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Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL SMSC EMC2102
36
Note: Although users can set the number of edges (in register 52h) to either 3, 5, 7 or 9, it is strongly
recommended using the default value 5 while driving a 2-pole DC fan. This equal to one
complete fan revolution for a 2-pole fan.
ChipMan translates TACH counts to the RPM value and displays the fan speed. For applications not
using ChipMan software, the following equation can be used to convert the TACH Reading values to
the real RPM speeds.
6.2 Characterizing a DC Fan with EVB-EMC2102 and ChipMan
As a very important component in the closed-loop, the DC fan’s characteristics have a great impact
on the control system’s performance. Using the EVB-EMC2102 and ChipMan software tool, a 5V DC
fan’s characteristics can be easily tested.
In general, the following parameters need to be characterized:
Minimum Startup Speed
Stall Speed
Minimum Valid TACH Speed
Maximum Fan Speed
Spin-up Level (i.e., rpm60 and rpm75. Refer to Section 5.3.1, "Fan Control Parameters" for more
details)
6.2.1 General Setup
Setup the system and make sure the hardware and software are working as discussed in Section
5.1, "Experiment 1 - Manual Fan Control".
Unplug the on-board DC fan from connector P1
Connect the DC fan to be tested to P1 (refer to EVB-EMC2102 User Manual for the DC fan
connector pinouts)
Figure 6.3 Fan TACH Measurement With EMC2102
where:
EDGES = number of edges set by register 52h[4:3]
COUNT = TACH Reading (58h) value [1]
Poles = number of pole pairs in the DC fan
m = factor defined by LIMIT2K (52h[6])
(m = 1 for 500 rpm and m = 4 for 2000 rpm)
Fan Tach Signal
3 EDGES
5 EDGES
7 EDGES
32.768kHz
Clock
n-EDGE WINDOW
T
WINDOW
m
COUN
T
P
oles
EDGES
RPM *
983040
*
1−
=