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PIC18 Timer2 weirdness

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.
Reply to
Toby Thain
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Your postscale bits are wrong.

Reply to
Gary Kato

Yup. Specifically, you're getting 1:15 rather than 1:8.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Others have pointed out the postscale problem. For future reference, here's an Excel spreadsheet for working out those pre/post-scaler issues:

formatting link

-- Dan Henry

Reply to
Dan Henry

Yes, I just found this out too. What a fool. Thanks! :-)

--

> 
> 
> Best regards, 
> Spehro Pefhany
Reply to
Toby Thain

Plus using that spreadsheet, you'll quickly discover that 1:16 pre/1:8 post with PR2 = 156 only gets you close to 500Hz and that 1:16/1:10 PR2 = 125 is spot-on.

-- Dan Henry

Reply to
Dan Henry

Yes, that imprecision is also due to my misreading of the manual; I overlooked that Timer2 actually allows all divisors 2-16 (more familiar with the other timer modules that only have powers-of-2 scales). You know how it is when you stare at something too long! :)

Reply to
Toby Thain

The comments that indicate what you *wanted* the code to do can impede troubleshooting. Maybe they should all left off 'til the end. ;-) ;-)

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

On 30 Oct, in article snipped-for-privacy@telegraphics.com.au "Toby Thain" wrote: ....

It stares back and makes silly faces at you :-^

--
Paul Carpenter          | paul@pcserviceselectronics.co.uk
    PC Services
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Reply to
Paul Carpenter

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