On Wed, Feb 2, 2011 at 4:09 PM, Gilles Chanteperdrix < [email protected]> wrote:
> 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? > I want to time stamp the samples from a 8 channel SPI-based ADC. I know that a conversion takes between 3 and 5 microseconds depending on the SPI clock. What I want is to adapt the code to acquire a fixed number of samples in user space (I knew it was a long shot). Plain, Xenomai-free, userland application was out of the question, so I figured that a userspace application in Xenomai run at highest priority should do the trick. On a second thought, a simple linux kernel module is what I want (this way I can use interrupts and one of the platform's six timers). R.
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