[ntp:questions] NTP on CubieBoard

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Oct 13 19:15:03 UTC 2014

On 2014-10-13 08:35, Rob wrote:
> David Woolley <david at ex.djwhome.demon.invalid> wrote:
>> On 13/10/14 09:23, Rob wrote:
>>> On the PC platform, with a recent development ntpd I can achieve
>>> PPS sync with offset within a couple of us on systems in normal
>>> environment, and well within 1us in a temperature conditioned room.
>> The correct quality measure is jitter, rather than offset.  offset
>> varies from sample to sample but still doesn't tell you the systematic
>> error in the time.
> I know that, the jitter is well below 1us on our systems.  But we
> require small offset as well.
> My point is not to discuss the requirement and viability of good time
> accuracy on ntpd.  I know I can achieve what I need on a PC.
> My question is if it can be done on a Cubieboard2.  I think one of
> the key requirements is the availability of a high-res (nanosecond)
> timer that can timestamp the PPS events and serves as a system
> realtime clock.  Not a 10ms interrupt driven realtime counter, as some
> systems have.
> Another is the availability of a programmable input bit that can issue
> a processor interrupt that will be rapidly served.  That means the system
> must not spend long time with interrupts disabled or at such a high
> load that it cannot perform quick actions.
> Our application should leave plenty of headroom on the 2 x 1GHz ARM-7
> processor.  So my question is if the other requirements are fulfilled,
> preferably on a default Linux install (Cubian).

The SoC implementation seems to be an AllWinner Technology A20
designed for low power mobile uses.
There is a lack of tech docs about these implementations available.
When I see low power mobile, I expect power and speed throttling,
so usability for NTP will depend on how well you can disable those
features to run with power always on.
But there is a reference to enabling Cubian Kernel PPS.
Whether that works or well seems to be answerable only by buy it and try it.
You may be better targeting a non-mobile platform or one of the Intel
embedded Linux boards designed to address the plethora of ARM boards.
Take care. Thanks, Brian Inglis

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