Re: [Linuxptp-devel] discrepancy between ptp4l output and oscilloscope readings on Beaglebone Black

[Juan S. <js...@ya...>]

On Wed, Jul 29, 2015, at 04:40 PM, Richard Cochran wrote:
> On Wed, Jul 29, 2015 at 02:46:34PM +0200, Juan Solano wrote:
> > I am learning about PTP and doing tests on 2 Beaglebone Blacks running a
> > recent stable linux (v3.18.16) with disabled dynamic ticks. They support
> > hardware timestamping and I have synchronized the system clocks to the
> > PHC. My logs show what seem to be good values:
> 
> ...
> 
> > However, when comparing the PPS GPIO outputs with an oscilloscope I can
> > see them varying in a range of +- 800 ns.
> 
> The CPTS in the AM335x does not have any hardware output signals.  The
> PTP clock cannot be adjusted because of silicon bugs, so we leave the
> hardware clock free running and use a software clock as a work around.
> 
> > Can anybody point me at something obvious that I am missing or how to
> > investigate further?
> 
> Tell us how you are generating the PPS.

Oops! time to ditch the BBB then. I have written a PPS generator driver
through GPIO:

https://github.com/jsln/pps-gen-gpio

Do you know of a cheap SBC we can use that supports HW timestamp? Has
anybody had success with Atmel SAMA5D3 chips?

Thanks,
Juan.