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Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL SMSC EMC2102
32
Using the EVB-EMC2102 to test this situation with rpm75 (~5650 rpm), simply set the Valid TACH
register (56h) with 4500 rpm, set the UPDATE (52h[2:0]) = 100ms and start the fan with 4700 rpm, the
spin-up routine starts over ‘and over (Figure 5.30, "Spin-up Case 3").
To fix this problem, change the UPDATE (52h[2:0]) back to 400 ms. It will give the fan more time to
reach the speed of the previous step and reduce the overshoot/undershoot. Another solution is to
decrease the Fan Step settings which will force the controller to take smaller steps to achieve the target
speed.
When the target speed is too close to the valid speed, the problem will not only occur when the fan
starts. It may also cause problems in the normal operation after the spin-up, since the fan speed
reading errors (caused by fan load changes, tachometer truncation errors, circuit noises, etc.) could
drop it below the valid speed and cause a spin-up routine restart. (Figure 5.31, "Spin-up Routine
Restarted During Normal Operation").
Figure 5.30 Spin-up Case 3
Figure 5.31 Spin-up Routine Restarted During Normal Operation
UPDATE = 100 ms
Undershoots cause
the fun runs below
valid speed
UPDATE = 400 ms