On 6/25/24 8:38 PM, Mike Easter wrote:
Jake M wrote:
I use 22.04 and need a free program that would provide time synchronization when offline. I have a program that needs such synchronization in order to function correctly, and I'm not always going to have net access. Thanks in advance.
I think we need some more specifics here.
Let us go from the normal situation and then try to expand it to your specific request.
The normal situation is that the computer has its own internal clock chip, and conventionally sync/s itself w/ online NTP, whereas the 'normal' situation for devices such as phones get their time sync/s from GPS + cell towers.
One would infer from your question that in your perception or analysis of the quality of timekeeping by your device's clock chip, you want to 'enhance' its accuracy w/o online NTP, but you haven't stated what kind of precision you want or why or 'how
often'.
Meinburg plus a GPS might work.
A GPS with a PPS (pulse per second) signal.
The tricky part, is getting the PPS signal to
come in, on a low latency interrupt.
If you refuse to wire up the PPS signal, and
use TX/RX/GND, then the synchronization is good
to "a fraction of a second". With the right hardware
for PPS, you could be good for 100usec or so. The
hardware could get you 100nsec, but it is difficult to
assign a jitter number for the interrupt handling.
https://www.adafruit.com/product/746 # pin header, VIN/TX/RX/GND to TTL level serial port
# (My attempt to modify a serial port card PCIe has been a flop...)
https://www.adafruit.com/product/4279 # USB interface, plug and play. PPS on Ring Indicator, needs
# a matching driver to make it work. This might help a purchaser
# avoid soldering iron work.
The GPS module can pick up a strong signal from only
two satellites, if you leave it in the house on
the window sill. They make an active antenna cable,
for extending the "reach" of the module, so more
satellites can be seen. This would require a hole
in the window (nope, not gonna do that :-) ).
I've had mine working, it's just I'm trying to
get to better than USB polling interval on
clock sync. USB2 polls every 125usec, which is
why the output is a bit sloppy. PCIe interrupts
(inband messages) have very low latency by comparison.
The module uses a 1GHz clock internally, so the
re-sampled edge of PPS should have a jitter of 1 nanosecond
or the like. This might be receive frequency divided by
two or so.
This is better than the 50 millisecond uncertainty of
NTP network sync.
The computer protocols, only need 1 second accuracy
for any sensitive network storage purposes. Any of the
schemes will do that for you (no PPS needed) -- the
rest of it is an ego trip :-) It's no fun having
a module with great potential, if you cannot get it
to deliver as intended.
Paul
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