Root/Documentation/i2c/i2c-protocol

1This 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
52We have found some I2C devices that needs the following modifications:
53
54  Flag I2C_M_NOSTART:
55    In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
56    point. For example, setting I2C_M_NOSTART on the second partial message
57    generates something like:
58      S Addr Rd [A] [Data] NA Data [A] P
59    If you set the I2C_M_NOSTART variable for the first partial message,
60    we do not generate Addr, but we do generate the startbit S. This will
61    probably confuse all other clients on your bus, so don't try this.
62
63  Flags I2C_M_REV_DIR_ADDR
64    This toggles the Rd/Wr flag. That is, if you want to do a write, but
65    need to emit an Rd instead of a Wr, or vice versa, you set this
66    flag. For example:
67      S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
68
69  Flags I2C_M_IGNORE_NAK
70    Normally message is interrupted immediately if there is [NA] from the
71    client. Setting this flag treats any [NA] as [A], and all of
72    message is sent.
73    These messages may still fail to SCL lo->hi timeout.
74
75  Flags I2C_M_NO_RD_ACK
76    In a read message, master A/NA bit is skipped.
77

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