On Mon, May 09, 2005 at 12:01:54PM +1200, Jinx wrote:
> > > Any pointers to good tutorials / examples about using a non-system
> > > clock for a 1 second delay would be extremely welcome.
> 
> If you're going to use a 32768 xtal on TMR1 -
> 
> 32768/4 = 8192Hz incrementation rate (DS30487B, Section 7.1)
> 
> Therefore, re-load TMR1 with 0x0000-0x2000 = 0xe000 (=
> - d8192) or just re-load TMR1H with 0xe0, as TMR1L has
> already gone through 0x00. When TMR1 counts up from 0xe000
> to pass through 0x0000 you can either have it generate an interrupt
> (by enabling TMR1IE) or poll TMR1IF to see if it's set. In either
> case, clear TMR1IF and reload. This is a very simple background
> 1 second timer. You don't need to do anything other than note that
> TMR1 has gone through 0x0000 (and of course re-load it before
> TMR1L gets to 0x00 again). Using binary counting like this means
> you don't have to worry about reload jitter that can happen when
> loading the timer low byte with non-zero values

Are you sure there is no jitter if you reset the upper byte only?

BTW I just checked my sunrise/sunset calendar program. It actually uses
the 1:8 prescale and rollover for a 19 bit count (512k). So it ticks every
16 seconds. With a 1:1 prescale it rolls over every 2 seconds.

> You can work out the maths if you're using a different xtal as the
> main oscillator and using another timer and/or pre-scalers
> 
> I'd go along with Byron about using mains cycles as the primary
> timing source (RA4/T0CKI - RB0/INT - RB6/T1CKI). Note
> though that mains frequency varies slightly during the day due to
> demand and you should look at the daily (ie a 24h period) count
> for accurate timing. IOW, the number of mains cycles coming in
> between 7am and 7pm is more than likely going to be be less
> (frequency would be 49.xx or 59.xx Hz) than the period 7pm to
> 7am (frequency will be catch-up 50.xx or 60.xx Hz)

But it remains accurate over the long term. You'll get 216000 ticks in a
24 hour period though the interval between them changes.

BAJ
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