EVAL-ADT7463EB ON Semiconductor, EVAL-ADT7463EB Datasheet - Page 28

• Manufacturer Part Number: EVAL-ADT7463EB
• Manufacturer: ON Semiconductor

Specifications of EVAL-ADT7463EB

• Lead Free Status / RoHS Status: Supplier Unconfirmed

ADT7463
If the fan has a strong pull-up (less than 1 kΩ) to 12 V or a totem-
pole output, then a series resistor can be added to limit the Zener
current, as shown in Figure 36c. Alternatively, a resistive
attenuator may be used, as shown in Figure 36d.
R1 and R2 should be chosen such that
The fan inputs have an input resistance of nominally 160 kΩ
to ground, so this should be taken into account when calculating
resistor values.
With a pull-up voltage of 12 V and pull-up resistor less than 1 kΩ,
suitable values for R1 and R2 would be 100 kΩ and 47 kΩ. 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 may 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
FAN
5V OR 12V
12V
12V
Figure 36b. Fan with TACH Pull-Up to Voltage
> 5 V, e.g., 12 V, Clamped with Zener Diode
Figure 36c. Fan with Strong TACH Pull-Up to
> V
and Resistor
Figure 36d. Fan with Strong TACH Pull-Up to
> V
2
CC
CC
PULL-UP TYP
TOTEM POLE
V
*CHOOSE ZD1 VOLTAGE APPROX 0.8
or Totem-Pole Output, Clamped with Zener
or Totem-Pole Output, Attenuated with R1/R2
<
PULL-UP
V
*CHOOSE ZD1 VOLTAGE APPROX 0.8
<1k
<1k
PULLUP
4.7k
OR
TYP
×
TACH
OUTPUT
TACH
OUTPUT
*SEE TEXT
R
TACH
OUTPUT
2
ZD1*
/
10k
R1
(
R1*
R
PULLUP
TACH
ZD1
ZENER*
R2*
TACH
TACH
+
R
1
V
CC
+
V
R
CC
ADT7463
FAN SPEED
2
COUNTER
ADT7463
FAN SPEED
ADT7463
FAN SPEED
)
COUNTER
COUNTER
V
CC
<
V
V
CC
CC
5
V
–28–
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 37), 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 settings
of Register 0x7B (fan pulses per revolution register). This
register contains two bits for each fan, allowing one, two
(default), three, or four TACH pulses to be counted.
Fan Speed Measurement Registers
The fan tachometer readings are 16-bit values consisting of a
2-byte read from the ADT7463.
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 ADT7463
If fan speeds are being measured, this 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 from. This prevents erroneous TACH
readings.
The fan tachometer reading registers report back 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). Since 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 that the fan has
stalled or is running very slowly (< 100 RPM).
Since the actual fan TACH period is being measured, exceeding
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
Figure 37. Fan Speed Measurement
1
2
3
4
REV. C