Block Diagram of BCD adder
A BCD adder is a circuit that adds two BCD digits in parallel and produces a sum digit which is also in BCD. It must include the correction logic in its internal construction. A block of BCD adder is shown below figure:
This adder has two 4bit BCD inputs X3 X2 X1 X0, Y3, Y2 Y1 Y0 and a carry input (Cin). It also has a 4 bit sum output (∑4, ∑3, ∑2, ∑1 ∑0) and a carry output (Cout). Here, the sum output is also in BCD form.
A BCD adder circuit must be able to do the following:
1. Add two 4-bit BCD numbers using straight binary addition.
2. If the four-bit sum is equal to or less than 9, the sum is in proper BCD form and no correction is needed.
3. If the four-bit sum is greater than 9 or if a carry is generated from the sum, the sum produces the BCD results. The carrier may be produced due to this addition and it is added to the next decimal position.