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diff --git a/Documentation/i2c/smbus-protocol.rst b/Documentation/i2c/smbus-protocol.rst new file mode 100644 index 000000000..64689d19d --- /dev/null +++ b/Documentation/i2c/smbus-protocol.rst @@ -0,0 +1,322 @@ +================== +The SMBus Protocol +================== + +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 :doc:`functionality` for +the details. + + +Key to symbols +============== + +=============== ============================================================= +S Start condition +P Stop condition +Rd/Wr (1 bit) Read/Write bit. Rd equals 1, Wr equals 0. +A, NA (1 bit) Acknowledge (ACK) and Not Acknowledge (NACK) 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:: + + S Addr Rd/Wr [A] P + +Functionality flag: I2C_FUNC_SMBUS_QUICK + + +SMBus Receive Byte +================== + +Implemented by 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 +=============== + +Implemented by 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 +=============== + +Implemented by 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 +=============== + +Implemented by 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 +================ + +Implemented by 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 +================ + +Implemented by i2c_smbus_write_word_data() + +This is the opposite of the Read Word operation. 16 bits +of data are 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 +================ + +Implemented by 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 +================= + +Implemented by 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_new_smbus_alert_device() to install 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 similar to the SMBus Block Read +and Write operations, except these do not have a Count byte. They are +supported by the SMBus layer and are described here for completeness, but +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 +============== + +Implemented by 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 +=============== + +Implemented by 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 |