Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 384

Step 5
The main program continues to perform other operations.
The interrupt service routine of OC0 performs the following operations:
If ( HiorLo 55 1)

Start an OC0 operation using DelayHi as the delay count

HiorLo ← 0

Return from interrupt

Else
Start an OC0 operation using DelayLo as the delay count
HiorLo ← 1
Return from interrupt.
The assembly program that implements this algorithm is as follows:
#include

“c:\miniide\hcs12.inc”

DelayHi equ 18000 ; pulse high interval duration

DelayLo equ 12000 ; pulse low interval duration

NN equ 10 ; number of pulses to be created

org $1000

pcount ds.b 1 ; number of OC0 operations remaining to be performed

HiorLo ds.b 1 ; flag to choose DelayHi(1) or DelayLo(0)

org $1500

lds #$1500

movw #oc0ISR,UserTimerCh0 ; set up OC0 interrupt vector

movb #$90,TSCR1

movb #$04,TSCR2

bset TIOS,OC0 ; enable OC0

movb #C0F,TFLG1 ; clear C0F flag

movb #$03,TCTL2 ; set OC0 pin action to pull high

ldd TCNT ; use OC0 operation to pull TC0 pin high

addd #12 ; quickly

std TC0 ; “

brclr TFLG1,C0F,* ; wait until C0F flag is set

movb #$01,TCTL2 ; set OC0 pin action to toggle

ldd TC0 ; start next OC0 operation with

addd #DelayHi ; delay set to DelayHi

std TC0 ; “

movb 2*NN-1,pcount ; prepare to perform pcount OC0 operations

clr HiorLo ; next OC0 operation use DelayLo as delay

bra $

oc0ISR ldaa HiorLo ; check the flag to choose delay count

beq pulseLo ; if flag is 0, then go and use DelayLo

ldd TC0 ; start an OC0 operation and use

addd #DelayHi ; DelayHi as delay count

std TC0 ; “

movb #0,HiorLo ; toggle the flag

bra decCnt

pulseLo ldd TC0 ; start an OC0 operation and use

addd #DelayLo ; DelayLo as delay count

std TC0 ; “

movb #1,HiorLo ; toggle the flag