Dual RPM-Based PWM Fan Controller
Datasheet
The Tach Target is not applied until the high byte is written. Once the high byte is written, the current
value of both high and low bytes will be used as the next Tach target.
5.18
TACH Reading Registers
Table 5.28 TACH Reading Registers
ADDR
R/W
REGISTER
B7
4096 2048 1024
16
4096 2048 1024
16
B6
B5
B4
B3
B2
B1
B0
DEFAULT
3Eh
R
Fan 1 TACH
512
256
128
64
32
FFh
Fan 1 TACH
Low Byte
3Fh
4Eh
4Fh
R
R
R
8
4
2
512
2
1
256
1
-
128
-
-
64
-
-
32
-
F8h
FFh
F8h
Fan 2 TACH
Fan 2 TACH
Low Byte
8
4
The TACH Reading Registers’ contents describe the current tachometer reading for each of the fans.
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 [2] shows the detailed conversion from TACH measurement (COUNT) to RPM while Equation
[3] 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.
Whenever the high byte register is read, the corresponding low byte data will be loaded to internal
shadow registers so that when the low byte is read, the data will always coincide with the previously
read high byte.
where:
poles = number of poles of the fan
(typically 2)
1
(n – 1)
-------------------- ---------------------------------
fTACH = the tachometer
measurement frequency (typically
32.768kHz)
RPM =
×
× fTACH × 60
[2]
[3]
(poles)
1
----
COUNT ×
m
n = number of edges measured
(typically 5 for a 2 pole fan)
m = the multiplier defined by the
RANGE bits
3,932,160 × m
--------------------------------------
RPM =
COUNT
COUNT = TACH Reading Register
value (in decimal)
Revision 1.1 (10-12-09)
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