Hi Simon,
Ah, slightly different question :)
I'm afraid I didn't write the code, so I can't really answer that
question. What I can say is that it does appear to be as good as Ian
(Lepore's) ntpq output shows.
To be honest, I just use the code :)
Iain
On 21/10/14 15:39, Simon Marsh wrote:
Sorry, I wasn't clear.
The /dev/pps0 devices output a timestamp corresponding to when the event
happened.
The GPIO driver does this very simply by waiting for an interrupt event
and then asking what (current) time it is. This leads to the problem
that there is a non-deterministic time between the event and when the
code gets to ask 'what time is it ?'
With HW capture, you can get an accurate view of when the event took
place but only relative to the counter in the particular timer/capture
unit that is being used. You have to synchronise between the counter
value and what the OS understands is 'system time' in order to create a
retrospective timestamp for when the event occured. Whilst you've solved
the problem with the interrupt approach, you've created a different one
of needing to synchronise counters.
My question is how do you convert between the timer value and system
time to get the timestamp ?
Cheers
Simon
On 21/10/2014 15:14, Iain Young wrote:
It just turns up as /dev/pps0 like any other PPS source, so you
configure ntp in the same way you would for any other PPS source,
or build ppsapitest to test it manually
(Although be aware you -may get a Invalid argument error from
ppsapitest after running it more than once. Reboot solves it,
and since the BBB's don't do anything else, and I don't restart ntp
too often, its not a big deal for me.)
Iain
On 21/10/14 14:58, Simon Marsh wrote:
Iain,
How do you map the timer counter value in to a PPS timestamp ?
(that is, how do you turn the HW counter value in to what the OS thought
the time was when the event occured ?)
Cheers
Simon
On 21/10/2014 13:54, Iain Young wrote:
It's been done on FreeBSD. See:
http://lists.freebsd.org/pipermail/freebsd-arm/2013-February/004769.html
Patch is now in recent FreeBSD releases/snapshots
And yes, it's far superior to than using the GPIOs, or UARTS
There was some work done on Linux, but I'm not sure it was ever
finished
or published.
All of my "Timing" Beaglebones run FreeBSD, with the exception of the
TIC stufff I wrote for the PRUSS's. As soon as the userspace bits of
that work on FreeBSD, I'll probably switch that to FreeBSD as well.
All the Best
Iain
On 21/10/14 13:33, Neil Schroeder wrote:
Andrew-
I'm actually referring to using either the eCAP function or one of the
integrated dmtimer triggers - which are, from some accounts, more
accurate
than a gpio.
Google beaglebone dmtimer pps.
NS
On Mon, Oct 20, 2014 at 11:56 PM, Andrew Rodland
<and...@cleverdomain.org>
wrote:
On Mon, Oct 20, 2014 at 10:50 PM, Neil Schroeder <gign...@gmail.com>
wrote:
The one thing that hasn't yet happened is making the beaglebone
timestamp
on the linux side in a way that works for ntp.
Custom code no problem. Freebsd PPSAPI no problem. Linux, nothing
there
yet.
I have been working on it but if anyone has some insight its
appreciated.
It appears to support gpio class devices, with interrupts, so the
pps-gpio driver (in-tree since 3.2) should work just fine. The only
thing that's needed (other than building the driver) is a bit of code
in the board support file to register the device. Various folks have
done it for the rpi (http://ntpi.openchaos.org/pps_pi/ for example),
and I've done it for the UDOO Dual
(https://gist.github.com/arodland/518f037e4f24b1984286). The BBB is
probably about as easy.
I'm not sure if there's other hardware that lets you do better than
grabbing an interrupt, but that will get you in the microsecond range
or a bit better, anyhow.
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