Reading analog inputs fast in beaglebone black

Question

I need to read all 7 analog pins in the BBB every 5 milliseconds. I'm doing so with the following C code:

void main(){
    char value_str[7];
    long int value_int = 0;

    FILE* f0 = fopen("/sys/bus/iio/devices/iio:device0/in_voltage0_raw", "r");

    while(1){
            fread(&value_str, 6, 6, f0);
            value_int = strtol(value_str,NULL,0);
            printf("0 %li\n", value_int);
            fflush(stdout);

            usleep(5000);
            rewind(f0);
    }

Hoever, the cpu usage goes up really high (20%). Is there any way to read the analog inputs differently so that it doesn't use as much CPU? Someone suggested "DMA" but I'm completely lost on that regard...

Any help will be appreciated.

Answer 1

This thread in the BBB forum was very useful and I ended up using libpruio. It uses the PRU to read the Beagle's built in io pins and analog to digital converters. The code I ended up using:

#include <stdio.h>
#include <unistd.h>
#include "pruio_c_wrapper.h"
#include "pruio_pins.h"

int main(int argc, const char *argv[]) { 
   PruIo *io = pruio_new(0, 0x98, 0, 1);
   if (io->Errr) {
      printf("Initialisation failed (%s)\n", io->Errr);
      return 1;
   }

   if(pruio_config(io, 0, 0x1FE, 0, 4, 0)){
      printf("Config failed (%s)\n", io->Errr); 
      return 1;
   }

   while(1){
      printf"\r%12o  %12o  %12o  %12o  %4X %4X %4X %4X %4X %4X %4X %4X\n"
         , io->Gpio[0].Stat, io->Gpio[1].Stat, io->Gpio[2].Stat, io->Gpio[3].Stat
         , io->Value[1], io->Value[2], io->Value[3], io->Value[4], io->Value[5]
         , io->Value[6], io->Value[7], io->Value[8]);
      usleep(1000);
    }

   pruio_destroy(io);

   return 0;
}

Answer 2

I suggest you use the PRU. It's very fast! This should get you started-> http://www.element14.com/community/community/knode/single-board_computers/next-gen_beaglebone/blog/2013/08/04/bbb--high-speed-data-acquisition-and-web-based-ui