@neel19 Ans given was 14.4units

Dark Mode

161 views

0 votes

**A 4-bit carry lookahead adder adds two 4-bit numbers. The adder is designed without making use of the EX-OR gates. The propagation delay for all gates is given as 2.4 time units. What will be the overall delay of adder if we assume that inputs are made available in both complemented and uncomplemented form and carry network has been implemented using AND, Or gates.**

**can someone explain me this in a deatiled manner as i am not able to find the appropriate solution for it ?**

0

@neel19 in options there was no option like 16.8units but my concern is that if it would have given 14.4 and none of these , then how to make sure that we have to consider the delays of NOT GATE or not as in question it hasn’t mentioned anything ??

0

@shikhar500 i would suggest you to watch digital electronics course of @GO Classes after watching it these questions will just be a cakewalk for you and all yours doubts like when to take not delay and when not will get cleared.

1

0 votes

**Required Knowledge:**

With the general assumption that a single-level gate implementation is always faster than a two-level gate implementation,

i.e., A single level of XOR will be faster than 2 levels of AND-OR

**In 4-bit Ripple Carry Adder, or Binary Parallel Adder**

Propagation delay = $\{4 \times (\text{ 2-level AND-OR })\} + (1 \times \text{ XOR})$

**In 4-bit CLA (Carry Lookahead Adder)**

Propagation Delay = $(2 \times \text{XOR}) + \{1 \times (\text{ 2-level AND-OR }) \}$

**Given:**

> Inputs are available in complemented and uncomplemented form

> **XOR** gates are not available

> Only **AND/OR** gates are available

> All gates have a propagataion delay of $2.4$ units

XOR can be constructed with 2-levels of AND-OR

since, $A \oplus B = AB’ + A’B = level_1\{A\times B’, A’\times B\} + level_2\{AB’ + A’B\}$

**Propagation delay of CLA** $= 2 \times \{level_1 + level_2\} + 1 \times \{level_1 + level_2\} = 3 \times \{level_1 + level_2\}$

$ = 6 \times \text{ANY-GATE} = 6 \times 2.4 = 14.4$