIRE/PLUS has an 8-bit parallel-loaded, serial shift register (SIO) using SI as the serial input and SO as the serial output. The contents of the SIO register may be accessed through any of the memory access instructions. SK is the clock for the SIO register (see Figure 2 ). The SK clock signal can be provided by an internal or external source. The internal clock rate is programmable by the DIVBY register. Data to be transmitted from the SIO register is shifted out on the falling edge of the SK clock. Serial data on the SI pin is latched in on the rising edge of the SK clock (see Figure 3 mWIRE Timing). HPC MICROWIRE/PLUS Master-Slave Handshaking Protocol HPC MICROWIRE/PLUS TM Master-Slave Handshaking Protocol TL/DD/9140 – 1 FIGURE 1. HPC mWIRE Block Diagram (Environmental Control System) AN-474 TRI-STATEÉ is a registered trademark of National Semiconductor Corporation. MICROWIRETM and MICROWIRE/PLUSTM are trademarks of National Semiconductor Corporation. C1995 National Semiconductor Corporation TL/DD/9140 RRD-B30M105/Printed in U. S. A. TL/DD/9140 – 2 Note: The most significant bit is shifted out first. The SO pin reflects the contents of the MSB in the SIO register. FIGURE 2. MICROWIRE/PLUS Block Diagram TL/DD/9140 – 3 Note: The first bit of every eight bits in the SIO register being shifted out will have a longer duration then the other bits. This results from the hardware implementation used for MICROWIRE. * This bit becomes valid immediately when the transmitting device loads its SIO register. ² Arrows indicate points at which SI is sampled. FIGURE 3. mWIRE Timing 2 master, resetting the bit will make it a slave. For the master, the DIVBY register has to be initialized to set the appropriate SK frequency (see Table I.). For example if the crystal frequency is 16 MHz and an SK frequency of 1 MHz is desired, load the least significant nibble of the DIVBY register with 2 (16 MHz/16 e 1 MHz). For a summary of the register and pin configurations for the master and slave modes see Table II. A mWDONE flag in the IRPD (Interrupt Pending) register indicates when the data shift is completed. The HPC can enter the MICROWIRE/PLUS mode as a master or a slave. The mWMODE control bit in the IRCD (Interrupt Condition) register determines whether the HPC is a master or slave. The shift clock is generated internally when the HPC is configured as a master. An externally generated shift clock on the SK pin is used when the HPC is configured as a slave. When the HPC is a master, the DIVBY register allows the SK clock frequency to be programmed in 14 selectable steps from 122 Hz to 1 MHz when CKI is 16 MHz (see Table I). TABLE I. HPC mWIRE DIVBY Register mWIRE SK Divisor MSB HOW TO USE MICROWIRE/PLUS To use MICROWIRE, start by setting up the B port appropriately for the MICROWIRE functions. The SO and SK functions are multiplexed onto Port B pins B5 and B6 respectively. For the master, set bits 5 and 6 in the DIRB register (direction register for Port B) to set SO and SK as outputs. For the slave, set bit 5 and reset bit 6 in the DIRB register to set SO as an output and SK as an input . The BFUN register (Port B function register) is used to set SO and SK as alternate functions in the master and only SO as an alternate function in the slave. The MICROWIRE/PLUS mode can be enabled or disabled any time under program control. This is done through the BFUN register. Placing a ‘‘1’’ in the corresponding bit location causes the alternate function to be activated, a ‘‘0’’ causes the alternate function to be disabled. It is good practice to initialize the output pins by setting PORTB (Port B data register) to a known state. The SI function is multiplexed onto Port I pin I5. This pin is always an input and the SI function is automatically selected when in the MICROWIRE mode. Setting the mWMODE control bit, bit 1, in the IRCD register