LM81CIMT-31 National Semiconductor, LM81CIMT-31 Datasheet - Page 15

• Manufacturer Part Number: LM81CIMT-31
• Description: Serial Interface ACPI-Compatible Microprocessor System
• Manufacturer: National Semiconductor

Functional Description
The power supply bypass, the parallel combination of 10 µF
(electrolytic or tantalum) and 0.1 µF (ceramic) bypass ca-
pacitors connected between pin 12 and ground, should also
be located as close as possible to the LM81.
6.0 FAN INPUTS
The FAN1 and FAN2 inputs accept signals from fans
equipped with tachometer outputs. These are logic-level in-
puts with an approximate threshold of V
ing in the LM81 accommodates the slow rise and fall times
typical of fan tachometer outputs. The maximum input signal
range is 0 to V
fan outputs which exceed 0 to V
diode clamping must be included to keep inputs within an ac-
ceptable range, as shown in Figure 7 . R2 is selected so that
it does not develop excessive voltage due to input leakage.
R1 is selected based on R2 to provide a minimum input of 2V
and a maximum of V
provide the maximum possible input up to V
immunity. Alternatively, use a shunt reference or zener diode
to clamp the input level.
+
. In the event these inputs are supplied from
+
. R1 should be as low as possible to
+
, either resistive division or
+
(Continued)
/2. Signal condition-
+
for best noise
15
If fans can be powered while the power to the LM81 is off,
the LM81 inputs will provide diode clamping. Limit input cur-
rent to the Input Current at Any Pin specification shown in the
ABSOLUTE MAXIMUM RATINGS section. In most cases,
open collector outputs with pull-up resistors inherently limit
this current. If this maximum current could be exceeded, ei-
ther a larger pull up resistor should be used or resistors con-
nected in series with the fan inputs.
The Fan Inputs gate an internal 22.5 kHz oscillator for one
period of the Fan signal into an 8-bit counter (maximum
count = 255). The default divisor, located in the VID/Fan Di-
visor Register, is set to 2 (choices are 1, 2, 4, and 8) provid-
ing a nominal count of 153 for a 4400 rpm fan with two
pulses per revolution. Typical practice is to consider 70% of
normal RPM a fan failure, at which point the count will be
219.
Determine the fan count according to:
Note that Fan 1 and Fan 2 Divisors are programmable via
the VID/Fan Divisor Register.
Fan tachometer outputs that provide one pulse per revolu-
tion should use a divisor setting twice that of outputs that
provide two pulses per revolution. For example, a 4400 RPM
fan that provides one pulse per revolution should have the
divisor set to 4 such that the nominal counter output is 153.
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