xref: /openbmc/linux/Documentation/i2c/i2c-protocol.rst (revision f6fcefa1)

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

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
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.