[linux.git] / Documentation / i2c / i2c-protocol

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

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 startbit 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.
Commit	Line	Data
1da177e4 LT	1	This document describes the i2c protocol. Or will, when it is finished :-)
	2
	3	Key to symbols
	4	==============
	5
	6	S (1 bit) : Start bit
	7	P (1 bit) : Stop bit
	8	Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
9f02fba8 WS	9	A, NA (1 bit) : Accept and reverse accept bit.
9f02fba8 WS	10	Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
1da177e4 LT	11	get a 10 bit I2C address.
	12	Comm (8 bits): Command byte, a data byte which often selects a register on
	13	the device.
	14	Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
	15	for 16 bit data.
	16	Count (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
	21	Simple send transaction
	22	======================
	23
	24	This corresponds to i2c_master_send.
	25
	26	S Addr Wr [A] Data [A] Data [A] ... [A] Data [A] P
	27
	28
	29	Simple receive transaction
	30	===========================
	31
	32	This corresponds to i2c_master_recv
	33
	34	S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
	35
	36
	37	Combined transactions
	38	====================
	39
	40	This corresponds to i2c_transfer
	41
	42	They are just like the above transactions, but instead of a stop bit P
	43	a start bit S is sent and the transaction continues. An example of
	44	a byte read, followed by a byte write:
	45
	46	S Addr Rd [A] [Data] NA S Addr Wr [A] Data [A] P
	47
	48
	49	Modified transactions
	50	=====================
	51
9f02fba8 WS	52	The following modifications to the I2C protocol can also be generated by
	53	setting these flags for i2c messages. With the exception of I2C_M_NOSTART, they
	54	are usually only needed to work around device issues:
1da177e4	55
9f02fba8 WS	56	I2C_M_IGNORE_NAK:
	57	Normally message is interrupted immediately if there is [NA] from the
	58	client. Setting this flag treats any [NA] as [A], and all of
	59	message is sent.
	60	These messages may still fail to SCL lo->hi timeout.
	61
	62	I2C_M_NO_RD_ACK:
	63	In a read message, master A/NA bit is skipped.
	64
	65	I2C_M_NOSTART:
1da177e4 LT	66	In a combined transaction, no 'S Addr Wr/Rd [A]' is generated at some
	67	point. For example, setting I2C_M_NOSTART on the second partial message
	68	generates something like:
	69	S Addr Rd [A] [Data] NA Data [A] P
	70	If you set the I2C_M_NOSTART variable for the first partial message,
	71	we do not generate Addr, but we do generate the startbit S. This will
	72	probably confuse all other clients on your bus, so don't try this.
	73
14674e70 MB	74	This is often used to gather transmits from multiple data buffers in
	75	system memory into something that appears as a single transfer to the
	76	I2C device but may also be used between direction changes by some
	77	rare devices.
	78
9f02fba8	79	I2C_M_REV_DIR_ADDR:
1da177e4 LT	80	This toggles the Rd/Wr flag. That is, if you want to do a write, but
	81	need to emit an Rd instead of a Wr, or vice versa, you set this
	82	flag. For example:
	83	S Addr Rd [A] Data [A] Data [A] ... [A] Data [A] P
	84
9f02fba8 WS	85	I2C_M_STOP:
	86	Force a stop condition (P) after the message. Some I2C related protocols
	87	like SCCB require that. Normally, you really don't want to get interrupted
	88	between the messages of one transfer.