| Re: [chrony-users] GPS/PSS hardware |
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On 05/08/2018 06:06 AM, Bill Unruh wrote:
On Tue, 8 May 2018, Nicolas Embriz wrote:
Hi, I would like to build/test a stratum-1 server, I have found
articles about how to do that with a raspberry PI and boards
like https://store.uputronics.com/index.php?route=product/product&path=60_64&product_id=81
But wondering if there is hardware/devices that could be attached via
USB and connect to an existing server.
USB is possible but not ideal. The interrupts are soft interrrupts (ie
done by
sending signals on the usb in software rather than some hardware line.)
Best
is if your system has some hardware line (eg serial port or parallel
port) to
use. Of course that depends on what kind of accuracy you want. If it is ms
then a serial to usb converter will certainly work, and even just the GPS
signal on the usb might be fine. If it is sub-ms you want, then a hardware
interrupt is needed.
Or your desktop server might already have a serial port, and you can use
that.
Or use an NTP appliance connected on a local link via Ethernet, and let
the NTP/chronyd standard filtering and latency calculations allow
(potentially) sub-microsecond. A lightly-loaded LAN would also work,
with a slight degradation in synchronization.
One thing to note: PPS into a serial port will be affected by the
interrupt request time from the UART. Some implementations of the
NS16550 in ASIC cell libraries can delay interrupts by some number of
baud clock ticks. This introduces what *should* be a fixed latency.
But, from a system viewpoint, it's not. Latency would be affected by
other things going on in the system: queued interrupts from other
sources, interrupt routines running with interrupts disabled.
So, using a serial port, you will get systemic jitter from PPS.
Parallel port? Because the printers are slow async devices, they are
normally put LAST in interrupt priority. Hence, jitter.
On appliances: I've not seen in the specifications where they declare
the system-induced jitter from their primary source of time. I think I
need to find a WWV-based NTP server, and compare apples to oranges on my
network. Another VLAN...
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