Datasheet

RPMRPMFan _2723
1983
000,400,51
_ =
u
=
,
)_(__
000,400,5
_
DecimalCountTachTotal
f
RPMFan
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.
Table 13. [Register]
HEX
and Setup Instructions
STEP [Register]
HEX
AND SETUP INSTRUCTIONS
(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
The Tach Count Registers 46
HEX
and 47
HEX
count the number of periods of the 90 kHz tachometer clock in the
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
Registers 46
HEX
and 47
HEX
. This can best be shown through an example.
EXAMPLE:
Given: the fan used has a tachometer output with 2 per revolution.
Let:
• Register 46 (LSB) is BF
HEX
= Decimal (11 x 16) + 15 = 191 and
• Register 47 (MSB) is 7
HEX
= Decimal (7 x 256) = 1792.
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)
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