RPM-Based Fan Controller with HW Thermal Shutdown
Datasheet
6.17
Valid TACH Count Register
Table 6.24 Valid TACH Count Register
ADDRESS
REGISTER
LIMIT2K
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
56h
Valid TACH
Count
‘0’
‘1’
2048 1024 512
512 256 128
256
64
128
32
64
16
32
8
16
4
F5h
The Valid TACH Count Register stores the maximum TACH count to indicate that the 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.
If the TACH count 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 Count 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.
6.18
TACH Target Register
Table 6.25 TACH Reading Registers
ADDRESS
REGISTER
LIMIT2K
B7
B6
B5
B4
B3
B2 B1 B0
DEFAULT
57h
TACH Target
‘0’
‘1’
2048 1024 512
512 256 128
256
64
128
32
64
16
32
8
16
4
FAh
The TACH Target Register holds the target TACH count that is maintained by the RPM based Fan
Control Algorithm.
If the algorithm is enabled, setting the Fan Target to FFh will immediately disable the High Side Fan
Driver. Setting the Fan Target to any other value will cause the algorithm to invoke the Spin Up Routine
after which it will function normally.
6.19
TACH Reading Register
Table 6.26 TACH Reading Register
ADDRESS
REGISTER
LIMIT2K
B7
B6
B5
B4
B3
B2 B1 B0
DEFAULT
58h
Fan TACH
‘0’
‘1’
2048 1024 512
512 256 128
256
64
128
32
64
16
32
8
16
4
FFh
The TACH Reading Register contents describe the current TACH setting of the fan. The data
represents the fan speed as the number of 32.768kHz clock periods that occur for a single revolution
of the fan.
Revision 1.95 (10-19-06)
SMSC EMC2102
DATA4S4HEET
SMSC [ SMSC CORPORATION ]
