at91_enthus wrote:
> 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).
You can use interrupts and the platform's six timers in a Xenomai driver
based on the RTDM skin. The thing is, what interrupt latency is your target?
--
Gilles.
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