1 second in 10 minutes = 1 second in 600 = .16% error, better than is to be
expected from a .5% precision timebase.  So your code is probably just fine.
If you want more accurate timing, you're going to have to start with a more
accurate timebase, such as a 20ppm crystal which should give more like .012
second error in 10 minutes.

Now, you must also understand that to get the 20ppm from your crystal, you
will have to, at least once, fiddle with the crystal loading caps and a
high-quality counter to adjust for the stray capacitance on your board.
Once you have found the right cap values, they will be pretty consistent for
a production run, but you still may need a tweaker on the crystal to get it
right on.

> -----Original Message-----
> From: Dean Biddle [mailto:d.biddle@MAILBOX.UQ.EDU.AU]
> Sent: Wednesday, March 08, 2000 11:39 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	
>