Provided by: i2c-tools_4.4-2_amd64 
NAME
i2ctransfer - send user-defined I2C messages in one transfer
SYNOPSIS
i2ctransfer [-a] [-b] [-f] [-v] [-y] i2cbus desc [data] [desc [data]] ...
i2ctransfer -h
i2ctransfer -V
DESCRIPTION
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.
OPTIONS
-a Allow using addresses between 0x00 - 0x07 and 0x78 - 0x7f. Not recommended.
-b Print data from read messages as binary data. Disables -v (verbose). Useful for piping into a
pretty printing tool like hexdump or for redirecting into a file to be analyzed later.
-f 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.
-h Display the help and exit.
-v Enable verbose output. It will print infos about all messages sent, i.e. not only for read
messages but also for write messages.
-V Display the version and exit.
-y 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.
ARGUMENTS
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:
{r|w}<length_of_message>[@address]
{r|w} specifies if the message is read or write
<length_of_message>
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.
[@address]
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, ...)
p use value as seed for an 8 bit pseudo random sequence (i.e. 0p means 0x00, 0x50, 0xb0, ...)
EXAMPLES
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-
WARNING
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.
BUGS
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>.
AUTHORS
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>.
SEE ALSO
i2cdetect(8),i2cdump(8),i2cget(8),i2cset(8)
May 2022 i2ctransfer(8)