RPM-Based Fan Controller with HW Thermal Shutdown
Datasheet
The PWM Divide Register determines the final frequency of the PWM driver. The driver base frequency
is divided by the value of the PWM Divide Register to determine the final frequency. The duty cycle
settings are not affected by these settings, only the final frequency of the PWM driver. A value of 00h
will be decoded as 01h.
The final PWM frequency is derived as the base frequency divided by the value of this register as
shown in Equation [2].
PWM base freqeuncy
PWM Divide Setting
--------------------------------------------------------------
fPWM
=
[2]
6.22
Fan Configuration 1 Register
Table 6.29 Fan Configuration 1 Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Fan
EN_
42h
R/W
RANGE[1:0]
EDGES[1:0]
UPDATE[2:0]
2Bh
Configuration 1 ALGO
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 6.32).
‘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 6.30.
Table 6.30 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 6.31 is used as a direct multiplier
SMSC EMC2103
Revision 0.85 (01-29-08)
DATA5S5HEET
SMSC [ SMSC CORPORATION ]
