LM78 National Semiconductor, LM78 Datasheet - Page 19

• Manufacturer Part Number: LM78
• Description: Microprocessor System Hardware Monitor
• Manufacturer: National Semiconductor

Divide by 1
Divide by 2
Divide by 4
Divide by 8
Mode Select
Functional Description
Counts are based on 2 pulses per revolution tachometer outputs.
(c) Fan with Tach Pull-Up to +12V and Diode Clamp
RPM
4400
3080
2640
(a) Fan with Tach Pull-Up to +5V
Nominal
RPM
8800
4400
2200
1100
Time per Revolution
13.64 ms
19.48 ms
22.73 ms
Time per Revolution
(Continued)
13.64 ms
27.27 ms
54.54 ms
6.82 ms
FIGURE 9. Alternatives for Fan Inputs
DS012873-14
DS012873-16
Given Speed in Decimal
(d) Fan with Strong Tach Pull-Up or Totem Pole Output
19
Counts for “Divide by 2”
(b) Fan with Tach Pull-Up to +12V, or Totem-Pole
Counts for the
(Default) in Decimal
(maximum counts)
153
153
153
153
153 counts
219 counts
255 counts
Output and Resistor Attenuator
and Diode Clamp
RPM
6160
3080
1540
70%
770
Time per Revolution
Typical RPM
Comments
for 70% RPM
70% RPM
60% RPM
19.48 ms
38.96 ms
77.92 ms
9.74 ms
DS012873-15
DS012873-17
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