Datasheet

ADT7470 Data Sheet

Rev. E | Page 24 of 40

Calculating Fan Speed and Tachometer Limits

Assuming that the measured number of tach pulses per rotation

corresponds to the number of pulses counted as set in register

0x43, fan speed is calculated by

Fan Speed (RPM) = (90,000 × 60)/Fan Tach Reading

where Fan Tach Reading is the 16-bit fan tachometer reading.

For example:

Tach 1 High Byte (Reg 0x2B) = 0x17

Tach 1 Low Byte (Reg 0x2A) = 0xFF

What is Fan 1 speed in RPM?

Fan 1 tach reading = 0x17FF = 6143 decimal

RPM = (f × 60)/Fan 1 tach reading

RPM = (90000 × 60)/6143

Fan Speed = 879 RPM

Calculate the tachometer maximum and minimum limits at

1000 RPM and 500 RPM as follows:

1000 RPM

(90,000 × 60)/1000 = 5400 decimal

5400 decimal = 1518 hex

Tach 1 maximum = 1518

500 RPM

(90,000 × 60)/500 = 10800 decimal

10800 decimal = 2A30 hex

Tach 1 minimum = 2A30

Fan Pulses per Revolution

Different fan models can output either 1, 2, 3, or 4 tach pulses

per revolution. The number of tach pulses per rotation for each

fan should be programmed into the fan pulses per revolution

then the fan speed cannot be determined using the equation in

the Calculating Fan Speed section.

Alternatively, if the number of tach pulses per rotation is not

know, this register can be used in determining the number

of pulses/revolution output by a given fan. By plotting fan

speed measurements at maximum speed with different pulses/

revolution settings, the smoothest graph with the lowest ripple

determines the correct pulses/revolution value.