Documentation/i2c/smbus-protocol.rst - linux - Git at Google

 ==================
 The SMBus Protocol
 ==================

 The following is a summary of the SMBus protocol. It applies to
 all revisions of the protocol (1.0, 1.1, and 2.0).
 Certain protocol features which are not supported by
 this package are briefly described at the end of this document.

 Some adapters understand only the SMBus (System Management Bus) protocol,
 which is a subset from the I2C protocol. Fortunately, many devices use
 only the same subset, which makes it possible to put them on an SMBus.

 If you write a driver for some I2C device, please try to use the SMBus
 commands if at all possible (if the device uses only that subset of the
 I2C protocol). This makes it possible to use the device driver on both
 SMBus adapters and I2C adapters (the SMBus command set is automatically
 translated to I2C on I2C adapters, but plain I2C commands can not be
 handled at all on most pure SMBus adapters).

 Below is a list of SMBus protocol operations, and the functions executing
 them.  Note that the names used in the SMBus protocol specifications usually
 don't match these function names.  For some of the operations which pass a
 single data byte, the functions using SMBus protocol operation names execute
 a different protocol operation entirely.

 Each transaction type corresponds to a functionality flag. Before calling a
 transaction function, a device driver should always check (just once) for
 the corresponding functionality flag to ensure that the underlying I2C
 adapter supports the transaction in question. See
 Documentation/i2c/functionality.rst for the details.


 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.
 =============== =============================================================


 SMBus Quick Command
 ===================

 This sends a single bit to the device, at the place of the Rd/Wr bit::

   S Addr Rd/Wr [A] P

 Functionality flag: I2C_FUNC_SMBUS_QUICK


 SMBus Receive Byte
 ==================

 Implemented by i2c_smbus_read_byte()

 This reads a single byte from a device, without specifying a device
 register. Some devices are so simple that this interface is enough; for
 others, it is a shorthand if you want to read the same register as in
 the previous SMBus command::

   S Addr Rd [A] [Data] NA P

 Functionality flag: I2C_FUNC_SMBUS_READ_BYTE


 SMBus Send Byte
 ===============

 Implemented by i2c_smbus_write_byte()

 This operation is the reverse of Receive Byte: it sends a single byte
 to a device.  See Receive Byte for more information.

 ::

   S Addr Wr [A] Data [A] P

 Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE


 SMBus Read Byte
 ===============

 Implemented by i2c_smbus_read_byte_data()

 This reads a single byte from a device, from a designated register.
 The register is specified through the Comm byte::

   S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P

 Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA


 SMBus Read Word
 ===============

 Implemented by i2c_smbus_read_word_data()

 This operation is very like Read Byte; again, data is read from a
 device, from a designated register that is specified through the Comm
 byte. But this time, the data is a complete word (16 bits)::

   S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P

 Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA

 Note the convenience function i2c_smbus_read_word_swapped() is
 available for reads where the two data bytes are the other way
 around (not SMBus compliant, but very popular.)


 SMBus Write Byte
 ================

 Implemented by i2c_smbus_write_byte_data()

 This writes a single byte to a device, to a designated register. The
 register is specified through the Comm byte. This is the opposite of
 the Read Byte operation.

 ::

   S Addr Wr [A] Comm [A] Data [A] P

 Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA


 SMBus Write Word
 ================

 Implemented by i2c_smbus_write_word_data()

 This is the opposite of the Read Word operation. 16 bits
 of data are written to a device, to the designated register that is
 specified through the Comm byte::

   S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P

 Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA

 Note the convenience function i2c_smbus_write_word_swapped() is
 available for writes where the two data bytes are the other way
 around (not SMBus compliant, but very popular.)


 SMBus Process Call
 ==================

 This command selects a device register (through the Comm byte), sends
 16 bits of data to it, and reads 16 bits of data in return::

   S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
                                S Addr Rd [A] [DataLow] A [DataHigh] NA P

 Functionality flag: I2C_FUNC_SMBUS_PROC_CALL


 SMBus Block Read
 ================

 Implemented by i2c_smbus_read_block_data()

 This command reads a block of up to 32 bytes from a device, from a
 designated register that is specified through the Comm byte. The amount
 of data is specified by the device in the Count byte.

 ::

   S Addr Wr [A] Comm [A]
              S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P

 Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA


 SMBus Block Write
 =================

 Implemented by i2c_smbus_write_block_data()

 The opposite of the Block Read command, this writes up to 32 bytes to
 a device, to a designated register that is specified through the
 Comm byte. The amount of data is specified in the Count byte.

 ::

   S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P

 Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA


 SMBus Block Write - Block Read Process Call
 ===========================================

 SMBus Block Write - Block Read Process Call was introduced in
 Revision 2.0 of the specification.

 This command selects a device register (through the Comm byte), sends
 1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return::

   S Addr Wr [A] Comm [A] Count [A] Data [A] ...
                                S Addr Rd [A] [Count] A [Data] ... A P

 Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL


 SMBus Host Notify
 =================

 This command is sent from a SMBus device acting as a master to the
 SMBus host acting as a slave.
 It is the same form as Write Word, with the command code replaced by the
 alerting device's address.

