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Manual Pages  — I2C


i2c – test I2C bus and slave devices



i2c -a address [-f device] [-d r|w] [-w 0|8|16] [-o offset] [-c count] [-m tr|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).
-b 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 tr|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.
tr complete-transfer
ss stop then start
rs repeated start
no none
Some I2C bus hardware does not provide control over the individual start, repeat-start, and stop operations. Such hardware can only perform a complete transfer of the offset and the data as a single operation. The tr mode creates control structures describing the transfer and submits them to the driver as a single complete transaction. This mode works on all types of I2C hardware.
-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). The default is zero. Use "-w 0" to disable writing of the offset to the slave.
-r reset the controller.
-s scan the bus for devices.
-v be verbose.
-w 0|8|16 device addressing width (in bits). This is used to determine how to pass offset specified with -o to the slave. Zero means that the offset is ignored and not passed to the slave at all.


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.



iic(4), iicbus(4)


The i2c utility appeared in FreeBSD 8.0 .


The i2c utility and this manual page were written by Bartlomiej Sieka <Mt tur@semihalf.com> and Michal Hajduk <Mt mih@semihalf.com>.

I2C (8) May 22, 2019

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