Advanced Configuration and Power Interface Specification

ACPI System Management Bus Interface Specification
SMBus operation regions require that all field elements be declared at command value granularity.
This means that each virtual register cannot be broken down to its individual bits within the field
definition.
Access to sub-portions of virtual registers can be done only outside of the field definition. This
limitation is imposed both to simplify the SMBus interface and to maintain consistency with the
physical model defined by the SMBus specification.
SMBus protocols are assigned to field elements using the AccessAs term within the field definition.
The syntax for this term (from Section 19.2.3, “ASL Root and SecondaryTerms”) is described
below.
AccessAs(
AccessType,
AccessAttribute
)
Where:
//AccessTypeKeyword
//Nothing | ByteConst | AccessAttribKeyword
• AccessType must be set to BufferAcc.
• AccessAttribute indicates the SMBus protocol to assign to command values that follow this
term. See Section 13.1.2, “SMBus Protocols,” for a listing of the SMBus protocol types and
values.
An AccessAs term must appear as the first entry in a field definition to set the initial SMBus protocol
for the field elements that follow. A maximum of one SMBus protocol may be defined for each field
element. Devices supporting multiple protocols for a single command value can be modeled by
specifying multiple field elements with the same offset (command value), where each field element
is preceded by an AccessAs term specifying an alternate protocol.
For example, the register at command value 0x08 for a Smart Battery device (illustrated below)
represents a word value specifying the battery temperature (in degrees Kelvin), while the register at
command value 0x20 represents a variable-length (0 to 32 bytes) character string specifying the
name of the company that manufactured the battery.
Smart Battery Device
ManufacturerAccess()
RemainingCapacityAlarm()
Temperature()
ManufacturerName()
DeviceName()
Command Value
0x00 (Word)
0x01 (Word)
:
0x08 (Word)
:
0x20 (Block)
0x21 (Block)
:
Register
Byte 0 Byte 1
Byte 0 Byte 1
Byte 0 Byte 1
Byte 0 ... Byte 31
Byte 0 ... Byte 31
Figure 13-75 Smart Battery Device Virtual Registers
Version 6.0 667