Timer Functions - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 403

The pulse accumulator system can be used to measure the signal frequency. The procedure
is as follows:
Step 1
Connect the unknown signal to the PT7 pin.
Step 2
Set up the PACA to operate in event-counting mode.
Step 3
Use one of the output-compare functions to create a 1-s time interval.
Step 4
Use a memory location to keep track of the number of pulse accumulator counter
overflow interrupts.
Step 5
Enable the PAOV interrupt.
Step 6
Disable the PAOV interrupt at the end of 1 s.

Example 8.14
▼ Write a program to measure the frequency of a signal connected to the PT7 pin using the
algorithm described previously.
Solution: We use the OC0 function to create a 1-s delay. Fifty OC0 operations will be
performed, with each OC0 operation creating a 20-ms delay. The service routine for the
PACNT overflow interrupt will increase the overflow count by 1. Let paov_cnt represent
the PACNT overflow count. At the end of 1 s, the frequency is equal to the following
expression:
Frequency 5 paov_cnt 3 2 16 1 PACNT
The assembly program that implements this procedure is as follows:
#include “c:\miniide\hcs12.inc”

org $1000

oc_cnt ds.b 1

paov_cnt ds.b 2 ; PACNT overflow count

frequency ds.b 4 ; signal frequency

org $1500

lds #$1500

movw #paov_isr,UserPAccOvf ; set up PAOV interrupt vector

movb #50,oc_cnt ; prepare to perform 50 OC0 actions

ldd #0

std PACNT ; let PACNT count up from 0

std paov_cnt ; initialize PACNT overflow count to 0

std frequency ; initialize frequency to 0

std frequency 1 2 ; “