﻿ Ripple carry adder_Circuit Diagram World ﻿
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2017-01-17 17:01
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Sum out S0 and carry out Cout of the Full Adder 1 is valid only after the propagation delay of Full Adder 1. In the same way, Sum out S3 of the Full Adder 4 is valid only after the joint propagation delays of Full Adder 1 to Full Adder 4. In simple words, the final result of the ripple carry adder is valid only after the joint propogation delays of all full adder circuits inside it.

To understand the working of a ripple carry adder completely, you need to have a look at the full adder too. Full adder is a logic circuit that adds two input operand bits plus a Carry in bit and outputs a Carry out bit and a sum bit.. The Sum out ?(Sout) of a full adder is the XOR of input operand bits A, B and the Carry in (Cin) bit.??Truth table and schematic of a 1 bit Full adder is shown below

There is a simple trick to find results of a full adder. Consider the second last row of the truth table, here the operands are 1, 1, 0 ie ?(A, B, Cin).?Add them together ie 1 1 0 = 10 . In binary system, ?the number order is 0, 1, 10, 11……. and so the result of 1 1 0 is 10 just like we get 1 1 0 =2 in decimal system. 2 in the decimal system corresponds to 10 in the binary system.?Swapping?the result “10″ will give S=0 and Cout = 1 and the second last row is justified. This can be applied to any row in the table.

1 bit full adder schematic and truth table

?A Full adder can be made by combining two half adder circuits together(a half adder is a circuit that adds two input bits and outputs a sum bit and a carry bit). ?

Full adder using NAND or NOR logic.

Alternatively the full adder can be made using NAND or NOR logic. These schemes are universally accepted and their circuit diagrams are shown below.