Daniel wrote:
> I am looking for code (or PIC programming help) for a clock with
> calendar and one channel controller.
>
> I am considering using a DS1307 RTC and 16F84 PIC or similar. It will
> be used outdoors, battery powered with no mains reference. The
> accuracy needs to be as accurate as possible but no more than 1 2
> minutes per month.

If you've already got a PIC in there, why not just connect a 32KHz watch
crystal to timer 1?  2min/month is about 45ppm.  These crystals are cheap
and readily available to 20ppm (< 1min/month).  It sounds like this PIC will
spend most of its time asleep, so that also provides an easy way for it to
wake up periodically.

The WWVB signal is a relatively simple way to automatically ensure long term
accuracy.  Activating the receiver once a day then free running from the
watch crystal in between should keep you within 2 seconds at all times
assuming a 20ppm crystal.  If you want to get really clever, you can use the
WWVB signal to compensate for the initial accuracy and long term drift of
the crystal.  That should keep you to well within a second at all times.

> I need to switch at specific times on certain days, plus it needs
> compensation / adjustment for daylight saving times.
>
> I can find plenty of code examples for regular clocks but not with
> dates (calendar).
>
> Does anyone have any guidance for this?

I had to write some time/date handling code a bunch of years ago.  The Naval
Observatory in Washington DC had useful information and publications about
issues with time zones, leap years, leap seconds, daylight savings time,
etc.  You don't need code samples.  That's easy once you've got a good spec.


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