SMSC EMC2102 User Manual

Fan Speed Control with the EMC2102 Device

SMSC EMC2102

Revision 0.2 (09-17-07)

35

 

An n-pole fan has n pairs of North-South magnetic poles which are generated by electromagnet coils.
At anytime, only one pair of coils are driven and which coil pair gets driven is determined by a
component called Hall Sensor. The architecture of a typical 2-pole DC fan is shown in 

.

The output of the hall sensor is also the TACH (or Tachometer) signal. When the magnetic field around
the Hall Sensor changes its direction, the sensor’s output level will follow the change to create a square
wave signal as shown in 

.

Assuming 2-pole fan is running at a speed of 6000 RPM, it will rotate 100 revolutions per second. With
2 pulses per revolution, the TACH pulse signal frequency will be 200Hz (

). Since a higher

RPM will yield a higher TACH frequency, or a shorter period between pulses, the TACH signal can be
used by the EMC devices to determine the speed of the fan. Generally speaking, we have:

TACH Pulse Frequency (in HZ) = (RPM / 60) x (# of Pole)

EMC2102 uses a clock (32.768KHz for example) to fill in a window between a programmable number
of Tachometer edges. A counter starts on a specific rising edge and keeps counting until it sees the
‘set’ number of edges, and then saves the counted pulse numbers into register 58h, the TACH Reading
register. 

Hall 

Sensor 

Motor 

Winding

Driver 

Motor 

Winding 

1 

Motor 

Winding 

2 

Protection

Diode 

VDD 

GND 

TACH 

1

N +

S -

+

N +

S -

+

-

-

Hall

Sensor

Fan Tach Signal

1 complete fan revolution 

= 2 pulses (5 edges)