Nico found this post on the PCEngines forum: [http://www.pcengines.info/forums/?page=post&id=DE352734-3123-41F2-AFA4-0CD1939BFDC7&fid=40251191-FF24-48A8-BB0E-995B04812ADE I2C-bus examples or a howto]
== Working example ==
The above helps, but it's not all. Looking at kernel Documentation/i2c/dev-interface helps some more, but not quite. Here's the smallest compiling example to use I²C in userspace linux ; it took me a while to get it together.
http:;example from from Documentation/i2c/wwwdev-interface: So let's say you want to access an i2c adapter from a C program.pcenginesThefirst thing to do is "#include <linux/i2c-dev.infoh>". Please note thatthere are two files named "i2c-dev.h" out there, one is distributedwith the Linux kernel and is meant to be included from kerneldriver code, the other one is distributed with i2c-tools and ismeant to be included from user-space programs. You obviously wantthe second one here. Now, you have to decide which adapter you want to access. You shouldinspect /forumssys/?page=post&id=DE352734class/i2c-3123dev/ or run "i2cdetect -41F2l" to decide this.Adapter numbers are assigned somewhat dynamically, so you can notassume much about them. They can even change from one boot to the next. The example values are taken from PCA9551 8 bit GPIO / LED Driver Next thing, open the device file, as follows: #include <fcntl.h> #include <linux/i2c-AFA4dev.h> #include <stdio.h> #include <stdlib.h> #include <sys/stat.h> #include <sys/types.h> int main(int argc, char *argv[]) { int file; int adapter_nr = 0; /* probably dynamically determined */ char filename[20]; snprintf(filename, 19, "/dev/i2c-0CD1939BFDC7&fid%d", adapter_nr); file =40251191open(filename, O_RDWR); if (file < 0) { /* ERROR HANDLING; you can check errno to see what went wrong */ exit(1); } When you have opened the device, you must specify with what deviceaddress you want to communicate: int addr = 0x67; /* The I2C address of PCA9551 */ if (ioctl(file, I2C_SLAVE, addr) < 0) { /* ERROR HANDLING; you can check errno to see what went wrong */ printf("error: ioctl on file failed\n"); exit(1); } Well, you are all set up now. You can now use SMBus commands or plainI2C to communicate with your device. SMBus commands are preferred ifthe device supports them. Both are illustrated below. /* Device register to access */ __u8 reg; __s32 res; __u8 value; char buf[10]; /* read a byte using SMBus commands */ // INPUT (read pin state) reg = 0x00; res = i2c_smbus_read_byte_data(file, reg); if (res < 0) { /* ERROR HANDLING: i2c transaction failed */ printf("error: i2c read failed\n"); } else { /* res contains the read word */ printf("LED status: %x\n",res); } /* write a byte using SMBus commands */ // LEDs 4-FF245 on PWM0, LEDs 6-48A8-BB0E-995B04812ADE7 on PWM1 reg = 0x06; // register we want to write to value = 0xfa; res = i2c_smbus_write_byte_data(file, reg, value); if (res < 0) { /* ERROR HANDLING: i2c transaction failed */ printf("error: i2c write failed\n"); } } Let the LEDs blink!
