Re: [Xenomai] Seeking recommendations for SBC with precision timer

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

On 10/26/2012 04:46 PM, Howard Lee Harkness wrote:
> Thank you for your fast responses!
> 
> 1) Not necessarily fanless, but that would be nice.
> 
> 2) Don't need a timestamp. All I need is a precision interval. I want to be
> able to send a signal to a device, and read a measurement exactly 11 us (or
> 5 us, or 17 us, or 31 us, etc.) later, plus or minus no more than about 1
> us. <1 us precision would be better, but I think for now 1 us resolution is
> sufficient. Reliably, repeatably.

What you are asking is a latency of less than 1us, not a resolution.
Here is a graph showing the (user-space wake-up) latencies we get with
various configurations of Xenomai 2.6.1 over Linux 3.4.6 on an atom 230:

http://xenomai.org/~gch/core-3.4-latencies/atom.png

These are user-space wake-up latencies, you get better result with irqs
handlers. But 1us, that is a bit hard. The only way I know to do this,
is, for instance, if you know that the maximum irq latency is 15us,
wake-up 15us before the wanted wake-up date, and spin in the timer
handler until the time you want, using a clocksource which has a better
resolution than 1us. x86 generally use the tsc which is such a clocksource.


-- 
					    Gilles.