| Re: [chrony-users] PPS Synchronization Verification |
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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-users] PPS Synchronization Verification
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Mon, 2 Nov 2020 17:31:43 +0100
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On Mon, Nov 02, 2020 at 08:02:12AM -0800, Bill Unruh wrote:
>
> On Mon, 2 Nov 2020, Miroslav Lichvar wrote:
>
> > On Mon, Nov 02, 2020 at 12:24:32PM +0000, Chang, Benjamin wrote:
> > > The PPS signal is coming from the NTP server (it's a syncserver S650) and is connected via a 232 line. I essentially did this https://www.crc.id.au/2016/09/24/adding-a-pps-source-to-ntpd/ but switched the ntpd kernel with the chrony kernel. Should I set an offset of .00002 for the PPS?
>
> 20 microseconds for an interrupt? That seems pretty high. One tests I ran
> about 10 years ago, (sending a timed signal over on the parallel port lines to
> the serial port interrupt line) the latency was closer to or less than 1 microsecond.
Modern hardware seems to be worse. There are other delays than the CPU
handling an interrupt, e.g. due to various bridges between the CPU and
UART. On one machine I have measured an offset of about 27
microseconds between PPS on serial port and hardware-timestamped NTP
(accurate to 1us).
--
Miroslav Lichvar
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