Page 19 - Embedded Microprocessor Systems Real World Design

EMC (electromagnetic interference/electromagnetic compatibility) certification,
safety agency approval (UL/IEC) , and environmental specifications (temperature,
humidity, salt spray, and so on).
Although we’ll discuss this further in Chapter 7, one problem with specifylng
requirements is verlfylng them. It is easy to determine whether the product meets
the EMI/EMC requirements-you can run tests to prove it. But how do you prove
you’ve met the requirement for “minimum switches and knobs”? Thus, keep in
mind the problem of verification when specifylng requirements.
A complex system may have another level of documentation, which I usually
refer to as the Engineering Specijication. This document describes the approach that
will be used to implement the design, including which boards will be included and
how the functions are partitioned onto those boards. I will return to this informa-
tion later, in Chapter 8. For now, assume that we have a simple product, which
makes this intermediate document unnecessary.
After the requirements are defined, the next step is to determine whether
a microprocessor is the best choice. For the pool timer, it is fairly obvious that a
microprocessor is the easiest way to do the job. Some other systems are not so
obvious. The following questions can help determine whether a microprocessor is
justified:

At what speed must the inputs and outputs be processed or updated? Although
the clock rates are ever increasing, there is a practical upper limit to the speed
at which a microprocessor can read an input or update an output and still do
any real work. At the time of this writing, an update rate of a few hundred kHz
is a practical upper limit for a simple microprocessor system with few processing
demands and running on a fast processor or digital signal processor (DSP). If
the system must do significant processing, buffer manipulation, or other com-
puting, the potential update rate will decrease.
Is there a single integrated circuit (IC) or a programmable logic device (PLD)
that will do the job? If so, a microprocessor is probably not justified.
Does the system have a lot of user I/O, such as switches or displays? If so, a micro-
processor usually makes the job much easier.
What are the interfaces to other external systems? If your system must talk to
something else using Synchronous Data Link Control (SDLC) or some other
complex communication protocol, a microprocessor may be the only practical
choice.
How complex is the computational burden on the system? Modern electronic
ignition systems, for example, have so many inputs (air sensors, engine rpm, and
so on) with complex relationships that few choices other than a microprocessor
are suitable.
Will the design need to be changed once it is finished, or will the requirements
be changing as the design progresses? Is there a need for customization of the

4 Embedded Microprocessor Systems