Page 219 - Programming Microcontrollers in C

204 Chapter 4 Small 8-Bit Systems

}
}
else /* must be that OCF1==1 rather than OCF2
*/
{
time_cnt.b.hi=OCHI1; /* output compare */
time_cnt.b.lo=OCLO1; /* get lo byte */
time_cnt.l +=count; /* time for interrupt */
OCHI1=time_cnt.b.hi;
OCLO1=time_cnt.b.lo;/* reset the OCF */
ms_10++;
return;
}
}

Listing 4-6: ISR For PWM System
Since the interrupt bits are set to allow output compare interrupts,
an interrupt will occur. The interrupt service routine will be entered,
and the instruction sequence

if( TSR.OCF2==1) /* accesses the TSR */
{
.
.
.
accomplishes two things. First, it determines if the output compare 2
caused the interrupt, and, secondly, it accesses the TSR and arms the
reset of the OCFX when the proper lower byte of the output compare
register is read later.
For the moment, assume that OCF2 bit is set: the interrupt was
caused by a compare in output compare register 2. In this case a test
is made to determine if flag.ON is reset. Since the initialization rou­
tine reset this bit, it will be 0. Therefore, the code sequence
flag.ON=1;
TCR.OLVL2=0; /* Timer Compare 2 to turn off*/
time_cnt.b.hi=OCHI2; /* get the start time */
time_cnt.b.lo=OCLO2; /* reset the OCF */
time_cnt.l +=pwm_number;