ADT7490 datasheet(33/76 Pages) AD

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ADT7490

Rev. 0 | Page 33 of 76

5V OR 12V

VCC

PULL-UP TYP

<1kΩ OR

TOTEM POLE

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7490

ZD1

ZENER*

FAN

*CHOOSE ZD1 VOLTAGE APPROXIMATELY 0.8 × VCC

10kΩ

Figure 45. Fan with Strong TACH Pull-Up to >VCC or Totem-Pole Output,

Clamped with Zener Diode and Resistor

Alternatively, a resistive attenuator can be used, as shown

in Figure 46. R1 and R2 should be chosen such that

2 V < VPULL-UP × R2/(RPULL-UP + R1 + R2) < 3.6 V

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 pull-up resistor less than

1 kΩ, suitable values for R1 and R2 are 100 kΩ and 40 kΩ,

respectively. This gives a high input voltage of 3.42 V.

12V

VCC

<1kΩ

TACH

OUTPUT

FAN SPEED

COUNTER

TACH

ADT7490

40kΩ

100kΩ

Figure 46. Fan with Strong TACH Pull-Up to >VCC or Totem-Pole Output,

Attenuated with R1/R2

The fan counter does not count the fan TACH output pulses

directly because the fan speed could be less than 1000 RPM,

and it takes 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 (see

Figure 47), 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 the TACH pulses per revolution register (0x7B). This register

contains two bits for each fan, allowing one, two (default), three,

or four TACH pulses to be counted.

CLOCK

PWM

TACH

Figure 47. Fan Speed Measurement

Fan Speed Measurement Registers

The fan tachometer registers are 16-bit values consisting of

a 2-byte read from the ADT7490.

Reading Fan Speed from the ADT7490

The measurement of fan speeds 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, preventing 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).

Because the device is essentially measuring the fan TACH

period, the higher the count value, the slower the fan is actu-

ally running. A 16-bit fan tachometer reading of 0xFFFF

indicates that either 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.

Fan TACH Limit Registers

The fan TACH limit registers are 16-bit values consisting of

two bytes.

ADT7490 Datasheet(PDF) 33 Page - Analog Devices

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