On Wed, Feb 2, 2011 at 3:39 PM, Gilles Chanteperdrix < [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. > > -- > Gilles. > Thank you.
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