EMC2302 datasheet(36/44 Pages) SMSC

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Dual RPM-Based PWM Fan Controller

Datasheet

Revision 1.1 (10-12-09)

SMSC EMC2302

DATASHEET

5.14

Fan Minimum Drive Registers

The Fan Minimum Drive registers store the minimum drive setting for each RPM-based Fan Speed

Control Algorithm. The RPM-based Fan Speed Control Algorithm will not drive the fan at a level lower

than the minimum drive unless the target Fan Speed is set at FFh (see Section 5.17).

During normal operation, if the fan stops for any reason (including low drive), the RPM-based Fan

Speed Control Algorithm will attempt to restart the fan. Setting the Fan Minimum Drive Register to a

setting that will maintain fan operation is a useful way to avoid potential fan oscillations as the control

circuitry attempts to drive it at a level that cannot support fan operation.

The Fan Minimum Drive Registers are software locked.

5.15

Valid TACH Count Registers

The Valid TACH Count registers store the maximum TACH Reading Register value to indicate that

each fan is spinning properly. The value is referenced at the end of the Spin Up Routine to determine

if the fan has started operating and decide if the device needs to retry. See Equation [2] in Section 5.18

for translating the count to an RPM.

If the TACH Reading Register value exceeds the Valid TACH Count Register (indicating that the Fan

RPM is below the threshold set by this count), then a stalled fan is detected. In this condition, the

algorithm will automatically begin its Spin Up Routine.

If a TACH Target setting is set above the Valid TACH Count setting, then that setting will be ignored

and the algorithm will use the current fan drive setting.

The Valid TACH Count registers are software locked.

Table 5.24 Minimum Fan Drive Registers

ADDR

R/W

B7B6B5

B2B1B0

DEFAULT

38h

R/W

Fan 1

Minimum

Drive

128

643216

66h

(40%)

48h

R/W

Fan 2

Minimum

Drive

128

643216

66h

(40%)

Table 5.25 Valid TACH Count Registers

ADDR

R/W

B7B6

B4B3B2

B1B0

DEFAULT

39h

R/W

Valid TACH

Count 1

4096

2048

1024

512

256

128

F5h

49h

R/W

Valid TACH

Count 2

4096

2048

1024

512

256

128

F5h

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