SMSC EMC2102 User Manual

Fan Speed Control with the EMC2102 Device

Revision 0.2 (09-17-07)

SMSC EMC2102

28

 

At the end of spin-up routine (see 

), the EMC2102 checks the

TACH Reading register (58h). If the value in this register is greater than the Valid TACH Count (56h),
which means the fan is running at a speed slower than the minimum valid speed, the spin-up routine
will be restarted (

).

Depending on the fan characters and EMC2102 settings, there are several situations at the end of
spin-up routine which will be discussed. 

For ease of understanding, all TACH values will be converted to RPM values in the following
examples.

In this case, the spin-up level rpm (rpm60 or rpm75) is smaller than both valid rpm (function of 56h)
and target rpm (function of 57h). The theoretical plot of this situation is shown in 

.

Since the fan speed at the end of spin-up routine is always below the minimum valid speed (a function
of 56h), the EMC2102 will think the fan is not running, and will try to restart the fan with spin-up routine
over and over.

Using the EVB-EMC2102 to test this situation with rpm60 (~4500 rpm), simply set the Valid TACH
register (56h) with 4800 rpm and start the fan with TACH Target (57h) = 5000 rpm. The repeating spin-
up routines can be observed (

For rpm75 (~5650 rpm), set 56h = 6000 rpm and 57h = 6500 rpm, as shown in 

.

There are mainly two ways to fix this problem. If the spin-up LEVEL in 53h is 60%, then change it to
75%, as shown in