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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Thu May 14 07:39:36 UTC 2009

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.)

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