Advanced Configuration and Power Interface Specification

Advanced Configuration and Power Interface Specification
4.8.2.4 Real Time Clock Alarm
If implemented, the Real Time Clock (RTC) alarm must generate a hardware wake event when in
the sleeping state. The RTC can be programmed to generate an alarm. An enabled RTC alarm can be
used to generate a wake event when the system is in a sleeping state. ACPI provides for additional
hardware to support OSPM in determining that the RTC was the source of the wake event: the
RTC_STS and RTC_EN bits. Although these bits are optional, if supported they must be
implemented as described here.
If the RTC_STS and RTC_EN bits are not supported, OSPM will attempt to identify the RTC as a
possible wake source; however, it might miss certain wake events. If implemented, the RTC wake
feature is required to work in the following sleeping states: S1-S3. S4 wake is optional and
supported through the RTC_S4 flag within the FADT (if set, then the platform supports RTC wake
in the S4 state) 1 .
When the RTC generates a wake event the RTC_STS bit will be set. If the RTC_EN bit is set, an
RTC hardware power management event will be generated (which will wake the system from a
sleeping state, provided the battery low signal is not asserted).
Real Time Clock
(RTC)
RTC_STS
PM1x_STS.10
RTC Wake-up
Event
RTC_EN
PM1x_EN.10
Figure 4-19 RTC Alarm
The RTC wake event status and enable bits are an optional fixed hardware feature and a flag within
the FADT (FIX_RTC) indicates if the register bits are to be used by OSPM. If the RTC wake event
status and enable bits are implemented in fixed hardware, OSPM can determine if the RTC was the
source of the wake event without loading the entire OS. This also gives the platform the capability of
indicating an RTC wake source without consuming a GPE bit, as would be required if RTC wake
was not implemented using the fixed hardware RTC feature. If the fixed hardware feature event bits
are not supported, then OSPM will attempt to determine this by reading the RTC’s status field. If the
platform implements the RTC fixed hardware feature, and this hardware consumes resources, the
_FIX method can be used to correlate these resources with the fixed hardware. See Section 6.2.5,
“_FIX (Fixed Register Resource Provide”, for details.
OSPM supports enhancements over the existing RTC device (which only supports a 99 year date and
24-hour alarm). Optional extensions are provided for the following features:
Day Alarm.
The DAY_ALRM field points to an optional CMOS RAM location that selects the
day within the month to generate an RTC alarm.
1. Notice that the G2/S5 “soft off” and the G3 “mechanical off” states are not sleeping states. The OS will disable
the RTC_EN bit prior to entering the G2/S5 or G3 states regardless.
82 April, 2015 Version 6.0