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Experiment 5 - The 2-bit Ripple-Carry Adder

However, a ripple-carry adder is slow when it has to add up large numbers. Why...? Consider again the circuit diagram of the 2-bit adder...

Illustration delay of a 2-bit ripple-carry adder.

No logic gate can provide immediate results. Only after a delay, the so-called gate delay time τ, the gate has executed the logic computation.

With the circuit diagram in mind, you can easily make the following statements...

Gate Delay

Information about the gate delay times can be found in the data sheets of the CMOS components. They are often indicated as "Propagation Delay Time".

Typical gate delay times are τ = 60 ns.

  • The output Q0 is delayed by 1 × τ = 60 ns.
  • The output Q1 is delayed by 2 × τ = 120 ns.
  • The Carry-OUT Bit CO is delayed by 3 × τ = 180 ns.

The full outcome of the 2-bit ripple carry adder is delayed by three times the gate delay time.

But what about a n-bit ripple-carry adder...?

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