Discrete transistor D Flip-Flop ring counter doesn't loop.

Thread Starter

ImD1G1TAL

Joined May 31, 2020
2
Hello,

I had designed an SR latch, and then built a D Flip-Flop out of it, which works perfectly fine, later I found out about edge triggering the circuit, and how to do that, but now I have the following problem: It won't loop back to the first one, (see below how it should work)
1 100
2 010
3 001
and then back to line one (which won't work in the actual circuit appearantly)
The loop works just fine in Logisim, but I can't get it to work in Circuit Wizard or NI Multisim.
If it doesn't work in a simulation, I don't think it would work when I make
Can anyone help me fix this problem?

Many Many Many thanks :)
 

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WBahn

Joined Mar 31, 2012
25,760
The problem may be that you are violating the hold-time requirement for your home-brew D flip flop. The real question is whether any of the simulators are using good enough models to simulate this issue accurately. Digital logic simulators are notorious for not modeling these issues well by default. What you normally need to do is to make an analog circuit for your basic gates and run analog simulations to characterize the AC characteristics of the gates and then build decent digital models using the results.
 

dl324

Joined Mar 30, 2015
10,747
It would be helpful if you drew your bistable elements more conventionally and single space the schematic so it wouldn't be so spread out.

1591028036382.png

Your circuit isn't going to work well when you connect an LED like this.

EDIT: Here's the circuit above drawn more conventionally:
clipimage.jpg
 
Last edited:

ci139

Joined Jul 11, 2016
1,463
? what is your led voltage drop ?? how such affects the rest of the circuit ...
... the fast fix could be setting the LED-s as collector load (would inverse the ON/OFF state for) with a parallel reverse diode and a parallel resistor R ≥ 8k2 ... something about http://tinyurl.com/ybg7drwm (you just have a pass gate = only very "narrow" !!! but long enough EN pulses make it work correctly . . . )

(( also :: some simulators don't like "sequential logic" nor analog "internal feedback" ... it's possible to force such simulators to work ... but they tend to be quite unstable or hang at some point of time ))
 

MrAl

Joined Jun 17, 2014
7,592
Hello,

I had designed an SR latch, and then built a D Flip-Flop out of it, which works perfectly fine, later I found out about edge triggering the circuit, and how to do that, but now I have the following problem: It won't loop back to the first one, (see below how it should work)
1 100
2 010
3 001
and then back to line one (which won't work in the actual circuit appearantly)
The loop works just fine in Logisim, but I can't get it to work in Circuit Wizard or NI Multisim.
If it doesn't work in a simulation, I don't think it would work when I make
Can anyone help me fix this problem?

Many Many Many thanks :)
Hi,

The parts are so small they are hard to see.
It looks like you might have LEDs connected to the outputs of each stage?
The LEDs go from the output to ground with no series resistor meaning they clamp the outputs to some maximum value like 1.2 to 3 volts. That may be too low.
Again the parts are hard to see so this is just a guess.
 
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