linux/Documentation/i2c/i2c-protocol
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   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
  52The following modifications to the I2C protocol can also be generated,
  53with the exception of I2C_M_NOSTART these are usually only needed to
  54work around device issues:
  55
  56  Flag I2C_M_NOSTART: 
  57    In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
  58    point. For example, setting I2C_M_NOSTART on the second partial message
  59    generates something like:
  60      S Addr Rd [A] [Data] NA Data [A] P
  61    If you set the I2C_M_NOSTART variable for the first partial message,
  62    we do not generate Addr, but we do generate the startbit S. This will
  63    probably confuse all other clients on your bus, so don't try this.
  64
  65    This is often used to gather transmits from multiple data buffers in
  66    system memory into something that appears as a single transfer to the
  67    I2C device but may also be used between direction changes by some
  68    rare devices.
  69
  70  Flags I2C_M_REV_DIR_ADDR
  71    This toggles the Rd/Wr flag. That is, if you want to do a write, but
  72    need to emit an Rd instead of a Wr, or vice versa, you set this
  73    flag. For example:
  74      S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
  75
  76  Flags I2C_M_IGNORE_NAK
  77    Normally message is interrupted immediately if there is [NA] from the
  78    client. Setting this flag treats any [NA] as [A], and all of
  79    message is sent.
  80    These messages may still fail to SCL lo->hi timeout.
  81
  82  Flags I2C_M_NO_RD_ACK
  83    In a read message, master A/NA bit is skipped.
  84
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