Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 376

The assembly program that implements this algorithm is as follows:

#include “c:\miniide\hcs12.inc”

org $1000

edge1 ds.w 1

ovcnt ds.w 1

PW ds.w 1

org $1500

movw #tov_isr,UserTimerOvf; set up TCNT overflow interrupt vector

lds #$1500 ; set up stack pointer

movw #0,ovcnt

movb #$90,TSCR1 ; enable TCNT and fast timer flag clear

movb #$05,TSCR2 ; disable TCNT interrupt, set prescaler to 32

bclr TIOS,IOS0 ; select IC0

movb #$01,TCTL4 ; capture the rising edge

movb #$01,TFLG1 ; clear the C0F flag

brclr TFLG1,C0F,* ; wait for the first rising edge

movw TC0,edge1 ; save the first edge, clear the C0F flag

movb #$80,TFLG2 ; clear the TOF flag

bset TSCR2,$80 ; enable TCNT overflow interrupt

cli ; “

movb #$02,TCTL4 ; capture the falling edge on PT0 pin

brclr TFLG1,C0F,* ; wait for the arrival of the falling edge

ldd TC0

subd edge1

std PW

bcc next ; is the second edge smaller?

ldx ovcnt ; second edge is smaller, so decrement

dex ; overflow count by 1

stx ovcnt ; “

next swi

tov_isr movb #$80,TFLG2 ; clear the TOF flag

ldx ovcnt

inx

stx ovcnt

rti

end

The C language version of this program to be compiled by the CodeWarrior C compiler is as
follows:

#include “c:\cwHCS12\include\hcs12.h”

unsigned diff, edge1, overflow;

unsigned long pulse_width;

void main(void)
{

overflow 5 0;

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

TSCR2 5 0x05; // set prescaler to 32, no timer overflow interrupt

TIOS & 5 , IOS0; // select input-capture 0