at91_enthus wrote:
>
>
> On Wed, Feb 2, 2011 at 3:39 PM, Gilles Chanteperdrix
> <[email protected]
> <mailto:[email protected]>> wrote:
>
> at91_enthus wrote:
> > Hi.
> >
> > In order to get better accuracy with Xenomai, I modified the I-pipe
> > timebase (MCK divider) in arch/arm.mach-at91/at91_ipipe_time.c.
> > Unfortunately, I cannot mount the rootfs on the MMC, since the MMC
> > controller is no longer functioning. I tried to change TCx in kernel
> > configuration to no avail.
> > When I switch back to a timebase of 1 MHz, the MMC works fine.
>
> The thing is that we are a bit tight on AT91. A 16 bits counter is used
> for both the timer and the tsc emulation, and this tsc must be refreshed
> at least once before it wraps. The problem is that since it is a 16 bits
> counter, it wraps really fast, on my AT91SAM9263 it wraps in 20 ms, and
> since the Linux default period is 10ms we are actually quite close to
> the limit.
>
> Anyway, trying to get a better accuracy than 1us is kind of useless on
> AT91, since even reading this counter takes more than 1us. So, you are
> not in fact improving anything. The 1MHz is even a bit overkill.
On the other hand, the AT91SAM9G20 may be running at a better frequency.
But with latencies in the order of tens of microseconds, having a better
accuracy than 1us still does not make really much sense. Why do you need
this accuracy?
--
Gilles.
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