[time-nuts] Raspberry Pi NTP server

[Petr Titěra]

On 15.07.2020 0:13, Trent Piepho 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.

I think you can have problems with that. At least in general. Look at 
this specification of system clock generation chip:

https://datasheet.datasheetarchive.com/originals/distributors/Datasheets-DGA19/372682.pdf

It seems to two independent PLLs to generate USB clocks and system 
clocks. Is common frequency source enough to synchronize these two?

There are more recent generators which claim that they use one common 
source for all generated frequencies but generally you can have separate 
timing of USB and rest of the system.

Petr Titera

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