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SMSC EMC2102 Figure 6.1 A Typical 2-poleDC Fan, Figure 6.2 Output Signal of a 2-poleFan

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Fan Speed Control with the EMC2102 Device

6 Appendix

6.1DC Fan Basics - Poles, Tach Meter Pulses and Edges

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 Figure 6.1, "A Typical 2-pole DC Fan".

Protection

Diode

		Hall	VDD
		Hall		Motor	Motor
	N +	Sensor		Motor	Motor
	N +			Winding	Winding
	-			Winding	Winding
	-		TACH	1	2
			TACH
1 S - +		+ S -
	-		Hall	Motor
			Hall	Winding
			Sensor	Winding
	N +		Sensor	Driver
				Driver

			GND

Figure 6.1 A Typical 2-pole DC Fan

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 Figure 6.2, "Output Signal of a 2-pole Fan".

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 (Figure 6.2). 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)

1 complete fan revolution

= 2 pulses (5 edges)

Fan Tach Signal

Figure 6.2 Output Signal of a 2-pole Fan

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.

SMSC EMC2102

USER MANUAL

Revision 0.2 (09-17-07)

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Contents

USER MANUAL Page Page Page Page Page Figure 4.1 EMC2102 Fan Control Evaluation System Figure 4.2 EVB-EMC2102Board 5.1.1ChipMan Installation 5.1.2Setting Up the ChipMan Figure 5.1 ChipMan Message for First Operation Note: Figure 5.2 ChipMan Device Selection Window Figure 5.3 Selecting the Device and Master Controller Type Figure 5.4 ChipMan Operation Window Notes: Figure 5.5 Changing Fan Speed 5.1.3Plotting From ChipMan Figure 5.6 Plotting from the ChipMan Figure 5.7 Plot Windows Figure 5.8 Plot Examples 5.2.1Getting Started 5.2.2Basic RPM Based Fan Control Figure 5.9 Plots for Proper Control Settings 5.2.3Effects of Loading Figure 5.10 Effects of Loading 5.2.4Register Change Summary Table 5.1 Register Change Summary for Experiment REGISTER DEFAULT 5.3.1Fan Control Parameters Figure 5.11 Fan Control Parameters 5.3.2Getting Started 5.3.3Forced Kick Function Figure 5.12 Forced Kick Function 5.3.4Spin-UpLevel Figure 5.13 60% Spin Level Setting 5.3.5Spin-UpTime Figure 5.14 Reduced Spin-upTime 5.3.6Register Change Summary Table 5.2 Register Change Summary for Experiment 5.4.1General Setup 5.4.2Controlling the Ramp Rate Figure 5.15 Default Ramp Rate Figure 5.16 Default Step Size with Different UPDATE Settings Figure 5.17 Default UPDATE with Different Step Size Settings Figure 5.18 Exceptionally Slow Rate 5.4.3Register Change Summary Table 5.3 Register Change Summary for Experiment 5.5.1General Setup 5.5.2Fan Configuration Register (52h) Figure 5.19 Update Time Modifications 5.5.3FAN Minimum Drive Register (55h) and Valid TACH Count (56h) Figure 5.20 Minimum Drive Setting Figure 5.21 Valid Tach Count Setting 5.5.4Register Change Summary Table 5.4 Register Change Summary for Experiment 5.6.1General Setup 5.6.2Fan Spin and Stall Interrupts Figure 5.22 Fan Spin and Stall 5.6.3Register Change Summary 5.7.1.1Case 1 - Spin-upLvel RPM < Valid RPM < Target RPM Figure 5.24 Theoretical Plot of Case Figure 5.25 Spin-upCase 1 Fix 5.7.1.2Case 2 - Valid RPM << Target RPM < Spin-upLevel RPM 5.7.1.3Case 3 - Valid RPM =< Target RPM << Spin-upLevel RPM Figure 5.30 Spin-upCase Figure 5.31 Spin-upRoutine Restarted During Normal Operation 5.7.1.4Case 4 - 4.Valid RPM < Spin-upLevel RPM (rpm60 or rpm75) < Target RPM 5.7.1.5Case 5 - Correct RPM Relationships with a Slow Response DC Fan Figure 6.1 A Typical 2-poleDC Fan Figure 6.2 Output Signal of a 2-poleFan Figure 6.3 Fan TACH Measurement With EMC2102 [1] 6.2.1General Setup 6.2.2Set the Correct LIMIT2K Value 6.2.3Determine the Number of Poles and the Maximum Fan Speed 6.2.4Determine the Minimum Startup Speed 6.2.5Determine the Stall Speed 6.2.6Determine the Minimum Valid TACH Speed 6.2.8.1Spin-upLEVEL (Register 53h[2]) 6.2.8.2Minimum Fan Speed 6.2.8.3Valid TACH Count Speed (Function of Register 56h) 6.2.8.4Maximum Fan Speed