SMSC EMC2102 - Manual (Page 18)

Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL
SMSC EMC2102
18
Note: When a fan starts, the first TACH count captured by the EMC2102 may not reflect the fan’s
speed correctly, since the counting clock (32.768 kHz) could only partially fill the counting
window, which will cause a higher speed reading. Also for some fans, the tachometer may not
work immediately after the fan starts therefore some invalid TACH signals (very short, noisy
pulses) could be sent out at the beginning. In either case, the invalid TACH counts could be
displayed on the ChipMan plots (
Figure 5.13 ), but will never be used to control the fan because
the EMC2102 will not look at the TACH reading until the end of spin-up routine.
5.3.5 Spin-Up Time
The EMC2102 FAN Spin Up Configuration register (53h) contains 2 bits to set the overall spin-up time.
This parameter allows for tailoring of rapid and slow response fans. Repeat the experiment in
Section 5.3.3 with 500 ms SPINUP_TIME. This will provide a quicker response with smaller overshoot,
as shown in Figure 5.14, "Reduced Spin-up Time" .
In general, spin-up time should be tailored for the fan type being used or the effect may be undesirable.
Figure 5.13 60% Spin Level Setting
Figure 5.14 Reduced Spin-up Time
Invalid TACH Count