Specifications

QSSC-S4R Technical Product Specification Processor Presence and Population Check
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Such oscillation can be prevented by specifying positive or negative hysteresis, or both. Each time the fan speed
contribution is calculated, the BMC uses the hysteresis values to create a window around the temperature value that
was used for the calculation. The fan speed and the hysteresis window remain unchanged until a new sensor reading
falls outside of the window. The process repeats with that new reading: it is used to recalculate the fan speed
contribution and define a new hysteresis window. This cycle is independent of the lookup table values and applies
regardless of whether the new temperature reading affects the fan speed contribution.
The BMC creates this window by applying the hysteresis values as follows:
1. A new reading is retrieved from the BMC’s sensor subsystem.
2. The last-applied reading, the reading that was used to calculate the sub-record’s current fan speed contribution, is
subtracted from the new reading.
3. Hysteresis is applied to the difference:
x If the difference is positive (the new reading is higher), the positive hysteresis is subtracted from the difference.
x Otherwise, the change is non-positive (negative or zero), and the negative hysteresis is added to the
difference.
4. The modified difference is evaluated:
x If factoring in the hysteresis changed the calculated difference from positive to negative or from non-positive to
positive, the new reading is ignored and the previously calculated fan speed contribution is used.
x Otherwise, the reading obtained in step 1 is used to recalculate the fan speed contribution, and it is used as
the last applied reading until the hysteresis window is exceeded again.
For example, if a stepwise linear sub-record specifies a positive hysteresis value of 3º C and a negative hysteresis of
2º C for an ambient temperature sensor. A sensor reading of 25º C is used to calculate the initial fan speed contribution
for the subrecord. With the given hysteresis window, the BMC does not recalculate the fan speed until the sensor reads
28º C or higher, due to the positive hysteresis, or 23º C or lower, due to the negative hysteresis. When one of these
temperature ranges is reached, the fan speed is recalculated and the hysteresis window is reset based upon the new
reading. See the figure below.
Figure 107. Stepwise Linear Control Hysteresis
This prevents oscillating fan speed behavior, although it is different from the IPMI sensor threshold interpretation of
hysteresis, which is applied to the thresholds not to the reading.
24.13.4.1.2 Domain Maximum
Stepwise linear Tcontrol sub-records might have a flag set that indicates that the instance provides the fan domain
maximum PWM value. These sub-records do not contribute to the fan speed. Instead, the fan speed obtained through