Dual RPM-Based PWM Fan Controller with Hardware Thermal Shutdown
Datasheet
When using the RPM based Fan Speed Control Algorithm (either FSC Mode or LUT with FSC
Mode), 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.
5.6.2
32kHz Clock Source
The EMC2104 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.
The external clock is provided on the CLK_IN. In order for the external clock to be used, the EXT_CLK
bit must be set in the Configuration Register.
5.6.3
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 incorrect fan speed settings.The EMC2104 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 then the DRIVE_FAIL status bits are set and the ALERT# pin
is asserted.
5.7
Spin Up Routine
The EMC2104 also contains programmable circuitry to control the spin up behavior of the fan driver
to ensure proper fan operation.
The Spin Up Routine is initiated in Direct Setting mode (with or without the Look Up Table - when
enabled) when the setting value changes from 00h to anything else.
When the Fan Speed Control Algorithm is enabled, the Spin Up Routine is initiated under the following
conditions when the Tach Period Measurement method of tach measurement is used:
1. The TACH Target Register value changes from a value of FFh to a value that is less than the Valid
TACH Count (see Section 6.28).
2. The RPM based Fan Speed Control Algorithm’s measured TACH Reading Register value is greater
than the Valid TACH Count setting.
When the Spin Up Routine is operating, the fan driver is set to full scale (optional) for one quarter of
the total user defined spin up time. For the remaining spin up time, the fan driver output is set a a user
defined level (30% through 65% drive).
After the Spin Up Routine has finished, the EMC2104 measures the TACHx signal. If the measured
TACH Reading Register value is higher than the Valid TACH Count Register setting, the FAN_SPIN
status bit is set and the Spin Up Routine will automatically attempt to restart the fan.
Figure 5.5 shows an example of the Spin Up Routine in response to a programmed fan speed change
based on the first condition above.
Revision 1.74 (05-08-08)
SMSC EMC2104
DATA3S2HEET
SMSC [ SMSC CORPORATION ]
