Advanced Configuration and Power Interface Specification

ACPI-Defined Devices and Device-Specific Objects
expected that the time on the platform will be consistent when different firmware interfaces are used
to query the platform time. For example, a UEFI call to get the time should return the same time as if
the OSPM used the time and alarm device at the same point in time.
The Time and Alarm device can optionally support power management objects (e.g. _PS0, _PS3) to
allow the OS to manage the device's power consumption.
The Time andAlarm device must support control method _PRW for being enabled to wake up the
system. It might support _DSW or _PSW to provide the functionality to enable or disable the
device's ability to wake a sleep system. On Hardware-reduced ACPI platforms, _PRW is only
required if the device depends on ACPI-defined power resources. _PRW’s GPEInfo structure is
ignored by OSPM. For enabling Wakeup, _DSW and _SxW are used, and the wakeup event is
signaled by the GPIO-signaled ACPI event mechanism (Section 5.6.5).
The Plug and Play ID of the Time and Wake Alarm device is ACPI000E.
Table 9-266 Time and Alarm Device
Object
_GCP
_GRT
_SRT
_GWS
_CWS
_STP
_STV
_TIP
_TIV
Description
Get the capabilities of the time and alarm device
Get the Real time
Set the Real time
Get Wake status
Clear Wake Status
Sets expired timer wake policy for the specified timer.
Sets the value in the specified timer.
Returns the current expired timer policy setting of the specified timer.
Returns the remaining time of the specified timer.
9.18.1Overview
The Time and Alarm device provides an alternative to the real time clock (RTC), which is defined as
a fixed feature hardware device. The wake timers allow the system to transition from the S3 (or
optionally S4/S5) state to S0 state after a time period elapses. In comparison with the Real Time
Clock (RTC) Alarm, the Time and Alarm device provides a larger scale of flexibility in the
operation of the wake timers, and allows the implementation of the time source to be abstracted from
the OSPM.
Time and Alarm device provides the OSPM with a firmware abstraction of time and alarm services
that can be applicable to a variety of hardware designs. The methods for setting and getting real time
provide an alternative to the (RTC). In addition the device provides two different levels of wake
services, depending on the platform capabilities, AC/DC wake or AC wake.
Time and Alarm devices that implement AC/DC wake service contain two programmable timers that
can be configured to wake the system depending on the platform's current power source (AC or DC)
when the timers expire. The two timers, which are referred to as the AC timer and the DC timer, are
independent in that they are individually programmable and applicable without interfering each
other. Each of the timers can be programmed with the number of seconds to elapse from the time the
timer is programmed until a wake is requested. When a timer expires, the Time and Alarm device
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