EMC2303 Datasheet, PDF (20/51 Pages) SMSC Corporation – Multiple RPM-Based PWM Fan Controller

4.4.2
4.5
4.5.1
4.5.2
Multiple RPM-Based PWM Fan Controller
Datasheet
„ 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
Register setting, the FAN_STALL status bit will be set.
„ 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.
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 EMC2303 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.
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.
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.
Table 4.2 CLK Pin Pull-Up Decode
PULL-UP RESISTOR
4.7k Ohm ±5%
6.8k Ohm ±5%
10k Ohm ±5%
15k Ohm ±5%
22k Ohm ±5%
33k Ohm ±5%
FAN DEFAULT DRIVE SETTING
0% - OFF
30%
50%
75%
100%
0% - OFF
External Clock
The EMC2303 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.
Revision 1.1 (10-12-09)
20
DATASHEET
SMSC EMC2303