On 16/06/2020 14:04, Miroslav Lichvar wrote:
On 2020-06-16, David Taylor <david-tay...@blueyonder.co.uk.invalid> wrote:
The clock on a Raspberry Pi ranges from 700 to 1500 MHz, so clock
resolution is in the nanosecond range.
For best timekeeping performance, you would want to set the CPU
frequency to a fixed value.
I would also like to see whether the characteristics of the GPS and its
location make a measurable difference to the RPi's timekeeping. For
example: is it better to have a GPS with 3 service capability at a
location where the signal is poor, or is it masked by the RPi's
performance? All this with kernel-mode PPS.
The interrupt latency of the PPS timestamping is probably much larger
than any errors related to GPS, so I'd say it doesn't matter.
Oh, sorry, that's the CPU performance range between the models, from
earliest to latest. Having said that, some of the [later?] models do
operate a varying CPU frequency. Perhaps that's why the offset varies
so little on my very earliest RPi:
https://www.satsignal.eu/mrtg/performance_raspi-1.php
I'm inclined to agree with you about interrupt latency, not to mention
the Ethernet being over USB in the earlier models! Still nice to make
measurements to see what can be achieved, and the principles are more
widely applicable.
Good lockdown learning!
--
Cheers,
David
Web: http://www.satsignal.eu
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