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Small Carry Delay Adder by Input Selection Disclosure Number: IPCOM000014888D
Original Publication Date: 2000-Mar-01
Included in the Prior Art Database: 2003-Jun-20

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Disclosed is a method to minimize carry delay of adder by optimizing adder's component using its input value. This method also makes the adder's hardware much simpler. According to this method, adder circuit constitution is different depend on its input value. The constitution is determined so that its carry line delay is minimized. The basic idea of this method is to cut down adder into 1 bit pieces and replace each bit adder with a optimized 1 bit half adder. Figure 1 shows the optimized 1 bit half adder's components proposed in this method. There are four different types of adders, that is, HA1e, HA1o, HA0e, and HA0o. The distinctions of these four adders are their input value (1 or 0) and their location (even bit or odd bit). These adder's carry line consists of only one NAND gate or one NOR gate. Thus the carry line delay per 1 bit is the average of one stage NAND and one stage NOR gate transfer delay. This makes carry line delay pretty short.