Designing 3-Stage Async Counter & Logic Circuit in PSpice

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Discussion Overview

The discussion revolves around the design of a three-stage asynchronous counter and its associated logic circuit using PSpice. Participants explore the implications of the counter's output states and how to effectively design the logic circuit to achieve specific outputs based on the counter's states.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant notes that the counter appears to reset at input pulse 8, questioning if this is correct.
  • Another participant clarifies that the counter is not resetting but is transitioning to the next state after reaching state 7.
  • A participant proposes using 10 different output states (0 to 9) for the logic circuit, seeking validation for this approach.
  • It is pointed out that with 3 D-flops, only 8 distinct states (0 to 7) can be achieved.
  • A participant expresses uncertainty about how to decode the 3 inputs into 4 outputs for the logic circuit design.
  • Another participant suggests starting with a truth table for the logic box and constructing boolean equations for the outputs, mentioning the potential for simplification using boolean math or Karnaugh maps.

Areas of Agreement / Disagreement

Participants generally agree on the limitation of the counter's states to 8, but there is no consensus on the best approach for designing the logic circuit or decoding the outputs.

Contextual Notes

Participants have not fully resolved how to effectively decode the outputs from the limited states of the counter, and there may be assumptions regarding prior knowledge of boolean algebra and Karnaugh maps that are not explicitly stated.

Who May Find This Useful

This discussion may be useful for students or practitioners interested in digital circuit design, particularly those working with asynchronous counters and logic circuit implementation in simulation software like PSpice.

Jerremy_S
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Homework Statement


The block diagram of FIGURE 3 shows a three-stage asynchrononous counter that is used to count a series of randomly occurring input pulses. The ‘Q’ outputs of the counter are used to drive a logic circuit that gives the output shown in TABLE 1.

(a) Design the counter using type D flip-flops and simulate your design in PSpice, producing waveforms to confirm the circuit’s operation.

(b) Design the logic circuit to realize the desired ABCD outputs and simulate your design in PSpice, again producing waveform to demonstrate the circuit’s operation.

upload_2017-5-15_22-26-14.png


Homework Equations



The Attempt at a Solution



Hi, could you please help.

I managed to run simulation regarding a), but looking closer at Table 1 looks like the counter could be reset at Input Pulse 8 as the state of outputs start repeating itself. Would this be correct?

Thanks
 
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You are correct in your observation. But the counter is not "resetting itself". It is simply going to the next state following state 7 (state 0).
 
Does it mean that using 10 different output states (0 to 9) as an input to the logic circuit would be a good approach?
 
With 3 D-flops, represented by their outputs Q1, Q2, and Q3 you can have only 8 distinct states.
 
Thanks! Just realized that! 2^3=8 states (0 to 7). Not sure how to decode 3 inputs into 4 outputs designing a logic circuit. Any hints?
 
Start by drawing a truth table for your Logic box. From there, you will construct 4 boolean equations, one for output D, one for C, and so on. Those equations can be simplified (if you want to minimize the number of logic gates needed to build) using boolean math or Karnaugh maps. I hope you have already been introduced to this. If not, you might have some self-study to do.
 

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