Page 80 - Embedded Microprocessor Systems Real World Design

Extended Data Hold Time
Occasionally you run into a peripheral IC that requires write data to be held for
some time after the WRITE strobe goes away. A typical example is the LM628 motor
controller IC from National Semiconductor. Most microprocessors do not guaran-
tee a data hold time long enough for parts that need an extended data hold time.
There are two ways to implement an interface to parts like this. The first method
is to latch the data and leave the buffer enabled to the peripheral device (see Figure
2.18). This provides the fastest transfer speed since the processor timing is unaf-
fected. However, it requires that you implement a bidirectional latching data buffer
for the peripheral part. Also, the data latch works only if the processor cannot do
another write quickly enough to change the latch contents too soon and violate the

ADDR. -CS '-Xp)
DATA * X X - '
-WR 1 DATA HOLD TIME
-WR -
NEEDED BY PERIPHERAL

ADDR. -CS =-x-'

PROCESSOR DATA X X - DATA HOLD TIMING FIX USING
BIDIRECTIONAL DATA LATCH
LATCHED PERIPHERAL DATA X X - TO HOLD DATA
f
DATABUFFER
LATCHES ON
RISING EDGE
OF WRITE STROBE

ADDR, -CS
DATA HOLD TIMING FIX USING
PROCESSOR DATA WAIT STATES TO EXTEND
PROCESSOR WRITE CYCLE
PROCESSOR -WR
PERIPHERAL -WR
-WAIT
4 &- Wa% state exlends processor
I I -WR signal by this amount
GATING LOGIC FOR -WR SIGNAL
PROCESSOR -WR
-WR TO PERIPHERAL
-WAIT

Figure 2.18
Extended Data Hold Time.

64 Embedded Micr@rocessm Systems