Hi James,

The only thing that I wasn't sure about was the lines:

 ;; subtract 1s from copy of counter
 movlw d'1000000'&0xff
 subwf t0,f
 btfss status,c
 decf t1,f

This is an eight bit subtract of a 24 bit value, but you are not bringing
the carry to the most significant byte ("t2").  I haven't done any kind of
analysis on what this can do to you, but I'm sure every once in a while it
will bite you.

I would suggest something like:

 ;; subtract 1s from copy of counter
 movlw d'1000000'&0xff
 subwf t0,f
 btfsc status,c
  goto $ + 5  ;; If No Borrow, Subtract from the Middle Byte
 movlw d'1'
 subwf t1,f
 btfss status,c
  decf t2,f ;;  "t1" was Zero upon Entry into Subtraction Routine, Decrement
t2

myke



----- Original Message -----
From: "James Cameron" <quozl@US.NETREK.ORG>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, March 07, 2001 11:25 PM
Subject: Re: [PIC]: Clock Tuning


> On Wed, Mar 07, 2001 at 11:52:40AM -0500, Dan Michaels wrote:
> > James Cameron wrote:
> > >Looking for archived discussions or sample code to adjust for
consistent
> > >drift in my PIC 16F877's 4MHz crystal oscillator.
> > .......
> > >The problem is that my clock is losing one second every three hours.
It
> > >is at least consistently doing this.  It is well within specifications
> > >for the crystal.
> > Your error is ~1/3600, while xtal error is <= 50/1000000 = 1/20000,
> > so the problem is no doubt in your code. 4 mhz is not an even power
> > of 2, so your method of adjusting for timer overflows probably needs
> > to be fixed. Look there.
>
> Ah, maybe I've got one of those off-by-one problems.  I checked most
> carefully ... TMR0 at 1:256, should give one T0IF every 65536uS exactly,
> which I add into a 24-bit counter and then subtract one second.  If the
> result is positive, I keep the new counter value and activate my 1Hz
> code path.  TMR0 runs free, never reset.  Does this logic seem sound?
>
> http://quozl.netrek.org/uptime/uptime.asm is the code.
> http://quozl.netrek.org/uptime/ has the rest of the code.
>
> --
> James Cameron    mailto:quozl@us.netrek.org     http://quozl.netrek.org/
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>
>

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