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 describes the I2C protocol. Or will, when it is finished :-)
6 APIs to perform them. <<
7 6
8 Key to symbols 7 Key to symbols
9 ============== 8 ==============
10 9
11 =============== ============================== 10 =============== =============================================================
12 S Start condition 11 S Start condition
13 P Stop condition 12 P Stop condition
14 Rd/Wr (1 bit) Read/Write bit. Rd equals 1, W 13 Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0.
15 A, NA (1 bit) Acknowledge (ACK) and Not Ackn 14 A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit
16 Addr (7 bits) I2C 7 bit address. Note that t !! 15 Addr (7 bits) I2C 7 bit address. Note that this can be expanded as usual to
17 get a 10 bit I2C address. 16 get a 10 bit I2C address.
18 Data (8 bits) A plain data byte. !! 17 Comm (8 bits) Command byte, a data byte which often selects a register on
>> 18 the device.
>> 19 Data (8 bits) A plain data byte. Sometimes, I write DataLow, DataHigh
>> 20 for 16 bit data.
>> 21 Count (8 bits) A data byte containing the length of a block operation.
19 22
20 [..] Data sent by I2C device, as op 23 [..] Data sent by I2C device, as opposed to data sent by the
21 host adapter. 24 host adapter.
22 =============== ============================== 25 =============== =============================================================
23 26
24 27
25 Simple send transaction 28 Simple send transaction
26 ======================= 29 =======================
27 30
28 Implemented by i2c_master_send():: 31 Implemented by i2c_master_send()::
29 32
30 S Addr Wr [A] Data [A] Data [A] ... [A] Data 33 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
31 34
32 35
33 Simple receive transaction 36 Simple receive transaction
34 ========================== 37 ==========================
35 38
36 Implemented by i2c_master_recv():: 39 Implemented by i2c_master_recv()::
37 40
38 S Addr Rd [A] [Data] A [Data] A ... A [Data] 41 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
39 42
40 43
41 Combined transactions 44 Combined transactions
42 ===================== 45 =====================
43 46
44 Implemented by i2c_transfer(). 47 Implemented by i2c_transfer().
45 48
46 They are just like the above transactions, but 49 They are just like the above transactions, but instead of a stop
47 condition P a start condition S is sent and th 50 condition P a start condition S is sent and the transaction continues.
48 An example of a byte read, followed by a byte 51 An example of a byte read, followed by a byte write::
49 52
50 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [ 53 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
51 54
52 55
53 Modified transactions 56 Modified transactions
54 ===================== 57 =====================
55 58
56 The following modifications to the I2C protoco 59 The following modifications to the I2C protocol can also be generated by
57 setting these flags for I2C messages. With the 60 setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they
58 are usually only needed to work around device 61 are usually only needed to work around device issues:
59 62
60 I2C_M_IGNORE_NAK: 63 I2C_M_IGNORE_NAK:
61 Normally message is interrupted immediatel 64 Normally message is interrupted immediately if there is [NA] from the
62 client. Setting this flag treats any [NA] 65 client. Setting this flag treats any [NA] as [A], and all of
63 message is sent. 66 message is sent.
64 These messages may still fail to SCL lo->h 67 These messages may still fail to SCL lo->hi timeout.
65 68
66 I2C_M_NO_RD_ACK: 69 I2C_M_NO_RD_ACK:
67 In a read message, master A/NA bit is skip 70 In a read message, master A/NA bit is skipped.
68 71
69 I2C_M_NOSTART: 72 I2C_M_NOSTART:
70 In a combined transaction, no 'S Addr Wr/R 73 In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
71 point. For example, setting I2C_M_NOSTART 74 point. For example, setting I2C_M_NOSTART on the second partial message
72 generates something like:: 75 generates something like::
73 76
74 S Addr Rd [A] [Data] NA Data [A] P 77 S Addr Rd [A] [Data] NA Data [A] P
75 78
76 If you set the I2C_M_NOSTART variable for 79 If you set the I2C_M_NOSTART variable for the first partial message,
77 we do not generate Addr, but we do generat 80 we do not generate Addr, but we do generate the start condition S.
78 This will probably confuse all other clien 81 This will probably confuse all other clients on your bus, so don't
79 try this. 82 try this.
80 83
81 This is often used to gather transmits fro 84 This is often used to gather transmits from multiple data buffers in
82 system memory into something that appears 85 system memory into something that appears as a single transfer to the
83 I2C device but may also be used between di 86 I2C device but may also be used between direction changes by some
84 rare devices. 87 rare devices.
85 88
86 I2C_M_REV_DIR_ADDR: 89 I2C_M_REV_DIR_ADDR:
87 This toggles the Rd/Wr flag. That is, if y 90 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 91 need to emit an Rd instead of a Wr, or vice versa, you set this
89 flag. For example:: 92 flag. For example::
90 93
91 S Addr Rd [A] Data [A] Data [A] ... [A] 94 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
92 95
93 I2C_M_STOP: 96 I2C_M_STOP:
94 Force a stop condition (P) after the messa 97 Force a stop condition (P) after the message. Some I2C related protocols
95 like SCCB require that. Normally, you real 98 like SCCB require that. Normally, you really don't want to get interrupted
96 between the messages of one transfer. 99 between the messages of one transfer.
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