Page 266 - Programming Microcontrollers in C

Timer Operations 251

prior to the expiration of the period. The following program can be
used to demonstrate this approach:
#include “hc11e9.h”
/* This program will provide a PWM output to OC3, or
PA5. The period will be the integer value
found in period,and the on time will be the
integer value found in time_on. Keep time_on
less than period. This program uses asynchro­
nous service of output compare occurrences. */

WORD period=0x1000, time_on=0x0800;
@port void OC3_Isr(void); /* need a prototype for the
ISR */
main()
{
OC1M.OC1M7=ON; /* sent OC1 tout to PA7 */
OC1M.OC1M5=ON; /* couple OC1 to OC3 */
TMSK1.OC3I=ON; /* enable the OC3 interrupt */
OC1D.OC1D5=ON; /* turn on OC3 when OC1 occurs */
TCTL1.OL3=ON; /* toggle OC3 when OC3 occurs */
PACTL.DDRA7=ON; /* make OC1 an output to PA7 */
TOC1=TCNT+period; /* set OC1 to the period */
TOC3=TOC1+time_on; /* set OC3 to the time on */
cli(); /* enable the system interrupts */
FOREVER
{ /* wait here forever */
}
}

@port void OC3_Isr( void)
{
TFLG1=OC1F; /* reset OC1 interrupt flag */
TOC1+=period;
OC1D.OC1D7 ^=ON; /* toggle the output */
TFLG1=OC3F; /* reset OC3 interrupt flag */
TOC3=TOC1+time_on;
}

Listing 5-5: System Using Asynchronous Time Service PWM1.C