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1================
2The I2C Protocol
3================
4
5This document is an overview of the basic I2C transactions and the kernel
6APIs to perform them.
7
8Key to symbols
9==============
10
11=============== =============================================================
12S Start condition
13P Stop condition
14Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0.
15A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit
16Addr (7 bits) I2C 7 bit address. Note that this can be expanded to
17 get a 10 bit I2C address.
18Data (8 bits) A plain data byte.
19
20[..] Data sent by I2C device, as opposed to data sent by the
21 host adapter.
22=============== =============================================================
23
24
25Simple send transaction
26=======================
27
28Implemented by i2c_master_send()::
29
30 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
31
32
33Simple receive transaction
34==========================
35
36Implemented by i2c_master_recv()::
37
38 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
39
40
41Combined transactions
42=====================
43
44Implemented by i2c_transfer().
45
46They are just like the above transactions, but instead of a stop
47condition P a start condition S is sent and the transaction continues.
48An example of a byte read, followed by a byte write::
49
50 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
51
52
53Modified transactions
54=====================
55
56The following modifications to the I2C protocol can also be generated by
57setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they
58are usually only needed to work around device issues:
59
60I2C_M_IGNORE_NAK:
61 Normally message is interrupted immediately if there is [NA] from the
62 client. Setting this flag treats any [NA] as [A], and all of
63 message is sent.
64 These messages may still fail to SCL lo->hi timeout.
65
66I2C_M_NO_RD_ACK:
67 In a read message, master A/NA bit is skipped.
68
69I2C_M_NOSTART:
70 In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
71 point. For example, setting I2C_M_NOSTART on the second partial message
72 generates something like::
73
74 S Addr Rd [A] [Data] NA Data [A] P
75
76 If you set the I2C_M_NOSTART variable for the first partial message,
77 we do not generate Addr, but we do generate the start condition S.
78 This will probably confuse all other clients on your bus, so don't
79 try this.
80
81 This is often used to gather transmits from multiple data buffers in
82 system memory into something that appears as a single transfer to the
83 I2C device but may also be used between direction changes by some
84 rare devices.
85
86I2C_M_REV_DIR_ADDR:
87 This toggles the Rd/Wr flag. That is, if you want to do a write, but
88 need to emit an Rd instead of a Wr, or vice versa, you set this
89 flag. For example::
90
91 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
92
93I2C_M_STOP:
94 Force a stop condition (P) after the message. Some I2C related protocols
95 like SCCB require that. Normally, you really don't want to get interrupted
96 between the messages of one transfer.