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ADT7470

Data Sheet

Rev. E | Page 26 of 40

AUTOMATIC FAN SPEED CONTROL

In automatic fan speed control mode, fan speed automatically

varies with temperature and without CPU intervention, once

initial parameters are set up. The advantage is that when a

system hangs, the user is guaranteed that the system is protected

from overheating.

Automatic fan speed control mode is recommended for use

only when temperatures > 8°C. In automatic fan control mode,

if the temperature drops below 0°C, the fans automatically

turn on.

For each thermal zone, when the temperature exceeds TMIN,

the fans turn on at PWMMIN duty cycle. When the temperature

reaches TMIN + 20°C, the fans increase in speed to PWMMAX.

To configure each fan into automatic fan control mode, the

BHVR bit for that fan must be set to 1. See Table 22 for more

details.

To control the fans in automatic fan control mode, a number of

parameters for each fan should be set up. The PWM minimum

and maximum duty cycles, as well as the minimum temperature

at which each fan turns on, should be configured. Which

TMP05 controls which fan also needs to be configured.

What follows are the automatic fan control configuration steps:

1. Put the fans into automatic fan control mode, by setting

the BHVR bits for each fan to 1.

2. Determine which TMP05 is to control the fan, by

configuring Registers 0x7C and 0x7D. Any TMP05, can

control any fan, or the hottest TMP05 can control the fan.

3. Set the minimum temperature for each fan, by writing to

Registers 0x6E to 0x70. When the temperature exceeds

TMIN, the fan runs at PWMMIN.

4. Set PWMMIN, the minimum PWM duty cycle, by writing to

Registers 0x6A to 0x6D.

5. Set PWMMAX, the maximum PWM duty cycle, by writing

to registers 0x38 to 0x3B.

6. Write to the STRT bit in Configuration Register 1 (0x40

Bit[0]) to start the ADT7470 monitoring cycle. Set Bit 7 in

this register to 1 to enable the TMP05 start pulse.

PWM Min Duty Cycle

The PWM min duty cycle registers, at address 0x6A to 0x6D,

set the PWM duty cycle at which the fans turn on in automatic

fan control mode.

The value to be programmed into the PWM Min Duty Cycle

registers can be calculated as follows:

Value (decimal) = Desired PWM duty cycle/0.39

Example: For a PWM Min Duty Cycle of 30%

Value (decimal) = 30/0.39 = 77 decimal

Value = 77 decimal or 4D hex

The PWM min duty cycle registers have a default value of

0x80, which corresponds to a duty cycle of 50% on the PWM

output pin.

PWN Max Duty Cycle

For each fan, the maximum PWM duty cycle can be set by

writing to Registers 0x38 to 0x3B.

The value to be programmed into the PWM max duty cycle

registers can be calculated as follows:

Value (decimal) = Desired PWM duty cycle/0.39

Example: For a PWM Max Duty Cycle of 90%

Value (decimal) = 90/0.39 = 230 decimal

Value = 230 decimal or E6 hex

The PWM max duty cycle registers have a default value of 0xFF,

which corresponds to a Logic 1 on the PWM output pin.

PWM Current Duty Cycle

In automatic fan control mode, the current PWM duty cycle for

each fan is recorded in the PWM current duty cycle registers,

(0x02 to 0x35). By reading these registers, the user can keep

track of the current duty cycle on each PWM output. During

fan start up, these registers report back 0x00.

If the FULLSPEED pin is activated, to blast the fans to the

maximum possible PWM ( logic high), the PWM current duty

cycle register is not updated.

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