need fast carry look ahead adder

[keremcant]

hi,
I need to implement an 18 bit carry look ahead adder using schematic editor. and my design has to be as fast as possible. what would you suggest me to do? what kind of an effect the number of levels of carry look aheads has on the design? what trick can be done to make the design faster?
thanks in advance

 

[lostinxlation]

One thing you can do is you design 2 identical carry-look ahead adders where the carry-in of each adders is 0 and 1 respectively. The products from each adders are muxed by the carry-in bit. Not much improvement, but it certainly improves. I guess carry-look ahead is carry-look ahead and you can't improve the timing more than it can do, however, using a good circuit technique can improve the timing dramatically... like using dynamic domino circuit..

 

[permute]

if this is for an FPGA, you might find yourself disappointed. The default addition "a + b" will typically end up being faster than CLA. This is due to the ability of the tools to make use of the dedicated routing and specialized logic for addition.

 

[keremcant]

I couldn't understand why multiplexing the sums would improve the circuit. and also as far as I've understood dynamic domin circuit is hardware, it's not about software, am I wrong? also this is not for an FPGA

 

[lostinxlation]

The most critical path in the adder starts at carry-in. If you can bring the carry-in to somewhere else instead of putting it in the bit[0], you can improve the timing a little bit. you can use the same technique to every 4 bits, or every 8 bits and so on..

I don't get what you mean by "dynamic domin circuit is hardware, it's not about software". The adder is hardware, isn't it ? A domino circuit is just like any other transistor level circuit. It just has a bit different concept from static circuit in the operation.

 

[keremcant]

what I meant by hardware/software is that if it is possible for me to change it is Ise Webpack, or not.( I mean I cannot decide to use GaAs mosfets instead of Si, or what ever, since the device is already fixed) but now I understood the trick with the carry-in, thanks for explaining it.