Dual RPM-Based PWM Fan Controller with Hardware Thermal Shutdown
Datasheet
5.5.1
Programming the RPM Based Fan Speed Control Algorithm
The RPM based Fan Speed Control Algorithm is disabled upon device power up. The following
registers control the algorithm. The EMC2104 fan control registers are pre-loaded with defaults that
will work for a wide variety of fans so only the TACH Target Register is required to set a fan speed.
The other fan control registers can be used to fine-tune the algorithm behavior based on application
requirements.
Note that steps 1 - 6 are optional and need only be performed if the default settings do not provide
the desired fan response.
1. Set the Spin Up Configuration Register to the Spin Up Level and Spin Time desired.
2. Set the Fan Step Register to the desired step size.
3. Set the Fan Minimum Drive Register to the minimum drive value that will maintain fan operation.
4. Set the Update Time, and Edges options in the Fan Configuration Register.
5. Set the Valid TACH Count Register to the highest tach count that indicates the fan is spinning.
6. Set the TACH Target Register to the desired tachometer count.
7. Enable the RPM based Fan Speed Control Algorithm by setting the EN_ALGO bit.
5.6
Tachometer Measurement
The tachometer measurement circuitry is used in conjunction with the RPM based Fan Speed Control
Algorithm to update the fan driver output. Additionally, it can be used in Direct Setting mode as a
diagnostic for host based fan control.
This method monitors the TACHx signal in real time. It constantly updates the tachometer
measurement by reporting the number of clocks between a user programmed number of edges on the
TACHx signal (see Table 6.30)
The tachometer measurement provides fast response times for the RPM based Fan Speed Control
Algorithm and the data is presented as a count value that represents the fan RPM period. When this
method is used, all fan target values must be input as a count value for proper operation.
APPLICATION NOTE: The tachometer measurement method works independently of the drive settings. If the
device is put into Direct Setting and the fan drive is set at a level that is lower than the fan
can operate (including zero drive), then the tachometer measurement may signal a Stalled
Fan condition and assert an interrupt.
5.6.1
Stalled Fan
A Stalled fan is detected if the tach counter exceeds the user-programmable Valid TACH Count setting
then it will flag the fan as stalled and trigger an interrupt.
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 Direct Setting with LUT Mode is enabled or whenever the
Spin Up Routine is enabled, the FAN_STALL interrupt will be masked for the duration of the
programmed Spin Up Time (see Table 6.40) to allow the fan an opportunity to reach a valid speed
without generating unnecessary interrupts.
In Direct Setting Mode or Direct Setting w/ LUT Mode, and the tachometer measurement is using
the Tach Period Measurement method, then whenever the TACH Reading Register value exceeds
the Valid TACH Count Register setting, the FAN_STALL status bit will be set.
SMSC EMC2104
Revision 1.74 (05-08-08)
DATA3S1HEET
SMSC [ SMSC CORPORATION ]
