ADT7463 Datasheet, PDF (27/53 Pages) Analog Devices – dB COOL Remote Thermal Controller and Voltage Monitor

ADT7463
Fan TACH Limit Registers
The Fan TACH Limit Registers are 16-bit values consisting of
two bytes.
Reg. 0x54 TACH1 Minimum Low Byte = 0xFF default
Reg. 0x55 TACH1 Minimum High Byte = 0xFF default
Reg. 0x56 TACH2 Minimum Low Byte = 0xFF default
Reg. 0x57 TACH2 Minimum High Byte = 0xFF default
Reg. 0x58 TACH3 Minimum Low Byte = 0xFF default
Reg. 0x59 TACH3 Minimum High Byte = 0xFF default
Reg. 0x5A TACH4 Minimum Low Byte = 0xFF default
Reg. 0x5B TACH4 Minimum High Byte = 0xFF default
Fan Speed Measurement Rate
The Fan TACH readings are normally updated once every
second.
The FAST bit (Bit 3) of Configuration Register 3 (Reg. 0x78),
when set, updates the Fan TACH readings every 250 ms.
If any of the fans are not being driven by a PWM channel but
are powered directly from 5 V or 12 V, its associated dc bit in
Configuration Register 3 should be set. This allows TACH
readings to be taken on a continuous basis for fans connected
directly to a dc source.
Calculating Fan Speed
Assuming a fan with a two pulses/revolution (and two pulses/rev
being measured) fan speed is calculated by:
Fan Speed (RPM) = (90,000 ؋ 60)/Fan Tach Reading
where,
Fan Tach Reading = 16-bit Fan Tachometer Reading
Example:
TACH1 High Byte (Reg 0x29) = 0x17
TACH1 Low Byte (Reg 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH reading = 0x17FF = 6143 decimal.
RPM = (f Ï« 60)/Fan 1 TACH reading
RPM = (90000 Ï« 60)/6143
Fan Speed = 879 RPM
Fan Pulses Per Revolution
Different fan models can output either 1, 2, 3, or 4 TACH
pulses per revolution. Once the number of fan TACH pulses
has been determined, it can be programmed into the Fan
Pulses Per Revolution Register (Reg. 0x7B) for each fan.
Alternatively, this register can be used to determine the num-
ber or pulses/revolution output by a given fan. By plotting fan
speed measurements at 100% speed with different pulses/rev
setting, the smoothest graph with the lowest ripple determines
the correct pulses/rev value.
Fan Pulses Per Revolution Register
<1:0> FAN1 default = 2 pulses per rev.
<3:2> FAN2 default = 2 pulses per rev.
<5:4> FAN3 default = 2 pulses per rev.
<7:6> FAN4 default = 2 pulses per rev.
00 = 1 pulse per rev.
01 = 2 pulses per rev.
10 = 3 pulses per rev.
11 = 4 pulses per rev.
2-Wire Fan Speed Measurements
The ADT7463 is capable of measuring the speed of 2-wire fans,
i.e., fans without TACH outputs. To do this, the fan must be
interfaced as shown in the Fan Drive Circuitry section of the
data sheet. In this case, the TACH inputs need to be repro-
grammed as analog inputs, AIN.
CONFIGURATION REGISTER 2 (REG. 0x73)
Bit 3 (AIN4) = 1, Pin 14 is reconfigured to measure the speed
of a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 2 (AIN3) = 1, Pin 9 is reconfigured to measure the speed
of a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 1 (AIN2) = 1, Pin 12 is reconfigured to measure the speed
of a 2-wire fan using an external sensing resistor and coupling
capacitor.
Bit 0 (AIN1) = 1, Pin 11 is reconfigured to measure the speed
of a 2-wire fan using an external sensing resistor and coupling
capacitor.
AIN Switching Threshold
Having configured the TACH inputs as AIN inputs for 2-wire
measurements, you can select the sensing threshold for the
AIN signal.
CONFIGURATION REGISTER 4 (REG. 0x7D)
<3:2> AINL These two bits define the input threshold for
2-wire fan speed measurements.
00 = ؎20 mV
01 = ؎40 mV
10 = ؎80 mV
11 = ؎130 mV
Fan Spin-Up
The ADT7463 has a unique fan spin-up function. It will spin
the fan at 100% PWM duty cycle until two TACH pulses are
detected on the TACH input. Once two pulses have been
detected, the PWM duty cycle will go to the expected running
value, e.g., 33%. The advantage of this is that fans have dif-
ferent spin-up characteristics and will take different times to
overcome inertia. The ADT7463 just runs the fans fast enough
to overcome inertia and will be quieter on spin-up than fans
programmed to spin-up for a given spin-up time.
Fan Start-Up Timeout
To prevent false interrupts being generated as a fan spins up
(since it is below running speed), the ADT7463 includes a Fan
Start-Up Timeout function. This is the time limit allowed for
two TACH pulses to be detected on spin-up. For example, if 2
seconds Fan Start-Up Timeout is chosen and no TACH pulses
occur within 2 seconds of the start of spin-up, a fan fault is
detected and flagged in the Interrupt Status Registers.
REV. 0
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