13.3. Using the SMBus Protocols¶
This section provides information and examples on how each of the SMBus protocols can be used to access SMBus devices from AML.
13.3.1. Read/Write Quick (SMBQuick)¶
The SMBus Read/Write Quick protocol (SMBQuick) is typically used to control simple devices using a device-specific binary command (for example, ON and OFF). Command values are not used by this protocol and thus only a single element (at offset 0) can be specified in the field definition. This protocol transfers no data.
The following ASL code illustrates how a device supporting the Read/Write Quick protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBQuick) // Use the SMBus Read/Write Quick protocol
FLD0, 8 // Virtual register at command value 0.
}
/* Create the SMBus data buffer */
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, OB1) // OB1 = Status (Byte)
/* Signal device (e.g. OFF) */
Store(FLD0, BUFF) // Invoke Read Quick transaction
If(LEqual(OB1, 0x00)) {...} // Successful?
/* Signal device (e.g. ON) */
Store(BUFF, FLD0) // Invoke Write Quick transaction
In this example, a single field element (FLD0) at offset 0 is defined to represent the protocol’s read/write bit. Access to FLD0 will cause an SMBus transaction to occur to the device. Reading the field results in a Read Quick, and writing to the field results in a Write Quick. In either case data is not transferred–access to the register is simply used as a mechanism to invoke the transaction.
13.3.2. Send/Receive Byte (SMBSendReceive)¶
The SMBus Send/Receive Byte protocol (SMBSendReceive) transfers a single byte of data. Like Read/Write Quick, command values are not used by this protocol and thus only a single element (at offset 0) can be specified in the field definition.
The following ASL code illustrates how a device supporting the Send/Receive Byte protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBSendReceive) // Use the SMBus Send/Receive Byte protocol
FLD0, 8 // Virtual register at command value 0.
}
// Create the SMBus data buffer
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateByteField(BUFF, 0x02, DATA) // DATA = Data (Byte)
// Receive a byte of data from the device
Store(FLD0, BUFF) // Invoke a Receive Byte transaction
If(LEqual(STAT, 0x00)) // Successful?
{
// DATA = Received byte...
}
// Send the byte '0x16' to the device
Store(0x16, DATA) // Save 0x16 into the data buffer
Store(BUFF, FLD0) // Invoke a Send Byte transaction
In this example, a single field element (FLD0) at offset 0 is defined to represent the protocol’s data byte. Access to FLD0 will cause an SMBus transaction to occur to the device. Reading the field results in a Receive Byte, and writing to the field results in a Send Byte.
13.3.3. Read/Write Byte (SMBByte)¶
The SMBus Read/Write Byte protocol (SMBByte) also transfers a single byte of data. But unlike Send/Receive Byte, this protocol uses a command value to reference up to 256 byte-sized virtual registers.
The following ASL code illustrates how a device supporting the Read/Write Byte protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBByte) // Use the SMBus Read/Write Byte protocol
FLD0, 8, // Virtual register at command value 0.
FLD1, 8, // Virtual register at command value 1.
FLD2, 8 // Virtual register at command value 2.
}
// Create the SMBus data buffer
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateByteField(BUFF, 0x02, DATA) // DATA = Data (Byte)
// Read a byte of data from the device using command value 1
Store(FLD1, BUFF) // Invoke a Read Byte transaction
If(LEqual(STAT, 0x00)) // Successful?
{
// DATA = Byte read from FLD1...
}
// Write the byte '0x16' to the device using command value 2
Store(0x16, DATA) // Save 0x16 into the data buffer
Store(BUFF, FLD2) // Invoke a Write Byte transaction
In this example, three field elements (FLD0, FLD1, and FLD2) are defined to represent the virtual registers for command values 0, 1, and 2. Access to any of the field elements will cause an SMBus transaction to occur to the device. Reading FLD1 results in a Read Byte with a command value of 1, and writing to FLD2 results in a Write Byte with command value 2.
13.3.4. Read/Write Word (SMBWord)¶
The SMBus Read/Write Word protocol (SMBWord) transfers 2 bytes of data. This protocol also uses a command value to reference up to 256 word-sized virtual device registers.
The following ASL code illustrates how a device supporting the Read/Write Word protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBWord) // Use the SMBus Read/Write Word protocol
FLD0, 8, // Virtual register at command value 0.
FLD1, 8, // Virtual register at command value 1.
FLD2, 8 // Virtual register at command value 2.
}
// Create the SMBus data buffer
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateWordField(BUFF, 0x02, DATA) // DATA = Data (Word)
// Read two bytes of data from the device using command value 1
Store(FLD1, BUFF) // Invoke a Read Word transaction
If(LEqual(STAT, 0x00)) // Successful?
{
// DATA = Word read from FLD1...
}
// Write the word '0x5416' to the device using command value 2
Store(0x5416, DATA) // Save 0x5416 into the data buffer
Store(BUFF, FLD2) // Invoke a Write Word transaction
In this example, three field elements (FLD0, FLD1, and FLD2) are defined to represent the virtual registers for command values 0, 1, and 2. Access to any of the field elements will cause an SMBus transaction to occur to the device. Reading FLD1 results in a Read Word with a command value of 1, and writing to FLD2 results in a Write Word with command value 2.
