Is it a good idea to use a usb-serial adapter for PPS input? Yes, it is.

[O'Connor, Daniel]

Hi Ian,

Firstly, this is a very cool test - thank you for running it :)

> On 3 Aug 2019, at 06:46, Ian Lepore <ian at freebsd.org> wrote:
> PPS(2) is an FTDI 232R, a USB 1.1 serial adapter, connected to a port
> on a USB 2.0 hub that's connected to a USB 2.0 host port on the
> Wandboard.  
> 
> PPS(3) is an FTDI 4232H, a USB 2.0 serial adapter, connected to a port
> on the same USB hub as PPS(2).  

<snip>

> Most people are not worried about their kernel clock being 200
> microseconds off from UTC, even if they're using the PPS signal from a
> GPS receiver.  So I think most people should feel completely at ease
> using a USB serial adapter as the input device for a PPS signal.  

Does the USB clock derive from the 10MHz Rb clock? If so that would mean you would see a lot less jitter than a 'normal' user where the USB clock is not locked too GPS.

Do you have a more detailed write up of things like the NTP configuration file? I think I could replicate your test here although I have a Beaglebone Black, not a Wanboard so I will need to check if it can take an external clock. (We have GPS modules & Rb oscillators at work to create reference clock for bi-static meteor applications).

Thanks again.

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