What's wrong with my I2C code?

Marc Nicholas

I hacked together some I2C code based on some things I found on the Interweb. I keep getting I2C read errors despite the fact I can see the I2C device with i2ctools.

root@Omega-1041:/# ./i2c 
HTU21D Test
error reading i2c device
: No such device or address
error reading i2c device
: No such device or address
eror reading i2c device
: No such device or address
eror reading i2c device
: No such device or address
Temperature 0x66 0xfc 0x6c\nHumidity 0x3f 0xfc 0x6c

But the values that are returned for temperature seem legit. Anyone got any ideas where I'm going wrong here?

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/i2c-dev.h>


int main( int argc, char **argv )
{
        int i;
        int r;
        int fd;
        unsigned char command[2];
        unsigned char value[3];
        useconds_t delay = 5000;

        char *dev = "/dev/i2c-0";
        int addr = 0x40;

        printf("HTU21D Test\n");

        fd = open(dev, O_RDWR );
        if(fd < 0)
        {
                perror("Opening i2c device node\n");
                return 1;
        }

        r = ioctl(fd, I2C_SLAVE, addr);
        if(r < 0)
        {
                perror("Selecting i2c device\n");
        }


        command[0] = 0xe3;
        r = write(fd, &command, 1);

        if(r != 1)
        {
                perror("error writing to i2c device\n");
        }
        usleep(delay);

                for(i = 0; i < 3; i++)
                {
                       
                        // the read is always one step behind the selected input
                        r = read(fd, &value[i], 1);
                        if(r != 1)
                        {
                                perror("error reading i2c device\n");
                        }
                        usleep(delay);
                }
                printf("Temperature 0x%02x 0x%02x 0x%02x\\n", value[0], value[1], value[2]);

                usleep(delay);


        command[0] = 0xe5;
        r = write(fd, &command, 1);
        usleep(delay);

                for(i = 0; i < 3; i++)
                {
                        // the read is always one step behind the selected input
                        r = read(fd, &value[i], 1);
                        if(r != 1)
                        {
                                perror("eror reading i2c device\n");
                        }
                        usleep(delay);
                }
                printf("Humidity 0x%02x 0x%02x 0x%02x\n", value[0], value[1], value[2]);
        

        close(fd);
        return(0);
}

Marc Nicholas

I busted out the logic analyzer and see the address (0x80 for write) and the byte (0xf3) for start conversion both get an ACK.

Reads get a NAK, however.

Przemyslaw Downar

@Marc-Nicholas Did you run HTU21d?

Marc Nicholas

@Przemyslaw-Downar I did. Although it doesn't seem 100% reliable on the Onion.

Chris Stratton

@Marc-Nicholas said in What's wrong with my I2C code?:

I busted out the logic analyzer and see the address (0x80 for write) and the byte (0xf3) for start conversion both get an ACK.

Reads get a NAK, however.

Is the direction bit being correctly set for a read, ie is 0x81 going down the wire?

Does the peripheral attempt to clock stretch?

Doug Petican

May not have anything to do with your problem but I noticed this: unsigned char command[2]; but later you only set the first command byte: command[0] = 0xe3;

Just thought I would point it out in case it helps.

Chris Stratton

@Doug-Petican said in What's wrong with my I2C code?:

May not have anything to do with your problem but I noticed this: unsigned char command[2]; but later you only set the first command byte: command[0] = 0xe3;

Not an issue. The only use of that array is write() calls with a count of 1.