On Mon, Jul 13, 2020 at 4:52 PM Hal Murray <[email protected]> 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.
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