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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