SMSC EMC2102 - Manual (Page 38)

Fan Speed Control with the EMC2102 Device
Revision 0.2 (09-17-07)
USER MANUAL
SMSC EMC2102
38
6.2.7 Determine the Spin-up Levels (rpm60 and rpm75)
„ Drive the fan with 60% of the maximum output (register 51h = 99h) and the rpm value in the TACH
Reading register (58h) will be the fan’s rpm60
„ Drive the fan with 75% of the maximum output (register 51h = BFh) and the rpm value in the TACH
Reading register (58h) will be the fan’s rpm75
6.2.8 Using the Tested Parameters
All parameters discussed above will vary from fan to fan and with fan aging, therefore some margins
have to be added when selecting the fan control settings. A minimum of 10% of full fan speed is
recommended for margins.
6.2.8.1 Spin-up LEVEL (Register 53h[2])
This parameter has to be set so that the corresponding fan speed is greater than Minimum Startup
Speed + Margin.
6.2.8.2 Minimum Fan Speed
To avoid stalling, the fan speed (a function of Fan Driver Setting [51h] or a function of TACH Target
[57h]) needs to be greater than Stall Speed + Margin at all times.
6.2.8.3 Valid TACH Count Speed (Function of Register 56h)
The EMC2102 will not respond to any TACH Target (57h) value that has a corresponding speed slower
than Minimum Valid TACH Speed + Margin, unless the value is FFh which will stop the fan. This
ensures that the RPM control algorithm will not drive too low.
6.2.8.4 Maximum Fan Speed
Use the measured maximum fan speed minus margin as the Maximum Fan Speed.