RPM-Based Fan Controller with HW Thermal Shutdown
Datasheet
3. Set the Fan Step Register to the desired step size.
4. Set the Fan Minimum Drive Register to the minimum drive value that will maintain fan operation.
5. Set the Update Time, and Edges options in the Fan Configuration Register.
6. Set the valid TACH count setting at the highest count that indicates that the fan is spinning.
7. Set the TACH Target Register to the desired tachometer count.
8. 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 TACH 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 TACH signal
(see Table 8.5)
Using the Tach Period Measurement method 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 Tach Period 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 differently based on which tach method is enabled. If the Tach Period
Measurement measurement method is implemented, and 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.
When the Direct Setting Mode or Direct Setting with LUT Mode are enabled or the Spin Up Routine
is initiated, the FAN_STALL interrupt will be masked for the duration of the programmed Spin Up
Time (see Table 8.16). This is to allow the fan opportunity to reach a valid speed without generating
unnecessary interrupts.
When the Direct Setting Mode or Direct Setting w/ LUT Mode are activated then 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 (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
Aging Fan or Invalid Drive Detection
The EMC2103 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 bit is and the ALERT pin is asserted. This is useful to detect aging
fan conditions (where the fan’s natural maximum speed degrades over time) or incorrect fan speed
settings.
SMSC EMC2103
Revision 0.85 (01-29-08)
DATA2S9HEET
SMSC [ SMSC CORPORATION ]
