Re: [chrony-users] GPS / Chrony NTP server config questions

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Bill Unruh wrote:

I am using chrony with serial PPS on a number of different machines, and I see the 6-7 us figure
on HP Proliant systems while on Dell PowerEdge sytems it is more like 1us.
No idea where the difference comes from.

How do you measure that latency? It does seem that the parallel port is a much
faster interrupt than the serial port, so it might have something to do with
the serial port hardware used. (in particular if the serial port is really a
phony serial->usb port I would expect large latency).
Well, I did not really measure the absolute latency very accurately, it is more the jitter in the latency what
is appearing in my measurements.
I compared two computers each running on a separate GPSDO but close together, using a scope,
and they were quite close in timing.  But those were both Dell computers.  Later we started using HP and the
results were a little worse.
However, my goal for this application is <10us between systems and it is always possible to achieve that.

All of those systems have an "onboard UART".  Setserial determines them as:
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
However there is of course no 16550A anywhere on those boards, it is a cell in some system support chip.

It is true that parallel is better than serial, if only due to the lack of RS232 linedrivers with their associated
filtering (slew rate limiting) as defined in the RS232 standard. Equipment manufacturers may add additional
filters on each line for EMI reasons.  The (emulated) 16550 may add other delays.
A parallel port traditionally was only some 74LS latch, and its current implementation probably still is faster than a UART.

For really accurate PPS handling one would want a dedicated PCI-E card with a clock oscillator and a counter,
latched in a register on PPS edge, and issuing the interrupt.  The driver would then read the latched counter
value and the current counter value on interrupt.  The interrupt response time becomes irrelevant because the
actual timestamp has been latched by the hardware.  I think some Meinberg products use this method.


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