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