 ::

   [S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]

 This is implemented in the following way in the Linux kernel:

 * I2C bus drivers which support SMBus Host Notify should report
   I2C_FUNC_SMBUS_HOST_NOTIFY.
 * I2C bus drivers trigger SMBus Host Notify by a call to
   i2c_handle_smbus_host_notify().
 * I2C drivers for devices which can trigger SMBus Host Notify will have
   client->irq assigned to a Host Notify IRQ if noone else specified an other.

 There is currently no way to retrieve the data parameter from the client.


 Packet Error Checking (PEC)
 ===========================

 Packet Error Checking was introduced in Revision 1.1 of the specification.

 PEC adds a CRC-8 error-checking byte to transfers using it, immediately
 before the terminating STOP.


 Address Resolution Protocol (ARP)
 =================================

 The Address Resolution Protocol was introduced in Revision 2.0 of
 the specification. It is a higher-layer protocol which uses the
 messages above.

 ARP adds device enumeration and dynamic address assignment to
 the protocol. All ARP communications use slave address 0x61 and
 require PEC checksums.


 SMBus Alert
 ===========

 SMBus Alert was introduced in Revision 1.0 of the specification.

 The SMBus alert protocol allows several SMBus slave devices to share a
 single interrupt pin on the SMBus master, while still allowing the master
 to know which slave triggered the interrupt.

 This is implemented the following way in the Linux kernel:

 * I2C bus drivers which support SMBus alert should call
   i2c_new_smbus_alert_device() to install SMBus alert support.
 * I2C drivers for devices which can trigger SMBus alerts should implement
   the optional alert() callback.


 I2C Block Transactions
 ======================

 The following I2C block transactions are similar to the SMBus Block Read
 and Write operations, except these do not have a Count byte. They are
 supported by the SMBus layer and are described here for completeness, but
 they are *NOT* defined by the SMBus specification.

 I2C block transactions do not limit the number of bytes transferred
 but the SMBus layer places a limit of 32 bytes.


 I2C Block Read
 ==============

 Implemented by i2c_smbus_read_i2c_block_data()

 This command reads a block of bytes from a device, from a
 designated register that is specified through the Comm byte::

   S Addr Wr [A] Comm [A]
              S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P

 Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK


 I2C Block Write
 ===============

 Implemented by i2c_smbus_write_i2c_block_data()

 The opposite of the Block Read command, this writes bytes to
 a device, to a designated register that is specified through the
 Comm byte. Note that command lengths of 0, 2, or more bytes are
 supported as they are indistinguishable from data.

 ::

   S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P

 Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK
	==================
	The SMBus Protocol
	==================

	The following is a summary of the SMBus protocol. It applies to
	all revisions of the protocol (1.0, 1.1, and 2.0).
	Certain protocol features which are not supported by
	this package are briefly described at the end of this document.

	Some adapters understand only the SMBus (System Management Bus) protocol,
	which is a subset from the I2C protocol. Fortunately, many devices use
	only the same subset, which makes it possible to put them on an SMBus.

	If you write a driver for some I2C device, please try to use the SMBus
	commands if at all possible (if the device uses only that subset of the
	I2C protocol). This makes it possible to use the device driver on both
	SMBus adapters and I2C adapters (the SMBus command set is automatically
	translated to I2C on I2C adapters, but plain I2C commands can not be
	handled at all on most pure SMBus adapters).

	Below is a list of SMBus protocol operations, and the functions executing
	them. Note that the names used in the SMBus protocol specifications usually
	don't match these function names. For some of the operations which pass a
	single data byte, the functions using SMBus protocol operation names execute
	a different protocol operation entirely.

	Each transaction type corresponds to a functionality flag. Before calling a
	transaction function, a device driver should always check (just once) for
	the corresponding functionality flag to ensure that the underlying I2C
	adapter supports the transaction in question. See
	Documentation/i2c/functionality.rst for the details.


	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.
	=============== =============================================================


	SMBus Quick Command
	===================

	This sends a single bit to the device, at the place of the Rd/Wr bit::

	S Addr Rd/Wr [A] P

	Functionality flag: I2C_FUNC_SMBUS_QUICK


	SMBus Receive Byte
	==================

	Implemented by i2c_smbus_read_byte()

	This reads a single byte from a device, without specifying a device
	register. Some devices are so simple that this interface is enough; for
	others, it is a shorthand if you want to read the same register as in
	the previous SMBus command::

	S Addr Rd [A] [Data] NA P

	Functionality flag: I2C_FUNC_SMBUS_READ_BYTE


	SMBus Send Byte
	===============

	Implemented by i2c_smbus_write_byte()

	This operation is the reverse of Receive Byte: it sends a single byte
	to a device. See Receive Byte for more information.

