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Figure 5.13 60% spin level setting, 5 spin-up time, Figure 5.14 reduced spin-up time – SMSC EMC2102 User Manual

Page 18: Spin-up time, Figure 5.13, "60% spin level setting, User manual

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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