Explain the working of BCD adder

One of the ways of adding decimal numbers in BCD is to use a 4-bit binary adder and perform arithmetic operation one digit at a time. The low-order pair of the BCD digits is first added to generate a binary sum if result is equal or greater than 1010, it is corrected by adding 0110 to the binary sum. The second operation produces an output- carry for the next pair of important digits. The next higher-order pair of digits, jointly with input-carry, is then added to produce their binary sum. If result is equal to or greater than 1010, it can corrected by adding 0110. The procedure is repeated until all the decimal digits are added.

The logic circuit which detects necessary correction can be derived from table entries. It is clear that a correction is needed when binary sum has an output carry K = 1. The other six combinations from 1010 to 1111 which need a correction have a 1 in position Z . To distinguish them from binary 1000 and 1001 that also have a8 1 in position Z , we specify further that either Z  or Z  must have the1. The condition for8 4 2 correction and an output-carry could be expressed by Boolean function

C = K + Z  Z  + Z  Z8 4 8 2

When C = 1, we require to add 0110 to binary sum and provide an output-carry for next stage.

BCD adder is circuit which adds two BCD digits in parallel and generates sum digit also in BCD. BCD adder must include correction logic in its internal construction