Budget GPSDO - complete

[MIS42N]

Tell me more about PICDIV.
Thanks
enut11

Uses assembler program for a pic12f675 or similar to create pulses from the OCXO 10MHz input. The processor runs without interrupts so relies on accurate loops to produce the desired output.

There are a few different 1PPS programs. The simple ones produce the 1PPS with no synchronisation so are not aligned with an external time source. More complex programs allow alignment to within ±200ns of another source with suitable external input. Due to the instruction time being a quarter of the clock, it would be difficult to adjust that last 200ns.

I don't know it is worth implementing. You want 1PPS, take it from the GPS module. As long as there is a reasonable sky view, they will continue to output 1PPS. The PIC in the GPSDO keeps stats on lost signal, rarely happens.

However, if 100% reliable (but not necessarily 100% accurate) 1PPS is required, for example to run a wall clock, then maybe PICDIV has a place. The OCXO is always running but with an extended loss of input there will be a CPU reboot and there may be a loss or gain of cycles before discipline is reestablished.

The GPSDO program could be written to give extended holdover (if input is lost then restored, the OCXO frequency is changed to compensate for any lost/gained cycles then discipline reestablished). However, that seemed like more complexity that a simple GPSDO was expected to deliver. Currently if signal is lost for around a minute, the CPU is rebooted and runs with the Vco established during calibration (unless the user interface is used to update the default Vco) until the signal returns.

http://www.leapsecond.com/pic/picdiv.htm