================
  The I2C Protocol
  ================

  This document describes the I2C protocol. Or will, when it is finished :-)

  
  Key to symbols
  ==============

  =============== =============================================================

  S               Start condition
  P               Stop condition
  Rd/Wr (1 bit)   Read/Write bit. Rd equals 1, Wr equals 0.

  A, NA (1 bit)   Acknowledge (ACK) and Not Acknowledge (NACK) 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

  =======================

  Implemented by i2c_master_send()::

  
    S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
  
  
  Simple receive transaction

  ==========================

  Implemented by i2c_master_recv()::

  
    S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
  
  
  Combined transactions

  =====================

  Implemented by i2c_transfer().

  They are just like the above transactions, but instead of a stop
  condition P a start condition 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
  =====================

  The following modifications to the I2C protocol can also be generated by

  setting these flags for I2C messages. With the exception of I2C_M_NOSTART, they

  are usually only needed to work around device issues:

  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.
  
  I2C_M_NO_RD_ACK:
      In a read message, master A/NA bit is skipped.
  
  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 start condition S.
      This will probably confuse all other clients on your bus, so don't
      try this.

      This is often used to gather transmits from multiple data buffers in
      system memory into something that appears as a single transfer to the
      I2C device but may also be used between direction changes by some
      rare devices.

  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

  I2C_M_STOP:
      Force a stop condition (P) after the message. Some I2C related protocols
      like SCCB require that. Normally, you really don't want to get interrupted
      between the messages of one transfer.