Question

Find the propagation delays for a 20 bit ripple carry adder Given the following propagation delays Component AND Propagation Delay 9 OR 8 XOR 7 And that each full adder is implemented as A x1 A BlX1 B Cin Sum Cinx1(cin Cin x1 Cout Hint Draw out at least a 4-bit ripple carry adder before trying to answer this question.

Answer 1

Here is the diagram of n-bit ripple carry adder

AN-1 BN-1 AN-2 BN-2 A1 B1 Ао Во CN-1 CN-2 CN-3 C1 Со Cin FAN-1 FAN-2 FA1 FAO SN-1 SN-2 S1 So

20 bit ripple carry adder will be implemented using 20 full adders

Propagation delay of 1 full adder = Propagation delay of AND gate (First level) + Propagation delay of AND gate (Second level) + Propagation delay of OR gate (third level)

Propagation delay of 1 full adder = 9 + 9 + 8 units = 26 units

Propagation delay of 20 full adders = 26*20 = 520 units

Explanation :

Delay at first level = max(Delay of AND gate,Delay of XOR gate)

Delay at second level = max(Delay of AND gate,Delay of XOR gate)

Delay at third level = Delay of OR gate