Notice that although accessing each field element transmits a word (16 bits) of data, the fields are listed as 8 bits each. The actual data size is determined by the protocol. Every field element is declared with a length of 8 bits so that command values and byte offsets are equivalent.
13.3.5. Read/Write Block (SMBBlock)¶
The SMBus Read/Write Block protocol (SMBBlock) transfers variable-sized (0-32 bytes) data. This protocol uses a command value to reference up to 256 block-sized virtual registers.
The following ASL code illustrates how a device supporting the Read/Write Block protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBBlock) // Use the SMBus Read/Write Block protocol
FLD0, 8, // Virtual register at command value 0.
FLD1, 8, // Virtual register at command value 1.
FLD2, 8 // Virtual register at command value 2.
}
// Create the SMBus data buffer
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateByteField(BUFF, 0x01, SIZE) // SIZE = Length (Byte)
CreateField(BUFF, 0x10, 256, DATA) // DATA = Data (Block)
// Read block data from the device using command value 1
Store(FLD1, BUFF) // Invoke a Read Block transaction
If(LEqual(STAT, 0x00)) // Successful?
{
// SIZE = Size (number of bytes)
// of the block data read from FLD1...
// DATA = Block data read from FLD1...
}
// Write the block 'TEST' to the device using command value 2
Store("TEST", DATA) // Save "TEST" into the data buffer
Store(4, SIZE) // Length of valid data in the data buffer
Store(BUFF, FLD2) // Invoke a Write Word transaction
In this example, three field elements (FLD0, FLD1, and FLD2) are defined to represent the virtual registers for command values 0, 1, and 2. Access to any of the field elements will cause an SMBus transaction to occur to the device. Reading FLD1 results in a Read Block with a command value of 1, and writing to FLD2 results in a Write Block with command value 2.
13.3.6. Word Process Call (SMBProcessCall)¶
The SMBus Process Call protocol (SMBProcessCall) transfers 2 bytes of data bi-directionally (performs a Write Word followed by a Read Word as an atomic transaction). This protocol uses a command value to reference up to 256 word-sized virtual registers.
The following ASL code illustrates how a device supporting the Process Call protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMBus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBProcessCall) // Use the SMBus Process Call protocol
FLD0, 8, // Virtual register at command value 0.
FLD1, 8, // Virtual register at command value 1.
FLD2, 8 // Virtual register at command value 2.
}
// Create the SMBus data buffer
Name(BUFF, Buffer(34){}) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateWordField(BUFF, 0x02, DATA) // DATA = Data (Word)
// Process Call with input value '0x5416' to the device using command value 1
Store(0x5416, DATA) // Save 0x5416 into the data buffer
Store(Store(BUFF, FLD1), BUFF) // Invoke a Process Call transaction
If(LEqual(STAT, 0x00)) // Successful?
{
// DATA = Word returned from FLD1...
}
In this example, three field elements (FLD0, FLD1, and FLD2) are defined to represent the virtual registers for command values 0, 1, and 2. Access to any of the field elements will cause an SMBus transaction to occur to the device. Reading or writing FLD1 results in a Process Call with a command value of 1. Notice that unlike other protocols, Process Call involves both a write and read operation in a single atomic transaction. This means that the Data element of the SMBus data buffer is set with an input value before the transaction is invoked, and holds the output value following the successful completion of the transaction.
13.3.7. Block Process Call (SMBBlockProcessCall)¶
The SMBus Block Write-Read Block Process Call protocol (SMBBlockProcessCall) transfers a block of data bi-directionally (performs a Write Block followed by a Read Block as an atomic transaction). The maximum aggregate amount of data that may be transferred is limited to 32 bytes. This protocol uses a command value to reference up to 256 block-sized virtual registers.
The following ASL code illustrates how a device supporting the Process Call protocol should be accessed:
OperationRegion(SMBD, SMBus, 0x4200, 0x100) // SMbus device at slave address 0x42
Field(SMBD, BufferAcc, NoLock, Preserve)
{
AccessAs(BufferAcc, SMBBlockProcessCall) // Use the Block Process Call protocol
FLD0, 8, // Virtual register representing a command value of 0
FLD1, 8 // Virtual register representing a command value of 1
}
// Create the SMBus data buffer as BUFF
Name(BUFF, Buffer(34)()) // Create SMBus data buffer as BUFF
CreateByteField(BUFF, 0x00, STAT) // STAT = Status (Byte)
CreateByteField(BUFF, 0x01, SIZE) // SIZE = Length (Byte)
CreateField(BUFF, 0x10, 256, DATA) // Data (Block)
// Process Call with input value "ACPI" to the device using command value 1
Store("ACPI", DATA) // Fill in outgoing data
Store(8, SIZE) // Length of the valid data
Store(Store(BUFF, FLD1), BUFF) // Execute the PC
if (LEqual(STAT, 0x00)) // Test the status
{
// BUFF now contains information returned
// from PC
// SIZE now equals size of data returned
}