[ntp:questions] GPS clock for Linux

Uwe Klein uwe_klein_habertwedt at t-online.de
Tue Sep 16 08:16:33 UTC 2008

Hi Martin,

Martin Burnicki wrote:

> Some times ago we had made some tests with some serial-to-USB converters
> which are connected to the PC via USB and provide a serial port to which
> serial devices can be connected. We have been using our GPS receivers which
> send a serial time string once per second as soon as possible after a
> second changeover. The jitter of the serial output is 1 bit time depending
> on the baud rate, i.e. ~52 microseconds @ 19200.
Yes, if one needs further improvements one would try to sync the baudrate
generator to the pps clock. with some tricks in the sending routine
one then could transmit in a coherent fashion.
Though you still retain the receiver uncertainty ( 1/16.. 1/64 of a baud cycle
depending on async receiver used )
> Some of the tested devices introduced a very low latency whereas others
> indroduced a much higher latency. So this depends in fact on the maybe the
> chip set and in any case the driver/firmware used for the converter.
Have you got vendor/product Id pairs for those? on windows/linux/bsd/* ?
I'd be interested in related information.
>>Thus my guess is that these latency may lie in the firmware running
>>on the sirf chipset.
> Of course, if the firmware already inserts some latency then the
> serial-to-USB converter is not to blame and can not eliminate it.
> Martin

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