GPSDO - Discipline an adjustable oscillator (typically OCXO) with GPS timing signals
Copyright (C) 2021  Chris Sullivan

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.

You may contact the author via his Github page: SullivanChrisJ

Under development

Using the 1PPS signal from a GPS receiver, this software, designed for a Microchip (f.k.a. AVR) ATMEGA 32A MCU, measures the frequency of its own clock. This allows measurement at the maximum speed of the CPU (16 MHz at 5V, and about 10.6 MHz at 3.3V), rather than 40% of the CPU speed if a timer input were used.

Things that work so far:

1. A non-precision timer function that allows the setting of one-shot and periodic timers in increments of 20ms. As it uses the 8-bit timer and clock divider, it is not entirely accurate. Timers cannot be cancelled although that may be added in future should there be a reason to do so. The main purpose of the timer is to update status messages.

2. SPI communications. Communications (to a Raspberry Pi 3B in my case) is asynchronous. Only the delivery of messages from the MCU to the Pi has been tested, but the other direction is coded and awaiting test. The MCU is configured as the slave. 25KHz clock works well at 4MHz CPU speed. 100KHz does not. Note that if messages are not being read, they will stop being added to the output queue. So when the Pi resume reading, it will get the oldest messages, not the latest.

3. Frequency measurement. The clock is measured at 16 second intervals and delivered to the SPI master. The program gpsdo.py will print these messages. Even with the GPS antenna placed near a window the measurements taken so far are good enough to see a diurnal pattern in a piezoelectric crystal frequency due to temperature effects.

4. Serial output. Messages can be sent to a serial port. This has been used for debugging and it is unlikely to be used in the final version. The code will be retained as in some configurations it may be preferable to have the MCU run standalone without the Pi. In my case I am also using the Pi/GPS combination as an NTP server for standalone use.

5. Other stuff: There is some simple code for turning LEDs on and off. This has been useful for debugging at times as it is simple enough to do inside an ISR. It was also useful diagnosing startup problems prior to the serial port initialization. The is also a module called ringbuf.c that was intended for serial input but is not referenced by the main program. It may be useful someday (or for some other purpose) so it remains for the time being.

The calculation of serial port speeds in the serial.h file using the preprocessor is way overkill. It probably should be converted to a runtime calculation. While the RAM space is fairly limited on the 32A, 32K of flash ROM is plenty and will support a lot more code.

FAQ

1. Why not use Arduino like everyone else? I'm not a fan of Arduino. More relevant is that Arduino configurations don't allow external CPU clocks.

2. Why is this needed? I like the idea of a high precision frequency reference. My Elecraft K3 has a 10MHz reference input and I plan to use it. In future I'll be working my way up from the 6 metre band to much higher frequencies and a frequency reference is a great asset in the microwave band.

3. Why isn't this project finished? The eBay OXCO that I bought several years ago is not only off-frequency, but the voltage output is way below spec. I found that by lowering the supply voltage to 8.24V I could drag the frequency down although the waveform was a bit distorted, but would need to build an amplifier to bring the voltage up. That seems like a waste of time so I'm shelving this for a few months then I will buy a new OCXO.