emc2106 Standard Microsystems Corp., emc2106 Datasheet - Page 79

• Manufacturer Part Number: emc2106
• Description: Emc2106 Dual Rpm-based Linear Fan Controller With Hardware Thermal Shutdown
• Manufacturer: Standard Microsystems Corp.

Dual RPM-Based Linear Fan Controller with Hardware Thermal Shutdown
Datasheet
SMSC EMC2106
6.34
ADDR
4Eh
4Fh
8Eh
8Fh
RPM
R/W
The TACH Target Registers hold the target tachometer value that is maintained each of the RPM based
Fan Speed Control Algorithms.
The value in the TACH Target Registers 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 one of the algorithms is enabled then 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. 3
The TACH Reading Registers’ contents describe the current tachometer reading for each of 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 [5]
[6]
measuring 5 edges, with a frequency of 32.768kHz.
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
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 6.49 TACH Reading Registers
4096
4096
)
×
B7
16
16
---- -
m
1
×
×
f
TACH
2048
2048
B6
DATASHEET
8
8
×
60
1024
1024
B5
79
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
-
-
RANGe bits
(typically 2)
32.768kHz)
= the tachometer
where:
B1
64
64
-
-
Revision 1.74 (05-08-08)
B0
32
32
-
-
DEFAULT
FFh
F8h
FFh
F8h
Equation
[5]
[6]