Page 73 - Embedded Microprocessor Systems Real World Design

It is necessary to gate the I/O strobes with the -RD or -WR signals because
the address typically is held longer than the data for a write. If a write strobe was
just an address decode (not gated with -WR), the register would not get a clock
until the after data were gone. If the read strobe were not gated with -RD, an out-
put buffer would be enabled too long, and there may be bus contention at the
end of the bus cycle when the next one starts and the microprocessor tries to drive
the data bus. A second reason for gating the strobes is that while the address
is changing at the start of a bus cycle, the address lines may not all change at the
same time. Consequently, the wrong address may momentarily appear on the
address lines, and the wrong device could be selected. The decoding logic could
produce a short pulse on a write strobe signal, clocking garbage data into a regis-
ter. Gating read and write strobes with the control signals makes sure the strobes
go active only when address and data signals are stable. Figure 2.13 shows this
timing.
The last circuit in Figure 2.13 shows how a 22V10 (or other PLD) can be used
to generate address decodes and read/write strobes fi-om a single IC. This example
decodes a 16-bit (64K) address space, producing a 32K EPROM chip select from
addresses 8000 through m, an 8K RAM chip select from 0000 through lFFI?,
and a peripheral chip select from 3000 through 3m. Read strobes are generated
at 4000 and 4004, and write strobes are generated at 4000, 4001, and 4002. Since
the EPROM, RAM, and our hypothetical peripheral IC have their own -WE
and -0E inputs, the chip selects for these parts will not be gated with the -RD
and -WR signals from the microprocessor. The read/write strobes will be gated
with the control signals, however, because they are intended for clocking data
into a latch or for enabling a buffer. The following equations implement this
PLD in CUPL/AE%EL format (& is the logical AND function, # is the logical OR
function, a ! prefix indicates a low-true signal, and a double slash [//I precedes
comments).

IEPROMCS = A15; // 8000-FFF'F
!RAMCS = lA15 Be 1814 Be 1A13; /I 0000-1FF'F
!PERIPRCS = 1A15 & !A14 & A13 & A12; // 3000-3F'FF
!WSTBl = 1A15 & A14 & IA13 & !A12 Be lAll & lAlO &
!A9 & 1A2 & IAl Be !A0 Be IWR; // 4000
IWS"B2 = !A15 & A14 & lA13 & IA12 & lAll & lAlO &
IAQ Be lA2 Be 1Al & A0 & IWR; // 4001
lWS"B3 = 1A15 Be A14 & 1A13 & IA12 Be lAll & lAlO &
IA9 Be !A2 & A1 & IAO & !WR; // 4002
!RsTBl = lA15 & A14 & IA13 Be 1A12 & lAll & !A10 &
!A9 & lA2 8e !A1 & !A0 & ID; // 4000
IRsTB2 = 1A15 Be A14 Be 1A13 Be IA12 & lAll Be lAlO &
IAQ & A2 Be 1Al Be IAO & IRD; // 4004

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