Page 201 - Programming Microcontrollers in C

186 Chapter 4 Small 8-Bit Systems

ferred to the proper output pin, TCMP1 or TCMP2. If the OCIE bit is
set in the timer control register, an interrupt will accompany the out­
put compare.
There are times when it is desirable to force an output compare
from a program. The FOLV1 and FOLV2 bits can be used for this
purpose. These bits will always read 0, but writing a 1 to these bits in
the TCR will cause transfer of the corresponding OLVL1 or OLVL2 bit
to the specified output compare bit, either TCMP1 or TCMP2. This
output does not affect the compare flags, so no interrupt is generated.

Programming the 16-bit Timer

We will examine several different uses of the 16-bit timer system
in this section. The first is merely a repeat of the simple timer pro­
grammed in the section on the 15-bit system. Here we merely want
to keep track of time, hours, minutes, and seconds in memory. No
provisions are made yet for reading the time values or to change the
values; these problems will be discussed later.
A listing of this program is shown below. In this case, the header
file for the M68HC05B6 is used. A listing of this file is found on the
CD-ROM. This program will make use of an output compare to gen­
erate periodic interrupts to the microcontroller. We will use output
compare register 1. It will be set up so that when the first output
compare interrupt occurs, the contents of the output compare regis­
ter will be incremented by 500. Since the clocking time of the counter
register is 2 microseconds, 500 2-microsecond periods will allow an
output compare every 1 millisecond. This occurrence will be treated
in the interrupt service routine.

#include “hc05b6.h”

int hrs, mts, sec; /* global variables */
long count=1000;

struct bothbytes /* 16 bit int structure */
{
int hi;
int lo;
};