1SMBus Protocol Summary
2======================
3
4The 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.
12
13If 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
20Below 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
26Each transaction type corresponds to a functionality flag. Before calling a
27transaction function, a device driver should always check (just once) for
28the corresponding functionality flag to ensure that the underlying I2C
29adapter supports the transaction in question. See
30<file:Documentation/i2c/functionality> for the details.
31
32
33Key to symbols
34==============
35
36S     (1 bit) : Start bit
37P     (1 bit) : Stop bit
38Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
39A, NA (1 bit) : Accept and reverse accept bit.
40Addr  (7 bits): I2C 7 bit address. Note that this can be expanded as usual to
41                get a 10 bit I2C address.
42Comm  (8 bits): Command byte, a data byte which often selects a register on
43                the device.
44Data  (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
45                for 16 bit data.
46Count (8 bits): A data byte containing the length of a block operation.
47
48[..]: Data sent by I2C device, as opposed to data sent by the host adapter.
49
50
51SMBus Quick Command
52===================
53
54This sends a single bit to the device, at the place of the Rd/Wr bit.
55
56A Addr Rd/Wr [A] P
57
58Functionality flag: I2C_FUNC_SMBUS_QUICK
59
60
61SMBus Receive Byte:  i2c_smbus_read_byte()
62==========================================
63
64This reads a single byte from a device, without specifying a device
65register. Some devices are so simple that this interface is enough; for
66others, it is a shorthand if you want to read the same register as in
67the previous SMBus command.
68
69S Addr Rd [A] [Data] NA P
70
71Functionality flag: I2C_FUNC_SMBUS_READ_BYTE
72
73
74SMBus Send Byte:  i2c_smbus_write_byte()
75========================================
76
77This operation is the reverse of Receive Byte: it sends a single byte
78to a device.  See Receive Byte for more information.
79
80S Addr Wr [A] Data [A] P
81
82Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE
83
84
85SMBus Read Byte:  i2c_smbus_read_byte_data()
86============================================
87
88This reads a single byte from a device, from a designated register.
89The register is specified through the Comm byte.
90
91S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P
92
93Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA
94
95
96SMBus Read Word:  i2c_smbus_read_word_data()
97============================================
98
99This operation is very like Read Byte; again, data is read from a
100device, from a designated register that is specified through the Comm
101byte. But this time, the data is a complete word (16 bits).
102
103S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P
104
105Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA
106
107Note the convenience function i2c_smbus_read_word_swapped is
108available for reads where the two data bytes are the other way
109around (not SMBus compliant, but very popular.)
110
111
112SMBus Write Byte:  i2c_smbus_write_byte_data()
113==============================================
114
115This writes a single byte to a device, to a designated register. The
116register is specified through the Comm byte. This is the opposite of
117the Read Byte operation.
118
119S Addr Wr [A] Comm [A] Data [A] P
120
121Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA
122
123
124SMBus Write Word:  i2c_smbus_write_word_data()
125==============================================
126
127This is the opposite of the Read Word operation. 16 bits
128of data is written to a device, to the designated register that is
129specified through the Comm byte.
130
131S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P
132
133Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA
134
135Note the convenience function i2c_smbus_write_word_swapped is
136available for writes where the two data bytes are the other way
137around (not SMBus compliant, but very popular.)
138
139
140SMBus Process Call:
141===================
142
143This command selects a device register (through the Comm byte), sends
14416 bits of data to it, and reads 16 bits of data in return.
145
146S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A]
147                             S Addr Rd [A] [DataLow] A [DataHigh] NA P
148
149Functionality flag: I2C_FUNC_SMBUS_PROC_CALL
150
151
152SMBus Block Read:  i2c_smbus_read_block_data()
153==============================================
154
155This command reads a block of up to 32 bytes from a device, from a
156designated register that is specified through the Comm byte. The amount
157of data is specified by the device in the Count byte.
158
159S Addr Wr [A] Comm [A]
160           S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P
161
162Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA
163
164
165SMBus Block Write:  i2c_smbus_write_block_data()
166================================================
167
168The opposite of the Block Read command, this writes up to 32 bytes to
169a device, to a designated register that is specified through the
170Comm byte. The amount of data is specified in the Count byte.
171
172S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P
173
174Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA
175
176
177SMBus Block Write - Block Read Process Call
178===========================================
179
180SMBus Block Write - Block Read Process Call was introduced in
181Revision 2.0 of the specification.
182
183This command selects a device register (through the Comm byte), sends
1841 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return.
185
186S Addr Wr [A] Comm [A] Count [A] Data [A] ...
187                             S Addr Rd [A] [Count] A [Data] ... A P
188
189Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL
190
191
192SMBus Host Notify
193=================
194
195This command is sent from a SMBus device acting as a master to the
196SMBus host acting as a slave.
197It is the same form as Write Word, with the command code replaced by the
198alerting device's address.
199
200[S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P]
201
202This is implemented in the following way in the Linux kernel:
203* I2C bus drivers which support SMBus Host Notify should report
204  I2C_FUNC_SMBUS_HOST_NOTIFY.
205* I2C bus drivers trigger SMBus Host Notify by a call to
206  i2c_handle_smbus_host_notify().
207* I2C drivers for devices which can trigger SMBus Host Notify will have
208  client->irq assigned to a Host Notify IRQ if noone else specified an other.
209
210There is currently no way to retrieve the data parameter from the client.
211
212
213Packet Error Checking (PEC)
214===========================
215
216Packet Error Checking was introduced in Revision 1.1 of the specification.
217
218PEC adds a CRC-8 error-checking byte to transfers using it, immediately
219before the terminating STOP.
220
221
222Address Resolution Protocol (ARP)
223=================================
224
225The Address Resolution Protocol was introduced in Revision 2.0 of
226the specification. It is a higher-layer protocol which uses the
227messages above.
228
229ARP adds device enumeration and dynamic address assignment to
230the protocol. All ARP communications use slave address 0x61 and
231require PEC checksums.
232
233
234SMBus Alert
235===========
236
237SMBus Alert was introduced in Revision 1.0 of the specification.
238
239The SMBus alert protocol allows several SMBus slave devices to share a
240single interrupt pin on the SMBus master, while still allowing the master
241to know which slave triggered the interrupt.
242
243This is implemented the following way in the Linux kernel:
244* I2C bus drivers which support SMBus alert should call
245  i2c_setup_smbus_alert() to setup SMBus alert support.
246* I2C drivers for devices which can trigger SMBus alerts should implement
247  the optional alert() callback.
248
249
250I2C Block Transactions
251======================
252
253The following I2C block transactions are supported by the
254SMBus layer and are described here for completeness.
255They are *NOT* defined by the SMBus specification.
256
257I2C block transactions do not limit the number of bytes transferred
258but the SMBus layer places a limit of 32 bytes.
259
260
261I2C Block Read:  i2c_smbus_read_i2c_block_data()
262================================================
263
264This command reads a block of bytes from a device, from a
265designated register that is specified through the Comm byte.
266
267S Addr Wr [A] Comm [A]
268           S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
269
270Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK
271
272
273I2C Block Write:  i2c_smbus_write_i2c_block_data()
274==================================================
275
276The opposite of the Block Read command, this writes bytes to
277a device, to a designated register that is specified through the
278Comm byte. Note that command lengths of 0, 2, or more bytes are
279supported as they are indistinguishable from data.
280
281S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P
282
283Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK
284