RPM-Based PWM Fan Controller
Datasheet
5.8
PWM Divide Register
Table 5.9 PWM Divide Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
31h
R/W
Fan Divide
128
64
32
16
8
4
2
1
01h
The PWM Divide registers determine the final frequency of the PWM Fan 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.
5.9
Fan Configuration 1 Register
Table 5.10 Fan Configuration 1 Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Fan
Configuration 1
EN_
ALGO
32h
R/W
RANGE[1:0]
EDGES[1:0]
UPDATE[2:0]
2Bh
The Fan Configuration 1 register controls the general operation of the RPM-based Fan Speed Control
Algorithm used for the Fan Driver.
Bit 7 - EN_ALGO - enables the RPM-based Fan Speed Control Algorithm.
‘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 5.11.
Table 5.11 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
TACHx 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
SMSC EMC2301
Revision 1.1 (10-12-09)
DATA2S9HEET
SMSC [ SMSC CORPORATION ]
