ADT7467 Analog Devices, Inc., ADT7467 Datasheet - Page 30

• Manufacturer Part Number: ADT7467
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

ADT7467
Alternatively, a resistive attenuator can be used as shown in
Figure 45. R1 and R2 should be chosen such that
The fan inputs have an input resistance of nominally 160 kΩ to
ground, which should be taken into account when calculating
resistor values.
With a pull-up voltage of 12 V and a pull-up resistor of less than
1 kΩ, suitable values for R1 and R2 are 100 kΩ and 47 kΩ,
respectively. This gives a high input voltage of 3.83 V.
FAN SPEED MEASUREMENT
The fan counter does not count the fan TACH output pulses
directly, because the fan speed could be less than 1,000 RPM,
and it would take several seconds to accumulate a reasonably
large and accurate count. Instead, the period of the fan
revolution is measured by gating an on-chip 90 kHz oscillator
into the input of a 16-bit counter for N periods of the fan TACH
output (Figure 46); therefore, the accumulated count is actually
proportional to the fan tachometer period and inversely
proportional to the fan speed.
The number of pulses counted, N, is determined by the settings
of Register 0x7B (TACH pulses per revolution register). This
register contains two bits for each fan, allowing counting of one,
two (default), three, or four TACH pulses.
FAN
5V OR 12V
Figure 44. Fan with Strong TACH Pull-Up to >VCC or Totem-Pole Output,
Figure 45. Fan with Strong TACH Pull-Up to >V
12V
2 V < V
PULL-UP TYP
TOTEM POLE
PULL-UP
<1kΩ OR
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V
<1kΩ
Clamped with Zener and Resistor
× R2/(R
Attenuated with R1/R2
TACH
OUTPUT
TACH
OUTPUT
10kΩ
R1*
R1
PULL-UP
*SEE TEXT
+ R1 + R2) < 5 V
ZD1
ZENER*
R2*
TACH
TACH
CC
or Totem-Pole Output,
FAN SPEED
ADT7467
FAN SPEED
ADT7467
COUNTER
COUNTER
V
V
CC
CC
CC
Rev. A | Page 30 of 80
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7467.
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 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 μs 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).
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 .
CLOCK
TACH
PWM
High limit: > comparison performed
1
2
3
4
Figure 46. Fan Speed Measurement