Page 360 - Programming Microcontrollers in C

Other MC68HC16 Considerations 345

The data array starts out empty, and the coefficients are each 0x100.
The output is as you would expect—it starts and increases linearly
from 0 to 64 in the first 32 samples. The output then remains at 64 until
the end of the test. This linear ramp is exactly what one would expect
when sending a step function into the constant weighting function.
There is one important point to be found in this program. It was
stated earlier that the programmer should attempt to keep the code in
C whenever possible. In the above case, we introduced a rather simple
function in assembly language. The fact that the program was a func­
tion, and it was necessary to use a large memory model, the code
required to handle the data as it was passed into the function is much
longer than you would expect. Also, the implied C code in preparing
the machine for the function call and the handling of the extension
registers in the main program makes the overall program larger than
expected. Unfortunately, it is not possible to access the DSP with C,
so if you wish to do DSP operations, assembly language access is all
that you can use.

Other MC68HC16 Considerations

The discussion in this chapter has been dedicated to the
MC68HC16 microcontroller. Of course, there are components of this
microcontroller that are not covered in this chapter. No attempt was
made to outline the access to the analog-to-digital converter, several
features of the general purpose timer, the serial peripheral interface,
or the static RAM. However, the programs shown here do cover
enough of the part to demonstrate that the on-board peripherals can
be accessed from the C language. In the approach used, the header
files contain all of the bit field definitions needed to access any bit or
bit field in any control register in the part.