In a message dated 97-03-10 01:03:50 EST, you write:

<< On Mon, 10 Mar 1997, TONY NIXON 54964 wrote:

 > I need to make a clock that is accurate. It will be powered by the
 > mains, and I thought of using this as a time base as it is a fairly
 > stable waveform.
 >
 > Does anyone else know of a way which will be more accurate?
 >
 > Regards
 >
 > Tony.
 >
 hello,
         i don't think using the mains for refrence frequency will give
 you a stable clock. if you are using PIC for the clock the best idea is
 to put it in some delay loop and keep on counting the number of passes.
 PIC being driven by a crystal oscillator will give much more stable clock.

 vishram.
  >>
vishram,
I disaggree.  Using the 60hz mains on the average is quite accurate.  It has
been used by many clock designs.  It depends on Tony's intended purpose for
the clock he is building.  If he wants a general purpose clock to tell time
by the average accuracy of the 60 cylcles is pretty closely controlled.
The other way is to use a separate osc. of 32768 hz and use a trimmer cap to
adjust the frequency to an exact known value.  getting this right can be
tricky but it would be better than just using a 32768 crystal without  the
cap (a technique used by mose cheap PC mother boards.  This explains why most
computers time is off).

My few cents worth
Dave Duley
V.P. DreiTek Inc.