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Linus Torvalds1da177e2005-04-16 15:20:36 -07001SMBus Protocol Summary
2======================
David Brownell1a31a882008-05-11 20:37:05 +02003
Linus Torvalds1da177e2005-04-16 15:20:36 -07004The following is a summary of the SMBus protocol. It applies to
5all revisions of the protocol (1.0, 1.1, and 2.0).
6Certain protocol features which are not supported by
7this package are briefly described at the end of this document.
8
9Some adapters understand only the SMBus (System Management Bus) protocol,
10which is a subset from the I2C protocol. Fortunately, many devices use
11only the same subset, which makes it possible to put them on an SMBus.
David Brownell1a31a882008-05-11 20:37:05 +020012
Linus Torvalds1da177e2005-04-16 15:20:36 -070013If you write a driver for some I2C device, please try to use the SMBus
14commands if at all possible (if the device uses only that subset of the
15I2C protocol). This makes it possible to use the device driver on both
16SMBus adapters and I2C adapters (the SMBus command set is automatically
17translated to I2C on I2C adapters, but plain I2C commands can not be
18handled at all on most pure SMBus adapters).
19
David Brownell1a31a882008-05-11 20:37:05 +020020Below is a list of SMBus protocol operations, and the functions executing
21them. Note that the names used in the SMBus protocol specifications usually
22don't match these function names. For some of the operations which pass a
23single data byte, the functions using SMBus protocol operation names execute
24a different protocol operation entirely.
25
Linus Torvalds1da177e2005-04-16 15:20:36 -070026
27Key to symbols
28==============
29
30S (1 bit) : Start bit
31P (1 bit) : Stop bit
32Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
33A, NA (1 bit) : Accept and reverse accept bit.
34Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
35 get a 10 bit I2C address.
36Comm (8 bits): Command byte, a data byte which often selects a register on
37 the device.
38Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
39 for 16 bit data.
40Count (8 bits): A data byte containing the length of a block operation.
41
42[..]: Data sent by I2C device, as opposed to data sent by the host adapter.
43
44
Jean Delvare67c2e662008-07-14 22:38:23 +020045SMBus Quick Command
46===================
Linus Torvalds1da177e2005-04-16 15:20:36 -070047
48This sends a single bit to the device, at the place of the Rd/Wr bit.
Linus Torvalds1da177e2005-04-16 15:20:36 -070049
50A Addr Rd/Wr [A] P
51
52
David Brownell1a31a882008-05-11 20:37:05 +020053SMBus Receive Byte: i2c_smbus_read_byte()
54==========================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070055
56This reads a single byte from a device, without specifying a device
57register. Some devices are so simple that this interface is enough; for
58others, it is a shorthand if you want to read the same register as in
59the previous SMBus command.
60
61S Addr Rd [A] [Data] NA P
62
63
David Brownell1a31a882008-05-11 20:37:05 +020064SMBus Send Byte: i2c_smbus_write_byte()
65========================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070066
David Brownell1a31a882008-05-11 20:37:05 +020067This operation is the reverse of Receive Byte: it sends a single byte
68to a device. See Receive Byte for more information.
Linus Torvalds1da177e2005-04-16 15:20:36 -070069
70S Addr Wr [A] Data [A] P
71
72
David Brownell1a31a882008-05-11 20:37:05 +020073SMBus Read Byte: i2c_smbus_read_byte_data()
74============================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070075
76This reads a single byte from a device, from a designated register.
77The register is specified through the Comm byte.
78
79S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P
80
81
David Brownell1a31a882008-05-11 20:37:05 +020082SMBus Read Word: i2c_smbus_read_word_data()
83============================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070084
David Brownell1a31a882008-05-11 20:37:05 +020085This operation is very like Read Byte; again, data is read from a
Linus Torvalds1da177e2005-04-16 15:20:36 -070086device, from a designated register that is specified through the Comm
87byte. But this time, the data is a complete word (16 bits).
88
89S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P
90
91
David Brownell1a31a882008-05-11 20:37:05 +020092SMBus Write Byte: i2c_smbus_write_byte_data()
93==============================================
Linus Torvalds1da177e2005-04-16 15:20:36 -070094
95This writes a single byte to a device, to a designated register. The
96register is specified through the Comm byte. This is the opposite of
David Brownell1a31a882008-05-11 20:37:05 +020097the Read Byte operation.
