Re: [Xenomai-help] [comedi] NI 6251 "wrong" samples

[Daniele Nicolodi <daniele@domain.hid>]

On 21/06/10 13:59, Alexis Berlemont wrote:
>> I have tried to look at Analogy buffer handling, but the lack of any
>> comment in the source code makes understanding how it is supposed to
>> work a complex task.
>>
> Please, tell me where comments are lacking, I will fix that.

I find buffer.c and buffer.h to be under documented to be easily
understood. For example it is not clear what the *_count members of the
a4l_buf_t struct are for without a quite tedious inspection of the code.

>> Alexis, can you see any case where there may be a synchronization issue
>> like the one I'm seeing in your code? Any suggestion in where to look
>> for the bug?
>>
> 
> You told you that the bug was occuring randomly and that there was no
> pattern in the repetition of the bad samples. But are you sure that
> the wrong samples do not occur every modulo x * 64KB ?

I convinced myself that this is not the case. For sure the error is not
there at any ring buffer wrap around. I increased the buffer size to
128KB and there hasn't been any noticeable difference. Am I right
supposing that those 64KB you are referring to are the buffer size?

> If the bug is located in Analogy buffer handling, it should not be
> specific to a driver. So, I will try to reproduce it with the testing
> drivers. I will compare the mapped and unmapped acquisition results.
> If I remember well, the "fake" driver outputs some deterministic
> content.  If nothing comes out, I will try to use a NI device.

To detect the problem you need to feed a deterministic signal to your
device. It looks like every now and then an old sample is returned (I
say so because the wrong value is always in the signal range). If you
feed a constant value you would be unable to distinguish the wrong
sample from the good ones. If you are missing a function generator the
DAC channels of the NI ADC board work just fine, as long as you provide
proper low pass filters.

My test case is a phase-meter implementation. I'm testing the phase
noise of the phase-meter and thus I acquire a zero mean sine wave. Every
now and then the average of a fixed number of samples, containing an
integer number of sine waves, is far from zero. When I detect this
situation I dump the content of the current buffer, and in each dump
there is always a single wrong sample.

My code is embedded into a data acquisition framework I'm developing,
but I can provide the code, however I can take a little bit to have it
to compile stand alone.

The bug shows usually after some minutes to half an hour of data taking
at 10 to 50 kHz. It would be unpractical to save all that data to disk
for inspection.

Thanks. Cheers,
-- 
Daniele