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carry look ahead adder
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CARRY LOOK AHEAD ADDER

 

A CLA (carry look-ahead adder) reduces the time necessary to compute the carry out after the arrival of a valid carry. The CLA does not require the additional information concerning the previous input dig. However, for large columns of bits, extremely large numbers of gates are required. Thus we may divide CLA into groups and have a separate adder in each group of 4 and each group can propagate as a Ripple Carry Adder .shows a four bit CLA structure. The delay and power calculation of various length carry look ahead adders (using standard 90nmCMOS technology) are shown. Let gi=ai.bi denotes the generated carry and let pi= ai xor  bi denote the propagation carry.

 

Ci+1 = ai.bi +bi.ci +ai.ci

       = aibi +ci (ai xor bi)                                                                                            (3.9)

       = gi + ci p                                                                                                                    (3.10)

 

Substituting Ci = gi-1+Ci-1pi-1 (2.3) in the above expression yields

 

Ci+1=gi+gi-1p1+ci-1pi-1p1                                                                                                                         (3.11)

 

 

 

Figure 3.2

A 4-bit Carry look-ahead adder