Designing a Carry Lookahead adder in Excel sheet

[beyondlight]

I'm designing a carry lookahead 8bit adder. My problem is that i don't know how to calculate the group generate signal.

I have 4 G signals:

First level:
G1:0
G3:2
G5:4
G7:6

And these have to be added together:

Second level:
G3:0
G7:4

Third level:
G7:0

Two different formulas for computing G:

Gi:0 = Gi +Pi * Gi-1:j eq. (1)

Gi:j = Gi:n+Pi:n * Gn-1:j eq. (2)I know how to use (1) for G1:0 but for G3:2 and further on i don't know how to do.

My book doesn't explain this well at all. (Integrated Cirucit Design, Pearson)

 

[berkeman]

Could you post a diagram of what you are designing? What is the difference between a standard adder and a carry-lookahead adder?

And an adder has two sets of inputs. What is your second set of inputs?

 

[beyondlight]

Could you post a diagram of what you are designing? What is the difference between a standard adder and a carry-lookahead adder?

And an adder has two sets of inputs. What is your second set of inputs?

I have a propagate signal as input and Carry in as input.

http://sv.tinypic.com/view.php?pic=9h1euo&s=8#.VED-NPl_vkg