Adder

Question

A one bit adder takes 75ns to produce sum and 50 ns to produce carry A 4 bit parallel adder is designed using this type of full adder .the maximum rate of addition per second can be provided by 4 bit parallel adder is A *10^6 , A ?

Comments

is $A=4.45$?

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4.44??

Answer 1

For one Flip flop

Sum Time = 75ns

Carry Time = 50ns

Consider this four bit parallel adder:-

Consider this Time-Action Table:-

Time (ns)	Action Performed
0	ff0 start working
50	ff0 generate carry for ff1, thus ff1 start working
100	ff1 generate carry for ff2, thus ff2 start working
150	ff2 generate carry for ff3, thus ff3 start working
225	ff3 generate the final result.

Calculation Part:-

From above table we can see that for performing 1 addition opr it require 225ns

1 addition ---- 225ns

1/(225 * 10^-9) -------- 1 second

4.44 * 10 ^6 addition ------- 1 second

So, the value of A is 4.44.

Comments

can it be 3.33 * 10^6 if we consider sum propagation?

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Yes instead of propagating carry if u propagate sum then final result that generated will be at 300ns. So yes it will be 3.33 *10^6.

But why?