EMC2302 Standard Microsystems Corp., EMC2302 Datasheet - Page 38

• Manufacturer Part Number: EMC2302
• Description: Rpm-based Pwm Fan Controller
• Manufacturer: Standard Microsystems Corp.

Revision 1.2 (03-22-10)
5.18
ADDR
3Eh
3Fh
4Eh
4Fh
RPM
R/W
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.
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]
[3]
measuring 5 edges, with a frequency of 32.768kHz. These equations are solved and tabulated for ease
of use in
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.
TACH Reading Registers
R
R
R
R
shows the simplified translation of TACH Reading Register count to RPM assuming a 2-pole fan,
=
------------------- -
(
poles
RPM
Fan 1 TACH
Fan 1 TACH
Fan 2 TACH
Fan 2 TACH
AN17.4 RPM to TACH Counts
1
REGISTER
Low Byte
Low Byte
shows the detailed conversion from TACH measurement (COUNT) to RPM while
)
×
=
--------------------------------- -
COUNT
3,932,160 m
------------------------------------- -
(
COUNT
n 1
–
Table 5.28 TACH Reading Registers
4096
4096
)
×
B7
16
16
---- -
m
1
×
×
f
TACH
2048
2048
B6
DATASHEET
8
8
×
60
Conversion.
1024
1024
B5
38
4
4
512
512
B4
2
2
poles = number of poles of the fan
COUNT = TACH Reading Register
measurement frequency (typically
256
256
m = the multiplier defined by the
B3
n = number of edges measured
1
1
(typically 5 for a 2 pole fan)
f
TACH
value (in decimal)
128
128
B2
-
-
Dual RPM-Based PWM Fan Controller
RANGE bits
(typically 2)
32.768kHz)
= the tachometer
where:
B1
64
64
-
-
B0
32
32
-
-
SMSC EMC2302
DEFAULT
Datasheet
FFh
F8h
FFh
F8h
Equation
[2]
[3]