EMC2305 datasheet(21/55 Pages) SMSC

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Multiple RPM-Based PWM Fan Controller for Five Fans

Datasheet

SMSC EMC2305

Revision 1.1 (10-12-09)

DATASHEET

If the RPM-based Fan Speed Control Algorithm is enabled, the algorithm will automatically attempt to

restart the fan until it detects a valid tachometer level or is disabled.

The FAN_STALL Status bit indicates that a stalled fan was detected. This bit is checked conditionally

depending on the mode of operation.

Whenever the Direct Setting Mode or the Spin Up Routine is enabled, the FAN_STALL interrupt

will be masked for the duration of the programmed Spin Up Time (see Table 5.22) to allow the fan

to reach a valid speed without generating unnecessary interrupts.

In Direct Setting Mode, whenever the TACH Reading Register value exceeds the Valid TACH Count

When using the RPM-based Fan Speed Control Algorithm, the stalled fan condition is checked

whenever the Update Time is met and the fan drive setting is updated. It is not a continuous check.

4.4.2

Aging Fan or Invalid Drive Detection

This is useful to detect aging fan conditions (where the fan’s natural maximum speed degrades over

time) or a speed setting that is faster than the fan is capable of. The EMC2305 contains circuitry that

detects that the programmed fan speed can be reached by the fan. If the target fan speed cannot be

reached within a user defined band of tach counts at maximum drive, the DRIVE_FAIL status bits are

set and the ALERT# pin is asserted.

4.5

CLK Pin

The CLK pin has multiple functionality as determined by the pull-up decode of the ADDR_SEL pin and

the settings of the Configuration register. The functionality associated with the CLK pin upon device

power up is independent of the CLK pin functionality after the device has been configured.

4.5.1

Pull Up Decode

If additional functionality is enabled via the ADDR_SEL pin (see Section 3.1.2), then the CLK pin

should be configured with a pull-up resistor to VDD and should not be used. The value of the pull-up

resistor on the CLK pin is used to determine the default drive state of all fan drivers as shown in

Table 4.2.

4.5.2

External Clock

The EMC2305 allows the user to choose between supplying an external 32.768kHz clock or use of

the internal 32kHz oscillator to measure the tachometer signal. This clock source is used by the RPM-

based Fan Speed Control Algorithm to calculate the current fan speed. This fan controller accuracy is

directly proportional to the accuracy of the clock source.

When this function is used, the external clock is driven into the device via the CLK pin.

Table 4.2 CLK Pin Pull-Up Decode

PULL-UP RESISTOR

FAN DEFAULT DRIVE SETTING

4.7k Ohm ±5%

0% - OFF

6.8k Ohm ±5%

30%

10k Ohm ±5%

50%

15k Ohm ±5%

75%

22k Ohm ±5%

100%

33k Ohm ±5%

0% - OFF

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