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NCT7491
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Table 73. REGISTER 0x62  Enhanced Acoustics Register 1 (PowerOn Default = 0x20)
Bit
Name
R/W
Description
<2:0>
ACOU
R/W
These bits define the maximum rate of change of the PWM1 output. Instead of the fan speed jumping
instantaneously to its newly determined speed, it ramps gracefully at the rate determined by these bits.
This feature ultimately enhances the acoustics of the fan.
Time Slot Increase
Time for 0% to 100%
000 = 1
37.5 sec
001 = 2
18.8 sec
010 = 3
12.5 sec
011 = 4
7.5 sec
100 = 8
4.7 sec
101 = 12
3.1 sec
110 = 24
1.6 sec
111 = 48
0.8 sec
<3>
EN1
R/W
When this bit is 1, smoothing is enabled on PWM1 output.
<4>
SYNC
R/W
SYNC = 1 synchronizes fan speed measurements on TACH2, TACH3, and TACH4 to PWM3. This
allows up to three fans to be driven from PWM3 output and their speeds to be measured.
SYNC = 0 synchronizes only TACH3 and TACH4 to PWM3 output.
<5>
MIN1
R/W
When the NCT7491 is in automatic fan control mode, this bit defines whether PWM1 is off (0% duty
cycle) or at PWM1 minimum duty cycle when the controlling temperature is below its T
MIN
  hysteresis
value.
0 = 0% duty cycle below T
MIN
  hysteresis.
1 = PWM1 minimum duty cycle below T
MIN
  hysteresis.
<6>
MIN2
R/W
When the NCT7491 is in automatic fan speed control mode, this bit defines whether PWM2 is off (0%
duty cycle) or at PWM2 minimum duty cycle when the controlling temperature is below its T
MIN
  hys-
teresis value.
0 = 0% duty cycle below T
MIN
  hysteresis.
1 = PWM 2 minimum duty cycle below T
MIN
  hysteresis.
<7>
MIN3
R/W
When the NCT7491 is in automatic fan speed control mode, this bit defines whether PWM3 is off (0%
duty cycle) or at PWM3 minimum duty cycle when the controlling temperature is below its T
MIN
  hys-
teresis value.
0 = 0% duty cycle below T
MIN
  hysteresis.
1 = PWM3 minimum duty cycle below T
MIN
  hysteresis.
Table 74. REGISTER 0x63  Enhanced Acoustics Register 2 (PowerOn Default = 0x00)
Bit
Name
R/W (Note 25)
Description
<2:0>
ACOU3
R/W
These bits define the maximum rate of change of the PWM3 output. Instead of the fan speed
jumping instantaneously to its newly determined speed, it ramps gracefully at the rate determ-
ined by these bits. This feature ultimately enhances the acoustics of the fan.
Time Slot Increase
Time for 0% to 100%
000 = 1
37.5 sec
001 = 2
18.8 sec
010 = 3
12.5 sec
011 = 4
7.5 sec
100 = 8
4.7 sec
101 = 12
3.1 sec
110 = 24
1.6 sec
111 = 48
0.8 sec
< 3 >
EN3
R/W
When this bit is 1, smoothing is enabled on the PWM3 output.
25.These registers become readonly when the NCT7491 is in automatic fan control mode.
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