At first, i tried to communicate with MCP3008 with raspberry using wiring Pi library and as it communicates through the spi driver, the data retrival of it is quite slow. Then, i opt for the next option that is to access the register of the BCM2835 Arm cortex chip and by using bcm2835.h, i communicate with MCP3008 and it successfully communicates. The code for communication is provided below.

#include <stdio.h>
#include <stdint.h>
#include <bcm2835.h>
#include <sched.h>
#include <string.h>
#include <sys/mman.h>
#include <inttypes.h>

static     uint8_t buff[3] = {0};
char value[] = {0x01, 0x80, 0x00};

int readadc(adcnum)
    char one[] = {'2', '2','3'};

    buff[0] = 1;
    buff[1] = (8+adcnum)<<4;
    buff[2] = 0;
    value[0] = buff[0];
    value[1] = buff[1];
    value[2] = buff[2];

   //send the value into function and declare the size of it inside
    bcm2835_spi_transfern(value, sizeof(value));

    buff[0] = value[0];
    buff[1] = value[1];
    buff[2] = value[2];

   //converting into output value upto 1024 supported by 10 bit resolution
   adcnum = ((buff[1]&3) << 8) + buff[2];
   return adcnum;

int main(int argc, char **argv)
    uint64_t a,b ;
    // If you call this, it will not actually access the GPIO
   // Use for testing
   // bcm2835_set_debug(1);

   if (!bcm2835_init())
       printf("bcm2835_init failed. Are you running as root??\n");
      return 1;

   if (!bcm2835_spi_begin())
      printf("bcm2835_spi_begin failed. Are you running as root??\n");
      return 1;
   bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);      

 int i = 0;

     a = bcm2835_st_read();
     printf("The value read as %d \n", readadc(0));
     printf("The value read as %d \n", readadc(1));
     printf("The value read as %d \n", readadc(2));
     printf("The value read as %d \n", readadc(3));
     printf("The value read as %d \n", readadc(4));
     printf("The value read as %d \n", readadc(5));
     printf("The value read as %d \n", readadc(6));
     printf("The value read as %d \n", readadc(7));
     printf("The value read as %d \n", readadc(4));
     printf("The value read as %d \n", readadc(5));
     printf("The value read as %d \n", readadc(6));
     printf("The value read as %d \n", readadc(7));


    return 0;


Using this piece of code, the clk of spi fluctuates a lot from 10khz to 20 khz. Then, i added this piece of line into above mentioned code to prevent process swapping and dramtically, speed increased from 10 to 20 khz to around 60 khz and best part of it is that the frequency value nearly remains constant.

struct sched_param sp;
memset(&sp, 0, sizeof(sp));
sp.sched_priority = sched_get_priority_max(SCHED_FIFO);
sched_setscheduler(0, SCHED_FIFO, &sp);

Can u provide me suggestion about more ways for increasing the speed of the clk of spi or u can optimize my code where it is required. Thank u.

  • 1
    If you want the code to run faster remove the printfs or redirect the output to a file rather than the screen.
    – joan
    Commented Jul 24, 2016 at 17:48
  • Ok,i will consider it. Can you provide me suggestion regarding killing the unused process in the raspberry os so that will help me to increase the speed. I will be overclocking my raspberry latter on. Commented Jul 24, 2016 at 18:06
  • That is pointless as it will not make any measurable difference.
    – joan
    Commented Jul 24, 2016 at 18:10

1 Answer 1


Try to use a lower Clock divider in this line:


For example use BCM2835_SPI_CLOCK_DIVIDER_2 for the fastest possible data rate.

  • Yes, i set to that value and what i observe is that the frequency get decreased rather than increasing. So, by doing experiment a few times, i found out that the setting i set out is the best option there. Can u provide me link about killing other unused process in raspberry ?? Commented Jul 24, 2016 at 18:04
  • If you are using Raspbian you could switch to a more lightweight OS, Minibian is a good start because you would still have access to Raspbian packages while it doesn't have as many preinstalled packages. Besides that, I don't know which processes you need and which not, you have to decide for yourself. You could however try to kill one process after another and check if your Pi is still operating as desired.
    – Domme
    Commented Jul 24, 2016 at 18:07
  • Ok, i will be try using htop which is interactive process viewer for unix. Tx. Commented Jul 24, 2016 at 18:19
  • with 12 ADC i am successfully able to sample around 100 values in 20 ms which will be enough for my work Commented Jul 26, 2016 at 21:23
  • That's good to hear, good luck =)
    – Domme
    Commented Jul 28, 2016 at 19:01

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