#include   "c:\miniide\hcs12.inc"
		org	$1000
paov_cnt	ds.b	1		; use to keep track of the PACNT overflow count
pulse_width	 ds.b	3		; hold the signal frequency
		org	$1500
		movw	#paov_isr,UserPAccOvf	 ; set up PAOV interrupt vector
		ldd	#0
		std	PACNT		; let PACNT count up from 0
		clr	paov_cnt	; initialize PACNT overflow count to 0
		movb	#$0,TSCR2 	; set TCNT timer prescaler to 1
; configure PA function: enable PA, select gated time accumulator mode, high level 
; of PAI signal enables PACNT counter, enable PAOV interrupt
		movb	#$72,PACTL	
		bclr	DDRT,$80	; configure PAI pin for input
		cli			; enable PAOV interrupt
		brclr	PAFLG,PAIF,* 	; wait for the arrival of the falling edge of PAI 
		movb	#0,PACTL	; disable PA function
		sei			; disable interrupt
		ldd	PACNT
		std	pulse_width+1
		ldaa	paov_cnt
		staa	pulse_width
		swi
paov_isr	movb	#PAOVF,PAFLG	; clear the PAOVF flag
		inc	paov_cnt	; increment PACNT overflow count by 1
		end