You will probably find that you are not accounting for the execution time of
the rest of your code, hence it runs slow.

I have run RTC code using the timer and a 3.568Mhz  xtal and got no drift
over many days.

unless your code is very simple/plain path then you need a regular interrupt
to keep track of time properly.

        cya,    Andrew...

> -----Original Message-----
> From: Dean Biddle [SMTP:d.biddle@MAILBOX.UQ.EDU.AU]
> Sent: Thursday, March 09, 2000 1:39 PM
> To:   PICLIST@MITVMA.MIT.EDU
> 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 µsecond 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