[ntp:questions] Setting the COM port for the NMEA driver

[David Woolley]

David J Taylor wrote:

> 
> I understand that USB alone will typically not have any PPS information, 
> so the timekeeping accuracy would be just that of the serial port 
> messages, degraded by whatever USB latency adds, but improved by the 
> fact that the messages are much shorter on USB than on serial.
>
The problem with USB latency is that it is unpredictable.  The 
uncertainty in the direct serial port latency is 1/16 th of the 
signalling unit length (about 6 microseconds at 9600) plus the 
uncertainty in the interrupt and process scheduling latencies.  (If the 
UART clock is free running on the GPS device, you might need to add 
another 1/16th, so about 12 microseconds.)