[time-nuts] GPS for ntp

Simon Marsh subscriptions at burble.com
Tue Oct 21 09:58:22 EDT 2014


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 
>> <andrew at cleverdomain.org>
>> wrote:
>>
>>> On Mon, Oct 20, 2014 at 10:50 PM, Neil Schroeder <gigneil at 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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