Block Diagram of BCD adder

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 diagram of the BCD adder is shown below:

Block Diagram of BCD adder

The two BCD numbers, together with input carry, are first added in the top 4-bit binary adder to produce a binary sum. When the output carry is equal to zero nothing (zero) is added to the binary sum. When it is equal to one, binary 0110 is added to the binary sum through the bottom 4-bit binary adder.

The output carry generated from the bottom binary adder can be ignored since it supplies information already available at the output-carry terminal. The logic circuit to detect a sum greater than 9 can be determined by simplifying the boolean expression of the given BCD Adder Truth Table:

Block Diagram of BCD adder

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.