Linus Torvalds1da177e2005-04-16 15:20:36 -070098
99S Addr Wr [A] Comm [A] Data [A] P
100
101
David Brownell1a31a882008-05-11 20:37:05 +0200102SMBus Write Word: i2c_smbus_write_word_data()
103==============================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700104
David Brownell1a31a882008-05-11 20:37:05 +0200105This is the opposite of the Read Word operation. 16 bits
Mike Frysinger3f9a4792007-02-13 22:08:59 +0100106of data is written to a device, to the designated register that is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700107specified through the Comm byte.
108
109S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P
110
111
112SMBus Process Call
113==================
114
115This command selects a device register (through the Comm byte), sends
11616 bits of data to it, and reads 16 bits of data in return.
117
118S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
119 S Addr Rd [A] [DataLow] A [DataHigh] NA P
120
121
David Brownell1a31a882008-05-11 20:37:05 +0200122SMBus Block Read: i2c_smbus_read_block_data()
123==============================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700124
125This command reads a block of up to 32 bytes from a device, from a
126designated register that is specified through the Comm byte. The amount
127of data is specified by the device in the Count byte.
128
129S Addr Wr [A] Comm [A]
130 S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P
131
132
David Brownell1a31a882008-05-11 20:37:05 +0200133SMBus Block Write: i2c_smbus_write_block_data()
134================================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700135
136The opposite of the Block Read command, this writes up to 32 bytes to
137a device, to a designated register that is specified through the
138Comm byte. The amount of data is specified in the Count byte.
139
140S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P
141
142
David Brownell1a31a882008-05-11 20:37:05 +0200143SMBus Block Write - Block Read Process Call
144===========================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700145
David Brownell1a31a882008-05-11 20:37:05 +0200146SMBus Block Write - Block Read Process Call was introduced in
147Revision 2.0 of the specification.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700148
149This command selects a device register (through the Comm byte), sends
1501 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return.
151
152S Addr Wr [A] Comm [A] Count [A] Data [A] ...
153 S Addr Rd [A] [Count] A [Data] ... A P
154
155
156SMBus Host Notify
157=================
158
159This command is sent from a SMBus device acting as a master to the
160SMBus host acting as a slave.
161It is the same form as Write Word, with the command code replaced by the
162alerting device's address.
163
164[S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]
165
166
167Packet Error Checking (PEC)
168===========================
David Brownell1a31a882008-05-11 20:37:05 +0200169
Linus Torvalds1da177e2005-04-16 15:20:36 -0700170Packet Error Checking was introduced in Revision 1.1 of the specification.
171
David Brownell1a31a882008-05-11 20:37:05 +0200172PEC adds a CRC-8 error-checking byte to transfers using it, immediately
173before the terminating STOP.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700174
175
176Address Resolution Protocol (ARP)
177=================================
David Brownell1a31a882008-05-11 20:37:05 +0200178
Linus Torvalds1da177e2005-04-16 15:20:36 -0700179The Address Resolution Protocol was introduced in Revision 2.0 of
180the specification. It is a higher-layer protocol which uses the
181messages above.
182
183ARP adds device enumeration and dynamic address assignment to
184the protocol. All ARP communications use slave address 0x61 and
185require PEC checksums.
186
187
188I2C Block Transactions
189======================
David Brownell1a31a882008-05-11 20:37:05 +0200190
Linus Torvalds1da177e2005-04-16 15:20:36 -0700191The following I2C block transactions are supported by the
192SMBus layer and are described here for completeness.
David Brownell1a31a882008-05-11 20:37:05 +0200193They are *NOT* defined by the SMBus specification.
194
Linus Torvalds1da177e2005-04-16 15:20:36 -0700195I2C block transactions do not limit the number of bytes transferred
196but the SMBus layer places a limit of 32 bytes.
197
198
David Brownell1a31a882008-05-11 20:37:05 +0200199I2C Block Read: i2c_smbus_read_i2c_block_data()
200================================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201
202This command reads a block of bytes from a device, from a
203designated register that is specified through the Comm byte.
204
205S Addr Wr [A] Comm [A]
206 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
207
208
209I2C Block Read (2 Comm bytes)
210=============================
211
212This command reads a block of bytes from a device, from a
213designated register that is specified through the two Comm bytes.
214
215S Addr Wr [A] Comm1 [A] Comm2 [A]
216 S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
217
218
David Brownell1a31a882008-05-11 20:37:05 +0200219I2C Block Write: i2c_smbus_write_i2c_block_data()
220==================================================
Linus Torvalds1da177e2005-04-16 15:20:36 -0700221
222The opposite of the Block Read command, this writes bytes to
223a device, to a designated register that is specified through the
224Comm byte. Note that command lengths of 0, 2, or more bytes are
225supported as they are indistinguishable from data.
226
227S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P