Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 402

8.7.6 Pulse Accumulator Applications
The pulse accumulator has a few interesting applications, such as interrupting after N
events, frequency measurement, and pulse duration measurement.

Example 8.13
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Suppose certain events are converted into pulses and connected to the PT7 pin. Write a pro-
gram so that the pulse accumulator generates an interrupt to the HCS12 when N events have
occurred. Assume that N is less than 65,536.
Solution: By writing the two's complement of N into PACNT, it will overflow after N events
and generate an interrupt. The assembly program is as follows:
#include “c:\miniide\hcs12.inc”

N equ 1350

org $1500

lds #$1500 ; set up stack pointer

movw #paov_isr,UserPAccOvf ; set up PAOV interrupt vector

ldd #N ; place the two's complement in PACNT

coma ; “

comb ; “

addd #1 ; “

std PACNT ; “

movb #$52,PACTL ; enable PACA, event counting mode, active

; edge is rising

cli ; enable PAOV interrupt

; . . .

swi

paov_isr movb #PAOVF,PAFLG ; clear the PAOVF flag

end
The C language version of the program for CodeWarrior C compiler is as follows:
#include “c:\cwHCS12\include\hcs12.h”
#define NN 1350
void main(void) {

PACNT 5 , N 1 1;

PACTL 5 0x52; //enable PACTL, event count mode,

increment on rising edge

asm(“cli”); //enable PACNT overflow interrupt

// . . .

}
interrupt void paovISR(void) {

PAFLG 5 PAOVF; //clear the flag and return

}

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