Page 39 - Embedded Microprocessor Systems Real World Design

consumer product, functions that cannot be performed in software may simply be
left out.

Distributed Processor Systems

We’ll cover multiprocessor systems in more detail in Chapter 8. Here, we summa-
rize the tradeoffs involved in choosing a multiprocessor architecture. A distributed
processor system might have a single CPU that communicates with a host computer
and distributes commands and data to lower-level processors that control motors,
collect data from sensors, or perform some other, simpler task. Distributed proces-
sor systems have the following advantages:
The actual processing hardware can be located near the device being controlled
or monitored. In large equipment, this may be a real advantage.
If some of the functionality is optional, the cost of the processor that controls
the option can be added or removed with the option.
In a distributed processor system, each of the distributed CPUs usually can be
a lower-performance (cheaper) part than would be required for one central
CPU.
A distributed system can be designed with a better match between the CPU and
the task it must perform. In a single-CPU system, the CPU must be fast enough
and have enough memory and so forth to perform all the tasks, whether they
are simple or complex.
The code for any given CPU in a distributed system usually is simpler.
It is easier to determine whether the CPU power is adequate in a distributed
system because fewer tasks are being swapped in and out and there is less inter-
action among the various processing that must be performed. For example, you
need not worry about how the motor control function affects the serial interface
throughput if the tasks are handled by separate processors.
Debug of distributed systems can be simpler since each processor performs a
limited set of tasks.

The advantages of a single-CPU system are:
Synchronization, when needed, is easier. For instance, it is easier for a single-CPU
system to synchronize motor startup to limit current surge simply by communi-
cation between tasks or by scheduling. In a distributed system, such synchro-
nization must be performed by CPU-t&PU communication or back through a
common control CPU.
All the data is in the same place, making communication with a host or other
systems easier. Fewer communication protocols are required to pass data around.

24 Embedded Microprocessor System