[time-nuts] NTP server using an OCXO, GPS chip and Raspberry Pi

[Hal Murray]

> I was basically planning on feeding the OCXO and the PPS from a GPS module
> into a simple microcontroller.

If you feed the PPS from a GPS into a microconroller, you can work out the 
clock speed on the microcontroller.

If you feed the serial signal from the GPS into the microcontroller, you can 
learn the time of each PPS and also know if the GPS is working or not.

If you feed the OCXO into a timer/counter block, you can work out the 
frequency of the OCXO.

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If you do that in a microcontroller next to the Pi, you are basically making a 
GPSDO.

You will need another counter/timer block to make a PPS, copying the one from 
the GPS when the GPS is working and tracking the OCXO when the GPS is not 
happy.

You will also need a serial output to tell ntpd the time of each PPS.

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You could also use the Pi for the microcontroller.  There is an API to the 
Kernel timekeeping routines.  Try man ntp_adjtime.  There are 2 knobs.  One is 
to adjust the speed of the clock, aka correct for the drift.  The other is to 
adjust the offset by adjusting the speed to be a bit fast or slow for the 
right amount of time.

If you get that working, you can run ntpd using the localclock driver.

That all sounds like fun without much soldering.

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The other approach is to use the OCXO to drive the CPU clock on the Pi, 
perhaps using a PLL chip to get the right frequency.  That will simplify the 
software.

Joe Bennett said:
> https://raspberrypi.stackexchange.com/questions/74482/switch-out-the-x1-oscill
> ator-on-a-rpi-2-3 

There is a crappy discussion in there about using an OCXO where the Pi is 
using a crystal.

If you look at the data sheet for the ARM chip on the board you will find that 
there are 2 pins for the crystal.  It's setup so you can feed an external 
clock into one of them.  The data sheet will tell you which one and what 
voltage level it expects.



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