Page 153 - Embedded Microprocessor Systems Real World Design

in the HLL and then writing the functions in assembler. You can waste a lot of time
trying to get this to work if the vendor does not document things well.
Another problem with microcontroller-based compilers is that the compiler
tends to generate code that is generic-the code produced for a do/while loop
may not take advantage of instructions that can speed up the code considerably if
you are ending when a variable reaches zero or something similar.
An example involves a design I did using a Microchip PIC device. The proces-
sor functions as a smart sensor, communicating with an external PLD that collects
time-based data. To make the PLD interface work, the processor needs one loop
to run very fast. The C compiler I was using simply would not generate any kind
of loop that terminated with a DECFSZ instruction, which decrements a memory
location, stores the result, and branches if the result is zero. In addition, a feature
of the PIC that involves a shift-and-test operation was implemented inefficiently. To
make matters worse, the assembly language interface for that compiler was poorly
documented, making the task of linking an assembly module very difficult.
The original code, prototyped in assembly, took less than a day to write, debug,
and verify. Attempting to write a C version and then to merge the original assem-
bly version with the final C program took three days and numerous email exchanges
with the technical support group at the vendor that supplied the compiler.
While the language is the tool that actually implements a design, other tools
document it. Flowcharting tools simplify flowchart generation, with linkages be-
tween blocks “rubberbanding” as the blocks are moved around on the page. Some
tools offer a complete development environment, from flowchart to finished code.

Debugging Tools

When selecting tools, give some thought to how the design will be debugged. If you
are developing an embedded system based on the PC architecture, you probably
will get some kind of debugger with the compiler. However, on a smaller design,
there are other considerations. The most severe restrictions occur when you are
dealing with microcontrollers. For example, some debugger software for micro-
controllers requires using the onchip serial port for communication with a host
PC. If your design uses the serial port to communicate with other processors in the
system, you must find another debugger solution.
Even some emulators require system resources (such as serial ports) that you
may need for your design. Check into this carefully before you choose your devel-
opment environment. Make sure you do not paint yourself into a corner.
Debugging systems on very small microcontrollers presents special challenges.
The best solution is an emulator. However, sometimes an emulator is not available
for cost reasons. I have worked on microcontroller-based designs in which an
emulator simply would not fit in the space available or the controller board was on
a moving robotic arm and attaching an emulator was out of the question.

134 Embedded Micrcprocessor Systems