Delay difference between two half adder and one full adder
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Another common and very useful combinational logic circuit which can be constructed using just a few basic logic gates allowing it to add together two or more binary numbers is the Binary Adder.
A basic Binary Adder circuit can be made from standard AND and Ex-OR gates allowing us to “add” together two single bit binary numbers, A and B.
The addition of these two digits produces an output called the SUM of the addition and a second output called the CARRY or Carry-out, ( COUT ) bit according to the rules for binary addition. One of the main uses for the Binary Adder is in arithmetic and counting circuits. Consider the simple addition of the two denary (base 10) numbers below.
123A(Augend)+ 789 B (Addend)912SUMFrom our maths lessons at school, we learnt that each number column is added together starting from the right hand side and that each digit has a weighted value depending upon its position within the columns.
When each column is added together a carry is generated if the result is greater or equal to 10, the base number. This carry is then added to the result of the addition of the next column to the left and so on, simple school math’s addition, add the numbers and carry.
The adding of binary numbers is exactly the same idea as that for adding together decimal numbers but this time a carry is only generated when the result in any column is greater or equal to “2”, the base number of binary. In other words 1 + 1 creates a carry.
Binary AdditionBinary Addition follows these same basic rules as for the denary addition above except in binary there are only two digits with the largest digit being “1”. So when adding binary numbers, a carry out is generated when the “SUM” equals or is greater than two (1+1) and this becomes a “CARRY” bit for any subsequent addition being passed over to the next column for addition and so on. Consider the single bit addition below.
Binary Addition of Two Bits0011+ 0+ 1+ 0+ 1011(carry) 1←0When the two single bits, A and B are added together, the addition of “0 + 0”, “0 + 1” and “1 + 0” results in either a “0” or a “1” until you get to the final column of “1 + 1” then the sum is equal to “2”. But the number two does not exists in binary however, 2 in binary is equal to 10, in other words a zero for the sum plus an extra carry bit.
Then the operation of a simple adder requires two data inputs producing two outputs, the Sum (S) of the equation and a Carry (C) bit as shown.