In: Electrical Engineering
A bcd adder is used to add 2 bcd number and produces a bcd number as output.It cannot be more than 9. If the sum of 2 bcd no. is less than or equal to 9 or carry is 0 than it is fine but if its sum is greater than 9 or carry is 1 than we must add 6(110) to the output. lets draw a binary table from 1 to 15 (s0,s1,s2,s3) and y as carry.From 1 to 9 carry i.e Y=0 and than from 10 to 15 carry i.e Y=1,
s3 | s2 | s1 | s0 | Y |
0 | 0 | 0 | 0 | 0 |
0 | 0 | 0 | 1 | 0 |
0 | 0 | 1 | 0 | 0 |
0 | 0 | 1 | 1 | 0 |
0 | 1 | 0 | 0 | 0 |
0 | 1 | 0 | 1 | 0 |
0 | 1 | 1 | 0 | 0 |
0 | 1 | 1 | 1 | 0 |
1 | 0 | 0 | 0 | 0 |
1 | 0 | 0 | 1 | 0 |
1 | 0 | 1 | 0 | 1 |
1 | 0 | 1 | 1 | 1 |
1 | 1 | 0 | 0 | 1 |
1 | 1 | 0 | 1 | 1 |
1 | 1 | 1 | 0 | 1 |
1 | 1 | 1 | 1 | 1 |
now the K MAP of this brinary table
s3 s2 s1 s0 | 00 | 01 | 11 | 10 |
---|---|---|---|---|
00 | 0 | 0 | 0 | 0 |
01 | 0 | 0 | 0 | 0 |
11 | 1 | 1 | 1 | 1 |
10 | 0 | 0 | 1 | 1 |
on solving we get Y=s3s2+s3s1, let b3b2b1b1 amd a3a2a1a0 are 2 bcd inputs
The output of combinational circuit is connected to b1b2 inputs of adder-2 and b3=b1+0 b3=b1+0 as they are connected to ground permanently. This makes b3b2b1Bb0= 0110 if Y' = 1.
The sum outputs of adder-1 are applied to a3a2a1a0 of adder-2. The output of combinational circuit is to be used as final output carry and the carry output of adder-2 is to be ignored.