[time-nuts] GPS message jitter (was GPS for Nixie Clock)

David J Taylor david-taylor at blueyonder.co.uk
Mon Jul 18 12:19:30 EDT 2016

I suppose it is one of those cases where, the GPS designers decided you
shouldn't ever use the serial data for sub-second timing, and consequently
spent no effort on serial latency and jitter.

Most UARTs I have come across have been synthesized with a 16x baud clock
and included flow control. It would not have been too much effort to spec
latency as some mu ±100 ns and jitter of ±1/(16*baud).

For 9600 baud, the jitter on the start bit would be ±6.5 us.

If CTS was resampled a 1 full bit time (9600 baud), the jitter would
be ±104 us.


You're right about the design priorities (and we have had to take Garmin to 
task on this, but they did fix the problem), but it's not the UART which is 
the major problem, but that the tiny CPU inside is taking a variable amount 
of time to have the serial data ready.  We're talking tens, possibly 
hundreds of milliseconds peak-to-peak jitter.

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