Re: [avr-chat] GPS based digital clock

[Chris Albertson]

What kind of a clock?  You mean one you'd put in a home or
office that displays the time and only needs to be accurate to a few
milliseconds

Or is this a lab grade "clock" used to say, provide a frequency
reference for a microwave transmitter?

If the first case it is almost trivial.  The GPS transmits the time of
day once per second as ASCII serial text.  You can find the details if
you
google for two terms "gps" and "nmea".  NMEA is the name of the
signal the GPS sends.  It's just 4800bps ascii but it's not quite
rs-232 voltage levels but half the time "close enough"

If you are making a lab instrument then google for "PPS" or 
"Pulse per Second" and "gps" these devices are much more complex
but can get close to the pico-second level

The basic theory is that you use the crystal to drive the clock
but you use the GPS as a check to see it the crystal is running
fast or slow and then the software adjusts the crystal.  Generally
you have to compare the crystal and gps for a long time to notice
drift


Chris Albertson
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  Office: 310-336-5189  address@hidden
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