[ntp:questions] GPS clock for Linux

Martin Burnicki martin.burnicki at meinberg.de
Tue Sep 23 14:16:53 UTC 2008


Uwe,

sorry for the delay.

Uwe Klein wrote:
> 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.

Unfortunately not. We have just made some quick tests under windows, using
some devices which you could buy at that time.

This has been some time ago, and I don't have those devices available
anymore. 

>>>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.

Yes, but what I meant is that the driver for the converter can insert some
latency which degrades accuracy even if the sending device behaves good.

We had tested some of our serial devices connected directly to a serial
port, and then connected to a serial-to-USB converter which implements an
additional COM port on the PC, which inserted several milliseconds of
jitter.

We have now some own USB devices which have USB support in the
microcontroller. They can be connected directly to an USB slot, so we have
more control on timing issues. However, there are still latencies in a
millisecond range due to the USB low level drivers which come with the
operating systems.

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




More information about the questions mailing list