Saturday, January 5, 2019

Carry look ahead adder | Fast carry adder logic circuit

Look Ahead Carry adder 

Fast Carry Adder 

In This Post, I will tell you about fast carry adder or look ahead carry adder. 
Fast Carry Adder 
Fast carry adder is a modulated form of n bit parallel adder. In this type of adder consume low summation time as compared to normal n bit parallel adder. 
Why n bit Parallel adder consume more time? 
Firstly, 
              we choose 4 - Bit Parallel Adder.  this Adder has is made with the help of 4 Full Adder respectively FA1 (Full Adder 1), FA2 (Full Adder 2), FA3 (Full Adder 3 ), FA4 (Full Adder 4). We add Two Binary Numbers . in this addition have to carry C1 are generated in FA 1  after some time.  this consume time is called propagation delay of FA1 ( This Delay depends on the speed of transistor). This carry goes on FA2. this carry changes the result of FA 2. After some time carry C2 generated. this consume time is called the propagation delay of FA2. this Consume time of carrying C2 is executed is equal to the sum of Propagation Delay of FA1 and Propagation delay of FA2. Similarly, Carry C3 is generated after the sum of the propagation delay of FA1, FA2, and FA3. And Carry C4 is generated after the sum of the propagation delay of FA1, FA2, FA3, and FA4. So Carry C4 has Generated After a Long Time.

Why Look Ahead Carry Adder is Fast?
This type of adder, each Carry are generated after the sum of the propagation delay of destination FA  and propagation delay of FA1.  So look ahead carry adder is fast . because its carry is generated only after two FA propagation delay.
IC Of Fast Carry Adder
Sn54 / 74LS83A 
Input First Number      =  A4  A3  A2  A1  , Direction - MSB --------> LSB
Input Second Number =  B4  B3  B2  B1  , Direction - MSB --------> LSB
Sum Output                 =  S4  S3  S2  S1  , Direction - MSB -------->LSB
Other Ic Carry Input     = C0
Over flow , 5 th bit carry output = C5
VCC , Ground = Power Supply 
Logic Circuit of Fast Carry Adder 
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