ripple carry adder

Question

Half Adder is implemented with XOR and AND gate. A. FA is implemented with 2 HA and 1 OR gate The propagation delay of XOR -gate is twice that of AND / OR gate. Propagation delay of AND/OR is 1.2 ms. A 4-bit ripple carry binary adder is implemented using 4 full adder. the total propagation time of this 4 bit binary adder in micro second is ________?

Answer 1

A half adder can be implemented using an XOR gate and an AND gate. Let's assume that the propagation delay of the XOR gate is 2 × 1.2 ms = 2.4 ms (twice that of the AND/OR gate).

A full adder (FA) is implemented using 2 half adders (HA) and 1 OR gate. Therefore, the propagation delay of a full adder is:

Propagation delay of FA = 2 × Propagation delay of HA + Propagation delay of OR

Propagation delay of FA = 2 × (Propagation delay of XOR + Propagation delay of AND) + Propagation delay of OR

Propagation delay of FA = 2 × (2.4 ms + 1.2 ms) + 1.2 ms = 7.2 ms

A 4-bit ripple carry binary adder is implemented using 4 full adders. The total propagation time of this 4-bit binary adder can be calculated as follows:

Total propagation delay of 4-bit ripple carry binary adder = Propagation delay of 1st full adder + Propagation delay of 2nd full adder + Propagation delay of 3rd full adder + Propagation delay of 4th full adder

Total propagation delay of 4-bit ripple carry binary adder = 4 × Propagation delay of FA

Total propagation delay of 4-bit ripple carry binary adder = 4 × 7.2 ms = 28.8 ms

Therefore, the total propagation time of the 4-bit ripple carry binary adder is 28.8 microseconds (µs).

Comments

Propagation delay of FA is wrong.

Propagation delay of FA = 2 × (2.4 ms + 1.2 ms) + 1.2 ms

                                            = 7.2 ms + 1.2 ms = 8.4 ms

Therefore,

Total Propagation delay = 8.4 ms x 4 = 33.6 ms

Answer 2

First, we have to calculate propagation delay of full adder. 

So the total propagation time of this 4 bit binary adder is (4 x propagation delay of FA) = (4 x 4.8ms) = 19.2ms

Answer is 19.2ms.