1================ 2The I2C Protocol 3================ 4 5This document describes the I2C protocol. Or will, when it is finished :-) 6 7Key to symbols 8============== 9 10=============== ============================================================= 11S Start condition 12P Stop condition 13Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0. 14A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit 15Addr (7 bits) I2C 7 bit address. Note that this can be expanded as usual to 16 get a 10 bit I2C address. 17Comm (8 bits) Command byte, a data byte which often selects a register on 18 the device. 19Data (8 bits) A plain data byte. Sometimes, I write DataLow, DataHigh 20 for 16 bit data. 21Count (8 bits) A data byte containing the length of a block operation. 22 23[..] Data sent by I2C device, as opposed to data sent by the 24 host adapter. 25=============== ============================================================= 26 27 28Simple send transaction 29======================= 30 31Implemented by i2c_master_send():: 32 33 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P 34 35 36Simple receive transaction 37========================== 38 39Implemented by i2c_master_recv():: 40 41 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P 42 43 44Combined transactions 45===================== 46 47Implemented by i2c_transfer(). 48 49They are just like the above transactions, but instead of a stop 50condition P a start condition S is sent and the transaction continues. 51An example of a byte read, followed by a byte write:: 52 53 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P 54 55 56Modified transactions 57===================== 58 59The following modifications to the I2C protocol can also be generated by 60setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they 61are usually only needed to work around device issues: 62 63I2C_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 69I2C_M_NO_RD_ACK: 70 In a read message, master A/NA bit is skipped. 71 72I2C_M_NOSTART: 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 75 generates something like:: 76 77 S Addr Rd [A] [Data] NA Data [A] P 78 79 If you set the I2C_M_NOSTART variable for the first partial message, 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. 83 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 89I2C_M_REV_DIR_ADDR: 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 92 flag. For example:: 93 94 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P 95 96I2C_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. 100