Page 21 - Embedded Microprocessor Systems Real World Design

separate them from the more general-purpose embedded processors. Since the
microcontroller does not need to generate signals to external memory, the device
pins are available for I/O. These pins are grouped as ports, and each pin may be
an input or an output. In our example system, one pin might turn on the pool
pump relay. Another pin might allow the processor to monitor the water level
sensor.
Most microprocessor manufacturers make a controller with internal memory
and external pins for controlling 1/0 devices. While it is impossible to list all the
variations and subtleties of these devices here, a brief list of typical devices follows:

Manufacturer Processor I/O Pins

Intel and others 8031/8051 family 32
Microchip PIC17C42 33
Motorola 68HC05 family varies
Zilog 28 (Z86E40) 32
Signetics/Philips 836751 19
Atmel AT9OS8515 32

This list cannot describe all the tradeoffs among the various parts. Some of these
parts include a bidirectional serial interface, for example, but you must give up two
port pins to use it. Some have internal timers that use a port pin for certain func-
tions. Some have highcurrent and opendrain outputs that are ideal for driving
relay or solenoid coils with no additional driver hardware. The specific IC that is
ideal for your application depends on the application.
When counting 1/0 pins, make sure that you take into account the use of inter-
nal functions, such as serial ports and timers, that restrict the use of certain pins.
Although we’ll discuss this in more detail in Chapter 2, keep in mind that some of
these parts support external RAM or ROM, but using that capability takes anywhere
from 8 to 19 1/0 pins to access the external memory.

Interfaces Required
The entire point of an embedded processor is to interact with some piece of real-
world hardware. Not only must the hardware be in place to handle the interface,
the processor must be fast enough to perform whatever processing must be done
on the data. In a singlechip system, processor selection may be highly dependent
on the interface requirements. For example, the Microchip PIC17C42 has two
pulse-width modulation (PWh4) outputs that simplify design of such things as
antilock braking systems and motor servos. One caveat: Study the data sheets care-
fully. Many processors have limitations that are not immediately obvious. You might
find, say, that the serial port is specified as being able to operate at a certain

6 Embedded MicroproGessor Systems