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1 | SMBus Protocol Summary |
2 | ====================== | |
1a31a88f | 3 | |
1da177e4 LT |
4 | The following is a summary of the SMBus protocol. It applies to |
5 | all revisions of the protocol (1.0, 1.1, and 2.0). | |
6 | Certain protocol features which are not supported by | |
7 | this package are briefly described at the end of this document. | |
8 | ||
9 | Some adapters understand only the SMBus (System Management Bus) protocol, | |
10 | which is a subset from the I2C protocol. Fortunately, many devices use | |
11 | only the same subset, which makes it possible to put them on an SMBus. | |
1a31a88f | 12 | |
1da177e4 LT |
13 | If you write a driver for some I2C device, please try to use the SMBus |
14 | commands if at all possible (if the device uses only that subset of the | |
15 | I2C protocol). This makes it possible to use the device driver on both | |
16 | SMBus adapters and I2C adapters (the SMBus command set is automatically | |
17 | translated to I2C on I2C adapters, but plain I2C commands can not be | |
18 | handled at all on most pure SMBus adapters). | |
19 | ||
1a31a88f DB |
20 | Below is a list of SMBus protocol operations, and the functions executing |
21 | them. Note that the names used in the SMBus protocol specifications usually | |
22 | don't match these function names. For some of the operations which pass a | |
23 | single data byte, the functions using SMBus protocol operation names execute | |
24 | a different protocol operation entirely. | |
25 | ||
a1681781 JD |
26 | Each transaction type corresponds to a functionality flag. Before calling a |
27 | transaction function, a device driver should always check (just once) for | |
28 | the corresponding functionality flag to ensure that the underlying I2C | |
29 | adapter supports the transaction in question. See | |
30 | <file:Documentation/i2c/functionality> for the details. | |
31 | ||
1da177e4 LT |
32 | |
33 | Key to symbols | |
34 | ============== | |
35 | ||
36 | S (1 bit) : Start bit | |
37 | P (1 bit) : Stop bit | |
38 | Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0. | |
39 | A, NA (1 bit) : Accept and reverse accept bit. | |
40 | Addr (7 bits): I2C 7 bit address. Note that this can be expanded as usual to | |
41 | get a 10 bit I2C address. | |
42 | Comm (8 bits): Command byte, a data byte which often selects a register on | |
43 | the device. | |
44 | Data (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh | |
45 | for 16 bit data. | |
46 | Count (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 | ||
67c2e665 JD |
51 | SMBus Quick Command |
52 | =================== | |
1da177e4 LT |
53 | |
54 | This sends a single bit to the device, at the place of the Rd/Wr bit. | |
1da177e4 LT |
55 | |
56 | A Addr Rd/Wr [A] P | |
57 | ||
a1681781 JD |
58 | Functionality flag: I2C_FUNC_SMBUS_QUICK |
59 | ||
1da177e4 | 60 | |
1a31a88f DB |
61 | SMBus Receive Byte: i2c_smbus_read_byte() |
62 | ========================================== | |
1da177e4 LT |
63 | |
64 | This reads a single byte from a device, without specifying a device | |
65 | register. Some devices are so simple that this interface is enough; for | |
66 | others, it is a shorthand if you want to read the same register as in | |
67 | the previous SMBus command. | |
68 | ||
69 | S Addr Rd [A] [Data] NA P | |
70 | ||
a1681781 JD |
71 | Functionality flag: I2C_FUNC_SMBUS_READ_BYTE |
72 | ||
1da177e4 | 73 | |
1a31a88f DB |
74 | SMBus Send Byte: i2c_smbus_write_byte() |
75 | ======================================== | |
1da177e4 | 76 | |
1a31a88f DB |
77 | This operation is the reverse of Receive Byte: it sends a single byte |
78 | to a device. See Receive Byte for more information. | |
1da177e4 LT |
79 | |
80 | S Addr Wr [A] Data [A] P | |
81 | ||
a1681781 JD |
82 | Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE |
83 | ||
1da177e4 | 84 | |
1a31a88f DB |
85 | SMBus Read Byte: i2c_smbus_read_byte_data() |
86 | ============================================ | |
1da177e4 LT |
87 | |
88 | This reads a single byte from a device, from a designated register. | |
