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f6fcefa1 LC |
1 | ================ |
2 | The I2C Protocol | |
3 | ================ | |
ccf988b6 | 4 | |
2f07c05f | 5 | This document describes the I2C protocol. Or will, when it is finished :-) |
1da177e4 LT |
6 | |
7 | Key to symbols | |
8 | ============== | |
9 | ||
ccf988b6 | 10 | =============== ============================================================= |
02622c88 LC |
11 | S Start condition |
12 | P Stop condition | |
13 | Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0. | |
db0d7424 | 14 | A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit |
02622c88 | 15 | Addr (7 bits) I2C 7 bit address. Note that this can be expanded as usual to |
1da177e4 | 16 | get a 10 bit I2C address. |
02622c88 | 17 | Comm (8 bits) Command byte, a data byte which often selects a register on |
1da177e4 | 18 | the device. |
02622c88 | 19 | Data (8 bits) A plain data byte. Sometimes, I write DataLow, DataHigh |
1da177e4 | 20 | for 16 bit data. |
02622c88 | 21 | Count (8 bits) A data byte containing the length of a block operation. |
1da177e4 | 22 | |
02622c88 | 23 | [..] Data sent by I2C device, as opposed to data sent by the |
ccf988b6 MCC |
24 | host adapter. |
25 | =============== ============================================================= | |
1da177e4 LT |
26 | |
27 | ||
28 | Simple send transaction | |
ccf988b6 | 29 | ======================= |
1da177e4 | 30 | |
f72beb8b | 31 | This corresponds to i2c_master_send():: |
1da177e4 LT |
32 | |
33 | S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P | |
34 | ||
35 | ||
36 | Simple receive transaction | |
ccf988b6 | 37 | ========================== |
1da177e4 | 38 | |
f72beb8b | 39 | This corresponds to i2c_master_recv():: |
1da177e4 LT |
40 | |
41 | S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P | |
42 | ||
43 | ||
44 | Combined transactions | |
ccf988b6 | 45 | ===================== |
1da177e4 | 46 | |
f72beb8b | 47 | This corresponds to i2c_transfer(). |
1da177e4 | 48 | |
f954731d LC |
49 | They are just like the above transactions, but instead of a stop |
50 | condition P a start condition S is sent and the transaction continues. | |
51 | An example of a byte read, followed by a byte write:: | |
1da177e4 LT |
52 | |
53 | S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P | |
54 | ||
55 | ||
56 | Modified transactions | |
57 | ===================== | |
58 | ||
9f02fba8 | 59 | The following modifications to the I2C protocol can also be generated by |
2f07c05f | 60 | setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they |
9f02fba8 | 61 | are usually only needed to work around device issues: |
1da177e4 | 62 | |
9f02fba8 WS |
63 | I2C_M_IGNORE_NAK: |
64 | Normally message is interrupted immediately if there is [NA] from the | |
65 | client. Setting this flag treats any [NA] as [A], and all of | |
66 | message is sent. | |
67 | These messages may still fail to SCL lo->hi timeout. | |
68 | ||
69 | I2C_M_NO_RD_ACK: | |
70 | In a read message, master A/NA bit is skipped. | |
71 | ||
72 | I2C_M_NOSTART: | |
1da177e4 LT |
73 | In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some |
74 | point. For example, setting I2C_M_NOSTART on the second partial message | |
ccf988b6 MCC |
75 | generates something like:: |
76 | ||
1da177e4 | 77 | S Addr Rd [A] [Data] NA Data [A] P |
ccf988b6 | 78 | |
1da177e4 | 79 | If you set the I2C_M_NOSTART variable for the first partial message, |
f954731d LC |
80 | we do not generate Addr, but we do generate the start condition S. |
81 | This will probably confuse all other clients on your bus, so don't | |
82 | try this. | |
1da177e4 | 83 | |
14674e70 MB |
84 | This is often used to gather transmits from multiple data buffers in |
85 | system memory into something that appears as a single transfer to the | |
86 | I2C device but may also be used between direction changes by some | |
87 | rare devices. | |
88 | ||
9f02fba8 | 89 | I2C_M_REV_DIR_ADDR: |
1da177e4 LT |
90 | This toggles the Rd/Wr flag. That is, if you want to do a write, but |
91 | need to emit an Rd instead of a Wr, or vice versa, you set this | |
ccf988b6 MCC |
92 | flag. For example:: |
93 | ||
1da177e4 LT |
94 | S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P |
95 | ||
9f02fba8 WS |
96 | I2C_M_STOP: |
97 | Force a stop condition (P) after the message. Some I2C related protocols | |
98 | like SCCB require that. Normally, you really don't want to get interrupted | |
99 | between the messages of one transfer. |