	::

	S Addr Wr [A] Data [A] P

	Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE


	SMBus Read Byte
	===============

	Implemented by i2c_smbus_read_byte_data()

	This reads a single byte from a device, from a designated register.
	The register is specified through the Comm byte::

	S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P

	Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA


	SMBus Read Word
	===============

	Implemented by i2c_smbus_read_word_data()

	This operation is very like Read Byte; again, data is read from a
	device, from a designated register that is specified through the Comm
	byte. But this time, the data is a complete word (16 bits)::

	S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P

	Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA

	Note the convenience function i2c_smbus_read_word_swapped() is
	available for reads where the two data bytes are the other way
	around (not SMBus compliant, but very popular.)


	SMBus Write Byte
	================

	Implemented by i2c_smbus_write_byte_data()

	This writes a single byte to a device, to a designated register. The
	register is specified through the Comm byte. This is the opposite of
	the Read Byte operation.

	::

	S Addr Wr [A] Comm [A] Data [A] P

	Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA


	SMBus Write Word
	================

	Implemented by i2c_smbus_write_word_data()

	This is the opposite of the Read Word operation. 16 bits
	of data are written to a device, to the designated register that is
	specified through the Comm byte::

	S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P

	Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA

	Note the convenience function i2c_smbus_write_word_swapped() is
	available for writes where the two data bytes are the other way
	around (not SMBus compliant, but very popular.)


	SMBus Process Call
	==================

	This command selects a device register (through the Comm byte), sends
	16 bits of data to it, and reads 16 bits of data in return::

	S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
	S Addr Rd [A] [DataLow] A [DataHigh] NA P

	Functionality flag: I2C_FUNC_SMBUS_PROC_CALL


	SMBus Block Read
	================

	Implemented by i2c_smbus_read_block_data()

	This command reads a block of up to 32 bytes from a device, from a
	designated register that is specified through the Comm byte. The amount
	of data is specified by the device in the Count byte.

	::

	S Addr Wr [A] Comm [A]
	S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P

	Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA


	SMBus Block Write
	=================

	Implemented by i2c_smbus_write_block_data()

	The opposite of the Block Read command, this writes up to 32 bytes to
	a device, to a designated register that is specified through the
	Comm byte. The amount of data is specified in the Count byte.

	::

	S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P

	Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA


	SMBus Block Write - Block Read Process Call
	===========================================

	SMBus Block Write - Block Read Process Call was introduced in
	Revision 2.0 of the specification.

	This command selects a device register (through the Comm byte), sends
	1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return::

	S Addr Wr [A] Comm [A] Count [A] Data [A] ...
	S Addr Rd [A] [Count] A [Data] ... A P

	Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL


	SMBus Host Notify
	=================

	This command is sent from a SMBus device acting as a master to the
	SMBus host acting as a slave.
	It is the same form as Write Word, with the command code replaced by the
	alerting device's address.

	::

	[S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]

	This is implemented in the following way in the Linux kernel:

	* I2C bus drivers which support SMBus Host Notify should report
	I2C_FUNC_SMBUS_HOST_NOTIFY.
	* I2C bus drivers trigger SMBus Host Notify by a call to
	i2c_handle_smbus_host_notify().
	* I2C drivers for devices which can trigger SMBus Host Notify will have
	client->irq assigned to a Host Notify IRQ if noone else specified an other.

	There is currently no way to retrieve the data parameter from the client.


	Packet Error Checking (PEC)
	===========================

	Packet Error Checking was introduced in Revision 1.1 of the specification.

	PEC adds a CRC-8 error-checking byte to transfers using it, immediately
	before the terminating STOP.


	Address Resolution Protocol (ARP)
	=================================

	The Address Resolution Protocol was introduced in Revision 2.0 of
	the specification. It is a higher-layer protocol which uses the
	messages above.

	ARP adds device enumeration and dynamic address assignment to
	the protocol. All ARP communications use slave address 0x61 and
	require PEC checksums.


	SMBus Alert
	===========

	SMBus Alert was introduced in Revision 1.0 of the specification.

	The SMBus alert protocol allows several SMBus slave devices to share a
	single interrupt pin on the SMBus master, while still allowing the master
	to know which slave triggered the interrupt.

	This is implemented the following way in the Linux kernel:

	* I2C bus drivers which support SMBus alert should call
	i2c_new_smbus_alert_device() to install SMBus alert support.
	* I2C drivers for devices which can trigger SMBus alerts should implement
	the optional alert() callback.


	I2C Block Transactions
	======================

	The following I2C block transactions are similar to the SMBus Block Read
	and Write operations, except these do not have a Count byte. They are
	supported by the SMBus layer and are described here for completeness, but
	they are NOT defined by the SMBus specification.

	I2C block transactions do not limit the number of bytes transferred
	but the SMBus layer places a limit of 32 bytes.


	I2C Block Read
	==============

	Implemented by i2c_smbus_read_i2c_block_data()

	This command reads a block of bytes from a device, from a
	designated register that is specified through the Comm byte::

	S Addr Wr [A] Comm [A]
	S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P

	Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK


	I2C Block Write
	===============

	Implemented by i2c_smbus_write_i2c_block_data()

	The opposite of the Block Read command, this writes bytes to
	a device, to a designated register that is specified through the
	Comm byte. Note that command lengths of 0, 2, or more bytes are
	supported as they are indistinguishable from data.

	::

	S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P

	Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK