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It is a circuit of ring counter. The clock here is negative triggered. but for set and clear it use bubble with bar.What is the need of bubble followed by bar. why it can not use a PRE and CLR (without bar) without bubble. Because both give the same result.

Is there any specific reason for choosing bubble and bar both?

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up vote 10 down vote accepted

You're asking why there is a double negative.

Typically in a circuit diagram, the elements will be real, purchaseable or manufactureable things. Many logic chips have active-low signals, such as CL(ea)R, so they come with that "bubble" baked in, indicating their inverted logic.

So, when a designer needs a flip-flop which has that active-low input, they will often label the net (wire) with what the signal means for the application, which could mean it too has negative, active-low logic.

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  • The "bubble" is part of the symbol and means the input is inverted, notice how the label inside the flip-flop symbol has no "bar" or overline, hence inside the symbol the signal is "normal", SET or CLR.
  • The "bar" or overline is part of the label given to the signal in the circuit diagram, sometimes referred to as net-name when working with a circuit editor.The overline indicates that the signal is inverted. So the signal is not(CLR) and not(SET).
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