89 | The register is specified through the Comm byte. | |
90 | ||
91 | S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P | |
92 | ||
a1681781 JD |
93 | Functionality flag: I2C_FUNC_SMBUS_READ_BYTE_DATA |
94 | ||
1da177e4 | 95 | |
1a31a88f DB |
96 | SMBus Read Word: i2c_smbus_read_word_data() |
97 | ============================================ | |
1da177e4 | 98 | |
1a31a88f | 99 | This operation is very like Read Byte; again, data is read from a |
1da177e4 LT |
100 | device, from a designated register that is specified through the Comm |
101 | byte. But this time, the data is a complete word (16 bits). | |
102 | ||
103 | S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P | |
104 | ||
a1681781 JD |
105 | Functionality flag: I2C_FUNC_SMBUS_READ_WORD_DATA |
106 | ||
06a67848 JC |
107 | Note the convenience function i2c_smbus_read_word_swapped is |
108 | available for reads where the two data bytes are the other way | |
109 | around (not SMBus compliant, but very popular.) | |
110 | ||
1da177e4 | 111 | |
1a31a88f DB |
112 | SMBus Write Byte: i2c_smbus_write_byte_data() |
113 | ============================================== | |
1da177e4 LT |
114 | |
115 | This writes a single byte to a device, to a designated register. The | |
116 | register is specified through the Comm byte. This is the opposite of | |
1a31a88f | 117 | the Read Byte operation. |
1da177e4 LT |
118 | |
119 | S Addr Wr [A] Comm [A] Data [A] P | |
120 | ||
a1681781 JD |
121 | Functionality flag: I2C_FUNC_SMBUS_WRITE_BYTE_DATA |
122 | ||
1da177e4 | 123 | |
1a31a88f DB |
124 | SMBus Write Word: i2c_smbus_write_word_data() |
125 | ============================================== | |
1da177e4 | 126 | |
1a31a88f | 127 | This is the opposite of the Read Word operation. 16 bits |
3f9a4790 | 128 | of data is written to a device, to the designated register that is |
1da177e4 LT |
129 | specified through the Comm byte. |
130 | ||
131 | S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P | |
132 | ||
a1681781 JD |
133 | Functionality flag: I2C_FUNC_SMBUS_WRITE_WORD_DATA |
134 | ||
06a67848 JC |
135 | Note the convenience function i2c_smbus_write_word_swapped is |
136 | available for writes where the two data bytes are the other way | |
137 | around (not SMBus compliant, but very popular.) | |
138 | ||
1da177e4 | 139 | |
c8110933 TB |
140 | SMBus Process Call: |
141 | =================== | |
1da177e4 LT |
142 | |
143 | This command selects a device register (through the Comm byte), sends | |
144 | 16 bits of data to it, and reads 16 bits of data in return. | |
145 | ||
146 | S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] | |
147 | S Addr Rd [A] [DataLow] A [DataHigh] NA P | |
148 | ||
a1681781 JD |
149 | Functionality flag: I2C_FUNC_SMBUS_PROC_CALL |
150 | ||
1da177e4 | 151 | |
1a31a88f DB |
152 | SMBus Block Read: i2c_smbus_read_block_data() |
153 | ============================================== | |
1da177e4 LT |
154 | |
155 | This command reads a block of up to 32 bytes from a device, from a | |
156 | designated register that is specified through the Comm byte. The amount | |
157 | of data is specified by the device in the Count byte. | |
158 | ||
159 | S Addr Wr [A] Comm [A] | |
160 | S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P | |
161 | ||
a1681781 JD |
162 | Functionality flag: I2C_FUNC_SMBUS_READ_BLOCK_DATA |
163 | ||
1da177e4 | 164 | |
1a31a88f DB |
165 | SMBus Block Write: i2c_smbus_write_block_data() |
166 | ================================================ | |
1da177e4 LT |
167 | |
168 | The opposite of the Block Read command, this writes up to 32 bytes to | |
169 | a device, to a designated register that is specified through the | |
170 | Comm byte. The amount of data is specified in the Count byte. | |
171 | ||
172 | S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P | |
173 | ||
a1681781 JD |
174 | Functionality flag: I2C_FUNC_SMBUS_WRITE_BLOCK_DATA |
175 | ||
1da177e4 | 176 | |
1a31a88f DB |
177 | SMBus Block Write - Block Read Process Call |
178 | =========================================== | |
1da177e4 | 179 | |
