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3 Complex Gates

Normally, an XOR gate can be formed by using other functions such as from AND and OR gates. However,

we can use a little trick to help us make the XOR and subsequently, the full adder more robust.

If we look at the basic equations for an xor gate, it is:

fzor = AⓇB

With some simple applications of DeMorgan's Theorem, the xor gate can be broken into the following:

fror

(A-B)-(A-B)

= (A+B)-(A + B)

= A-B+A-B

This can be easily be built in CMOS using the techniques we utilized during the first part of the semester.

Figure 1: Sample Floorplan.

A B XOR

0 0

0

0

1

1

1

0

1

1 1 0

Table 1: XOR Truth table.

Fig: 1