will enable the part to be a 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 LSB CLOCK 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 not allowed not recommended* CKI/16 CKI/32 CKI/64 CKI/128 CKI/256 CKI/512 CKI/1024 CKI/2048 CKI/4096 CKI/8192 CKI/16384 CKI/32768 CKI/65536 CKI/131072 *This option uses timer T3 output, but does not generate a square wave. (See HPC users manual for more details.) TABLE II. mWIRE Register and Pin Conditions for Master and Slave Operation Operation mWMODE bit BFUN B5 BFUN B6 DIRB B5 DIRB B6 PIN B5 PIN B6 PIN I5 MICROWIRE Master 1 1 1 1 1 SO INT. SK SI MICROWIRE Master 1 1 1 0 1 TRISTATEÉ INT. SK SI MICROWIRE Slave 0 1 0 1 0 SO EXT. SK SI MICROWIRE Slave 0 1 0 0 0 TRISTATE EXT. SK SI 3 (PSW), disabling interrupts, setting the DIVBY register as necessary, initializing Port B, and enabling the appropriate MICROWIRE mode (IRCD). Then the slave continues with its main code (a wait loop) until interrupted. When the master decides it’s ready to send MICROWIRE data, it signals the slave by setting the slave interrupt pin on Port B, then it waits for the slave to respond. Meanwhile, the slave goes into action. It clears the mWDONE flag and loads the SIO register (X A, SIO), then notifies the master that it is ready to continue. Once the master realizes the slave is ready to continue, it removes the interrupt signal to the slave (RESET PORTB.SLAVI), reads in the data to be sent (LD A, PORTD), and starts transmitting it (X A, SIO). At the same time the master reads in the data received at the last data exchange with the slave. Then the master loops until it is done transferring data and loops again until the slave is finished with its interrupt routine. In a real program the master would be off executing code and not having to wait in these loops. Once the transmission is complete the slave reads in the new data (LD A, SIO), lets the master know it is done with its interrupt routine (RESET PORTB.MASTR), and re-enables interrupts as it returns to the main routine (RETI). DEFINING THE MASTER/SLAVE HANDSHAKING PROTOCOL There are a few things to keep in mind when defining a handshaking protocol for the HPC: 1) Only the master can generate SK clocks. 2) As 8 bits are shifted into the SIO register, the 8 bits already in there are shifted out. 3) After 8 bits are shifted into (or out of) the SIO register the MICROWIRE done (mWIRE DONE) flag gets set. 4) ANY access to the SIO register in the master that performs a write operation causes the contents of SIO to be shifted out. 5) No data will be shifted into or out of the slave’s SIO register if its mWIRE DONE flag is set. 6) Any write to the SIO register in the master or slave resets its mWIRE DONE flag. Keeping the above six points in mind, let’s look at one possible handshaking protocol between a master HPC and a slave HPC. Number two above tells us we can send and receive data at the same time, however since only the master initiates data transfer we want to be sure the slave is ready before we get started with the exchange. Since the master initiates the transfer process there is no need for the master’s MICROWIRE routine to be interrupt driven (though it can be if it is desired to have the slave initiate data transfers also). On the other hand, since the slave will be off doing other tasks it is most effective to have its MICROWIRE routine be interrupt driven. In the master’s code there is only one access to the SIO register and that access is an exchange. Remember point Ý4, we can take advantage of the exchange instruction (X A, SIO), which is a read-modify-write instruction. Therefore, with one instruction, we can read the data from the previous transfer into the accumulator, and write the data to be transferred into the SIO register. If this method is not practical, then separate read and write instructions must be used. When