Why does i2c not work on my raspberry pi 3 model B?

Question

I have the raspberry pi 3 model B and would like to communicate over i2c. I have heard that the bcm2835 and bcm2837 chip ar almost identical, so I should be able to use the bcm2835 library to communicate over i2c, yet I don't see anything happening.

• I have 2 pull ups from 10k
• this code shows me that I always get a NACK

• a logic analyzer that shows me that the serial lines are always high and never go low

• yes I did enable i2c in raspberry pi configurations > interfaces > i2c

Could anybody explain me what I am doing wrong?

EDIT:

- i2cdetect -y 1

    pi@raspberrypi:~ $ i2cdetect -y 1

         0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f

    00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 

    10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

    20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

    30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

    40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

    50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

    60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- -- 

    70: -- -- -- -- -- -- -- --                         

    pi@raspberrypi:~ $ 

• This is the code I use to communicate, reason is always equal to 1 (=NACK):

  #include <bcm2835.h>
  
  #include <stdio.h>
  
  
  #define slaveAddress 0x68
  
  int main(int argc, char **argv) {
  
      char commands[1];
      uint8_t reason;
  
      bcm2835_init();
      bcm2835_i2c_begin();
  
      printf("i2c busy\n");
      bcm2835_i2c_setSlaveAddress(slaveAddress);
      bcm2835_i2c_set_baudrate(1000);
      bcm2835_delay(500);
  
      commands[0] = 0x35;
      reason = bcm2835_i2c_write(commands, 1);
  
      printf("reason: %d\n", reason);
      return 0;
  }
  

In the BCM2835 header file, I modified the base address from 0x20000000 to 0x3F00000000 (neither of them worked).

• this is my setup

• I tried to communicate with the module using the wiringPi library. That worked. So the issue has to do with my code somehow....

Answer 1

First of all, your code is missing pin initialization

Do so by using void bcm2835_gpio_fsel(uint8_t pin, uint8_t mode)

Also consider your access level

If the library runs with any other effective UID (ie not root), then bcm2835_init() will attempt to open /dev/gpiomem, and, if successful, will only permit GPIO operations. In particular, bcm2835_spi_begin() and bcm2835_i2c_begin() will return false and all other non-gpio operations may fail silently or crash.

Check for bcm_init's return value if you are unsure. Nonzero means you are good to go (=root).

Modifying the base address to 0x3F00000000 should not be necessary as of the RPi3 (although it is correct that they do differ):

On RPI 2, the peripheral addresses are different and the bcm2835 library gets them from reading /proc/device-tree/soc/ranges. This is only availble with recent versions of the kernel on RPI 2.

(2 and 3 have the same base address)

This mechanism depends on the presence of /proc/device-tree/soc/ranges in your filesystem. So go check that and look up how to enable device-tree support in case it is not visible.

If you are done checking all of these and it is still not working, the next step is to consider void bcm2835_i2c_setClockDivider(uint16_t divider)