EMC2305 Datasheet, PDF (47/55 Pages) SMSC Corporation – Multiple RPM-Based PWM Fan Controller for Five Fans

Multiple RPM-Based PWM Fan Controller for Five Fans
Datasheet
Table 5.28 TACH Reading Registers (continued)
ADDR
R/W
REGISTER B7 B6 B5 B4 B3 B2 B1 B0 DEFAULT
4Fh
R
Fan 2 TACH
Low Byte
16
8
4
2
1
-
-
-
F8h
5Eh
R
Fan 3 TACH 4096 2048 1024 512 256 128 64 32
FFh
5Fh
R
Fan 3 TACH
Low Byte
16
8
4
2
1
-
-
-
F8h
6Eh
R
Fan 4 TACH 4096 2048 1024 512 256 128 64 32
FFh
6Fh
R
Fan 4 TACH
Low Byte
16
8
4
2
1
-
-
-
F8h
7Eh
R
Fan 5 TACH 4096 2048 1024 512 256 128 64 32
FFh
7Fh
R
Fan 5 TACH
Low Byte
16
8
4
2
1
-
-
-
F8h
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.
RPM
=
(^---p^----o^--1-l^--e^----s^---)^-
×
---------⁽--ⁿ-----–-----¹---⁾---------
COUNT × -¹---
×
f_T
A
C
H
×
60
m
RPM
=
3_----,_--9_---3_----2_---,_--1_---6_----0_----×_-----m_----
COUNT
where:
poles = number of poles of the fan
(typically 2)
f_TACH = the tachometer
[2]
measurement frequency (typically
32.768kHz)
n = number of edges measured
(typically 5 for a 2 pole fan)
m = the multiplier defined by the
RANGE bits
[3]
COUNT = TACH Reading Register
value (in decimal)
SMSC EMC2305
47
DATASHEET
Revision 1.1 (10-12-09)