adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 33

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

Calculating Fan Speed
Assuming a fan with two pulses per revolution, and with the
ADT7476A programmed to measure two pulses per revolution,
fan speed is calculated by
where Fan TACH Reading is the 16-bit fan tachometer reading.
Example:
What is Fan 1 speed in RPM?
TACH Pulses per Revolution
Different fan models can output either one, two, three, or four
TACH pulses per revolution. Once the number of fan TACH
pulses has been determined, it can be programmed into the
TACH Pulses per Revolution Register (0x7B) for each fan.
Alternatively, this register can be used to determine the number
of pulses per revolution output by a given fan. By plotting fan
speed measurements at 100% speed with different pulses per
revolution settings, the smoothest graph with the lowest
ripple determines the correct pulses per revolution value.
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.
Fan Spin-Up
The ADT7476A 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 have been detected,
the PWM duty cycle goes to the expected running value, for
example, 33%. Fans have different spin-up characteristics and
take different times to overcome inertia. The advantage of the
ADT7476A is that it runs the fans just fast enough to overcome
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
TACH1 High Byte (0x29) = 0x17
TACH1 Low Byte (0x28) = 0xFF
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90,000 × 60)/6143
Fan Speed = 879 RPM
00 = 1 pulse per revolution.
01 = 2 pulses per revolution.
10 = 3 pulses per revolution.
11 = 4 pulses per revolution.
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inertia and is quieter on spin-up than fans that are programmed
to spin up for a given time.
Fan Startup Timeout
To prevent the generation of false interrupts as a fan spins up
(because it is below running speed), the ADT7476A includes a
fan startup timeout function. During this time, the ADT7476A
looks for two TACH pulses. If two TACH pulses are not
detected, an interrupt is generated.
Fan startup timeout can be disabled by setting Bit 5 (FSPDIS) of
Configuration Register 1 (0x40).
PWM1, PWM2, PWM3 Configuration (0x5C, 0x5D, 0x5E)
[2:0] SPIN, startup timeout for PWM1 = 0x5C, PWM2 = 0x5D,
and PWM3 = 0x5E.
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
(0x40) disables the spin-up for two TACH pulses. Instead, the
fan spins up for the fixed time as selected in Register 0x5C to
Register 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 (0x5C)
[4] INV.
PWM2 Configuration (0x5D)
[4] INV.
0 = Logic high for 100% PWM duty cycle
1 = Logic low for 100% PWM duty cycle
0 = Logic high for 100% PWM duty cycle
1 = Logic low for 100% PWM duty cycle
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
ADT7476A