accessing the SIO register be sure the mWIRE DONE flag is set so you know the data is not changing. At other times we have to be sure the flag is reset or no data will ever be transferred (shifted in or out). Notice that the ‘‘X A, SIO’’ was used to reset the mWIRE DONE flag as well as load the register with the data to be sent. A FEW THINGS TO NOTE ABOUT THE PROGRAMS The following programs refer to the system configuration shown in Figure 1. This example code does a simple data transfer. The master reads in data on Port D, sends it via MICROWIRE to the slave, and reads it back. They both start by initializing the chip mode and number of wait states 4 MASTER’S Flow Chart SLAVE’S Flow Chart TL/DD/9140 – 5 TL/DD/9140 – 4 MASTER’s SAMPLE CODE ; ;VARIABLE DECLARE ; PSW 4 M(00C0) BFUN DIRB PORTB 4 W(0F4) 4 W(0F2) 4 W(0E2) ;Port B ALTERNATE FUNCTION REGISTER ;Port B DIRECTION REGISTER ;Port B DATA REGISTER PORTD 4 M(0104) ;Port D (INPUT PORT) ENIR IRPD IRCD SIO PORTI DIVBY 4 4 4 4 4 4 M(0D0) M(0D2) M(0D4) M(0D6) M(0D8) W(018E) ;INTERRUPT ENABLE REGISTER ;INTERRUPT PENDING REGISTER ;INTERRUPT CONDITION REGISTER ;SERIAL I/O REGISTER ;INTERRUPT (AND uWIRE SERIAL IN) INPUT PORT ;TIMER DIVIDE BY REGISTER SLAVI uWDONE uWMODE SK SLAVR 4 4 4 4 4 4 0 1 6 2 ;SLAVE ;uWIRE ;uWIRE ;uWIRE ;SLAVE INTERRUPT BIT (IN Port B) DONE BIT (IN IRPD) MASTER/SLAVE BIT (IN IRCD) SERIAL CLOCK (IN Port B) RESPONSE BIT (IN Port B) 5 MASTER’s SAMPLE CODE (Continued) .40F800 ;START PROGRAM BEGIN: LD LD LD PSW,008 ENIR,00 DIVBY,02222 ;SINGLE CHIP MODE, 1 WAIT STATE ;DISABLE ALL INTERRUPTS ;uWIRE CLOCK 4 /16 LD LD LD DIRB,0FFFF BFUN,00060 PORTB,00000 ;Port B ALL OUTPUTS ;ONLY SO & SK HAVE ALTERNATE FUNCTIONS ;INIT PORTB TO ALL ZEROs SET IRCD.uWMODE ;SET THIS HPC AS MASTER SET PORTB.SLAVI IF JP JP PORTI.SLAVR SLAVRS WAIT DOITAG: ;JUMP TO HERE TO DO IT AGAIN ;NOTIFY SLAVE (INTERRUPT THE SLAVE) WAIT: ;SLAVE READY? ;GO SEND/RECEIVE uWIRE DATA ;NO IT IS NOT READY YET SLAVRS: RESET PORTB.SLAVI LD A,PORTD X A,SIO ;REMOVE SLAVE NOTIFIER ;LOAD A W/ DATA TO SEND ;SEND NEW DATA AND READ DATA FROM ;...LAST uWIRE EXCHANGE IF JP JP IRPD.uWDONE CONT DONE ;WAIT TILL DONE EXCHANGING ;uWIRE IS DONE ;uWIRE NOT DONE (KEEP TESTING) IF JP JP PORTI.SLAVR CONT DOITAG ;IS SLAVE READY TO CONTINUE? ;NO ;START ALL OVER (DO IT AGAIN) DONE: CONT: .END BEGIN SLAVE’s SAMPLE CODE ; ;VARIABLE DECLARE ; PSW 4 M(00C0) BFUN DIRB PORTB 4 W(0F4) 4 W(0F2) 4 W(0E2) ;Port B ALTERNATE FUNCTION REGISTER ;Port B DIRECTION REGISTER ;Port B DATA REGISTER 6 SLAVE’s SAMPLE CODE (Continued) ENIR 4 M(0D0) ;INTERRUPT ENABLE REGISTER IRPD 4 M(0D2) ;INTERRUPT PENDING REGISTER IRCD 4 M(0D4) ;INTERRUPT CONDITION REGISTER SIO SO MASTR uWDONE uWMODE INT2 4 4 4 4 4 4 ;SERIAL I/O REGISTER ;uWIRE SERIAL OUTPUT PIN (ON Port B) ;MASTER RESPONSE BIT (IN Port B) ;uWIRE DONE BIT (IN IRPD) ;uWIRE MASTER/SLAVE BIT (IN IRCD) ;INTERRUPT 2 BIT M(0D6) 5 4 0 1 2 .40FFFA .WORD MASNOT .40F800 ;INT2 - INTERRUPT VECTOR ;...MASTER NOTIFICATION ;START PROGRAM BEGIN: LD LD PSW,008 ENIR,01 ;SINGLE CHIP MODE, 1 WAIT STATE ;DISABLE ALL INTERRUPTs, BUT ENABLE GIE LD DIRB,0FF10 LD BFUN,00020 LD PORT B,00000 ;Port B UPPER, & MASTR ARE OUTPUTS ;...(use LD DIRB,0FF30 to set SO as an ;...output if not using any peripherals) ;ONLY SO HAS ALTERNATE FUNCTION ;...NOTE: SK is NOT an alternate ;...function in the slave! ;INIT PORTB TO ALL ZEROS RESET IRCD.uWMODE SET IRCD.INT2 SET ENIR.INT2 ;SET THIS HPC AS A SLAVE ;SET INT2 INTERRUPT (0) POLARITY ;ENABLE EXTERNAL INTERRUPT TO ;...RECEIVE SLAVE RESPONSE JP PAU ;WAIT HERE FOR INTERRUPT FROM MASTER X A,SIO SET SET PORTB.SO PORTB.MASTR ;CLEAR uWDONE FLAG (AND LOAD DATA FROM ;...ACCUMULAT0R TO SEND) ;ENABLE SO (needed only if using a peripheral) ;NOTIFY MASTER THAT READY TO CONTINUE IF JP JP IRPD.uWDONE DONE NOTDN ;WAIT TILL DONE SHIFTING ;DONE, GO CONTINUE ;NOT DONE, CONTINUE LOOPING LD