Propagation delay is the time delta between the input and the output.
Design of a full adder #
Carry Propagate Adder (CPA)
- Full adder is an arithmetic logic unit or circuit to add two single digit numbers with Carry-in
- It is still combinational circuit with inputs
AxandBxSxis the sum (1 bit)C-incarry inCxis the carry out
- Adder is also called Ripple Carry Adder (RCA)
- Fundamental circuit for an adder and has longest propagation delay that is proportional to the number of the carry bits
To estimate the propagation delay of an \( n \) -bit CPA:
\[\begin{aligned} \Delta \text{ CPA } (n) = \left[ (n-1) \cdot \Delta FA_C \right] + \Delta FA_S \end{aligned}\]
eg calculating propagation delay #
Design of a full adder #
A |
B |
S |
C |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 |
Lets add
A = 0111
B = 1111
Ai |
Bi |
C-in |
S |
C-out |
|---|---|---|---|---|
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 |