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Linus Torvalds1da177e2005-04-16 15:20:36 -07001This document describes the i2c protocol. Or will, when it is finished :-)
2
3Key to symbols
4==============
5
6S (1 bit) : Start bit
7P (1 bit) : Stop bit
8Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
9A, NA (1 bit) : Accept and reverse accept bit.
10Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
11 get a 10 bit I2C address.
12Comm (8 bits): Command byte, a data byte which often selects a register on
13 the device.
14Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
15 for 16 bit data.
16Count (8 bits): A data byte containing the length of a block operation.
17
18[..]: Data sent by I2C device, as opposed to data sent by the host adapter.
19
20
21Simple send transaction
22======================
23
24This corresponds to i2c_master_send.
25
26 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
27
28
29Simple receive transaction
30===========================
31
32This corresponds to i2c_master_recv
33
34 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
35
36
37Combined transactions
38====================
39
40This corresponds to i2c_transfer
41
42They are just like the above transactions, but instead of a stop bit P
43a start bit S is sent and the transaction continues. An example of
44a byte read, followed by a byte write:
45
46 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
47
48
49Modified transactions
50=====================
51
Mark Brown14674e72012-05-30 10:55:34 +020052The following modifications to the I2C protocol can also be generated,
53with the exception of I2C_M_NOSTART these are usually only needed to
54work around device issues:
Linus Torvalds1da177e2005-04-16 15:20:36 -070055
56 Flag I2C_M_NOSTART:
57 In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
58 point. For example, setting I2C_M_NOSTART on the second partial message
59 generates something like:
60 S Addr Rd [A] [Data] NA Data [A] P
61 If you set the I2C_M_NOSTART variable for the first partial message,
62 we do not generate Addr, but we do generate the startbit S. This will
63 probably confuse all other clients on your bus, so don't try this.
64
Mark Brown14674e72012-05-30 10:55:34 +020065 This is often used to gather transmits from multiple data buffers in
66 system memory into something that appears as a single transfer to the
67 I2C device but may also be used between direction changes by some
68 rare devices.
69
Linus Torvalds1da177e2005-04-16 15:20:36 -070070 Flags I2C_M_REV_DIR_ADDR
71 This toggles the Rd/Wr flag. That is, if you want to do a write, but
72 need to emit an Rd instead of a Wr, or vice versa, you set this
73 flag. For example:
74 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
75
76 Flags I2C_M_IGNORE_NAK
77 Normally message is interrupted immediately if there is [NA] from the
Marcin Garskidb9551702007-10-19 23:22:11 +020078 client. Setting this flag treats any [NA] as [A], and all of
Linus Torvalds1da177e2005-04-16 15:20:36 -070079 message is sent.
80 These messages may still fail to SCL lo->hi timeout.
81
82 Flags I2C_M_NO_RD_ACK
83 In a read message, master A/NA bit is skipped.