EMC2302 datasheet(30/44 Pages) SMSC

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Dual RPM-Based PWM Fan Controller

Datasheet

Revision 1.1 (10-12-09)

SMSC EMC2302

DATASHEET

frequency. The duty cycle settings are not affected by these settings, only the final frequency of the

PWM driver. A value of 00h will be decoded as 01h.

5.9

Fan Configuration 1 Registers

The Fan Configuration 1 registers control the general operation of the RPM-based Fan Speed Control

Algorithm used for the respective Fan Driver.

Bit 7 - EN_ALGOx - enables the RPM-based Fan Speed Control Algorithm.

‘0’ - (default) the control circuitry is disabled and the fan driver output is determined by the Fan

Driver Setting Register.

‘1’ - the control circuitry is enabled and the Fan Driver output will be automatically updated to

maintain the programmed fan speed as indicated by the TACH Target Register.

Bits 6- 5 - RANGEx[1:0] - Adjusts the range of reported and programmed tachometer reading values.

The RANGE bits determine the weighting of all TACH values (including the Valid TACH Count, TACH

Target, and TACH reading) as shown in Table 5.11.

Bits 4-3 - EDGESx[1:0] - determines the minimum number of edges that must be detected on the

TACHx signal to determine a single rotation. A typical fan measured 5 edges (for a 2-pole fan). For

more accurate tachometer measurement, the minimum number of edges measured may be increased.

Increasing the number of edges measured with respect to the number of poles of the fan will cause

the TACH Reading registers to indicate a fan speed that is higher or lower than the actual speed. In

order for the FSC Algorithm to operate correctly, the TACH Target must be updated by the user to

accommodate this shift. The Effective Tach Multiplier shown in Table 5.12 is used as a direct multiplier

term that is applied to the Actual RPM to achieve the Reported RPM. It should only be applied if the

number of edges measured does not match the number of edges expected based on the number of

poles of the fan (which is fixed for any given fan).

Contact SMSC for recommended settings when using fans with more or less than 2 poles.

Table 5.10 Fan Configuration 1 Registers

ADDR

R/W

DEFAULT

32h

R/W

Fan 1

Configuration 1

EN_

ALGO1

RANGE1[1:0]

EDGES1[1:0]

UPDATE1[2:0]

2Bh

42h

R/W

Fan 2

Configuration 1

EN_

ALGO2

RANGE2[1:0]

EDGES2[1:0]

UPDATE2[2:0]

2Bh

Table 5.11 Range Decode

RANGEX[1:0]

REPORTED MINIMUM

RPM

TACH COUNT

MULTIPLIER

500

1000 (default)

2000

4000

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