1i2ctransfer(8) System Manager's Manual i2ctransfer(8)
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6 i2ctransfer - send user-defined I2C messages in one transfer
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10 i2ctransfer [-f] [-y] [-v] i2cbus desc [data] [desc [data]] ...
11 i2ctransfer -V
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15 i2ctransfer is a program to create I2C messages and send them combined
16 as one transfer. For read messages, the contents of the received buf‐
17 fers are printed to stdout, one line per read message.
18 Please note the difference between a transfer and a message here. A
19 transfer may consist of multiple messages and is started with a START
20 condition and ends with a STOP condition as described in the I2C speci‐
21 fication. Messages within the transfer are concatenated using the
22 REPEATED START condition which is described there as well. There are
23 some advantages of having multiple messages in one transfer. First,
24 some devices keep their internal states for REPEATED START but reset
25 them after a STOP. Second, you cannot get interrupted during one
26 transfer, but it might happen between multiple transfers. Interruption
27 could happen on hardware level by another I2C master on the bus, or at
28 software level by another I2C user who got its transfer scheduled
29 between yours. This program helps you to create proper transfers for
30 your needs.
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34 -f Force access to the device even if it is already busy. By
35 default, i2ctransfer will refuse to access addresses marked as
36 reserved by the I2C standard or to a device which is already
37 under the control of a kernel driver. Using this flag is dan‐
38 gerous, it can seriously confuse the kernel driver in question.
39 It can also cause i2ctransfer to silently write to the wrong
40 register. So use at your own risk and only if you know what
41 you're doing.
42 -y Disable interactive mode. By default, i2ctransfer will wait for
44 a confirmation from the user before messing with the I2C bus.
45 When this flag is used, it will perform the operation directly.
46 This is mainly meant to be used in scripts.
47 -v Enable verbose output. It will print infos about all messages
49 sent, i.e. not only for read messages but also for write mes‐
50 sages.
51 -V Display the version and exit.
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56 The first parameter i2cbus indicates the number or name of the I2C bus
57 to be used. This number should correspond to one of the busses listed
58 by i2cdetect -l.
59 The next parameter is one or multiple desc blocks. The number of
62 blocks is limited by the Linux Kernel and defined by
63 I2C_RDWR_IOCTL_MAX_MSGS (42 as of v4.10). desc blocks are composed
64 like this:
65 {r|w}<length_of_message>[@address]
67 {r|w} specifies if the message is read or write
70 <length_of_message>
72 specifies the number of bytes read or written in this message.
73 It is parsed as an unsigned 16 bit integer, but note that the
74 Linux Kernel applies an additional upper limit (8192 as of
75 v4.10).
76 [@address]
78 specifies the 7-bit address of the chip to be accessed for this
79 message, and is an integer. If omitted, reuse the previous
80 address. Normally, addresses outside the range of 0x03-0x77 and
81 addresses with a kernel driver attached to them will be blocked.
82 With -f (force), all addresses can be used. Be very careful
83 when using that! 10-bit addresses are currently not supported
84 at all.
85 If the I2C message is a write, then a data block with the data to be
88 written follows. It consists of <length_of_message> bytes which can be
89 marked with the usual prefixes for hexadecimal, octal, etc. To make it
90 easier to create larger data blocks easily, the data byte can have a
91 suffix.
92 = keep value constant until end of message (i.e. 0= means 0, 0, 0,
95 ...)
96 + increase value by 1 until end of message (i.e. 0+ means 0, 1, 2,
98 ...)
99 - decrease value by 1 until end of message (i.e. 0xff- means 0xff,
101 0xfe, 0xfd, ...)
102 p use value as seed for an 8 bit pseudo random sequence (i.e. 0p
104 means 0x00, 0x50, 0xb0, ...)
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108 On bus 0, from an EEPROM at address 0x50, read 8 byte from offset 0x64
109 (first message writes one byte to set the memory pointer to 0x64, sec‐
110 ond message reads from the same chip):
111 # i2ctransfer 0 w1@0x50 0x64 r8
112 For the same EEPROM, at offset 0x42 write 0xff 0xfe ... 0xf0 (one write
114 message; first byte sets the memory pointer to 0x42, 0xff is the first
115 data byte, all following data bytes are decreased by one):
116 # i2ctransfer 0 w17@0x50 0x42 0xff-
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120 i2ctransfer can be extremely dangerous if used improperly. It can con‐
121 fuse your I2C bus, cause data loss, or have more serious side effects.
122 Writing to a serial EEPROM on a memory DIMM (chip addresses between
123 0x50 and 0x57) may DESTROY your memory, leaving your system unbootable!
124 Be extremely careful using this program.
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128 Wolfram Sang, based on i2cget by Jean Delvare
129 This manual page was originally written by Wolfram Sang based on the
131 manual for i2cset by David Z Maze <dmaze@debian.org>.
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135 i2cdetect(8),i2cdump(8),i2cget(8),i2cset(8)
136 February 2017 i2ctransfer(8)