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ADT7475
input but less than 3.6 V, allowing for the voltage tolerance of
the Zener. A value between 3.0 V and 3.6 V is suitable.
12 V V CC
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
PULLUP
4.7 k W
TYP
TACH
OUTPUT
ZD1*
TACH
FAN SPEED
COUNTER
ADT7475
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 (see Figure 39), so the accumulated
count is actually proportional to the fan tachometer period
and inversely proportional to the fan speed.
N, the number of pulses counted, is determined by the
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V CC
Figure 36. Fan with TACH Pullup to Voltage > 3.6 V
(Example, 12 V) Clamped with Zener Diode
If the fan has a strong pullup (less than 1 k W ) to 12 V or
a totem-pole output, then a series resistor can be added to
limit the Zener current, as shown in Figure 37.
5 V OR12 V V CC
FAN
settings of Register 0x7B (TACH Pulses per Revolution
register). This register contains two bits for each fan,
allowing one, two (default), three, or four TACH pulses to
be counted.
Measuring Fan TACH
When the ADT7475 starts up, TACH measurements are
locked. In effect, an internal read of the low byte has been
made for each TACH input. The net result of this is that all
TACH readings are locked until the high byte is read from the
corresponding TACH registers. All TACH related interrupts
PULLUP
TYP < 1 k W
OR TOTEM-POLE
R1
10 k W
TACH
OUTPUT
TACH
ZD1*
ZENER
FAN SPEED
COUNTER
ADT7475
are also ignored until the appropriate high byte is read.
Once the corresponding high byte has been read, TACH
measurements are unlocked and interrupts are processed as
normal.
*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × V CC
Figure 37. Fan with Strong TACH. Pullup to > V CC or
Totem - Pole Output, Clamped with Zener and Resistor
Alternatively, a resistive attenuator can be used, as shown
CLOCK
PWM
in Figure 38. R1 and R2 should be chosen such that:
TACH
1
2 V t V PULLUP R2 R PULLUP ) R1 ) R2 t 3.6 V (eq. 3)
The fan inputs can have an input resistance of 160 k W to
5.1 k W to ground, which should be taken into account when
calculating resistor values.
With a pullup voltage of 12 V and pullup resistor less than
1 k W , suitable values for R1 and R2 would be 100 k W and
33 k W , respectively. This gives a high input voltage of
2.95 V.
2
3
4
Figure 39. Fan Speed Measurement
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting
of a 2-byte read from the ADT7475.
12 V
V CC
Table 31. FAN SPEED MEASUREMENT REGISTERS
Register
Description
Default
< 1 k W
TACH
OUTPUT
R1*
TACH
R2*
FAN SPEED
COUNTER
ADT7475
0x28
0x29
0x2A
0x2B
0x2C
TACH1 Low Byte
TACH1 High Byte
TACH2 Low Byte
TACH2 High Byte
TACH3 Low Byte
0x00
0x00
0x00
0x00
0x00
*SEE TEXT
Figure 38. Fan with Strong TACH. Pullup to > V CC or
Totem-Pole Output, Attenuated with R1/R2
0x2D
0x2E
0x2F
TACH3 High Byte
TACH4 Low Byte
TACH4 High Byte
0x00
0x00
0x00
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