A simple but effective project for your perusal. Laureline is an embedded SNTP server that receives time from a GPS receiver in the form of a pulse-per-second (PPS) input as well as the usual serial data in whatever format it comes (NMEA, Oncore, TSIP, etc.). I call it SNTP and not NTP because time is served from GPS alone and no other NTP servers are consulted. However, laureline is perfectly suitable as a Stratum 1 reference for NTP and SNTP clients alike.
The processor is a STM32F107 which includes a built-in 100Mbit Ethernet MAC, which I have paired with an inexpensive transceiver to get a good Ethernet solution with predictable and minimal latency unlike the USB-based MACs found in e.g. Raspberry Pi. Also onboard is a small VCXO which is disciplined by the microcontroller to precisely 10MHz using the GPS pulses, in effect making laureline a low-grade GPSDO. The board is powered by a buck converter that accepts 8-40VDC, allowing for direct power from a 12V or 24V lead acid battery. Other than that, the rest of the magic is in software: observing the frequency of the local oscillator by way of incoming GPS pulses, calculating NTP time from the 70MHz timer when queried and handling network administrivia.
Because almost all STM32 IOs are 5V-tolerant, the serial interface works with both 3.3V and 5V GPS receivers. I've designed but not yet fabricated some tiny adapters to connect two types of receivers to the 6-pin header on laureline, one being Oncore GT+/UT+ and probably other Motorola receivers as well, and the other being Trimble's Resolution T and Resolution SMT carrier boards. Both types are presently available used on Ebay for about $30, half that in the case of UT+. As you can see in the pictures, jumper wires also work perfectly well in lieu of an adapter. There's enough current at either 3.3V or 5V to power a typical receiver with active antenna, as well.
Here is laureline paired with an Oncore GT+, the board at lower left is an isolated USB UART I'm using for debugging:
A (blurry) closeup of the board:
Current status: rev 1 of the hardware works flawlessly, except for the power supply which needed extra capacitance. rev 2 will have a separate PSU module of my own design, so that when I screw it up again at least I can test it separately and/or swap it out.
The software is still in early development. All of the disciplining/GPSDO stuff is not written yet, although I have written such things before for an earlier project that actually was a GPSDO. Presently I hacked together a simple mechanism that just resets the counter each time a PPS arrives and does no disciplining but it's still performing quite well. It also needs to handle leap seconds, both internally and also for presenting the leap flag to NTP clients. I haven't added the code to the repository but I'll do so in the next day or two.
Repository (design docs, code, etc.): http://hg.partiallystapled.com/circuits/laureline/
In zip format: http://hg.partiallystapled.com/circuits/laureline/archive/tip.zip
Also download the library and unzip it to lib/: http://hg.partiallystapled.com/circuits/alib/archive/tip.zip
Schematic PDF: http://hg.partiallystapled.com/circuits/laureline/raw-file/tip/out/production.PDF