[time-nuts] Raspberry Pi NTP server

[Trent Piepho]

On Mon, Jul 13, 2020 at 8:54 AM Petr Titěra <petr at titera.eu> wrote:
>
> All Prolific  chips I saw claimed to be USB 2.0 Full-speed. That means
> they are polled only once in 1ms and there is no way how to change it
> (poll rate is selected at hardware level).

Looking at the UHCI specification, for USB 1.1 HCIs, while there poll
rate is fixed at about 1 ms, it is possible to tweak it slightly via
the start of frame modify register.

If one is measuring a PPS, then the pulses are not random.  It's
possible to try to align the USB frames with the PPS and do quite a
bit better than just a 1 msec poll rate would do at random.  Certainly
would be hard to fit this into the software stack.  And I don't see
this ability in the EHCI spec, though I'm certainly no expert with USB
HCI drivers, so perhaps it's gone now because no one could figure out
how to use it.