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
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