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ADT7467
a chassis fan (on PWM2) use Remote 1 temperature.
Because the Remote 1 temperature is smoothed, both fans
are updated at exactly the same rate. If the chassis fan is
PWM_OUT
33% Duty
Cycle
85
Time Slots
170 Time Slots
much louder than the CPU fan, there is no way to improve
its acoustics without changing the thermal solution of the
CPU cooling fan.
The fan-centric approach to system acoustic enhancement
controls the PWM duty cycle, driving the fan at a fixed rate
(for example, 6%). Each time the PWM duty cycle is
updated, it is incremented by a fixed 6%. As a result, the fan
ramps smoothly to its newly calculated speed. If the
temperature starts to drop, the PWM duty cycle immediately
decreases by 6% at every update. Therefore, the fan ramps
up or down smoothly without inherent system delay.
Consider, for example, controlling the same CPU cooler fan
(on PWM1) and chassis fan (on PWM2) using Remote 1
temperature. The T MIN and T RANGE settings have been
defined in automatic fan speed control mode; that is, thermal
characterization of the control loop has been optimized.
Now the chassis fan is noisier than the CPU cooling fan.
Using the fan-centric approach, PWM2 can be placed into
acoustic enhancement mode independently of PWM1. The
PWM Output (One Period)
= 255 Time Slots
Figure 81. 33% PWM Duty Cycle, Represented in
Time Slots
The ramp rates in the enhanced acoustics mode are
selectable from the values 1, 2, 3, 5, 8, 12, 24, and 48. The
ramp rates are discrete time slots. For example, if the ramp
rate is 8, eight time slots are added or subtracted to increase
or decrease, respectively, the PWM high duty cycle.
Figure 82 shows how the enhanced acoustics mode
algorithm operates.
Read
Temperature
Calculate
New PWM
Duty Cycle
acoustics of the chassis fan can, therefore, be adjusted
without affecting the acoustic behavior of the CPU cooling
fan, even though both fans are controlled by Remote 1
temperature.
Is New
PWM Value >
Previous
Value?
YES
NO
Decrement
Previous
PWM Value
by Ramp Rate
Enabling Acoustic Enhancement for Each PWM Output
Enhanced Acoustics Register 1 (0x62)
<3> = 1 enables acoustic enhancement on PWM1 output.
Enhanced Acoustics Register 2 (0x63)
<7> = 1 enables acoustic enhancement on PWM2 output.
<3> = 1 enables acoustic enhancement on PWM3 output.
Effect of Ramp Rate on Enhanced Acoustics Mode
The PWM signal driving the fan has a period, T, given by
the PWM drive frequency, f, because T = 1/f. For a given
PWM period, T, the PWM period is subdivided into 255
equal time slots. One time slot corresponds to the smallest
possible increment in the PWM duty cycle. A PWM signal
of 33% duty cycle is, therefore, high for 1/3 ? 255 time slots
and low for 2/3 ? 255 time slots. Therefore, a 33% PWM
duty cycle corresponds to a signal that is high for 85 time
slots and low for 170 time slots.
Increment
Previous
PWM Value
by Ramp Rate
Figure 82. Enhanced Acoustics Algorithm
The enhanced acoustics mode algorithm calculates a new
PWM duty cycle based on the temperature measured. If the
new PWM duty cycle value is greater than the previous
PWM value, the previous PWM duty cycle value is
incremented by 1, 2, 3, 5, 8, 12, 24, or 48 time slots,
depending on the settings of the enhanced acoustics
registers. If the new PWM duty cycle value is less than the
previous PWM value, the previous PWM duty cycle is
decremented by 1, 2, 3, 5, 8, 12, 24, or 48 time slots. Each
time the PWM duty cycle is incremented or decremented, its
value is stored as the previous PWM duty cycle for the next
comparison. A ramp rate of 1 corresponds to one time slot,
which is 1/255 of the PWM period. In enhanced acoustics
mode, incrementing or decrementing by 1 changes the PWM
output by 1/255 ? 100%.
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