[time-nuts] Raspberry Pi NTP server

[Bob kb8tq]

Hi



> On Jul 14, 2020, at 6:13 PM, Trent Piepho <tpiepho at gmail.com> wrote:
> 
> On Mon, Jul 13, 2020 at 4:52 PM Hal Murray <hmurray at megapathdsl.net> wrote:
>>> 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 ?
> 
> It seems maybe this was thought about in USB 1.1.  From the UHCI spec:
> 
>        Minor adjustments can be made to this value [ 1-ms frame time]
> ...  This feature permits minor changes to be made to the frame time
> period, if necessary, to maintain real-time synchronization throughout
> a USB system.
> 
> It seems like it would not be that hard to get the USB frame sequence
> phase locked to the system clock.  One would need a way to measure the
> phase offset of the USB S-o-F vs the system clock, and then it's a
> standard process to phase lock, with the necessary control to do this
> described above.

To make this work, both ends of the USB link would need to “understand” what is going 
on. With part of the link buried in a USB <-> serial chip, that might not be very easy to do.

Bob


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