1This document describes the i2c protocol. Or will, when it is finished :-) 2 3Key to symbols 4============== 5 6S (1 bit) : Start bit 7P (1 bit) : Stop bit 8Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0. 9A, NA (1 bit) : Accept and reverse accept bit. 10Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to 11 get a 10 bit I2C address. 12Comm (8 bits): Command byte, a data byte which often selects a register on 13 the device. 14Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh 15 for 16 bit data. 16Count (8 bits): A data byte containing the length of a block operation. 17 18[..]: Data sent by I2C device, as opposed to data sent by the host adapter. 19 20 21Simple send transaction 22====================== 23 24This corresponds to i2c_master_send. 25 26 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P 27 28 29Simple receive transaction 30=========================== 31 32This corresponds to i2c_master_recv 33 34 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P 35 36 37Combined transactions 38==================== 39 40This corresponds to i2c_transfer 41 42They are just like the above transactions, but instead of a stop bit P 43a start bit S is sent and the transaction continues. An example of 44a byte read, followed by a byte write: 45 46 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P 47 48 49Modified transactions 50===================== 51 52We have found some I2C devices that needs the following modifications: 53 54 Flag I2C_M_NOSTART: 55 In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some 56 point. For example, setting I2C_M_NOSTART on the second partial message 57 generates something like: 58 S Addr Rd [A] [Data] NA Data [A] P 59 If you set the I2C_M_NOSTART variable for the first partial message, 60 we do not generate Addr, but we do generate the startbit S. This will 61 probably confuse all other clients on your bus, so don't try this. 62 63 Flags I2C_M_REV_DIR_ADDR 64 This toggles the Rd/Wr flag. That is, if you want to do a write, but 65 need to emit an Rd instead of a Wr, or vice versa, you set this 66 flag. For example: 67 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P 68 69 Flags I2C_M_IGNORE_NAK 70 Normally message is interrupted immediately if there is [NA] from the 71 client. Setting this flag treats any [NA] as [A], and all of 72 message is sent. 73 These messages may still fail to SCL lo->hi timeout. 74 75 Flags I2C_M_NO_RD_ACK 76 In a read message, master A/NA bit is skipped. 77