i2ctransfer(8) System Manager's Manual i2ctransfer(8)
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 speci-
fication. Messages within the transfer are concatenated using the RE-
PEATED START condition which is described there as well. There are some
advantages of having multiple messages in one transfer. First, some de-
vices 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 (ver-
bose). 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 de-
fault, i2ctransfer will refuse to access a device which is al-
ready under the control of a kernel driver. Using this flag is
dangerous, it can seriously confuse the kernel driver in ques-
tion. 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 mes-
sages.
-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 ad-
dress. 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 con-
fuse 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 Del-
vare <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)
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