Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
decides whether to wake the system based on the current power source. If the current power source
is consistent with the timer type that expired, a wake signal will be asserted. Otherwise, the wake
signal will not be asserted.
Time and Alarm devices that implement the AC only (power independent) wake contain one
programmable timer that can be configured to wake up the system regardless of the platform's power
source when the timer expires. To simplify the programming interface the AC wake will use the AC
timer portion of the AC/DC wake; writes to the DC timer when AC only wake is supported will be
ignored.
To simplify the programming interface for the time and alarm device, timer expiration events will
persist. This means that if the OSPM programs a wake timer that expires before the OSPM
completes the transition into S3 (or S4/S5 if supported) the time and alarm device will wake the
system immediately after the OSPM completes the transition. Figure 9-56 illustrates this behavior.
OSPM programs
the wake timer
Wake timer
expires
OSPM
completes
transition
into S3
Wake device
immediately
wakes the
system
S0
S3
Time
Figure 9-55 Persistence of expired timer events
The time and alarm device will provide the OSPM with an interface to query the status of the wake
timers and discover what timers have expired. This interface enables the OSPM to discover the wake
source. The status of wake timers can be reset by setting the wake alarm; the OSPM may clear the
alarm status using the clear wake status method. All expired wake timer must be cleared if the
OSPM requires the platform to stay in S3 (S4/S5), otherwise the expired timers will immediately
wake up the system.
For the AC/DC wake services, and in case the current power source is inconsistent with the timer
type that expires, an expired timer wake policy value, in units of seconds, is defined that enables the
time and alarm device to wake the system when the power source corresponding to the expired timer
becomes active (wake either immediately, after some time period, or never). The expired timer wake
policy is applicable only on devices that support AC/DC wake and only when the timer expires and
540 April, 2015 Version 6.0