LPC47M192 datasheet(124/228 Pages) SMSC

124 / 228 page

SMSC DS – LPC47M192

Page 124

Rev. 03/30/05

DATASHEET

Note: If the fan is working the Fan Tachometer register will be latched by the Fan Tachometer

input. If the fan fails, the first time the overflow bit is generated the Fan Tachomter register will

be updated. The Fan Tachometer Registerwill not be updated again until after the Fan

Tachometer input resets the counter.

Preload

Logic to generate

PME event

Programmable

Divider

1, 2, 4, 8

Counter

32kHz

Latch

Fan Tach Register

Fan Tach input pulse

Counter=0xFF

FIGURE 9

− CONCEPTUAL BLOCK DIAGRAM OF FAN MONITORING LOGIC

Fan Tachometer Register

The Fan Tachometer registers are used to store the value of the counter that monitors the speed of the associated

fan. This value may be used by the operating system to monitor how the fan is functioning.

The ACPI BIOS polls this register periodically to determine the speed at which the fan rotor is revolving. See the

Metalious ACPI/Manageability Specification

, v1.0, dated April 30, 1999. The BIOS looks for particular values in this

the count value must correspond to a particular rate of functioning, the Preload register is used to offset any

differences. Therefore, the equation below has been derived to calculate the desired values for Count.

The fan count is determined according to the following equation:

1.966 x 10

Count =

RPM x Divisor

+ Preload

(Equation 1)

(Term 1)

In addition to the ACPI BIOS polling the register, a PME or SMI is generated at a count of 192, if enabled through the

PME or SMI enable register, which corresponds to the “upper limit” for the fan count. This value is made to

correspond to the “lower limit” of the RPM for the fan by programming the divisor and preload value accordingly.

Typical practice is to consider 70% of normal RPM a fan failure.

Fan Preload Register

The preload value is programmable via the FAN1 Preload Register and FAN2 Preload Register. The preload is the

initial value for the fan count, which is used to adjust the count such that the value of 192 corresponds to the “lower

limit” of the RPM. By setting the preload value and divisor properly, a PME or SMI will be generated when the RPM

reaches the percentage of the nominal RPM that indicates a fan failure.

Notice in the examples shown below the Preload value was determined to be 32 for a fan that normally operates at

4400 RPMs. This preload value was determined for the case that a PME should be initiated at 70% of normal

operation. A PME is generated when the counter reaches 192, but at 70% in this example only 160 counts are

generated during one Fan Tachometer input pulse leaving a difference of 32.

Examples

Term 1 in the equation above is ½ of the value determined by multiplying the clock source of 32.768kHz by

60sec/min and dividing by the product of the revolutions per minute times the divisor. The factor of ½ in Term 1

corresponds to two pulses per revolution. The default divisor, located in the Fan Control Register, is two. This results

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