1a31a88f DB |
180 | SMBus Block Write - Block Read Process Call was introduced in |
181 | Revision 2.0 of the specification. | |
1da177e4 LT |
182 | |
183 | This command selects a device register (through the Comm byte), sends | |
184 | 1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return. | |
185 | ||
186 | S Addr Wr [A] Comm [A] Count [A] Data [A] ... | |
187 | S Addr Rd [A] [Count] A [Data] ... A P | |
188 | ||
a1681781 JD |
189 | Functionality flag: I2C_FUNC_SMBUS_BLOCK_PROC_CALL |
190 | ||
1da177e4 LT |
191 | |
192 | SMBus Host Notify | |
193 | ================= | |
194 | ||
195 | This command is sent from a SMBus device acting as a master to the | |
196 | SMBus host acting as a slave. | |
197 | It is the same form as Write Word, with the command code replaced by the | |
198 | alerting device's address. | |
199 | ||
200 | [S] [HostAddr] [Wr] A [DevAddr] A [DataLow] A [DataHigh] A [P] | |
201 | ||
202 | ||
203 | Packet Error Checking (PEC) | |
204 | =========================== | |
1a31a88f | 205 | |
1da177e4 LT |
206 | Packet Error Checking was introduced in Revision 1.1 of the specification. |
207 | ||
1a31a88f DB |
208 | PEC adds a CRC-8 error-checking byte to transfers using it, immediately |
209 | before the terminating STOP. | |
1da177e4 LT |
210 | |
211 | ||
212 | Address Resolution Protocol (ARP) | |
213 | ================================= | |
1a31a88f | 214 | |
1da177e4 LT |
215 | The Address Resolution Protocol was introduced in Revision 2.0 of |
216 | the specification. It is a higher-layer protocol which uses the | |
217 | messages above. | |
218 | ||
219 | ARP adds device enumeration and dynamic address assignment to | |
220 | the protocol. All ARP communications use slave address 0x61 and | |
221 | require PEC checksums. | |
222 | ||
223 | ||
b5527a77 JD |
224 | SMBus Alert |
225 | =========== | |
226 | ||
227 | SMBus Alert was introduced in Revision 1.0 of the specification. | |
228 | ||
229 | The SMBus alert protocol allows several SMBus slave devices to share a | |
230 | single interrupt pin on the SMBus master, while still allowing the master | |
231 | to know which slave triggered the interrupt. | |
232 | ||
233 | This is implemented the following way in the Linux kernel: | |
234 | * I2C bus drivers which support SMBus alert should call | |
235 | i2c_setup_smbus_alert() to setup SMBus alert support. | |
236 | * I2C drivers for devices which can trigger SMBus alerts should implement | |
237 | the optional alert() callback. | |
238 | ||
239 | ||
1da177e4 LT |
240 | I2C Block Transactions |
241 | ====================== | |
1a31a88f | 242 | |
1da177e4 LT |
243 | The following I2C block transactions are supported by the |
244 | SMBus layer and are described here for completeness. | |
1a31a88f DB |
245 | They are *NOT* defined by the SMBus specification. |
246 | ||
1da177e4 LT |
247 | I2C block transactions do not limit the number of bytes transferred |
248 | but the SMBus layer places a limit of 32 bytes. | |
249 | ||
250 | ||
1a31a88f DB |
251 | I2C Block Read: i2c_smbus_read_i2c_block_data() |
252 | ================================================ | |
1da177e4 LT |
253 | |
254 | This command reads a block of bytes from a device, from a | |
255 | designated register that is specified through the Comm byte. | |
256 | ||
257 | S Addr Wr [A] Comm [A] | |
258 | S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P | |
259 | ||
a1681781 | 260 | Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK |
1da177e4 LT |
261 | |
262 | ||
1a31a88f DB |
263 | I2C Block Write: i2c_smbus_write_i2c_block_data() |
264 | ================================================== | |
1da177e4 LT |
265 | |
266 | The opposite of the Block Read command, this writes bytes to | |
267 | a device, to a designated register that is specified through the | |
268 | Comm byte. Note that command lengths of 0, 2, or more bytes are | |
269 | supported as they are indistinguishable from data. | |
270 | ||
271 | S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P | |
a1681781 JD |
272 | |
273 | Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK |