linux/Documentation/i2c/smbus-protocol
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   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
 202
 203Packet Error Checking (PEC)
 204===========================
 205
 206Packet Error Checking was introduced in Revision 1.1 of the specification.
 207
 208PEC adds a CRC-8 error-checking byte to transfers using it, immediately
 209before the terminating STOP.
 210
 211
 212Address Resolution Protocol (ARP)
 213=================================
 214
 215The Address Resolution Protocol was introduced in Revision 2.0 of
 216the specification. It is a higher-layer protocol which uses the
 217messages above.
 218
 219ARP adds device enumeration and dynamic address assignment to
 220the protocol. All ARP communications use slave address 0x61 and
 221require PEC checksums.
 222
 223
 224SMBus Alert
 225===========
 226
 227SMBus Alert was introduced in Revision 1.0 of the specification.
 228
 229The SMBus alert protocol allows several SMBus slave devices to share a
 230single interrupt pin on the SMBus master, while still allowing the master
 231to know which slave triggered the interrupt.
 232
 233This 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
 240I2C Block Transactions
 241======================
 242
 243The following I2C block transactions are supported by the
 244SMBus layer and are described here for completeness.
 245They are *NOT* defined by the SMBus specification.
 246
 247I2C block transactions do not limit the number of bytes transferred
 248but the SMBus layer places a limit of 32 bytes.
 249
 250
 251I2C Block Read:  i2c_smbus_read_i2c_block_data()
 252================================================
 253
 254This command reads a block of bytes from a device, from a 
 255designated register that is specified through the Comm byte.
 256
 257S Addr Wr [A] Comm [A] 
 258           S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P
 259
 260Functionality flag: I2C_FUNC_SMBUS_READ_I2C_BLOCK
 261
 262
 263I2C Block Write:  i2c_smbus_write_i2c_block_data()
 264==================================================
 265
 266The opposite of the Block Read command, this writes bytes to 
 267a device, to a designated register that is specified through the
 268Comm byte. Note that command lengths of 0, 2, or more bytes are
 269supported as they are indistinguishable from data.
 270
 271S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P
 272
 273Functionality flag: I2C_FUNC_SMBUS_WRITE_I2C_BLOCK
 274
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