Section: Maintenance Commands (8)
Updated: February 2017
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i2ctransfer - send user-defined I2C messages in one transfer



i2ctransfer [-f] [-y] [-v] [-a] i2cbus desc [data] [desc [data]] ...
i2ctransfer -V



i2ctransfer is a program to create I2C messages and send them combined as one transfer. For read messages, the contents of the received buffers are printed to stdout, one line per read message.
Please note the difference between a transfer and a message here. A transfer may consist of multiple messages and is started with a START condition and ends with a STOP condition as described in the I2C specification. Messages within the transfer are concatenated using the REPEATED START condition which is described there as well. There are some advantages of having multiple messages in one transfer. First, some devices keep their internal states for REPEATED START but reset them after a STOP. Second, you cannot get interrupted during one transfer, but it might happen between multiple transfers. Interruption could happen on hardware level by another I2C master on the bus, or at software level by another I2C user who got its transfer scheduled between yours. This program helps you to create proper transfers for your needs.



Force access to the device even if it is already busy. By default, i2ctransfer will refuse to access a device which is already under the control of a kernel driver. Using this flag is dangerous, it can seriously confuse the kernel driver in question. It can also cause i2ctransfer to silently write to the wrong register. So use at your own risk and only if you know what you're doing.
Disable interactive mode. By default, i2ctransfer will wait for a confirmation from the user before messing with the I2C bus. When this flag is used, it will perform the operation directly. This is mainly meant to be used in scripts.
Enable verbose output. It will print infos about all messages sent, i.e. not only for read messages but also for write messages.
Display the version and exit.
Allow using addresses between 0x00 - 0x07 and 0x78 - 0x7f. Not recommended.



The first parameter i2cbus indicates the number or name of the I2C bus to be used. This number should correspond to one of the busses listed by i2cdetect -l.

The next parameter is one or multiple desc blocks. The number of blocks is limited by the Linux Kernel and defined by I2C_RDWR_IOCTL_MAX_MSGS (42 as of v4.10). desc blocks are composed like this:


specifies if the message is read or write
specifies the number of bytes read or written in this message. It is parsed as an unsigned 16 bit integer, but note that the Linux Kernel applies an additional upper limit (8192 as of v4.10). For read messages to targets which support SMBus Block transactions, it can also be '?', then the target will determine the length.
specifies the 7-bit address of the chip to be accessed for this message, and is an integer. If omitted, reuse the previous address. Normally, addresses outside the range of 0x08-0x77 and addresses with a kernel driver attached to them will be blocked. This can be overridden with -a (all) or -f (force). Be very careful when using these! 10-bit addresses are currently not supported at all.

If the I2C message is a write, then a data block with the data to be written follows. It consists of <length_of_message> bytes which can be marked with the usual prefixes for hexadecimal, octal, etc. To make it easier to create larger data blocks easily, the data byte can have a suffix.

keep value constant until end of message (i.e. 0= means 0, 0, 0, ...)
increase value by 1 until end of message (i.e. 0+ means 0, 1, 2, ...)
decrease value by 1 until end of message (i.e. 0xff- means 0xff, 0xfe, 0xfd, ...)
use value as seed for an 8 bit pseudo random sequence (i.e. 0p means 0x00, 0x50, 0xb0, ...)



On bus 0, from an EEPROM at address 0x50, read 8 byte from offset 0x64 (first message writes one byte to set the memory pointer to 0x64, second message reads from the same chip):

# i2ctransfer 0 w1@0x50 0x64 r8

For the same EEPROM, at offset 0x42 write 0xff 0xfe ... 0xf0 (one write message; first byte sets the memory pointer to 0x42, 0xff is the first data byte, all following data bytes are decreased by one):

# i2ctransfer 0 w17@0x50 0x42 0xff-



i2ctransfer can be extremely dangerous if used improperly. It can confuse your I2C bus, cause data loss, or have more serious side effects. Writing to a serial EEPROM on a memory DIMM (chip addresses between 0x50 and 0x57) may DESTROY your memory, leaving your system unbootable! Be extremely careful using this program.



To report bugs or send fixes, please write to the Linux I2C mailing list <linux-i2c@vger.kernel.org> with Cc to the current maintainer: Jean Delvare <jdelvare@suse.de>.



Wolfram Sang, based on i2cget by Jean Delvare

This manual page was originally written by Wolfram Sang based on the manual for i2cset by David Z Maze <dmaze@debian.org>.