Page 277 - Programming Microcontrollers in C

262 Chapter 5 Programming Large 8-Bit Systems

If the counts are converted to RPM prior to calculation of the error
signal, the reduction of sensitivity for large errors would not be a
problem. The problem that occurs when the motor is stopped still exists.
In this case, no input capture would occur when the motor is not rotating,
and the count value would be undefined. There seems to be no clear-cut
reason to choose the time or the velocity measurement. Each has
advantages and each has drawbacks. The drawbacks are about the same
for each, so we will choose the case that has the simplest program.
Therefore, the time-based or count-based system will be used.
Now comes the interesting problem of the calculation of 3000000/
RPM. Each time the desired speed of the motor is input to the system,
this calculation must be completed. The straightforward calculation
of this value will yield code as follows:

#include “hc11e9.h”

WORD count(WORD RPM)
{
unsigned long num = 30000000;

return num/RPM;
}

The listing file of the compiled version of this function is

1 ; Compilateur C pour MC68HC11 (COSMIC-France)
2 .include”macro.h11"
3 .list +
4 .psect _text
5 ; 1 #include “hc11e9.h”
6 .psect _data
7 _Register_Set:
8 0000 1000 .word 4096
9;2
10 ; 3 WORD count(WORD RPM)
11 ; 4 {
12 .psect _text
13 _count:
14 0000 BD0000 jsr c_kents