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




> On 17 Jun 2022, at 23:38, David Taylor <david-taylor@xxxxxxxxxxxxxxxx.invalid> wrote:
> 
> On 17/06/2022 03:03, Daniel O'Connor wrote:
>>> 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).
> 
> If a 125 us uncertainty in the PPS is something you can tolerate, so be it.  If you are bothering with PPS then presumably you want better accuracy than can be achieved without it.

I think for standard PC timing in a network it is more than adequate.
Using PPS over USB does give you an improvement over just serial traffic so why not use it since it's free.

> No need for DMA hackery.  Standard NTP with the Raspberry Pi can handle PPS on a GPIO signal with a couple of edits to allow the PPS support already built into the kernel to be attached to the appropriate GPIO pin.  Not out of the box, but very little effort required.
> 
> The Raspberry Pi can act as a server for hundreds of clients.  If you mean a PC-based Windows server, that's not something I would immediately recommend, but if you must a £20 serial card may be all you need to add.

Sure, but I would think something with an input capture timer will produce a better result again. It is just unfortunate the RPi can't do it.

--
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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