EMC2301 Standard Microsystems Corp., EMC2301 Datasheet - Page 36

• Manufacturer Part Number: EMC2301
• Description: Rpm-based Pwm Fan Controller
• Manufacturer: Standard Microsystems Corp.

Revision 1.2 (03-22-10)
5.17
5.18
ADDR
ADDR
3Ch
3Dh
3Eh
3Fh
R/W
R/W
R/W
R/W
The TACH Target Registers hold the target tachometer value that is maintained by the RPM-based
Fan Speed Control Algorithm.
The value in the TACH Target Registers will always reflect the current TACH Target value.
If the algorithm is enabled, setting the TACH Target Register to FFh will disable the fan driver (set the
fan drive setting 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.
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 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 [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 Target Registers
TACH Reading Registers
R
R
shows the simplified translation of TACH Reading Register count to RPM assuming a 2-pole fan,
TACH Target
TACH Target
AN17.4 RPM to TACH Counts
REGISTER
REGISTER
Fan TACH
Fan TACH
High Byte
Low Byte
Low Byte
shows the detailed conversion from TACH measurement (COUNT) to RPM while
Table 5.28 TACH Reading Registers
4096
4096
Table 5.27 TACH Target Registers
B7
B7
16
16
2048
2048
B6
B6
DATASHEET
8
8
Conversion.
1024
1024
B5
B5
36
4
4
512
512
B4
B4
2
2
256
256
B3
B3
1
1
128
128
B2
B2
-
-
RPM-Based PWM Fan Controller
B1
B1
64
64
-
-
B0
B0
32
32
-
-
SMSC EMC2301
DEFAULT
DEFAULT
Datasheet
F8h
FFh
FFh
F8h
Equation