SMBus Fan Control with 1°C Accurate Temperature Monitoring
Datasheet
Table 6.15 CPU Beta Values (continued)
ENABLE
B2
B1
B0
MINIMUM BETA
0.33
0
0
0
0
0
1
0
1
1
1
1
X
1
0
0
1
1
X
1
0
1
0
1
X
0.43
1.00
2.33
Disabled
Automatic detection
6.14
TACH Reading Registers
Table 6.16 TACH Reading Registers
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
TACH Reading
Low Byte
TACH7
(128)
TACH6
(64)
TACH5
(32)
TACH4
(16)
TACH3
(8)
TACH2
(4)
TACH1
(2)
TACH0
(1)
46h
47h
R
R
FFh
TACH Reading
High Byte
TACH15
(32768)
TACH14
(16384)
TACH13
(8192)
TACH12
(4096)
TACH11
(2048)
TACH10
(1024)
TACH9
(512)
TACH8
(256)
FFh
The TACH Registers hold the 16-bit TACH Reading. This reading represents the number of TACH
counts detected. The RPM of the fan can be determined by Equation [3] (see also Appendix B). The
bit weighting of each TACH[15:0] bit is shown in parenthesis after the value. When determining the
final fan speed, the TACH[15:0] bits need to be decoded into an equivalent decimal number.
Where: TACH_COUNT is the
decimal representation of the
TACH[13:0] bits.
5, 400, 000
TACH_COUNT
-------------------------------------------
RPM =
[3]
6.15
TACH Limit Registers
Table 6.17 TACH Reading Low Byte Register
ADDR
R/W
REGISTER
B7
B6
B5
B4
B3
B2
B1
B0
DEFAULT
TACH Limit
Low Byte
TACH_
L7
TACH_
L6
TACH_
L5
TACH_
L4
TACH_
L3
TACH_
L2
TACH_
L1
TACH_
L0
48h
49h
R/W
FFh
FFh
TACH Limit
High Byte
TACH_
L15
TACH_
L14
TACH_
L13
TACH_
L12
TACH_
L11
TACH_
L10
TACH_
L9
TACH_
L8
R/W
The TACH Limit Registers store the maximum TACH count that the fan is expected to operate at.
TACH count is inversely proportional to the actual fan speed. This limit is used to guarantee that the
fan has spun up properly. If the measured TACH is higher than this limit (indicating that the fan speed
is lower than the minimum RPM value), then the TACH bit is set in the Status Register.
Additionally if the measured TACH count exceeds this limit, depending on the status of the
TACH_M[1:0] bits (see Section 6.16), the TACH reading registers may be forced to FFFFh.
SMSC EMC2101
Revision 2.53 (03-13-07)
DATA3S9HEET
SMSC [ SMSC CORPORATION ]
