Datasheet

Data Sheet ADT7470
Rev. E | Page 25 of 40
MANUAL FAN SPEED CONTROL
Manual fan speed control on the ADT7470 allows the user to
control the PWM duty cycle for each fan via the registers. The
ADT7470 powers-up in manual fan control mode, with all
PWM duty cycles set to maximum. The PWM Configuration
registers determine whether the fans are in manual or automatic
fan control mode.
SETTING THE PWM DUTY CYCLE
The ADT7470 allows the duty cycle of any PWM output to be
manually adjusted. This can be useful if users want to change
fan speed in software or want to adjust PWM duty cycle output
for test purposes. The PWM current duty cycle registers
(Register 0x32 to Register 0x35) can be written with 8-bit
values in manual fan speed control mode to manually adjust
the speeds of the cooling fans.
The PWM duty cycle for each output can be set anywhere from
0% to 100%, in steps of 0.39%.
The value to be programmed into the PWM Current Duty
Cycle registers can be calculated as follows:
Value (decimal) = Desired PWM duty cycle/0.39
Example 1: For a PWM Duty Cycle of 50%
Value (decimal) = 50/0.39 = 128 decimal
Value = 128 decimal or 80 hex
Example 2: For a PWM Duty Cycle of 33%
Value (decimal) = 33/0.39 = 85 decimal
Value = 85 decimal or 54 hex
Table 21. PWM Current Duty Cycle Registers
Register Address Description Default
0x32 PWM1 duty cycle 0xFF (100%)
0x33 PWM2 duty cycle 0xFF (100%)
0x34
PWM3 duty cycle
0xFF (100%)
0x35 PWM4 duty cycle 0xFF (100%)
Table 22.Fan Control Mode Configuration
Register/Bit
Mnenonic
Description
0x68 Bit[6] BHVR2 This bit determines fan behavior for PWM2 output.
0 = Manual mode (PWM2 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM2 (automatic fan control mode).
0x68 Bit[7] BHVR1 This bit determines fan behavior for PWM1 output.
0 = Manual mode (PWM1 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM1 (automatic fan control mode).
0x69 Bit[6]
BHVR4
This bit determines fan behavior for PWM4 output.
0 = Manual mode (PWM4 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM4 (automatic fan control mode).
0x69 Bit[7] BHVR3 This bit determines fan behavior for PWM3 output.
0 = Manual mode (PWM3 duty cycle controlled in software).
1 = Fastest speed calculated by all temperatures control PWM3 (automatic fan control mode).