If PPS output is generated by Dual-Mode Timer, its accuracy doesn't depend on any software latencies, so there is no need in RT patches. However the resolution of DMTimer is limited to 26 MHz (~39 ns).
In our device, the AM335x core is clocked by 19.2 MHz adjustable oscillator, so the PPS output resolution is ~53 ns. In practice, DMTimer lowers PPS readings by 20-30 ns. For example, under the ideal conditions (two devices are connected directly with a single patch cord), ptp4l reports master offset to be in the range of +- 15 ns, while the oscilloscope shows +- 30 ns. 2015-08-04 10:49 GMT+03:00 Juan Solano <jsol...@yahoo.es>: > thanks for your suggestion, Kim. What kind of values are you getting? > Juan. > > > On Wed, Jul 29, 2015, at 07:41 PM, Yursen Kim wrote: > > We implemented PPS output signal through AM335x Dual-Mode Timer running in > the PWM mode with SCLK frequency. > > This is more accurate than software GPIO output because the CPTS on the > AM335x > can generate hardware timestamp event on the rising edge of the DMTimer, > making it possible to synchronize PPS with the CPTS internal clock counter. > > > 2015-07-29 20:08 GMT+03:00 Chris Healy <cphe...@gmail.com>: > > The Freescale MX6 based Sabre boards are inexpensive and have good PTP > support now, don't they? IIRC, recently there was some changes made to > support the HW 1588 block in the MAC that allows the HW to drive a 1588 > output pin correctly. > I have not tested this myself yet though but will be doing so in the near > future as I'm working on a design that will depend on it.... ;-) > > On Wed, Jul 29, 2015 at 9:17 AM, Richard Cochran <richardcoch...@gmail.com > > wrote: > > On Wed, Jul 29, 2015 at 05:28:12PM +0200, Juan Solano wrote: > > Oops! time to ditch the BBB then. I have written a PPS generator driver > > through GPIO: > > > > https://github.com/jsln/pps-gen-gpio > > Right, so the jitter you are seeing is mostly due to latencies in the > OS. You can improve this a bit by using a PREEMPT_RT kernel. > > > Do you know of a cheap SBC we can use that supports HW timestamp? Has > > anybody had success with Atmel SAMA5D3 chips? > > I can't think of any cheap SBC that also has good PTP support. > > Sorry, > Richard > > > ------------------------------------------------------------------------------ > _______________________________________________ > Linuxptp-devel mailing list > Linuxptp-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/linuxptp-devel > > > > > ------------------------------------------------------------------------------ > > _______________________________________________ > Linuxptp-devel mailing list > Linuxptp-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/linuxptp-devel > > > >
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