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SMBus Protocol Summary

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

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 commands.

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.

SMBus Write Quick

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

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

There is no equivalent Read Quick command.

A Addr Rd/Wr [A] P

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

SMBus Write Byte

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

This is the reverse of Read Byte: it sends a single byte to a device.

See Read Byte for more information.

S Addr Wr [A] Data [A] P

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

SMBus Read Word Data

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

This command is very like Read Byte Data; 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

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 Data command.

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

SMBus Write Word Data

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

This is the opposite operation of the Read Word Data command. 16 bits

of data is read from a device, from a designated register that is

specified through the Comm byte.

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

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

SMBus Block Read

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

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

SMBus Block Write

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

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

SMBus Block Process Call

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

SMBus Block 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] ... NA P

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]

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 all transfers.

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.

I2C Block Transactions

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

The following I2C block transactions are supported by the

SMBus layer and are described here for completeness.

I2C block transactions do not limit the number of bytes transferred

but the SMBus layer places a limit of 32 bytes.

I2C Block Read

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

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

I2C Block Read (2 Comm bytes)

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

This command reads a block of bytes from a device, from a

designated register that is specified through the two Comm bytes.

S Addr Wr [A] Comm1 [A] Comm2 [A]

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

I2C Block Write

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

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