Page 25 - Embedded Microprocessor Systems Real World Design

seems that we must go to a much faster, more expensive processor to keep up with
the motor, thus raising the cost of the system.
There is another solution, however. Many microprocessors have PWM outputs
or timers that can be configured to operate as PWM outputs. Typical examples are
the Microchip PIC 16C/17C family, the Atmel ATSOS family, and the Intel 80C196
series. Using the internal PWM controller relieves the microprocessor of the
burden of generating every motor current change. Instead, the processorjust sends
changes in the duty cycle (or frequency) to the PWM controller.
This is just one example of how picking the right processor can solve a real-time
problem. Other examples include selecting a processor with a built-in, high-speed
serial port for interprocessor communications; selecting a processor with an on-
chip direct memory access (DMA) controller (more about that in a later chapter);
or selecting a processor with special memory manipulation registers that will speed
things up. Sometimes you can find a microcontroller that has exactly the right inter-
face for your application, such as an onchip LCD controller.

Development Environment
The development environment often is a key consideration. New development tools
require a learning curve, and with a tight development schedule there often is no
time to research, acquire, and become proficient with a new set of tools. If the
company has several tens of thousands of dollars (a not unrealistic figure) invested
in emulators for a specific processor, and if all the software engineers are com-
fortable with those tools, someone usually objects to changing processors just so an
enthusiastic engineer can tinker with the latest chip. That is not much fun for the
frustrated engineer, but it is an economic fact of life. This is why some companies
(or subsidiaries within very large companies) expend a great deal of effort to pick
a processor family they can live with for a long time.
Even if a design starts with a blank slate, however, the development tools can be
a major consideration. For example, selecting a widely used processor, such as the
8031, allows you to select from a wide array of tools from a number of vendors. The
capability of these tools (such as emulators) can be matched to whatever budget
you have. On the other hand, the tools for some specialized processors are avail-
able only from the manufacturer, and the cost can be prohibitive.
Tools can be a major factor. If the processor choice gets down to just two,
researching the cost of tools may make the decision obvious. In any event, be sure
you know the cost of these tools, especially emulators from the IC manufacturer,
before you make the final selection.
If you’re planning to use an RTOS (real-time operating system), the choice of
which one to use also may drive your processor selection. RTOSs come in various
flavors, with some charging a onetime fee and others charging a license fee or

10 Embedded Microprocessor Systems