RPM-Based Fan Controller with Multiple Temperature Zones & Hardware Thermal Shutdown
Datasheet
The Fan Configuration 1 Register controls the general operation of the RPM-based Fan Speed Control
Algorithm used on the PWM pin.
Bit 7 - EN_ALGO - enables the RPM-based Fan Speed Control Algorithm. Based on the setting of the
RPM / PWM bit, this bit is automatically set or cleared when the LUT_LOCK bit is set (see Section
7.34, "Look Up Table Configuration Register").
‘0’ - (default) the control circuitry is disabled and the fan driver output is determined by the Fan
Driver Setting Register.
‘1’ - the control circuitry is enabled and the Fan Driver output will be automatically updated to
maintain the programmed fan speed as indicated by the TACH Target Register.
Bits 6- 5 - RANGE[1:0] - Adjusts the range of reported and programmed tachometer reading values.
The RANGE bits determine the weighting of all TACH values (including the Valid TACH Count, TACH
Target, and TACH reading) as shown in Table 7.33, "Range Decode".
Table 7.33 Range Decode
RANGE[1:0]
REPORTED MINIMUM
RPM
TACH COUNT
MULTIPLIER
1
0
0
0
1
1
0
1
0
1
500
1000 (default)
2000
1
2
4
8
4000
Bits 4-3 - EDGES[1:0] - determines the minimum number of edges that must be detected on the TACH
signal to determine a single rotation. A typical fan measured 5 edges (for a 2-pole fan). For more
accurate tachometer measurement, the minimum number of edges measured may be increased.
Increasing the number of edges measured with respect to the number of poles of the fan will cause
the TACH Reading registers to indicate a fan speed that is higher or lower than the actual speed. In
order for the FSC Algorithm to operate correctly, the TACH Target must be updated by the user to
accommodate this shift. The Effective Tach Multiplier shown in Table 7.34, "Minimum Edges for Fan
Rotation" is used as a direct multiplier term that is applied to the Actual RPM to achieve the Reported
RPM. It should only be applied if the number of edges measured does not match the number of edges
expected based on the number of poles of the fan (which is fixed for any given fan).
Contact SMSC for recommended settings when using fans with more or less than 2 poles.
Table 7.34 Minimum Edges for Fan Rotation
EDGES[1:0]
EFFECTIVE TACH
MULTIPLIER (BASED ON 2
POLE FANS)
MINIMUM TACH
EDGES
1
0
NUMBER OF FAN POLES
0
0
1
1
0
1
0
1
3
5
7
9
1 pole
0.5
1
2 poles (default)
3 poles
1.5
2
4 poles
Bit 2-0 - UPDATE - determines the base time between fan driver updates. The Update Time, along
with the Fan Step Register, is used to control the ramp rate of the drive response to provide a cleaner
SMSC EMC2113
Revision 1.2 (10-08-09)
DATA5S9HEET
SMSC [ SMSC CORPORATION ]
