PIC18 Timer2 weirdness

From: Toby Thain (toby_at_telegraphics.com.au)
Date: 10/29/04 

Date: 29 Oct 2004 10:32:03 -0700

Hi,

I'm seeing something unexpected in the running time of a Timer2 based
delay loop. With the combined prescale, period, and postscale, should
be setting TMR2IF at 500Hz? I expect the polling loop below to fall
out after approx 2ms. But it's taking closer to 3.8ms* and I can't
think of an explanation.

Part is PIC18F6680, Fosc = 40 MHz.

; THIS DOESN'T WORK...
; Timer2 input = Fosc/4 = 10,000,000 Hz
; divide by 1/16 prescale = 625,000 Hz
; divide by period (156) = ~4006 Hz
; divide by 1/8 postscale = ~500 Hz = approx 2ms!
delay2ms: ; set up Timer2
        movlw b'01110010' ; 1:8 postscale,timer off,1:16 prescale
        movwf T2CON
        clrf TMR2
        movlw 156 ; 156 x 8 x 16 cycles = ~1996.8us = approx 2ms
        movwf PR2
        bcf PIR1,TMR2IF
        bsf T2CON,TMR2ON ; start it!
          btfss PIR1,TMR2IF ; check for timer overflow
          bra $-2
        return

*The running time is measured using Timer0, and I have confidence in
the correctness of that timer, having verified expected durations for
sanity with simple cycle wasting loops.

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