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$