LM80 National Semiconductor, LM80 Datasheet - Page 14

• Manufacturer Part Number: LM80
• Description: Serial Interface ACPI-Compatible Microprocessor System Hardware Monitor
• Manufacturer: National Semiconductor

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Functional Description
6.0 FAN INPUTS
Inputs are provided for signals from fans equipped with ta-
chometer outputs. These are logic-level inputs with an ap-
proximate threshold of V
accommodates the slow rise and fall times typical of fan ta-
chometer outputs. The maximum input signal range is 0 to
V
which exceed 0 to V+, either resistive division or diode
clamping must be included to keep inputs within an accept-
able range, as shown in Figure 7 . R2 is selected so that it
does not develop excessive error voltage due to input leak-
age. R1 is selected based on R2 to provide a minimum input
of 2V and a maximum of V
to provide the maximum possible input up to V
noise immunity. Alternatively, use a shunt reference or zener
diode to clamp the input level.
If fans can be powered while the power to the LM80 is off,
the LM80 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.
+
. In the event these inputs are supplied from fan outputs
+
/2. Signal conditioning in the LM80
+
. R1 should be as low as possible
(Continued)
+
for best
14
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 Fan Divisor/RST_OUT/OS Register.
FAN1 and FAN2 inputs can also be programmed to be level
sensitive digital inputs.
Fans that provide only one pulse per revolution would re-
quire a divisor set twice as high as fans that provide two
pulses, thus maintaining a nominal fan count of 153. There-
fore the divisor should be set to 4 for a fan that provides 1
pulse per revolution with a nominal RPM of 4400.