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Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL SMSC EMC2102
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5.5.3 FAN Minimum Drive Register (55h) and Valid TACH Count (56h)
These two registers assist the user in defining the operational environment for a given fan. The
Minimum Drive register is an absolute minimum value the RPM controller may drive to in an attempt
to achieve low RPM settings. The Valid Tach Count register is used to compare against the Tach Target
register. No value less than the Valid Tach Count will be accepted by the controller. To examine this
feature, set the minimum drive to C0h, and command an RPM setting of less than 5500 RPM. The fan
will start at about 5600 rpm (51h = C0h). Set the target to 6000 rpm, and the fan speed follows. Drop
the fan target speed from 6000 rpm to 4500 rpm, the real fan speed will only drop to about 5560 rpm,
as shown in Figure 5.20, "Minimum Drive Setting".
Likewise, the Valid TACH Count register operation is simple to show. Start the fan at 6000 RPM with
all default setting, and then set the Valid TACH register to 5000 RPM. Enter a command of 4800 RPM,
and the fan will not respond as shown in Figure 5.21, "Valid Tach Count Setting".
Figure 5.20 Minimum Drive Setting
Figure 5.21 Valid Tach Count Setting
Fan will not run below
Minimum Drtive
Fan starts at
Minimum Drive
Target of 4800 rpm