Advanced Configuration and Power Interface Specification

ACPI System Management Bus Interface Specification
// Receive a byte of data from the device
Store(FLD0, BUFF)
If(LEqual(STAT, 0x00))
{
// DATA = Received byte…
}
// Send the byte ‘0x16’ to the device
Store(0x16, DATA)
Store(BUFF, FLD0)
// Invoke a Receive Byte transaction
// Successful?
// Save 0x16 into the data buffer
// 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.
}
Name(BUFF, Buffer(34){})
CreateByteField(BUFF, 0x00, STAT)
CreateByteField(BUFF, 0x02, DATA)
// Create the SMBus data buffer
// Create SMBus data buffer as BUFF
// STAT = Status (Byte)
// 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.
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