https://www.ebay.com/itm/384876455621
which is pretty nice because it has an onboard RTC and battery backup. I then added on a timing specific GPS module that was made to fit onto the GPIO pins (https://www.tindie.com/products/nsayer/gps-timing-module-bre...). The CM4 is interesting because it supports hardware timestamping on the ethernet so it can actually run PTP for ultra-precise synchronization. Also added one of those SPI 8 digit seven segment displays and wrote a crappy little program to update the time as fast as possible for some bling. It sucks the CM4 ended up being the biggest cost of the project since I ended up paying ~80 bucks for it. Still far cheaper than the cost of a "real" PTP host.
