Page 74 - Embedded Microprocessor Systems Real World Design

Peripheral UO Integrated Circuits
The advantage of using discrete latches and buffers for 1/0 is simplicity. The
disadvantages are:
Unidirectional operation. The latch outputs cannot be read to determine
whether a particular bit is set.
Not programmable. The inputs are always inputs; the outputs are always
outputs. If you need nine inputs instead of eight, but only seven outputs, you
cannot use a latch output as an input-you must add another 74AC244 buffer.
PC board real estate. Each new set of eight inputs or outputs requires another
IC and another output from the decoding logic.
Interface. The requirement for discrete read/write strobes to each device
complicates interface with 68000- or Z8type processors that generate a
common data strobe and direction signal.
In addition to these, another problem is that this type of discrete 1/0 is limited
to just that-digital 1/0 bits. A design often requires other functions, such as a
timer, serial interface, or ADC, which cannot be implemented with simple latches.

Peripheral ICs

Most microprocessors intended for multichip designs have peripheral ICs as part
of the product family. These include timer/counters, serial interface chips, and
port expansion. A few examples are described here.
Timers

A timer peripheral consists of a counter that decrements or increments at some
clock rate. The processor can read the count, and the timer may generate an inter-
rupt or pulse an output pin when the count rolls over to zero. Some timer ICs allow
one timer to be cascaded from another for long delays. The timer output varies
with the particular IC used; many have outputs that can be programmed for a
square wave, single pulse on output, or variable duty cycle. In addition to the count,
the processor can control timer start/stop and modes of operation. Typical uses
for a timer IC are to generate a delay, usually for scheduling some real-time event,
controlling motors (DC PWM or stepper), and generating a regular timekeeping
interrupt. We’ll cover timers in more detail in Chapter 3.

UO Ports
These ICs provide a multichip design with the same programmable 1/0 port capa-
bility as a microcontroller. A typical 1/0 port IC may provide three or four &bit

58 Embedded Microprocessor Systems