Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 404

movb #$90,TSCR1 ; enable TCNT and fast timer flag clear
bset TIOS,OC0 ; select OC0 function
movb #$03,TSCR2 ; set prescaler to TCNT to 8
bclr DDRT,$80 ; configure PT7 for input
; configure PA function: enable PA, select event-counting mode, rising edge
; of PAI signal increments the PACNT counter, enable PAOV interrupt

movb #$52,PACTL

cli ; enable PAOV interrupt

ldd TCNT

sec_loop addd #60000

std TC0

brclr TFLG1,C0F,* ; wait for 20 ms here

ldd TC0

dec oc_cnt

bne sec_loop

movb #0,PACTL ; disable PA function

sei ; disable interrupt

ldd PACNT

std frequency 1 2

ldd paov_cnt

std frequency

swi

paov_isr movb #PAOVF,PAFLG ; clear the PAOVF flag

ldx paov_cnt ; increment PACNT overflow

inx ; count by 1

stx paov_cnt ; “

end
The C language version of the program is as follows:
#include “c:\cwHCS12\include\hcs12.h”
unsigned long int frequency;
unsigned int paov_cnt;
void main (void)
{

int oc_cnt;

PACNT 5 0;

frequency 5 0;

paov_cnt 5 0;

TSCR1 5 0x90; // enable TCNT and fast flag clear

TIOS 5 OC0; // select OC0 function

TSCR2 5 0x03; // set prescale factor to 8

PACTL 5 0x52; // enable PA function, enable PAOV interrupt

DDRT & 5 0x7F; // configure the PT7 pin for input

asm(“cli”); // enable interrupt globally

oc_cnt 5 50;

TC0 5 TCNT 1 60000u;

while (oc_cnt) {

while(!(TFLG1 & C0F));

TC0 5 TC0 1 60000u;