Analog Devices dBCool ADT7473 Manual page 29

Register 0x57, TACH2 Minimum H
Register 0x58, TACH3 Minimum Low Byte = 0xFF default
Register 0x59, TACH3 Minimum High Byte = 0xFF default
Register 0x5A, TACH4 Minimum Low Byte = 0xFF default
Register 0x5B, T ACH4 Minimum High Byte = 0xFF default
Fan Speed Measur ement Rate
The fan TACH readings are normally updated once every
second.
The FAST bit (Bit 3) of Configuration Regist
set, updates t he fan TACH readings every 250 ms.
If any of the fans are not being driven by a P
are powered directly from 5 V or 12 V, their
Configuration Register 3 should be set. This
readings to be taken on a continuous basis f
directly to a dc source. For optimal results, t
should always be set when using 4-wire fans
Calculating Fan Speed
Assuming a fan has two pulses per revolutio
per revoluti on being measured), fan speed is calculated by
Fan Speed (RPM) = (90,000 × 60)/Fan T
where Fan TACH Reading is the 16-bit fan tachome
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TA CH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90000 × 60)/6143
Fan Speed = 879 RP M
Fan Pulses per Re volution
Diff erent fan models can o utput either one, two, three, or four
TAC H pulses per re volution. Once the number of fan TACH
pulses has b een determined,
puls es per revolution register
Alter natively, this re gister can be used to determine the number
or p ulses per revolu tion output by a given fan. By plotting fan
speed measuremen ts at a 100% speed with different pulses per
revo lution setting, t he smoothest graph with the lowest ripple
determines the correct pulses per revolution value.
igh Byte = 0xFF default
er 3 (0x78), when
WM channel but
associated dc bit in
allows TACH
or fans connected
he associated dc bit
.
n (and two pulses
ACH Reading
ter reading.
it can be programmed into the fan
(Register 0x7B) for each fan.
TACH Pulses per Rev
Bits [1:0] Fan 1 default = 2 pulses per revolution
Bits [3:2] Fan 2 default = 2 pulses per revolution
Bits [5:4] Fan 3 default = 2 pulses per revolution
Bits [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 tim
overcome inertia. The ADT7473 runs the fans just fast
to overcome inertia and is quieter on spin-up than fans
grammed for a gi
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 start-up timeout function. During this time, the ADT7473
looks for two TACH pulses. If
detected, an interrupt is generated. Using Configuration
Register 1 (0x40), Bit 5 (FSPDIS), this functionality can be
changed (see the Disabling Fan Startup Timeout section).
PWM1, PWM2, PWM3 Configura
(Register 0x5C, Register 0x5D, and Re
Bits [2:0] SPIN, start-up timeout for PWM1 = 0x5C,
PW M2 = 0x5D, and PWM3 = 0x5E.
000 = No start-up timeout
001 = 100 ms
010 = 250 ms default
011 = 400 ms
100 = 667 ms
101 = 1 sec
110 = 2 sec
111 = 4 sec
Rev. A | Page 29 of 76
olution Register
ven spin-up time.
two TACH pulses are not
tion Registers
gister 0x5E)
ADT7473
es to
enough
pro-