Interfacing The DP8420A/21A/22A To Z280/Z80000/Z8000 Microprocessor AN 0546

User Manual: AN-0546

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I INTRODUCTION
This application note describes how to interface the Z280
microprocessor to the DP8422A DRAM controller (also applicable to DP8420A/21A). It is assumed that the reader is
already familiar with Z280 and the DP8422A modes of operation. The interface to the Z80000 and Z8000 is similar to
the interface described in this application note.
II DESCRIPTION OF DESIGN, ALLOWING OPERATION
AT 10 MHz (AND ABOVE) WITH 1 WAIT STATE IN NORMAL ACCESSES AND 1 WAIT STATE DURING BURST
ACCESSES
The block diagram of this design is shown driving two banks
of DRAM, each bank being 16 bits in width, giving a maximum memory capacity of up to 16 Mbytes (using 4 M-bit c
1 DRAMs). By choosing a different RAS and CAS configuration mode (see programming mode bits section of DP8422A
data sheet) this application can support 4 banks of DRAM,
giving a memory capacity of up to 32 Mbytes (using 4 M-bit
c 1 DRAMs).
The memory banks are interleaved on every four word (16bit word) boundry. This means that the address bit (A3) is
tied to the bank select input of the DP8422A (B1).
Address bits A2,1 are tied to the most significant row and
column address inputs (R9,C9 for 1 Mbit DRAMs) to support
burst accesses using static column mode DRAMs. Since
this application assumes the use of static column DRAMs
the column address strobe (CAS) is left low throughout the
entire burst access. If the user desires to use nibble mode
or page mode DRAMs the CAS outputs must be toggled,
the ECAS inputs the DP8422A can be used for this purpose
(DS of the Z280 could be ‘‘OR’’ed with the current ECAS
inputs). If nibble mode DRAMs are used the COLINC input
of the DP8422A need not be driven.
Address bit A0 is used to produce the two byte select data
strobes along with the byte/word signal (B/W). These byte
selects (Byte 0 ECAS and Byte 1 ECAS) are used in byte
reads and writes as well as selects for the transceivers.
If the majority of accesses made by the Z280 are sequential, the Z280 can be doing burst accesses most of the time.
Each burst of four words can alternate memory banks, allowing one memory bank to be precharging (RAS precharge) while the other bank is being accessed (Bank select, B1, tied to address A3). This is a higher performance
memory system then a non-interleaved memory system
(bank select on the higher address bits). Each separate
memory access to the same memory bank may require extra wait states to be inserted into the CPU access cycles to
allow for the RAS precharge time, if two periods or more of
RAS precharge were programmed.

National Semiconductor
Application Note 546
Webster (Rusty) Meier, Jr. and Joe Tate
May 1989

This application allows 1 or more wait states to be inserted
in normal accesses and 1 or more wait states to be inserted
during burst accesses of the Z280. The number of wait
states can be adjusted through the WAITIN input of the
DP8422A.
The logic shown in this application note forms a complete
Z280 memory sub-system, no other logic is needed. This
sub-system automatically takes care of:
A. arbitration between Port A, Port B, and refreshing the
DRAM;
B. the insertion of wait states to the processor (Port A and
Port B) when needed (i.e., if RAS precharge is needed,
refresh is happening during a memory access, the other
Port is currently doing an access . . .etc);
C. performing byte writes and reads to the 16-bit words in
memory;
D. normal and burst access operations.
The external wait logic (U1, U2, U3, U4; see Figure 1 ) is
needed to support burst accesses of the Z280. During burst
accesses the Z280 WAIT input is sampled every falling
clock edge. What is worse is that the WAIT input needs one
half clock period setup time and the DS signal (used to toggle ECAS0 – 3 and thereby toggle the DP8422A WAIT output) takes close to one half of a clock period to transition
high. This leaves no time for the DP8422A WAIT output to
transition between states. The external flip-flop is used to
provide extra fast response time for normal access wait
states and to toggle when doing a burst mode access. If the
user is not going to do burst accesses the WAIT output can
be tied directly to the WAIT input of the Z280 (U1, U2, U3,
U4 would not be needed). Also all this logic could easily be
put into a PALÉ if desired.
By using the ‘‘output control’’ pins of some external latches
(74ALS373’s), this application can easily be used in a dual
access application. The addresses could be TRI-STATEÉ
through these latches, the write input (WIN), lock input
(LOCK), and ECAS0 – 3 inputs must also be able to be TRISTATE (a 74AS244 could be used for this purpose). By multiplexing the above inputs (through the use of the above
parts and similar parts for Port B) the DP8422A can be used
in a dual access application. If this design is used in a dual
access application the tRAC and tCAC (required RAS and
CAS access time required by the DRAM) will have to be
recalculated since the time to RAS and CAS is longer for
the dual access application (see TIMING section of this application note).