A,SIO ;READ IN NEW DATA PAU: MASNOT: ;uWIRE INTERRUPT ROUTINE NOTDN: DONE: RESET PORT B.SO RESET PORTB.MASTR RETI ;TRI-STATE SO (needed only if ; using a peripheral) ;REMOVE SIGNAL TO MASTER .END BEGIN 7 HPC MICROWIRE/PLUS Master-Slave Handshaking Protocol ADDING PERIPHERALS OR ANOTHER SLAVE Adding another slave HPC or a peripheral to the above Microwire configuration can add more power to your design with minimal extra cost and design time. In Figure 1, an extra peripheral is shown in dotted outline form. The hardware and software modifications are straightforward, however there are a few considerations to keep in mind: Ð Tri-state the SO pin on the slave HPC by resetting B5 in the DIRB register when the slave is not ‘chip-selected’ by the master. Ð When adding more HPC slaves, the master’s and slave’s routines remain the same. Only different B port pins for chip select and I or B port pins for slave acknowledge need to be used. Ð For peripherals the principals of operation are still the same and so are the initialization procedures, however some of the code will have to be modified to accommodate the specific handshaking required by the peripheral. (Note: some of the peripherals require 16 or more consecutive bits without interruption of the SK clock. To provide continuous SK clocks, set up the accumulator with next byte of data to send, loop until mWDONE is set, then exchange the contents of the accumulator and the SIO register (X A, SIO). The above steps will provide nearly continuous SK clocksÐthe slower the SK clock is set for, the more continuous they will appear.) APPLICATIONS Now that you are more familiar with MICROWIRE/PLUS, where can you get experience using it? Ð It can be used in a security system where the on-site master lets the periphery slaves know which security codes they can now let in, while at the same time the slaves monitor fire alarms and smoke detectors. Ð It can be used in automotive brakes to allow all the wheels to communicate with each other. The wheels can trade information on road conditions and a master can monitor all four wheels to coordinate them and check for malfunctions. Ð It can be used in a robot arm to allow each joint to make the decision as to how it will help the entire arm reach its final position. This application is one example of how MICROWIRE/PLUS can be used for system task partitioning. Ð It can be used in a MUX-WIRING system. When using MICROWIRE to communicate between two chips on the same board, a high data rate can be used. When communicating over longer distances, slower speeds should be used. SUMMARY MICROWIRE/PLUS can be a very powerful tool that can easily add power to a microcontroller based system. It is easy to use and does not require much hardware to implement. To add a new feature to your current design, choose a peripheral and add a small amount of code. To start using MICROWIRE, define the handshake protocol best suited for your application keeping in mind the six points given above in the ‘Defining the Master/Slave Handshaking Protocol’ section. Then initialize the appropriate registers: BFUN, DIRB, PORTB, DIVBY, and IRCD. The MICROWIRE circuitry will then run independent of the CPU except to exchange data between the SIO register and the CPU, and to initiate the data exchange between the master and slaves. With a CPU clock of 16 MHz, MICROWIRE/PLUS may achieve a maximum data rate of 1 MHz. MICROWIRE can be used to add display controllers, A/D’s, memories, timers, and even other microcontrollers to an HPC microcontroller based design. Remember MICROWIRE/PLUS is not a trivial piece of very fine wire, it is a high speed two way serial communications interface! Lit. Ý100474 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: AN-474 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. 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