Binary ripple carry adder:-
To calculate delay find the longest path from the carry in to carry out
Taking 4-bit ripple carry adder:-
$sum=a0\Theta b0\Theta cin$
$carry out=a0*b0+(a0\Theta b0) * cin$
Now you can calculate a0 exor b0= $a0\Theta b0$ and a0*b0 => delay of maximum (Exor gate, AND) as both are working parallely
after getting a0 exor b0 find ( $a0\Theta b0$ ) * cin => delay AND gate delay.
And now perform one OR operation to get carry out
$carry out=a0*b0+(a0\Theta b0) * cin$ =>> delay OR gate
First stage delay => Exor+AND+OR
now in second stage computing a1 and b1 sum,
a1 exor b1 and a1 and b1 is parallely computed as it is independent of previous stage. so delay in second stage is basically :- AND to compute ( a1 exor b1 ) AND previous stage carry + OR to compute cout.
Second stage delay =>> AND+OR
similarly Third stage delay =>> AND+OR
Fourth stage delay ==> AND+OR
Total delay= one EXOR delay + 4(AND+OR)