In Auto Fan Mode, when the temperature for a zone is above the Temperature Limit (Registers 67-69h) and below its Absolute

Temperature Limit (Registers 6A-6Ch), the speed of a fan assigned to that zone is determined as follows.

When the temperature reaches the Fan Temp Limit for a zone, the PWM output assigned to that zone will be Fan PWM Minimum.

Between Fan Temp Limit and (Fan Temp Limit + Range), the PWM duty cycle will increase linearly according to the temperature

as shown in the figure below. The PWM duty cycle will be 100% at (Fan Temp Limit + Range).

Example for PWM1 assigned to Zone 1:

– Zone 1 Fan Temp Limit (Register 67h) is set to 50˚C (32h).

– Range (Register 5Fh) is set to 8˚C (6xh).

– Fan 1 PWM Minimum (Register 64h) is set to 50% (32h).

In this case, the PWM1 duty cycle will be 50% at 50˚C.

Since (Zone 1 Fan Temp Limit) + (Zone 1 Range) = 50˚C + 8˚C = 58˚C, the fan will run at 100% duty cycle when the temperature

of the Zone 1 sensor reaches 58˚C.

Since the midpoint of the fan control range is 54˚C, and the median duty cycle is 75% (Halfway between the PWM Minimum and

100%), PWM1 duty cycle would be 75% at 54˚C.

Above (Zone 1 Fan Temp Limit) + (Zone 1 Range), the duty cycle will be 100%.

PWM frequency bits [3:0]

The PWM frequency bits [3:0] determine the PWM frequency for the fan.

PWM Frequency Selection (Default = 011 = 30.04 Hz)

TABLE 6. Register Setting vs PWM Frequency

Freq [2:0]

PWM Frequency (Exact frequencies vary by

manufacturer)

National Actual PWM Frequency

000

10 Hz

10.01 Hz

001

15 Hz

15.02 Hz

010

23 Hz

23.14 Hz

011

30 Hz

30.04 Hz

100

38 Hz

38.16 Hz

101

47 Hz

47.06 Hz

110

62 Hz

61.38 Hz

111

94 Hz

94.12 Hz

Range Selection RAN [3:0]

RAN [3:0]

Range (˚C)

0000

2

0001

2.5

0010

3.33

20035306

FIGURE 1. Fan Activity above Fan Temp Limit

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