Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 387

The assembly program that implements this algorithm for an HCS12DP256 running with
a 24-MHz E-clock is as follows:
#include “c:\miniide\hcs12.inc”
hi_freq equ

1250 ; delay count for 1200 Hz (with 1:8 prescaler)

lo_freq equ 5000 ; delay count for 300 Hz (with 1:8 prescaler)

toggle equ $04 ; value to toggle the TC5 pin

org $1000

delay ds.w 1 ; store the delay for output-compare operation

org $1500

lds #$1500

movw #oc5_isr,UserTimerCh5 ; initialize the interrupt vector entry

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

movb #$03,TSCR2 ; set main timer prescaler to 8

bset TIOS,OC5 ; enable OC5

movb #toggle,TCTL1 ; select toggle for OC5 pin action

ldd #hi_freq

std delay ; use high-frequency delay count first

ldd TCNT ; start the high-frequency sound

addd delay ; “

std TC5 ; “

bset TIE,OC5 ; enable OC5 interrupt

cli ; “

forever ldy #5 ; wait for half a second

jsr delayby100ms ; “

movw #lo_freq,delay ; switch to low-frequency delay count

ldy #5

jsr delayby100ms

movw #hi_freq,delay ; switch to high-frequency delay count

bra forever

oc5_isr ldd TC5

addd delay

std TC5

rti
#include c:\miniide\delay.asm”
end
In this program, both the OC0 and OC5 channels rely on the TCNT for timing control. One
should use the same prescaler throughout all the timing operations. The C language version of
the program to generate the siren to be compiled by CodeWarrior is as follows:
#include “c:\cwHCS12\include\hcs12.h”
#include “c:\cwHCS12\include\delay.h”
#define HiFreq 1250
#define LoFreq 5000
int

dlycnt; // delay count for OC5 operation

void oc5ISR(void);

void main(void)

{

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

TSCR2 5 0x03; // set prescaler to TCNT to 1:8