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Help with demultiplexer simultaneous outputs problem?

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Electstud

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Help with simultaneous outputs problem?

Hi people,
I got a silly problem here,

Here is a simplified circuit I created in multisim.
Look at the picture first.
**broken link removed**

First the 4-bit counters gets a clock signal of 0.25Hz.
When the 4-bit counters counts to 1110(15), the output o14 (demultiplexer) gets a pulse.
After 4 seconds, the 4-bit counters counts to 1111(16), and the output o15 (demultiplexer) gets another pulse.

The circuit operation is easy, first the output o14 triggers the D-flipflop once, then 4 seconds after the output o15 triggers the same D-flipflop.

Also binary value 1110(15) triggers once, and 1111(16) triggers once.
1110= output o14 and 1111=output o15


But here is where the problem begins.
Output o15 gets a "high" when o14 is about to go "low"(like there is no propagation delay time between them)
So o14 and o15 either short-circuit or they got counted as "one" pulse, instead of two.

In Multisim there are many ways to solve the problem.
But none worked in reality.

Any ideas folks?
 
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I don't know is this may help, maybe the TTL have an open collector output. Then pullup resistor should be added to those outputs.
 
I don't know is this may help, maybe the TTL have an open collector output. Then pullup resistor should be added to those outputs.

Are you pointing at the 74LS163N? The datasheet doesn't mention open collector output.
The 4514BD is CMOS.
Anyway, when I use output o13 and o15, it works.
But are the o14 and o15 I must use to obtain the right timing.

In multisim I could connect o14 and o15 thru diodes and problem solved.
I tried using two diodes one for o14 and one for o15, it seems like there is not enough voltage or current flowing thru the diodes to trigger the D-FF
 
Clock

If I understand correctly you want to clock the FF with 14 or 15. The other problem you have is the transitions are to close together for the FF to recognize them as 2 pulses. So try something like the attached.
 

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If I follow what your doing, your trying to make a pulse for count 14 and a pulse for count 15. If so, try this configuration. Simulation and schema attached.
 

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  • 1415pulsesch.gif
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Last edited:
If I understand correctly you want to clock the FF with 14 or 15. The other problem you have is the transitions are to close together for the FF to recognize them as 2 pulses. So try something like the attached.


I want to clock the FF with 1110(15)=output o14, then 4 seconds after,clock the FF with 1111(16)=output o15.
But your idea may work, since o15 gets delayed.
Thank u anyway!
 
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If I follow what your doing, your trying to make a pulse for count 14 and a pulse for count 15. If so, try this configuration. Simulation and schema attached.

You almost got it, I want the 4-bit binary counters to count to (1110)15 then give a pulse, and (1111)16 then give a pulse.

Out of demux it would be outputs o14 and o15, since output start at o0.
But can something be done using less IC?
Diodes, resistors and such would be perfect, as they do not count as IC.
At last resort I can replace the demux with another IC, like the quad AND.

Thank in advance!
 
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The cap and resistor is the cheap and dirty method. Mike has the clean implementation. I just ran the simulation at the higher speed so you could see the transistions.
 
The cap and resistor is the cheap and dirty method. Mike has the clean implementation. I just ran the simulation at the higher speed so you could see the transistions.


Ok, I'll try it out tomorrow.
Expect to hear from me:)
 
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