1 ================ 2 The I2C Protocol 3 ================ 4 5 This document is an overview of the basic I2C transactions and the kernel 6 APIs to perform them. 7 8 Key to symbols 9 ============== 10 11 =============== ============================================================= 12 S Start condition 13 P Stop condition 14 Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0. 15 A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit 16 Addr (7 bits) I2C 7 bit address. Note that this can be expanded to 17 get a 10 bit I2C address. 18 Data (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 25 Simple send transaction 26 ======================= 27 28 Implemented by i2c_master_send():: 29 30 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P 31 32 33 Simple receive transaction 34 ========================== 35 36 Implemented by i2c_master_recv():: 37 38 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P 39 40 41 Combined transactions 42 ===================== 43 44 Implemented by i2c_transfer(). 45 46 They are just like the above transactions, but instead of a stop 47 condition P a start condition S is sent and the transaction continues. 48 An 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 53 Modified transactions 54 ===================== 55 56 The following modifications to the I2C protocol can also be generated by 57 setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they 58 are usually only needed to work around device issues: 59 60 I2C_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 66 I2C_M_NO_RD_ACK: 67 In a read message, master A/NA bit is skipped. 68 69 I2C_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 86 I2C_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 93 I2C_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.
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