RPM-Based Fan Controller with Multiple Temperature Zones & Hardware Thermal Shutdown
Datasheet
7.32
TACH Target Register
Table 7.49 TACH Target Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
Fan TACH
Target Low
Byte
4Ch
4Dh
R/W
R/W
16
8
4
2
1
-
-
-
F8h
FFh
TACH Target
4096 2048 1024
512
256
128
64
32
The TACH Target Register holds the target tachometer value that is maintained by the RPM-based
Fan Speed Control Algorithm.
The value in the TACH Target Register will always reflect the current TACH Target value. If the Look
Up Table is active and configured to operate in RPM Mode, then this register will be read only. Writing
to this register will have no affect and the data will not be stored.
If the algorithm is enabled then setting the TACH Target Register to FFh will disable the fan driver (set
the PWM duty cycle to 0%). Setting the TACH Target to any other value (from a setting of FFh) will
cause the algorithm to invoke the Spin Up Routine after which it will function normally.
7.33
TACH Reading Register
Table 7.50 TACH Reading Register
ADDR
R/W
REGISTER
B7
4096 2048 1024
16
B6
B5
B4
B3
B2
B1
B0
DEFAULT
4Eh
R
Fan TACH
512
256
128
64
32
FFh
Fan TACH
Low Byte
4Fh
R
8
4
2
1
-
-
-
F8h
The TACH Reading Register contents describe the current tachometer reading for the fan. By default,
the data represents the fan speed as the number of 32kHz clock periods that occur for a single
revolution of the fan.
Equation [4] shows the detailed conversion from TACH measurement (COUNT) to RPM while Equation
[5] shows the simplified translation of TACH Reading Register count to RPM assuming a 2-pole fan,
measuring 5 edges, with a frequency of 32.768kHz.
These equations are solved and tabulated for ease of use in AN17.4 RPM to TACH Counts
Conversion.
Revision 1.2 (10-08-09)
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