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ADT7473
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7473.
Reg. 0x28 TACH1 Low Byte = 0x00 default
Reg. 0x29 TACH1 High Byte = 0x00 default
Reg. 0x2A TACH2 Low Byte = 0x00 default
Reg. 0x2B TACH2 High Byte = 0x00 default
Reg. 0x2C TACH3 Low Byte = 0x00 default
Reg. 0x2D TACH3 High Byte = 0x00 default
Reg. 0x2E TACH4 Low Byte = 0x00 default
Reg. 0x2F TACH4 High Byte = 0x00 default
Reading Fan Speed from the ADT7473
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 µ 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 either the fan 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.
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, 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
Fan Speed (RPM) = (90,000 × 60)/Fan TACH Reading
where:
Fan TACH Reading is the 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 × 60)/Fan 1 TACH Reading
RPM = (90000 × 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 number or pulses per
revolution output by a given fan. By plotting fan speed measure-
ments at 100% speed with different pulses per revolution
setting, the smoothest graph with the lowest ripple determines
the correct pulses per revolution value.
Rev. 0 | Page 28 of 76
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