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diff --git a/Documentation/i2c/i2c-protocol b/Documentation/i2c/i2c-protocol new file mode 100644 index 000000000000..b4022c914210 --- /dev/null +++ b/Documentation/i2c/i2c-protocol @@ -0,0 +1,76 @@ +This document describes the i2c protocol. Or will, when it is finished :-) + +Key to symbols +============== + +S (1 bit) : Start bit +P (1 bit) : Stop bit +Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0. +A, NA (1 bit) : Accept and reverse accept bit. +Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to + get a 10 bit I2C address. +Comm (8 bits): Command byte, a data byte which often selects a register on + the device. +Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh + for 16 bit data. +Count (8 bits): A data byte containing the length of a block operation. + +[..]: Data sent by I2C device, as opposed to data sent by the host adapter. + + +Simple send transaction +====================== + +This corresponds to i2c_master_send. + + S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P + + +Simple receive transaction +=========================== + +This corresponds to i2c_master_recv + + S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P + + +Combined transactions +==================== + +This corresponds to i2c_transfer + +They are just like the above transactions, but instead of a stop bit P +a start bit S is sent and the transaction continues. An example of +a byte read, followed by a byte write: + + S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P + + +Modified transactions +===================== + +We have found some I2C devices that needs the following modifications: + + Flag I2C_M_NOSTART: + In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some + point. For example, setting I2C_M_NOSTART on the second partial message + generates something like: + S Addr Rd [A] [Data] NA Data [A] P + If you set the I2C_M_NOSTART variable for the first partial message, + we do not generate Addr, but we do generate the startbit S. This will + probably confuse all other clients on your bus, so don't try this. + + Flags I2C_M_REV_DIR_ADDR + This toggles the Rd/Wr flag. That is, if you want to do a write, but + need to emit an Rd instead of a Wr, or vice versa, you set this + flag. For example: + S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P + + Flags I2C_M_IGNORE_NAK + Normally message is interrupted immediately if there is [NA] from the + client. Setting this flag treats any [NA] as [A], and all of + message is sent. + These messages may still fail to SCL lo->hi timeout. + + Flags I2C_M_NO_RD_ACK + In a read message, master A/NA bit is skipped. |