I2C(8) FreeBSD System Manager's Manual I2C(8)


i2ctest I2C bus and slave devices


i2c -a address [ -f device][ -d r|w][ -w 0|8|16][ -o offset][ -c count][ -m ss|rs|no][ -b][ -v]

i2c -s [ -f device][ -n skip_addr][ -v]

i2c -r [ -f device][ -v]


The i2c utility can be used to perform raw data transfers (read or write) with devices on the I2C bus. It can also scan the bus for available devices and reset the I2C controller.

The options are as follows:

-a address
7-bit address on the I2C device to operate on (hex).
binary mode - when performing a read operation, the data read from the device is output in binary format on stdout; when doing a write, the binary data to be written to the device is read from stdin.
-c count
number of bytes to transfer (dec).
-d r|w
transfer direction: r - read, w - write.
-f device
I2C bus to use (default is /dev/iic0).
-m ss|rs|no
addressing mode, i.e., I2C bus operations performed after the offset for the transfer has been written to the device and before the actual read/write operation. rs - repeated start; ss - stop start; no - none.
-n skip_addr
skip address - address(es) to be skipped during bus scan. There are two ways to specify addresses to ignore: by range 'a..b' or using selected addresses 'a:b:c'. This option is available only when "-s" is used.
-o offset
offset within the device for data transfer (hex).
reset the controller.
scan the bus for devices.
be verbose.
-w 0|8|16
device addressing width (in bits).


Great care must be taken when manipulating slave I2C devices with the i2c utility. Often times important configuration data for the system is kept in non-volatile but write enabled memories located on the I2C bus, for example Ethernet hardware addresses, RAM module parameters (SPD), processor reset configuration word etc.

It is very easy to render the whole system unusable when such configuration data is deleted or altered, so use the “-d w” (write) command only if you know exactly what you are doing.

Also avoid ungraceful interrupting of an ongoing transaction on the I2C bus, as it can lead to potentially dangerous effects. Consider the following scenario: when the host CPU is reset (for whatever reason) in the middle of a started I2C transaction, the I2C slave device could be left in write mode waiting for data or offset to arrive. When the CPU reinitializes itself and talks to this I2C slave device again, the commands and other control info it sends are treated by the slave device as data or offset it was waiting for, and there's great potential for corruption if such a write is performed.


  • Scan the default bus (/dev/iic0) for devices:

    i2c -s

  • Scan the default bus (/dev/iic0) for devices and skip addresses 0x56 and 0x45.

    i2c -s -n 0x56:0x45

  • Scan the default bus (/dev/iic0) for devices and skip address range 0x34 to 0x56.

    i2c -s -n 0x34..0x56

  • Read 8 bytes of data from device at address 0x56 (e.g., an EEPROM):

    i2c -a 0x56 -d r -c 8

  • Write 16 bytes of data from file data.bin to device 0x56 at offset 0x10:

    i2c -a 0x56 -d w -c 16 -o 0x10 -b < data.bin

  • Copy 4 bytes between two EEPROMs (0x56 on /dev/iic1 to 0x57 on /dev/iic0):

    i2c -a 0x56 -f /dev/iic1 -d r -c 0x4 -b | i2c -a 0x57 -f /dev/iic0 -d w -c 4 -b

  • Reset the controller:

    i2c -f /dev/iic1 -r


iic(4), iicbus(4)


The i2c utility appeared in FreeBSD 8.0.


The i2c utility and this manual page were written by Bartlomiej Sieka <tur@semihalf.com> and Michal Hajduk <mih@semihalf.com>.
January 23, 2009 FreeBSD