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ADT7467
register). This register contains two bits for each fan,
allowing counting of one, two (default), three, or four TACH
pulses.
Fan TACH Limit Registers
The fan TACH limit registers are 16-bit values consisting
of two bytes.
Table 27. FAN TACH LIMIT REGISTERS
CLOCK
PWM
Register
0x54
0x55
0x56
Description
TACH1 Minimum Low Byte
TACH1 Minimum High Byte
TACH2 Minimum Low Byte
Default
0xFF
0xFF
0xFF
TACH
1
2
3
4
Figure 45. Fan Speed Measurement
0x57
0x58
0x59
0x5A
0x5B
TACH2 Minimum High Byte
TACH3 Minimum Low Byte
TACH3 Minimum High Byte
TACH4 Minimum Low Byte
TACH4 Minimum High Byte
0xFF
0xFF
0xFF
0xFF
0xFF
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting
of a 2-byte read from the ADT7467.
Table 26. FAN SPEED MEASUREMENT REGISTERS
Register Description Default
0x28 TACH1 Low Byte 0x00
Fan Speed Measurement Rate
The fan TACH readings are normally updated once every
second.
When set, the FAST bit (Bit 3) of Configuration
Register 3 (0x78) updates the fan TACH readings every
250 ms.
If a fan is powered directly from 5 V or 12 V and is not
driven by a PWM channel, its associated dc bit in
0x29
0x2A
0x2B
0x2C
0x2D
0x2E
0x2F
TACH1 High Byte
TACH2 Low Byte
TACH2 High Byte
TACH3 Low Byte
TACH3 High Byte
TACH4 Low Byte
TACH4 High Byte
0x00
0x00
0x00
0x00
0x00
0x00
0x00
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 two pulses per revolution (and two
pulses per revolution being measured), fan speed is
Reading Fan Speed from the ADT7467
The measurement of fan speeds involves a 2-register read
for each measurement. The low byte should be read first.
This freezes the high byte until both high and low byte
registers are read, preventing erroneous TACH readings.
The fan tachometer reading registers report 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 runs.
A 16-bit fan tachometer reading of 0xFFFF indicates either
that 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.
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 (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 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 fan pulses per revolution register (0x7B) for each
fan. Alternatively, this register can be used to determine the
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