Timing analysis for carry-skip adders?

Timing analysis for carry-skip adders?

Post by Shoushen » Sat, 21 Sep 1996 04:00:00


Does anyone knows how to do timing analysis for carry-skip adders?

I remember there were some discussions on carry-skip adders a while back. But
I don't recall if someone had made some points on timing analysis or not. It
seems to me that timing analysing tools (we are using Synopsis) can not
recgonize this type of state-sensitive delays. This kind of "observatory don't
cares" are plainly ignored. On the other hand, if one want to simulate the
performance, how can one get all the testing vectors that can excersize all the
potential critical paths?

Any enlighting is appreciated!



| /______~___/_ | HE, Shousheng.  OR, still remember the "Old River"? |
| / o_/ // _%_  | TdE, Lund Univ. Tel.|Fax: +46-46-{2229469 | 129948} |


1. Explain Carry-skip Adders???

Can anybody give a concise explanation of exactly why carry-skip adders
are faster than ripple-carry? I'm having a hard time finding material on
the subject.

Carry-skip adders function on the principle that, if the two input bits to
an adder are different, the carry-in can skip directly to the next most
significant adder.

Of course, using this method, addition time is data dependent, but
carry-skip adders still out-perform ripple-carry adders. Why?

It seems to me that there would always be some case where the carry would
be forced to ripple up through the adder and not skip any adders,
resulting if the same performance as a ripple-carry adder for that
particular data.

Can anybody explain this to me?

I am looking into carry-skip adders for an ASIC design, and unfortunately,
there seems to be a limited number of people who understand carry-skip

 - Jeff

P.S. Anybody who wants to "tell" me about other adder architectures -
Don't Bother.
I'm already familiar with them, and wouldn't actually use a ripple-carry
adder in this design. The reason I am posting this is because I want to
understand carry-skip adders in particular so that I can make an informed

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