EMC2302 datasheet(38/44 Pages) SMSC

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Dual RPM-Based PWM Fan Controller

Datasheet

Revision 1.1 (10-12-09)

SMSC EMC2302

DATASHEET

The Tach Target is not applied until the high byte is written. Once the high byte is written, the current

value of both high and low bytes will be used as the next Tach target.

5.18

TACH Reading Registers

The TACH Reading Registers’ contents describe the current tachometer reading for each of the fans.

By default, the data represents the fan speed as the number of 32kHz clock periods that occur for a

single revolution of the fan.

Equation [2] shows the detailed conversion from TACH measurement (COUNT) to RPM while Equation

[3] shows the simplified translation of TACH Reading Register count to RPM assuming a 2-pole fan,

measuring 5 edges, with a frequency of 32.768kHz. These equations are solved and tabulated for ease

of use in AN17.4 RPM to TACH Counts Conversion.

Whenever the high byte register is read, the corresponding low byte data will be loaded to internal

shadow registers so that when the low byte is read, the data will always coincide with the previously

read high byte.

Table 5.28 TACH Reading Registers

ADDR

R/W

DEFAULT

3Eh

Fan 1 TACH

4096

2048

1024

512

256

128

FFh

3Fh

Fan 1 TACH

Low Byte

F8h

4Eh

Fan 2 TACH

4096

2048

1024

512

256

128

FFh

4Fh

Fan 2 TACH

Low Byte

F8h

where:

[2]

poles = number of poles of the fan

(typically 2)

fTACH = the tachometer

measurement frequency (typically

32.768kHz)

n = number of edges measured

(typically 5 for a 2 pole fan)

m = the multiplier defined by the

RANGE bits

[3]

COUNT = TACH Reading Register

value (in decimal)

RPM

poles

()

--------------------

–

()

COUNT

-----

----------------------------------

TACH

RPM

3,932,160

COUNT

--------------------------------------

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