AN-546

PALÉ is a registered trademark of and is used under license from Monolithic Memories, Inc.
TRI-STATEÉ is a registered trademark of National Semiconductor Corporation.
C1995 National Semiconductor Corporation

TL/F/9740

Interfacing the DP8420A/21A/22A to the Z280/Z80000/Z8000 Microprocessor

Interfacing the
DP8420A/21A/22A to the
Z280/Z80000/Z8000
Microprocessor

RRD-B30M115/Printed in U. S. A.

2A. Minimum address setup time to ALE low (DP8422A-20
needs 3 ns, Ý306):

III Z280 DESIGN, 10 MHz WITH 1 WAIT STATE DURING
NORMAL ACCESSES AND 1 WAIT STATE DURING
BURST ACCESSES, PROGRAMMING MODE BITS
Programming
Bits
R0 e 0
R1 e 1

R2 e 0
R3 e 1
R4 e 0
R5 e 0
R6 e 0

R7 e 1
R8 e 1
R9 e X
C0 e X
C1 e X
C2 e X

C3
C4
C5
C6

e
e
e
e

X
0
0
1

C7 e 1
C8 e 1
C9 e 1
B0 e 0
B1 e 0
ECAS0 e 0

25 ns (address setup to AS high, Ý20 Z280 data sheet)
a 1 ns (74ALS04B min delay) e 26 ns

Description

2B. Minimum address hold time to ALE low (DP8422A-20
needs 10 ns, Ý305):

RAS low two clocks, RAS precharge
of two clocks, this setup
will only guarantee 93.5 ns RAS
precharge (at 10 MHz) from refresh
RAS high to access RAS low. If more
RAS precharge is desired the user
should program three periods of RAS
precharge.
DTACK one half is chosen. DTACK
low first rising CLK edge
after access RAS is low.
No WAIT states during burst accesses

20 ns (address hold from AS high, Ý22 of Z280 data
sheet) a 1 ns (74ALS04B min delay) e 21 ns
2C. Minimum address setup to CLOCK high (DP8422A-20
needs bank address setup to CLOCK of 20 ns, Ý303):
100 ns (one clock period) b 20 ns (max clock to address valid, Z280 data sheet Ý2) e 80 ns
3. Minimum CS setup time to clock high (DP8422A-20
needs 14 ns, Ý300): 80 ns (Ý2C above) b 22 ns (max
74ALS138 decoder) e 58 ns
4. Determining tRAC during a normal access (RAS access
time needed by the DRAM):
250 ns (two and one half clock periods to do the access) b 32 ns (CLK to RAS low max, DP8422A-20
Ý307) b 30 ns (Z280 data setup time, Ý9) b 10 ns
(74ALS245A max delay) e 178 ns
Therefore the tRAC of the DRAM must be 178 ns or
less. (One can see that if zero wait states would have
been programmed the tRAC would have been 84 ns (using DP8422A-25, has faster CLK to RAS low of 26 ns)
184 – 100 (one clock)).
5. Determining tCAC during a normal access (CAS access
time) and column address access time needed by the
DRAM:
250 ns b 89 ns (CLK to CAS low on DP8422A-20,
Ý308a) b 30 ns b 10 ns e 121 ns
Therefore the tCAC of the DRAM must be 121 ns or
less.
6. Determining the column address access time needed
during a static column mode burst access:
20 ns (two clocks to do the access, Ex. mid T3 to mid
TBW to mid T4) b 35 ns (DS high, Z280 parameter Ý8)
b 43 ns (COLINC asserted to address outputs of
DP8420A-20 incremented, Ý27) b 30 ns (Z280 data
setup time, Ý9) b 10 ns (74ALS245A max delay) e 82
ns
Therefore the column address access time of the
DRAM must be 82 ns or less. (One can see that if zero
wait states would have been programmed the column
address access time would have been less then 0 ns
(82 b 100 (one clock))).
7. Maximum time to DTACK one half low (74ALS374 D
type flip-flop needs 10 ns setup to CLK):
100 ns (One clock, mid T2 in mid TW) b 33 ns (DTACK
one half low from CLK high on DP8422A-20, Ý18) b 12
ns (max delay on 74ALS02 e 55 ns
8. Minimum WAIT setup time to CLK low (Z280 WAIT input
needs 50 ns, Ý14):
100 ns (one clock period) b 16 ns (74ALS374 max delay) b 14 ns (74ALS08 max delay) e 70 ns
9. Minimum RAS precharge (DP8422A programmed with 2
clock periods of RAS precharge):
Since the AREQ input of the DP8422A will go high from
DS and IE both being high the AREQ high setup to
clock rising edge (DP8422A parameter Ý29b, 19 ns)
parameter is violated. This means that the rising clock
edge following AREQ high may or may not be counted.

