summaryrefslogtreecommitdiffstats
path: root/Documentation/i2c/smbus-protocol.rst
blob: e30eb1d274c6841c7ab3f7aa1bad6a696c8cdc17 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
======================
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 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
<file:Documentation/i2c/functionality.rst> for the details.


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 Quick Command
===================

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

  A Addr Rd/Wr [A] P

Functionality flag: I2C_FUNC_SMBUS_QUICK


SMBus Receive Byte:  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:  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:  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:  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:  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:  i2c_smbus_write_word_data()
==============================================

This is the opposite of the Read Word operation. 16 bits
of data is 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:  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:  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_setup_smbus_alert() to setup 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 supported by the
SMBus layer and are described here for completeness.
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:  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:  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