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[SOLVED] Did anybody know an "easy" way to generate/keep a clock from GPS signals ?

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jorgealdo

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Do anybody know an "easy" way to generate/keep a clock from GPS signals ?

I'm thinking about a circuit.

I want to do serial comm without clock regeneration (no need for preamble).

I was thinking if with current technology its possible to use a GPS receiver (Those with rs232 output) to generate a clock source, removing the need for separate clock line or clock regeneration techniques.

Is this possible at low cost ? Or still too high tech ?

I would want this to feed the clock input of a microcontroller.
 

jorgealdo

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Re: Did anybody know an "easy" way to generate/keep a clock from GPS signals ?

What i had in mind was to lock the clock to the GPS.

I'm planning to do the following :

1 - Get a tunable crystal clock and wire its tune pin to an DAC pin of the microcontroller.
2 - Use a prescaler to count the clock pulses (Wire it to a microcontroller pin, triggering an interrupt).
3 - Get the PPS (Pulse-per-second) output of the GPS receiver and wire it to a pin of the microcontroller (Triggering an interrupt).
4 - When a PPS trigger happens, check the difference to the value counted from the clock (IE, if at X MHz the counting would have gone to X Million pulses per second, the actual counting value is the drift).
5 - Alter the DAC value to make the crystal compensate for the drift.

Is this feasible ?
 

KJ6EAD

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Re: Did anybody know an "easy" way to generate/keep a clock from GPS signals ?

Sure, but that's like looking at the pendulum on a clock and counting the swings instead of looking at the face. Why don't you just read the ZDA sentence from the GPS? For a more primitive method you can count water drops falling from one container into another and then create containers calibrated by size for minutes, hours, etc.;-)
 

RCinFLA

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Re: Did anybody know an "easy" way to generate/keep a clock from GPS signals ?

I read this as you are trying to have source and destination of serial data with third party (in this case GPS) providing clock for both ends.

Unless you are running relatively low bit rate, direct reading of clock will likely have too much instantaneous delay variation in processing the message.

You don't mention the comm path length which will introduce a delay getting from source to destination, slewing the sampling even if both sides were given the same clock. You would still have to detect edges and adjust phase offset for path delay. With this you have already done most of the work of a bit syncronizer clock recovery circuit.

Attached is a simple clock recovery circuit that can be build from a few 74HC or 4000 series CMOS chips. The high freq clock Fo does not have to be an exact multiple of serial bit rate as long as the divider ratio is at least 10 to 20. The higher the divider ratio the better. Fo can be the uP clock oscillator.
 

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  • Data Clock Recovery Circuit.pdf
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jorgealdo

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Re: Did anybody know an "easy" way to generate/keep a clock from GPS signals ?

So i can feed the output clock to the SCLK pin as usually i would do if i had a separate clock line ?

then i keep pumping the Sync char (01010101 or 10101010) when i have nothing to send, right ?

nice... very helpfull, i forgot that such a simple clock recovery circuit existed...
 

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