Dual RPM-Based PWM Fan Controller with Hardware Thermal Shutdown
Datasheet
6.27
Fan Minimum Drive Registers
Table 6.42 Minimum Fan Drive Registers
B7 B6 B5 B4 B3
ADDR
R/W
REGISTER
B2
B1
B0
DEFAULT
Fan 1
Minimum
Drive
66h
(40%)
48h
88h
R/W
128
64
32
16
8
4
2
1
Fan 2
Minimum
Drive
66h
(40%)
R/W
128
64
32
16
8
4
2
1
The Fan Minimum Drive Register stores the minimum drive setting for each RPM based Fan Speed
Control Algorithm. The RPM based Fan Speed Control Algorithm will not drive the fan at a level lower
than the minimum drive unless the target Fan Speed is set at FFh (see Section 6.30)
During normal operation, if the fan stops for any reason (including low drive), the RPM based Fan
Speed Control Algorithm will attempt to restart the fan. Setting the Fan Minimum Drive Registers to a
setting that will maintain fan operation is a useful way to avoid potential fan oscillations as the control
circuitry attempts to drive it at a level that cannot support fan operation.
The Fan Minimum Drive Register is software locked.
6.28
Valid TACH Count Registers
Table 6.43 Valid TACH Count Registers
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Valid TACH
Count 1
49h
R/W
4096
2048
1024
512
256
128
64
32
F5h
Valid TACH
Count 2
89h
R/W
4096
2048
1024
512
256
128
64
32
F5h
The Valid TACH Count Register stores the maximum TACH Reading Register value to indicate that
the each fan is spinning properly. The value is referenced at the end of the Spin Up Routine to
determine if the fan has started operating and decide if the device needs to retry. See Equation [3] for
translating the count to an RPM. This register is only used when the FSC is active.
If the TACH Reading Register value exceeds the Valid TACH Count Register (indicating that the Fan
RPM is below the threshold set by this count), then a stalled fan is detected. In this condition, the
algorithm will automatically begin its Spin Up Routine.
If a TACH Target setting is set above the Valid TACH Count setting, then that setting will be ignored
and the algorithm will use the current fan drive setting.
The Valid TACH Count Register is software locked.
Revision 1.74 (05-08-08)
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