Dean, 1 second in 10 minutes = 1/600 = 0.16667 %...

I don't have an answer to your question as to how to fix it though...

Cheers
 Roland


----- Original Message -----
From: Dean Biddle <d.biddle@MAILBOX.UQ.EDU.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, March 09, 2000 7:38 AM
Subject: Why doesn't 1+1=2?


> Hi,
>
> The timing of the code (infamed by Myke Predko?) at the end of this mail
> simulates to the nearest 5second using MPLAB.  However, when executed on a
> P16F84 using a 4 MHz 3 pin resonator marked
> [4.00 (M x], I loose about 1 second every 10 minutes.  Same occurs at
> different supply voltages between 3.5 and 6V.
>
> Presumably this is because of the +/-0.5% error (or greater) in the
crystal
> and my too frequent reliance on 1 microsecond being accurate?
>
> If this magnitude of error is expected for this type of delay code, could
> someone please point me to another technique (e.g. TMR0 + prescaler) with
> accurate equations which allow accurate pre-calculation of the interval.
>
> I'm using RA5 hence can't use a 32k crystal for timing.  Is there any
other
> external timing techniques (sleep or not) for a P16F84?
>
> Is the life of a microcomputer reduced if it is allowed to continuously
loop
> compared to a sleeping chip?
>
> ;
> ; Delay rountine for P16F84 using 4 MHz crystal.  Sum individual
expressions
> for total and add
> ; extra cycles where required
> ;
> MOVLW   0X00?
> MOVWF   COUNT1
> MOVLW   0X00?
> MOVWF   COUNT2
> MOVLW   0X00?
> MOVWF   COUNT3
> MOVLW   0X00?
> MOVWF   COUNT4
> DECFSZ  COUNT4,F  ;2+(3*1)*(COUNT4-1)
> GOTO    $-1
> DECFSZ  COUNT3,F  ;2+(3*255+3+2)*(COUNT3-1)
> GOTO    $-3
> DECFSZ  COUNT2,F  ;2+(3*255*256+5*255+3+2+2)*(COUNT2-1)
> GOTO    $-5
> DECFSZ  COUNT1,F  ;2+(3*255*256*256+5*255*256+7*255+3+2+2+2)*(COUNT1-1)
> GOTO    $-7
>