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ADT7475
Reading Fan Speed from the ADT7475
The measurement of fan speeds involves a 2-register read
for each measurement. The low byte should be read first.
This causes the high byte to be frozen until both high and low
byte registers have been read, preventing erroneous TACH
readings. The fan tachometer reading registers report back
the number of 11.11 m s period clocks (90 kHz oscillator)
gated to the fan speed counter, from the rising edge of the
first fan TACH pulse to the rising edge of the third fan TACH
pulse (assuming two pulses per revolution are being
counted). Because the device is essentially measuring the
fan TACH period, the higher the count value, the slower the
fan is actually running. A 16-bit fan tachometer reading of
0xFFFF indicates that the fan either has stalled or is running
very slowly (<100 RPM).
High Limit > Comparison Performed
Because the actual fan TACH period is being measured,
falling below a fan TACH limit by 1 sets the appropriate
status bit and can be used to generate an SMBALERT.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Example
TACH1 High Byte (Register 0x29) = 0x17
TACH1 Low Byte (Register 0x28) = 0xFF
What is Fan 1 speed in RPM?
Fan 1 TACH Reading = 0x17FF = 6143 (decimal)
RPM = (f × 60)/Fan 1 TACH Reading
RPM = (90,000 × 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 TACH
Pulses per Revolution register (Register 0x7B) for each fan.
Alternatively, this register can be used to determine the
number or pulses per revolution output by a given fan. By
plotting fan speed measurements at 100% speed with
different pulses per revolution setting, the smoothest graph
with the lowest ripple determines the correct pulses per
revolution value.
Table 33. TACH PULSES/REVOLUTION REGISTER
(REG. 0X7B)
Table 32. FAN TACH LIMIT REGISTERS
Bit
Mnemonic
Description
Register
0x54
0x55
0x56
Description
TACH1 Minimum Low Byte
TACH1 Minimum High Byte
TACH2 Minimum Low Byte
Default
0xFF
0xFF
0xFF
<1:0>
<3:2>
<5:4>
<7:6>
FAN1 Default
FAN2 Default
FAN3 Default
FAN4 Default
2 Pulses per Revolution
2 Pulses per Revolution
2 Pulses per Revolution
2 Pulses per Revolution
0x57
0x58
TACH2 Minimum High Byte
TACH3 Minimum Low Byte
0xFF
0xFF
Table 34. TACH PULSES/REVOLUTION REGISTER
BIT VALUES
0x59
0x5A
0x5B
TACH3 Minimum High Byte
TACH4 Minimum Low Byte
TACH4 Minimum High Byte
0xFF
0xFF
0xFF
Value
00
01
Description
1 Pulse per Revolution
2 Pulses per Revolution
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
10
11
3 Pulses per Revolution
4 Pulses per Revolution
second. The FAST bit (Bit 3) of Configuration Register 3
(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 5.0 V or 12 V, their 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. For optimal results, the
associated dc bit should always be set when using 4-wire
fans.
Calculating Fan Speed
Assuming a fan with a two pulses per revolution (and two
pulses per revolution being measured), fan speed is
calculated by the following:
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
where Fan TACH Reading is the 16-bit fan tachometer
reading.
Fan Spin-up
The ADT7475 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%. The advantage is that fans
have different spin-up characteristics and take different
times to overcome inertia. The ADT7475 runs the fans just
fast enough to overcome inertia and is quieter on spin-up
than fans programmed 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 ADT7475
includes a fan startup timeout function. During this time, the
ADT7475 looks for two TACH pulses. If two TACH pulses
are not detected, an interrupt is generated. Using
Configuration Register 1 (0x40), Bit 5 (FSPDIS), this
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