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Software Functional Overview
• If a standard single-speed fan is the Active cooling device, then the policy is
represented by the temperature to which _AC0 evaluates, and the fan is listed in
_AL0.
• If the zone uses two independently-controlled single-speed fans to regulate the
temperature, then _AC0 will evaluate to the maximum cooling temperature using two
fans, and _AC1 will evaluate to the standard cooling temperature using one fan.
• If a zone has a single fan with a low speed and a high speed, the _AC0 will evaluate
to the temperature associated with running the fan at high-speed, and _AC1 will
evaluate to the temperature associated with running the fan at low speed. _AL0 and
_AL1 will both point to different device objects associated with the same physical fan,
but control the fan at different speeds.
3.6.5 Passive Cooling Equation
Unlike the case for _ACx, during passive cooling the OS takes the initiative to actively
monitor the temperature in order to cool the platform. On an ACPI-compatible platform that
properly implements CPU throttling, the temperature transitions will be similar to the
following figure.
CPU Performance
Time
Temperature
100%
50%
Δ
P
_TSP (Sampling period)
T
t
T
n - 1
T
n
For the OS to assess the optimum CPU performance change required to bring the temperature
down, the following equation must be incorporated into the OS.
ΔP [%] = _TC1 * ( Tn - Tn-1 ) + _TC2 * (Tn - Tt)
where
Tn = current temperature
Tt = target temperature (_PSV)
The two coefficients _TC1 and _TC2 and the sampling period _TSP are hardware-dependent
constants the OEM must supply to the OS (for more information, see section 12.3). The
object _TSP contains a time interval that the OS uses to poll the hardware to sample the
temperature. Whenever _TSP time has elapsed, the OS will run _TMP to sample the current
temperature (shown as Tn in the above equation). Then the OS will use the sampled
temperature and _PSV (which is the target temperature Tt) to evaluate the equation for ΔP.
3-30 FIC M295 / M296 Service Manual