Hi Tony,
        In the US, the frequency of the mains is regulated by law.  Both
short-term and long-term error are required to be within certain limits.
The long-term error limit is far better than most crystal controlled clocks.
        If you clean up and current limit a signal from the mains, then keep tra
ck
of the average number of instruction cycles for each mains pulse, you can
_also_ keep very accurate time during a power outage.  (Assuming you have a
rechargeable back-up battery on floating standby-- great for smoothing the
input power also.)
        As for greater accuracy, the US government has short wave radio stations
WWV and WWVB in Colorado, and WWVH in Hawaii which broadcast atomically
accurate time signals.  Some of their signals are formatted for computer
reception.  Even the verbal time channels have beeps at regular intervals
which can be used to correct for crystal drift.  The main problem with the
beeps method is that the time it takes radio signals to propagate from the
transmitter to you fluctuates.  You would need a continuous, moving average
over a long period to get a good measurement.
        Another source for greater accuracy is the internet.  There are several
locations around the world from which you can request the exact time.
There is a lag for transmission through the network of servers, but the
long-term error of your clock would be nil.
        For periodic resetting methods, you can use the trick of self-adaptive
precision.  Each time the clock is reset, figure how long it has been since
it was reset and what the error was.  You then adjust the number of
instruction cycles you wait between increments of the clock's smallest
unit.  You can also have a daily correction factor to increase the
precision. (Pause or jump ahead x number of instruction cycles by modifying
the delay counter.)

Frequencies:
WWV:    (male voice - Fort Collins, Colorado)
         2,500 kHz     2.5 kWatts
         5,000 kHz      10 kW
        10,000 kHz      10 kW
        15,000 kHz      10 kW
        20,000  kHz      2.5 kW
WWVH:   (female voice - Kekaha, Kauai, Hawaii)
         2,500 kHz       5kW
         5,000 kHz      10kW
        10,000 kHz      10kW
        15,000  kHz     10kW
WWVB: (continuous BCD time code - Fort Collins, Colorado)
        60kHz   13kW
Many other countries have similar transmitters, often on similar frequencies.

Best Wishes,
Eric.

>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.