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Fan Speed Control with the EMC2102 Device
SMSC EMC2102
USER MANUAL Revision 0.2 (09-17-07)
37
Verify the device is in Manual Mode (52h[7] = 0)
6.2.2 Set the Correct LIMIT2K Value
Register 52h (FAN Configuration) bit 6 (LIMIT2K) is a fan dependent parameter as discussed in
Section 5.5.2, "Fan Configuration Register (52h)". When reading the TACH Reading register (58h),
ChipMan uses the default LIMIT2K value (2000 rpm) to convert it to an rpm speed
If the RANGE is set to 500 rpm, the TACH Reading value on the ChipMan display should be
multiplied by 0.5
6.2.3 Determine the Number of Poles and the Maximum Fan Speed
Set the device in Manual Mode (52h[7] = 0)
Start the fan with 100% output drive (register 51h = 255h)
EMC2102 assumes that it is driving a 2-pole fan by default. If the fan speed in register 58h does
not match the fan’s maximum speed specified in its datasheet, then it is not a 2-pole fan. The
EDGES value in register 52h has to be modified, or all speed readings displayed by ChipMan have
to be re-calculated using Equation 1
The value in 58h also can be compared to the fan’s tachometer output frequency obtained by an
oscilloscope to verify its correctness
Record the rpm values in the TACH Reading register (58h) as the maximum fan speed
The maximum fan speed for the EVB-EMC2102 on-board DC fan is about 7000 rpm
6.2.4 Determine the Minimum Startup Speed
Set the device in Manual Mode (52h[7] = 0)
Write a value (for example 80h) to Fan Driver Setting register (51h)
If the fan cannot start, then increase the drive value until the fan start
If the fan starts at the first drive value, then stop the fan (51h = 0) and write a smaller value to
register 51h
Repeat the above steps until the minimum startup speed is determined. Record the drive value in
51h and the fan speed in register 58h
The minimal start speed for the EVB-EMC2102 on-board DC fan is about 4500 rpm with output
drive of 9Eh
6.2.5 Determine the Stall Speed
While the fan is running, reduce the fan drive value in register 51h, step by step, until the fan stops
Write down the drive value in 51h and the fan speed in register 58h before the fan stops
The stall speed for the EVB-EMC2102 on-board DC fan is about 3000 rpm (~70h)
6.2.6 Determine the Minimum Valid TACH Speed
While the fan is running in non-RPM mode, reduce the fan drive value in register 51h, step by step
Monitor the rpm values in the TACH Reading register (58h)
Using an oscilloscope to observe the fan’s Tachometer signal
When invalid, the TACH signal will become erratic with incorrect values (usually much less TACH
counts or very high rpm) in register 58h
The TACH signal may be valid until the fan stalls. If it happens, the fan’s stall speed could be used
as the minimum valid TACH speed