Morris Mano Edition 3 Exercise 5 Question 6 (Page No. 198)

Question

Assume that the EXCLUSIVE-OR gate has a propagation delay of 20ns and that the AND and OR gates have a Propagation delay of 10ns. What is the total Propagation delay time in the four-bit adder of the figure given below?

Answer 1

Total delay will be due to 3 levels as above

• P&G generator
• Carry generator
• Sum Generator

P&G generator has a single level of EX-OR & AND gates they will work in parallel 

delay1 =max(20,10) =20

Carry generator has a two levels  AND + OR  gates they will work one after another 

delay2 =10+10 =20

Sum generator has a single level of EX-OR gates they will work all  in parallel 

delay3 =20

Total delay  =delay1+delay2+delay3

                     20 + 20 +20 = 60ns

Answer 2

20ns+10ns+10ns+20ns=60ns.

Answer 3

If you Know the Structure of Carry LookaHead Genrator Then it Is Easy

Propagation delay = Max( 20,10) + 10 +10 + 20

                                = 20+10 +10+20

                                 = 40