From: <[email protected]>
Reply-To: <[email protected]>
Date: Monday, November 25, 2013 at 3:19 AM
To: <[email protected]>
Subject: [beagleboard] Help: "Fast" ADC sampling on the beagleboneblack
> Hi!
>
> I am currently using my Beaglebone Black with the following KERNEL:
> 3.8.13xenomai-bone28.1 (including the ti_am335_adc driver).
>
> Part of my Project is reading signals from an ADC and then processing them. I
> would love to use the onboard ADC for that, but I require to sample 5 channels
> @ 2 kHz each. (I have installed the Xenomai Kernel mainly for processing
> purposes but it might be useful for sampling as well ;) )
>
> My first simplistic try was to just read the voltage values from the files
> provided in "/sys/bus/iio/devices/iio:device0/in_voltage0_raw" .
> This works fine so far, but is just not fast enough. ( I get about 500 sps on
> ONE channel)
>
> Here is that part of my CODE:
> for(;;) {
>
> rt_task_wait_period(NULL); // Xenomai
>
> now=rt_timer_read(); // Xenomai
>
> // Work for the current period //
>
> FILE *read = fopen (
> "/sys/bus/iio/devices/iio:device0/in_voltage0_raw", "r");
>
>
> fscanf (read, "%f", &value);
> float voltage = value * (1800.0/4095.0);
> fprintf (write, "Value: %f Voltage: %f\n", value, voltage);
>
> printf("Time since last turn: %ld,%06ld ms Value:%f Voltage:
> %f\n",
> (long)(now - previous) / 1000000,
> (long)(now - previous) % 1000000, value, voltage);
> previous = now;
>
> fclose ( read );
> fclose ( write );
>
> }
> (I know that leaving out the printf part and the voltage calculation increases
> performance, but still not enough)
>
> Is there any other way reading the values, directly from the memory location
> or a register? Perhaps using mmap?
> Or is there a Xenomai function for reading from an iio device? (I couldn't
> find any...)
>
> I have seen two interesting links about continuous sampling:
> -
> http://beagleboard-gsoc13.blogspot.de/2013/07/sampling-analogue-signals-using-
> adc-on.html
> - http://processors.wiki.ti.com/index.php/AM335x_ADC_Driver%27s_Guide
>
> But i can't get the generic_buffer.c from those sites to work correctly (as
> the trigger handling seems to be outdated), plus i might need to know a kind
> of "oneshot" read function anyway to mux the 5 channels i want to sample. Or
> is it possible to continuous sample multiple channels? The Driver guide says
> 'no', but the Author from the other link seems to have worked a lot on the adc
> Driver since.
>
> I am currently trying to figure out the code in generic_buffer.c and the used
> iio_utils.h and the adc driver, but it is a lot of code and drivers are not my
> strong suit.
> So far I think I need to setup the ADC to my needs (somewhat similar to
> generic_buffer.c) and then read from it somehow, but I am really struggling
> with that!
>
>
> So now you know the state of my project :) but to sum it up:
>
> Are my demands (5 channels @ 2 kHz) even possible using only the on-board ADC?
> And if yes how do i read the values fast enough? Any tips?
Reading the ADC values via sysfs isn¹t going to be fast. Also, it doesn¹t
help to have a Xenomai kernel if you don¹t use it. You have to write a
Xenomai kernel driver and then create a user space app that uses the Xenomai
API to communicate with the Xenomai kernel driver. Xenomai comes with
several examples to help you get started. You could also look at Charles
Steinkuehler¹s LinuxCNC work where he uses nmap to control GPIO with the
Xenomai kernel.
Regards,
John
>
> Thank you in advance!!
>
>
>
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