[time-nuts] Raspberry Pi NTP server

[Adrian Godwin]

Is there any way for a USB device to synchronise with the CPU
clock (perhaps via the USB framing) so that a special-purpose device could
timestamp the PPS occurrence with respect to CPU time ?


On Mon, Jul 13, 2020 at 9:51 PM Trent Piepho <tpiepho at gmail.com> wrote:

> 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.
>
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