I2C bus interface

I2C is a multi-master serial single-ended bus invented by Philips that is used to attach low-speed peripherals to an embedded system. SMBus, defined by Intel in 1995, is a subset of I2C. This article is a practical guide to use the I2C bus on the Acme Systems Linux embedded boards

• Read more about I2C bus on Wikipedia...

To have a more in depth idea of how the I2C works under Linux please read the Linux Kernel documentation on:

• I2C bus Kernel documentation

Using i2c-tools

The faster way to do the first experiments with this board is by installing and using the i2c-tools.
i2c-tools is a package contains a heterogeneous set of I2C tools for Linux such as:

• a bus probing tool
• a chip dumper
• a register-level access helpers
• an EEPROM decoding scripts
• ...and more

To install i2c-tools on the FOX Board just type:

debarm:~# apt-get update
debarm:~# apt-get install i2c-tools

Using i2cdetect

i2cdetect is an userspace program to scan an I2C bus for devices. It outputs a table with the list of detected devices on the specified bus.

• i2cdetect man page

Example:

debarm:~# i2cdetect -y 0 
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f                             
00:          -- -- -- -- -- -- -- -- -- -- -- -- --                             
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                             
70: -- -- -- -- -- -- -- --                                                     

In this case a device has been detected on address 20 hex.

Using i2cset

i2cset is a small helper program to set registers visible through the I2C bus.

• i2cset man page

The following simple command writes the byte value 255 to the I2C device at address 20 hex on the i2c bus 0 (/dev/i2c-0).

debarm:~# i2cset -y 0 0x20 255

If for example you are using a DAISY-22 module with a PCF8574 I2C I/O expander this command will set all the GPIO lines to 1.

Python code example

python-smbus is a Python module allows SMBus access through the I2C /dev interface on Linux hosts. The host kernel must have I2C support, I2C device interface support, and a bus adapter driver.
The following example sends a sequence of values from 0 to 255 to the PCF8574 I2C I/O expander at address 0x20.

• write.py
  This code example is downloadable from the Acme playground repository. File: python/i2c/write.py

  
  

  import smbus
  import time
   
  bus = smbus.SMBus(0)
   
  for a in range(0,256):
   bus.write_byte(0x20,a)
   time.sleep(0.1)

C code example

The following example sends a sequence of values from 0 to 255 to the PCF8574 I2C I/O expander at address 0x20 in C language.

• write.c
  This code example is downloadable from the Acme playground repository. File: c/i2c/write.c

  
  

  #include 
  #include 
  #include 
  #include 
  
  #define I2C_ADDR 0x20
   
  int main (void) {
   int value;
   int fd;
  
   fd = open("/dev/i2c-0", O_RDWR);
  
   if (fd < 0) {
    printf("Error opening file: %s\n", strerror(errno));
    return 1;
   }
  
   if (ioctl(fd, I2C_SLAVE, I2C_ADDR) < 0) {
    printf("ioctl error: %s\n", strerror(errno));
    return 1;
   }
  
   for (value=0; value<=255; value++) {
    if (write(fd, &value, 1) != 1) {
     printf("Error writing file: %s\n", strerror(errno));
    }
    usleep(100000);
   }
   return 0;
  }