We have been using a Beaglebone Black for a while now using the PRU(s) for time sensitive experiments. In our estimate the PRU has shown a very stable clock, running at 200MHz with tight control. The clock does run off a very stable TCXO and the two PRUs are synchronized to within mHz of offset (relative to each other) likely attributable to any clock skew effects. Getting day and time stamp does need synching up to a network time server via NTP. I hope that helps.
Venky On Sat, Feb 29, 2020, 12:05 AM Dennis Lee Bieber <[email protected]> wrote: > On Fri, 28 Feb 2020 01:35:54 -0800 (PST), in > gmane.comp.hardware.beagleboard.user Jaka Koren > <[email protected]> wrote: > > > >-Is using a PRU even suitable for this kind of experiment? I am aiming > for > >sub-microsecond accuracy. > >-My task for example would require obtaining global time via selected > >protocol, then synchronizing the hardware clock and PRU to the obtained > >timestamp. Is this possible, considering PRU is a separate unit on the > >board? > > The processor clock(s) will be only as reliable as the crystal(s) > used > on the board. These crystals are not, to my knowledge, temperature > compensated, nor are they in an "oven" -- so some drift as the circuit > warms up or the environment changes temperature is to be expected. > > >-Is PRU clock accessible to programs in same way as processor clock (via > >hwclock on linux)? Is there a way to send pulses on pins periodically, > but > >starting on a predefined moment on clock? > > The PRUs do not have a time-of-day clock. The main processor > time-of-day is periodically synchronized using NTP, and I suspect is > incremented using an interrupt from a 32.768kHz crystal. The main processor > itself appears to be driven by a 24MHz clock (probably via some multiplier > to get to the 1GHz internal cycle). > > The PRU processor clock is 200MHz, which I believe gives a 5ns > instruction timing (you'll have to figure out how many instructions will be > needed to handle GPIO toggling and any time delays). I expect this clock, > as with the processor main clock, are conditioned on the 24MHz crystal > frequency. > > The time-of-day clock likely is not synchronized to the ARM > processor > clock (except as an artifact of interrupt signal gating). > > > > -- > Dennis L Bieber > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/mani5f5fhkhuvtinqsurh7tep79458toik%404ax.com > . > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/CAHW0bcx%3DSdYNFMPURxHsxFp7gjHn5CuwBTLmFNL-ft8EVL3Opw%40mail.gmail.com.
