RPM

RPM

Fan

_

2723

1983

000

,

400

,

5

1

_

=

u

=

,

)

_(

_

_

000

,

400

,

5

_

Decimal

Count

Tach

Total

f

RPM

Fan

u

=

LM63

SNAS190E – SEPTEMBER 2002 – REVISED MAY 2013

www.ti.com

REQUIRED INITIAL FAN CONTROL REGISTER SEQUENCE

Important! The BIOS must follow the sequence listed in Table 13 to configure the following Fan Registers for the

LM63 before using any of the Fan or Tachometer or PWM registers.

STEP

(1)

[4A] Write bits 0 and 1; 3 and 4. This includes tach settings if used, PWM internal clock select (1.4 kHz or 360 kHz) and PWM

1

Output Polarity.

2

[4B] Write bits 0 through 5 to program the spin-up settings.

3

[4D] Write bits 0 through 4 to set the frequency settings. This works with the PWM internal clock select.

Choose, then write, only one of the following:

4

A. [4F–5F] the Lookup Table, or

B. [4C] the PWM value bits 0 through 5.

5

If Step 4A, Lookup Table, was chosen and written then write [4A] bit 5 = 0.

(1)

All other registers can be written at any time after the above sequence.

COMPUTING RPM OF THE FAN FROM THE TACH COUNT

LM63 for the tachometer input from the fan assuming a 2 pulse per revolution fan tachometer, such as the fans

supplied with the Pentium 4 boxed processors. The RPM of the fan can be computed from the Tach Count

EXAMPLE:

Given: the fan used has a tachometer output with 2 per revolution.

Let:

•

•

The total Tach Count, in decimal, is 191 + 1792 = 1983.

The RPM is computed using the formula

where

•

f = 1 for 2 pulses/rev fan tachometer output;

•

f = 2 for 1 pulse/rev fan tachometer output, and

•

f = 2 / 3 for 3 pulses/rev fan tachometer output

(2)

For our example

(3)

30

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