GATE | GATE-CS-2015 (Set 2) | Question 65
A half adder is implemented with XOR and AND gates. A full adder is implemented with two half adders and one OR gate. The propagation delay of an XOR gate is twice that of an AND/OR gate. The propagation delay of an AND/OR gate is 1.2 microseconds. A 4-bit ripple-carry binary adder is implemented by using full adders. The total propagation time of this 4-bit binary adder in microseconds is
(A) 19.2 microseconds
Explanation: A Ripple Carry Adder allows to add two n-bit numbers. It uses half and full adders. Following diagram shows a ripple adder using full adders.
Let us first calculate propagation delay of a single 1 bit full adder. Propagation Delay by n bit full adder is (2n + 2) gate delays. [See this for formula]. Here n = 1, so total delay of a 1 bit full adder is (2 + 2)*1.2 = 4.8 ms Delay of 4 full adders is = 4 * 4.8 = 19.2 ms