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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