Page 86 - Embedded Microprocessor Systems Real World Design

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HIGH
DATA BUS DATA BUS DRIM
UNBUFFERED PERIPHERAL
PROCESSOR
DATA BUS
BIDIRECTIONAL - HIGH

Figure 2.20
Buffering a Microprocessor Data Bus.

using a buffer between the processor and a peripheral that requires write data
to be stable at the leading edge of the write strobe, make sure that propagation
delay through the buffer does not delay data far enough to cause problems.
Although usually fewer devices are tied directly to the processor address bus
(especially with a multiplexed bus), the same considerations apply. Use buffers if
the load exceeds the processor’s capacity. One simplifylng factor in these cases is
that the address bus usually needs only unidirectional buffers.

Nonvolatile Memory

In many designs, it is necessary for the processor to remember certain parameters
when the power is removed. Typical examples are the calibration parameters for
some kinds of sensors, the enable/disable code for a burglar alarm, and the last
channel selected on a television. In a multichip design, this can be accomplished
by either nonvolatile memory, described earlier, or with an EEPROM. The

70 Embedded Microprocessor Systems