ADT7463ARQZ-R7 datasheet(28/52 Pages) ONSEMI

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REV. C

–28–

ADT7463

FAN SPEED

COUNTER

TACH

OUTPUT

VCC

ZD1*

*CHOOSE ZD1 VOLTAGE APPROX 0.8 � VCC

12V

PULL-UP

4.7k�

TYP

Figure 36b. Fan with TACH Pull-Up to Voltage

> 5 V, e.g., 12 V, Clamped with Zener Diode

If the fan has a strong pull-up (less than 1 k

Ω) 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

() <

PULLUP

The fan inputs have an input resistance of nominally 160 k

Ω

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 k

Ω,

suitable values for R1 and R2 would be 100 k

Ω and 47 kΩ. This

gives a high input voltage of 3.83 V.

5V OR 12V

ADT7463

FAN SPEED

COUNTER

TACH

OUTPUT

PULL-UP TYP

<1k�

TOTEM POLE

VCC

FAN

ZD1

ZENER*

*CHOOSE ZD1 VOLTAGE APPROX 0.8 � VCC

10k�

Figure 36c. Fan with Strong TACH Pull-Up to

> VCC or Totem-Pole Output, Clamped with Zener

and Resistor

12V

ADT7463

FAN SPEED

COUNTER

TACH

OUTPUT

<1k�

VCC

R2*

*SEE TEXT

R1*

Figure 36d. Fan with Strong TACH Pull-Up to

> VCC or Totem-Pole Output, Attenuated with R1/R2

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

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.

CLOCK

PWM

TACH

Figure 37. Fan Speed Measurement

N, the number of pulses counted, is determined by the settings

of Register 0x7B (fan pulses per revolution register). This

(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).

HIGH LIMIT: > COMPARISON PERFORMED

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.

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