Analog Devices ADT7473 Manual page 29

Fan Pulses per Revolution Register
<1:0> Fan 1 default = 2 pulses per revolution.
<3:2> Fan 2 default = 2 pulses per revolution.
<5:4> Fan 3 default = 2 pulses per revolution.
<7:6> Fan 4 default = 2 pulses per revolution.
00 = 1 pulse per revolution.
01 = 2 pulses per revolution.
10 = 3 pulses per revolution.
11 = 4 pulses per revolution.
Fan Spin-Up
The ADT7473 has a unique fan spin-up function. It spins
the fan at 100% PWM duty cycle until two TACH pulses are
detected on the TACH input. Once two TACH pulses are
detected, the PWM duty cycle goes to the expected running
value, for example, 33%. The advantage is that fans have
different spin-up characteristics and take different times to
overcome inertia. The ADT7473 runs the fans just fast enough
to overcome inertia and is quieter on spin-up than fans pro-
grammed to spin up for a given spin-up time.
Fan Startup Timeout
To prevent the generation of false interrupts as a fan spins up
(because it is below running speed), the ADT7473 includes a
fan startup timeout function. During this time, the ADT7473
looks for two TACH pulses. If two TACH pulses are not
detected, an interrupt is generated. Using Configuration
Register 4 (0x40) Bit 5 (FSPDIS), this functionality can be
changed (see the Disabling Fan Startup Timeout section).
PWM1, 2, 3 Configuration (Reg. 0x5C, 0x5D, 0x5E)
<2:0> SPIN, startup timeout for PWM1 = 0x5C,
PWM2 = 0x5D and PWM3 = 0x5E.
000 = No startup timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
Disabling Fan Startup Timeout
Although fan startup makes fan spin-ups much quieter than
fixed-time spin-ups, the option exists to use fixed spin-up
times. Setting Bit 5 (FSPDIS) to 1 in Configuration Register 1
(Reg. 0x40) disables the spin-up for two TACH pulses. Instead,
the fan spins up for the fixed time as selected in Reg. 0x5C to
Reg. 0x5E.
PWM Logic State
The PWM outputs can be programmed high for 100% duty
cycle (noninverted) or low for 100% duty cycle (inverted).
PWM1 Configuration (Reg. 0x5C)
<4> INV.
0 = Logic high for 100% PWM duty cycle.
1 = Logic low for 100% PWM duty cycle.
PWM2 Configuration (Reg. 0x5D)
<4> INV .
0 = Logic high for 100% PWM duty cycle.
1 = Logic low for 100% PWM duty cycle.
PWM3 Configuration (Reg. 0x5E)
<4> INV.
0 = Logic high for 100% PWM duty cycle.
1 = Logic low for 100% PWM duty cycle.
Low Frequency Mode PWM Drive Frequency
The PWM drive frequency can be adjusted for the application.
Reg. 0x5F to Reg. 0x61 configure the PWM frequency for
PWM1 to PWM3, respectively. In high frequency mode, the
PWM drive frequency is always 22.5 kHz.
High Frequency Mode PWM Drive
Setting Bit 3 of Register 0x5F enables high frequency mode for
all fans.
PWM1 Frequency Registers (Reg. 0x5F to Reg. 0x61)
<2:0> FREQ
000 = 11.0 Hz
001 = 14.7 Hz
010 = 22.1 Hz
011 = 29.4 Hz
100 = 35.3 Hz (default)
101 = 44.1 Hz
110 = 58.8 Hz
111 = 88.2 Hz
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ADT7473