Testing driving a SwitecX25 stepper over i2c using an MCP23008 I/O chip

i2ctest.c

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

int main(int argc, char **argv)
{
    const char *i2cDevice = "/dev/i2c-0";
    const int i2cAddress = 0x20;
    unsigned char buf[2];
    unsigned char i;
    int fd;

    /*                                                                                                       
     * ref: http://www.kernel.org/doc/Documentation/i2c/dev-interface                                        
     */

    if ((fd = open(i2cDevice, O_RDWR)) < 0) {
        printf("Failed to open i2c port\n");
        exit(1);
    }

    // set address                                                                                           
    if (ioctl(fd, I2C_SLAVE, i2cAddress) < 0) {
        printf("Unable to configure i2c address\n");
        exit(1);
    }

    for (i=0;i<=10;i++) {
        buf[0] = i;  // set register                                                                         
        if (write(fd, buf, 1) != 1) {
            printf("error writing i2c register address\n");
            exit;
        }

        // I could of course read all 10 registers in one read here instead.                                 
        if (read(fd, buf, 1) != 1) { // read back data into buf                                              
            printf("Short read from i2c\n");
            exit(1);
        }

        printf("Register %u = %u\n", i, buf[0]);
    }
    printf("[done]\n");

#define REG_GPIO  0x0A
#define REG_IODIR 0x00

    buf[0] = REG_IODIR;
    buf[1] = 0x00;  // all pins set to output
    if (write(fd, buf, 2) != 2) {
        printf("error writing i2c iodir register\n");
        exit;
    }
    
    static unsigned char stateMap[] = {0x9, 0x1, 0x7, 0x6, 0xE, 0x8};
    unsigned char state = 0;
    while (1) {
        buf[0] = REG_GPIO;
        buf[1] = stateMap[state];
        //printf("state %d bits %u\n", state, buf[1]);
        if (write(fd, buf, 2) != 2) {
            printf("error writing i2c gpio register\n");
            exit;
        }
        usleep(400);
        state = (state + 1) % 6;
    }
}