How to use the I2C ports

This article illustrates how to use the I2C ports on Acme Systems Road Runner SOM based on Microchip SAMA5D27 MPU

The I2C bus lines are located on the following pins:

Signal		Per A	Per B	Per C	Per D	Per E	Per F
TWD0	SDA		PD21		PB31	PC27	PC29
TWCK0	SCL		PD22		PC0	PC28	PC30
TWD1	SDA	PD4	PD19	PC6
TWCK1	SCL	PD5	PD20	PC7

Roadrunner pinout

Kernel configuration

Device Drivers  --->
  I2C support  ---> 
    <*>   I2C device interface
          I2C Hardware Bus support  ---> 
          <*> Atmel AT91 I2C Two-Wire interface (TWI)

Device tree

i2c0: i2c@f8028000 {
    dmas = <0>, <0>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_i2c0_default>;
    i2c-sda-hold-time-ns = <350>;
    status = "okay";
};

i2c1: i2c@fc028000 {
    dmas = <0>, <0>;
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_i2c1_default>;
    status = "okay";
};

pinctrl_i2c0_default: i2c0_default {
    pinmux = <PIN_PD21__TWD0>,
         <PIN_PD22__TWCK0>;
    bias-disable;
};

pinctrl_i2c1_default: i2c1_default {
    pinmux = <PIN_PD4__TWD1>,
         <PIN_PD5__TWCK1>;
    bias-disable;
};

Using I2C from user space

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:

apt-get update
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 of use:

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.

i2cset and i2cset

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

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

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.

i2cget i2cget is a small helper program to read registers visible through the I2C bus.

The follow simple command read a byte from an I2C device at address 20 hex on the i2c bus 0 (/dev/i2c-0).

i2cget -y 0 0x20 0x01