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Rastin February 2016

Accurately measuring Time for an event ( relay contact closure) using GPS PPS

I have a relay contact closure event that needs to be timestamped accurately ( 1 msec) with a GPS and the PPS output... I am not sure how to feed the relay contact output to a microcontroller and then synchronize the microcontroller clock to the GPS ...plus how to get the UTC afterall? Can you please help me. thanks


Jorge Torres February 2016

If your microcontroller has at least two interrupts based on hardware pins, you could connect the relay to one of the interrupt-generating pins, and the PPS to the other interrupt-generating pin.

You will need to connect the NMEA (or other proprietary protocol of your GPS) to the corresponding port in your microcontroller. Some common buses are UART or SIP.

Then, every time that you get a PPS interrupt, you enable a global flag that can be used in the main loop to reset a counter. This counter will tell you how far apart from the PPS the relay switched (if it happens within that second). If you know the base frequency of your counter, you can convert the counter into fractions of seconds. Note that if both edges of the relay state change have to be detected, you will need an interrupt source capable to interrupt on both edges (or use two interrupts)

Then, if the Relay interrupt goes off, you can get the value of the counter upon interrupt, and save it in storage, send it to host, etc. (Note, it would be best to save the value in RAM, lift a flag of "value present", and leave the sending/storing to the main loop, then turning off the flag).

Finally, when you receive a complete NMEA message (this could be being parsed in your main loop by a state machine for instance), you can send this information to the host or storage along with the counter that you saved to time your relay state change. Note please that the NMEA message will be generated and decoded with a certain delay from the PPS, so you will need to compensate for that.

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Asked in February 2016
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