Look-Ahead Carry

The circuit is a lookahead carry generator which is used for avoiding the long propagation delay associated with carry ripple. This circuit is used for a reasonable width of the addends. As the addends get wider the circuit gets increasingly larger and will also operate slower. A2..A0 and B2..B0 are the addends and Cin is the carry input. C0, C1, and C2 are the carries corresponding to bit positions 0, 1, and 2. These will then be applied as carry input to each of the full adders for every bit positions. Each of the adder then don't need to wait for its carry input to ripple as carry output from the preceding adder. C0 is simply equal to Cin, buffering can be employed when necessary, direct wiring is used here for simplicity. Cout is the overall carry output (from the most significant bit) which is also obtained by lookahead for cascading purpose.