Dual RPM-Based PWM Fan Controller
Datasheet
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 Registers
Table 5.10 Fan Configuration 1 Registers
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Fan 1
Configuration 1
EN_
ALGO1
32h
R/W
RANGE1[1:0] EDGES1[1:0]
RANGE2[1:0] EDGES2[1:0]
UPDATE1[2:0]
UPDATE2[2:0]
2Bh
Fan 2
Configuration 1
EN_
ALGO2
42h
R/W
2Bh
The Fan Configuration 1 registers control the general operation of the RPM-based Fan Speed Control
Algorithm used for the respective Fan Driver.
Bit 7 - EN_ALGOx - 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 - RANGEx[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
RANGEX[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 - EDGESx[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
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 5.12 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.
Revision 1.1 (10-12-09)
SMSC EMC2302
DATA3S0HEET
SMSC [ SMSC CORPORATION ]
