[time-nuts] time transfer over USB

[Dan Drown]

On May 14, Peter Monta wrote:
> So a USB-based GPS should:
> - maintain a cycle count of its local crystal oscillator (e.g. with  
> a timer peripheral)
> - report this count when requested
> - timestamp PPS edges from the GPS module, and report these timestamps when
requested


I was already tinkering with stuff along these lines, so I threw  
together some code that just prints to a USB emulated serial port on  
every PPS pulse as well as a simple program on the PC side to  
timestamp it.  I know this isn't ideal, I just wanted to get a  
starting point.  My goal is to get the error under 1ms ("NTP" grade  
instead of "timenut" grade :).

This is the result (red "bar" of 41k samples), compared to NTP  
loopstats info (green line, cable modem connection):
http://dan.drown.org/stm32/run2/time-drift.png

 From that picture, you can see that it's around 2ms worth of  
inaccuracy.  I'm working on adding your recommendations to see if I  
can get that number lower.

More info and graphs if anyone's interested:
http://dan.drown.org/stm32/run2.html