[time-nuts] How to get PPS from ublox mini-PCI GPS to APU2 SoC serial port for ntpd

Paul tic-toc at bodosom.net
Tue Aug 16 19:34:38 EDT 2016

On Tue, Aug 16, 2016 at 3:36 PM, Attila Kinali <attila at kinali.ch> wrote:

> I disagree here. With the APU and its IRQ system you get already to
> sub-us PPS time stamping (probably in the 10ns-100ns range).

An APU1 post said they had 50ms of jitter (as measured by Chrony) so they
assumed the PPS driver was broken on the hardware.  Pascal suggested
elsewhere that the combined LPC plus system IRQ jitter would be too much
for NTP.  While I don't believe this I'm disinclined to test it .  So I was
-- clearly too briefly -- implying that there are solutions that should be
less expensive* and known to achieve micro-second jitter.

I had no reason to suspect Pascal would overestimate the issues however
another APU1 owner** used an mPCIe serial card and an mPCIe GPS card and
got expected microsecond jitter (as measured by NTP) results.  Again this
is not quite what the OP is proposing but it's encouraging.

These are time sync. protocol results and don't speak to sub-microsecond
time-stamping in the hardware.

* which is not to say pcengine hardware is expensive.  It's not.
** https://wiki.polaire.nl/doku.php?id=versalogic_vl-mpeu-g2_ntp

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