1ms or better accuracy on a USB-connected GPS.
stm32f4 discovery board using a 10.5mhz internal timer is measuring the PPS interval from a Fastrax UP501 GPS reciever (claims "+/- 50 ns (RMS) accuracy"). The stm32f4 is connected via full-speed usb to a pc
The microcontroller's timer is now sampled on USB poll, instead of saving a buffered copy
This compares NTP's local clock offset (from loopstats) to the GPS timestamps. Since the NTP timestamps will always be offset from the actual time due to how my internet connection works, I've shifted NTP's time by 2.5ms
This change lowered the window between the actual time and the PC's reception of it from 2ms wide to under 500µs wide.