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 <tpie...@gmail.com> wrote: > On Mon, Jul 13, 2020 at 8:54 AM Petr Titěra <p...@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. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.