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Re: [questions] Re: GPS+PPS vs NTP server, why a huge offset ?




> On 17 Jun 2022, at 00:07, David Taylor <david-taylor@xxxxxxxxxxxxxxxx.invalid> wrote:
> 
> On 16/06/2022 10:00, Thiebaud HUMBERT wrote:
>> To do the inversion, I just changed the "Pulse Mode" parameter to "Falling edge" from "Rising edge".
>> The offset induced by the "pulse length" has disappeared.
>> But there is still an offset of around 10.3ms, which I think is induced by USB as explained in this article about other chipsets (https://lists.freebsd.org/pipermail/freebsd-usb/2019- August/016078.html)
> 
> Yes, Thiebaud, USB is not good enough for PPS signals!

This is absolutely false.

If you are using it for NTP then GPS+PPS over USB is quite adequate (from personal experience).

Ian Lepore (RIP) who worked for Micro Semi and worked on FreeBSD did a bunch of tests on a PPS over USB setup and found it more than 
acceptable for keeping a PC in (good) time. Here's the thread: https://lists.freebsd.org/pipermail/freebsd-arm/2019-August/020263.html

> See if your motherboard has a true serial port - perhaps just as a header but not a back connector.  If not, just set the offset of the PPS to ~10.3 milliseconds  (10.3 - IIRC the offsets are in milliseconds but please check). Plus or minus 10.3, try it and see!   Not perfect, but better than nothing.
> 
> You might find better results using that GPS/PPS with a Raspberry Pi as a stratum-1 server and offering that as a server on your LAN.

The next level would be something where you can do an input capture on the PPS I don't think there are any pre canned solutions. I made one with a Beagle Bone Black and a uBox GPS module but it's not exactly turn key. Or for a server then you would need a fancy (ie $$$$) internal card.

The Raspberry Pi does not have an input capture timer, but I believe you can do better with DMA hackery (I haven't tried though).

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
-- Andrew Tanenbaum
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