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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
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
Marcin Garskidb955172007-10-19 23:22:11 +020071 client. Setting this flag treats any [NA] as [A], and all of
Linus Torvalds1da177e2005-04-16 15:20:36 -070072 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.