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1================2The I2C Protocol3================4 5This document is an overview of the basic I2C transactions and the kernel6APIs to perform them.7 8Key to symbols9==============10 11=============== =============================================================12S Start condition13P Stop condition14Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0.15A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) bit16Addr (7 bits) I2C 7 bit address. Note that this can be expanded to17 get a 10 bit I2C address.18Data (8 bits) A plain data byte.19 20[..] Data sent by I2C device, as opposed to data sent by the21 host adapter.22=============== =============================================================23 24 25Simple send transaction26=======================27 28Implemented by i2c_master_send()::29 30 S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P31 32 33Simple receive transaction34==========================35 36Implemented by i2c_master_recv()::37 38 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P39 40 41Combined transactions42=====================43 44Implemented by i2c_transfer().45 46They are just like the above transactions, but instead of a stop47condition P a start condition S is sent and the transaction continues.48An example of a byte read, followed by a byte write::49 50 S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P51 52 53Modified transactions54=====================55 56The following modifications to the I2C protocol can also be generated by57setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they58are usually only needed to work around device issues:59 60I2C_M_IGNORE_NAK:61 Normally message is interrupted immediately if there is [NA] from the62 client. Setting this flag treats any [NA] as [A], and all of63 message is sent.64 These messages may still fail to SCL lo->hi timeout.65 66I2C_M_NO_RD_ACK:67 In a read message, master A/NA bit is skipped.68 69I2C_M_NOSTART:70 In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some71 point. For example, setting I2C_M_NOSTART on the second partial message72 generates something like::73 74 S Addr Rd [A] [Data] NA Data [A] P75 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't79 try this.80 81 This is often used to gather transmits from multiple data buffers in82 system memory into something that appears as a single transfer to the83 I2C device but may also be used between direction changes by some84 rare devices.85 86I2C_M_REV_DIR_ADDR:87 This toggles the Rd/Wr flag. That is, if you want to do a write, but88 need to emit an Rd instead of a Wr, or vice versa, you set this89 flag. For example::90 91 S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P92 93I2C_M_STOP:94 Force a stop condition (P) after the message. Some I2C related protocols95 like SCCB require that. Normally, you really don't want to get interrupted96 between the messages of one transfer.97