Advanced Configuration and Power Interface Specification

ACPI-Defined Devices and Device-Specific Objects
0x00000001 – 0xFFFFFFFE: Time between the power source changes and the timer wakes up the
system ( in units of seconds)
0xFFFFFFFF – The timer will never wake up the system after the power source changes
9.18.10 _TIV (Timer Values)
This object is required and returns the remaining time of the specified timer before it expires.
Arguments: (1)
Arg0 – TimerIdentifier (Integer(DWORD)): indicates the timer to be read:
0x00000000 – AC Timer
0x00000001 – DC Timer
Return Value:
An Integer containing the current timer value. A value of 0xFFFFFFFF indicates that the timer is
disabled.
9.18.11 ACPI Wakeup Alarm Events
The Wake Alarm, device as a generic hardware, supports control methods _PSW and _PRW to wake
up the system and issues a Notify(, 0x2) on the wakeup alarm device.
9.18.12 Relationship to Real Time Clock Alarm
Though both of the devices support wakeup timers to wake up system from sleeping state, they work
independently. The Real Time Clock Alarm is defined as a fixed feature hardware whereas Time and
Alarm device is defined as a generic hardware and can replace or coexist with the real time clock.
OSPM may choose which device to utilize to provide timed wake capability.
9.18.13 Time and Alarm device as a replacement to the RTC
The Time and Alarm device can be an alternative to the RTC on some platforms where the legacy
RTC hardware is not available, on these platforms the OSPM can use the Time and Alarm device to
obtain time and set wake alarms. For platforms that don't require AC/DC wake service (e.g. a
platform that have one power source only) the AC timer can be used to provide all the functions that
were traditionally provided by the RTC. Using the capabilities object the Time and Alarm device
can provide a scalable range of services to the OSPM.
9.18.14 Relationship to UEFI time source
The Time and Alarm device must be driven from the same time source as UEFI time services. This
ensures that the platform has a consistent value of real time (time of day) and wake alarms. The
OSPM can interact with this value using either ACPI or UEFI.
• OSPM must use only one runtime interface to configure/query the platform alarm(s); undefined
behavior may occur if the two wakeup interfaces are used on the same hardware.
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