ADT7467ARQZ-R7 datasheet(30/77 Pages) ONSEMI

30 / 77 page

ADT7467

Rev. 3 | Page 30 of 77 | www.onsemi.com

Clamped with Zener and Resistor

Alternatively, a resistive attenuator can be used, as shown in

Figure 45. R1 and R2 should be chosen such that

2 V < VPULL-UP × R2/(RPULL-UP + R1 + R2) < 5 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 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.

12V

VCC

<1kΩ

TACH

OUTPUT

FAN SPEED

COUNTER

TACHx

ADT7467

R2*

*SEE TEXT.

R1*

Figure 45. 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 could be less than 1000 RPM,

and it would take several seconds to accumulate a reasonably

large and accurate count. Instead, the period of the fan revolu-

tion 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 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

two (default), three, or four TACH pulses.

CLOCK

PWM

TACH

Figure 46. Fan Speed Measurement

Fan Speed Measurement Registers

The fan tachometer readings are 16-bit values consisting of a

2-byte read from the ADT7467.

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).

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.

ADT7467ARQZ-R7 Datasheet(PDF) 30 Page - ON Semiconductor

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