Re: Clock synchronisation using the frame counter

On Sat, 3 Mar 2007, Laurent Pinchart wrote:

> Hi everybody,
> 
> I'm trying to find a way to synchronise the Linux system clock with a device 
> clock using the USB frame counter as a shared clock.
...
> I end up with three time sources: the device clock, the USB clock (through the 
> SOF counter), and the Linux system clock. The jitter between the device clock 
> and the USB clock is reported in the Source Clock Reference, and the jitter 
> between the USB clock and the Linux system clock should be computed somehow. 
> I then need to use all those parameters to estimate the jitter between the 
> device clock and the Linux system clock, to convert the PTS values into Linux 
> system clock units.
> 
> I'd like to know if anyone has ever solved the same problem, or thought about 
> how to solve it. I know there's a way to retrieve the current SOF counter, 
> but there doesn't seem to be a way to get the Linux system clock at the time 
> the SOF packet is sent.
> 
> Any help would be appreciated.

Section 5.12 of the USB 2.0 spec contains an extensive discussion of clock 
synchronization.

Alan Stern


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