Need some help using I2C on Beaglebone

Question

I'm a beginner at I2C communication and I am trying to hook up communication with an I2C device a (mpu6050) using librobotcontrol http://strawsondesign.com/docs/librobotcontrol/group___i2_c.html i2c library in C. The code Excluding stuff not I2C related I have:

#define I2C_ADDRESS 0x68
#define I2C_BUS 1

void setupMPU();
float recordAccelRegisters();

int main(){

    float ax = 0;
    while(1){
        ax = recordAccelRegisters()
    }

    float recordAccelRegisters(){
        float Xg = 0;
        float Xa = 0, Ya, Za;
        unsigned char data;

        printf("Recording Accel Data\n");

        rc_i2c_init (I2C_BUS,I2C_ADDRESS);
        rc_i2c_read_bytes(I2C_BUS, 0x3B, 6, data);
        rc_i2c_close(I2C_BUS);

        //Xa = 8 << data;
        //Xa = (float)Xa;
        //Xg = Xa/16384.0;
        return Xg;
        }

    void setupMPU(){
        printf("Initalizing Mpu\n");

        // init I2C Bus
        rc_i2c_init (I2C_BUS,I2C_ADDRESS);

        //Wake up setup
        rc_i2c_write_byte(I2C_BUS, 0x6B, 0x00);
        rc_i2c_close(I2C_BUS);

        printf("Configuring Accelerometer\n");
        printf("Initalizing Mpu\n");

        //Accel Config
        rc_i2c_init (I2C_BUS,I2C_ADDRESS);
        rc_i2c_write_byte(I2C_BUS, 0x1C, 0x00);

        printf("Done with Accel Config\n");
        rc_i2c_close(I2C_BUS);
        }
   }

I am trying to figure out if I need to close and reopen the port every time like in an Arduino using wire library or if I can just initialize it once. I have tried many different types of code configurations. For instance only initializing the communication only once, using send words instead of send bytes, sending only 1 byte at a time ect... but I am still unable to get data from the device. I am also struggling with weather or not to use read words or read bytes. It keeps returning a -1 bytes as seen by this error message when using

read bytes:

ERROR: in rc_i2c_read_bytes, received -1 bytes from device, expected 1

and this error when using read words:

ERROR: Segmentation Fault
Fault address: (nil)
Address not mapped.
Segmentation fault

Any help on this will be much appreciated.

Answer 1

According to the documentation you linked, the function rc_i2c_read_bytes takes a pointer where it will store the bytes read:

int rc_i2c_read_bytes (int bus, uint8_t regAddr, size_t count, uint8_t *data);
Reads multiple bytes from a device register

Thus, in your recordAccelRegisters function, you should define an array to store the bytes:

float recordAccelRegisters(){
        float Xg = 0;
        float Xa = 0, Ya, Za;
        unsigned char data[6];

        printf("Recording Accel Data\n");

        rc_i2c_init (I2C_BUS,I2C_ADDRESS);
        rc_i2c_read_bytes(I2C_BUS, 0x3B, 6, data);
        rc_i2c_close(I2C_BUS);

        //Xa = 8 << data;
        //Xa = (float)Xa;
        //Xg = Xa/16384.0;
        return Xg;
}

Same applies if you use rc_i2c_read_word, it also requires a pointer (to a uint16_t):

float recordAccelRegisters(){
        float Xg = 0;
        float Xa = 0, Ya, Za;
        uint16_t data;

        printf("Recording Accel Data\n");

        rc_i2c_init (I2C_BUS,I2C_ADDRESS);
        rc_i2c_read_word(I2C_BUS, 0x3B, &data);
        rc_i2c_close(I2C_BUS);

        //Xa = 8 << data;
        //Xa = (float)Xa;
        //Xg = Xa/16384.0;
        return Xg;
}