If WAITIN e 0, add one clock to
DTACK. WAITIN may be tied high or
low in this application depending upon
the number of wait states the user
desires to insert into the access.
Select DTACK
Non-interleaved Mode
Select based upon the input
‘‘DELCLK’’ frequency. Example: if the
input clock frequency is 10 MHz then
choose C0,1,2 e 1,0,1 (divide by five,
this will give a frequency of 2 MHz).
RAS groups selected by ‘‘B1’’. This
mode allows two RAS outputs to go
low during an access, and allows byte
writing in 16- or 32-bit words.
Column address setup time of 0 ns
Row address hold time of 15 ns
Delay CAS during write accesses to
one clock after RAS transitions low
Latches latch on ALE input low
Access mode 0
CASn not extended beyond RASn

0 e Program with low voltage level
1 e Program with high voltage level
X e Program with either high or low voltage level
(don’t care condition)
IV Z280 TIMING CALCULATIONS FOR DESIGN AT
10 MHz WITH 1 WAIT STATE DURING NORMAL
ACCESSES AND 1 WAIT STATE DURING BURST
ACCESSES
1. Minimum ALE high setup time to CLOCK high if using the
on-chip latches and more then one RAS bank (DP8422A20 needs 29 ns, Ý301b):
100 ns (one clock period) b 20 ns (AS valid maximum
delay, Ý3 of Z280 data sheet) b 11 ns (74ALS04B max
delay) e 69 ns

2

Therefore, the user should guarantee that the DRAM
he is using needs a RAS precharge time of 93.5 ns or
less. If more RAS precharge time is needed the user
should program the DP8422A with 3 periods of RAS
precharge (R0, R1) during programming.

Since that first rising clock edge could be counted, and
would give less RAS precharge time, we must assume
this condition in the calculation of the minimum RAS
precharge. Therefore:
200 ns (2 clock periods) b 50 ns (half clock period
before both IE and DS transition high) b 35 ns (IE and
DS high, Z280 parameters Ý8 and Ý19) b 5.5 ns
(74AS08 max delay) b 16 ns (DP8422A RAS high to
RAS low difference parameter Ý50) e 93.5 ns

Note: Calculations can be performed for different frequencies and/or different combinations of wait states by substatuting the appropriate values
into the above equations.

TL/F/9740 – 1

*The user may want to gate CS (‘‘OR’’ Gate) with the signals that produce OE to the DRAMs and EN to the transceivers

FIGURE 1. 10 MHz Z280 Design (Z-bus Interface), 1 Wait State in Normal Accesses, 1 Wait State in Burst Accesses

TL/F/9740 – 2

FIGURE 2. Z280 Access Cycles and Refresh (1 Wait State during Normal Access Cycles)

3

Interfacing the DP8420A/21A/22A to the Z280/Z80000/Z8000 Microprocessor
AN-546

TL/D/9740 – 4

FIGURE 3. Z280 Burst Access Cycle (1 Wait State in Normal and Burst Accesses)
DRAM Speed vs Processor Speed (DRAM Speed
References the RAS Access Time, tRAC, of the DRAM.
Using DP8422A-25 Timing Specifications)

TL/D/9740 – 5

Lit. Ý 100546

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File Type                       : PDF
File Type Extension             : pdf
MIME Type                       : application/pdf
PDF Version                     : 1.3
Linearized                      : Yes
Create Date                     : 1995:10:17 08:59:31
Producer                        : Acrobat Distiller 2.0 for Windows
Title                           : Interfacing the DP8420A/21A/22A to the Z280/Z80000/Z8000 Microprocessor
Subject                         : AN-546
Author                          : 
Keywords                        : Application, Notes
Modify Date                     : 2001:11:23 11:45:01+05:30
Page Count                      : 4

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