Rainbow Electronics LM80 - Manual (Page 14)

Functional Description
(Continued)
6.0 FAN INPUTS
Inputs are provided for signals from fans equipped with
tachometer outputs. These are logic-level inputs with an
approximate threshold of V
+ /2. Signal conditioning in the
LM80 accommodates the slow rise and fall times typical of
fan tachometer outputs. The maximum input signal range is
0 to V
+ . In the event these inputs are supplied from fan
outputs which exceed 0 to V+, either resistive division or
diode clamping must be included to keep inputs within an
acceptable range, as shown in
Figure 7. R2 is selected so
that it does not develop excessive error voltage due to input
leakage. R1 is selected based on R2 to provide a minimum
input of 2V and a maximum of V
+ . R1 should be as low as
possible to provide the maximum possible input up to V + for
best 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
current 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 inher-
ently limit this current. If this maximum current could be
exceeded, either a larger pull up resistor should be used or
resistors connected 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
Divisor Register, is set to 2 (choices are 1, 2, 4, and 8)
providing 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.
LM80
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