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PASS TRANSISTOR Realization of a General Boolean Function

IP.com Disclosure Number: IPCOM000046116D
Original Publication Date: 1983-Jun-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 42K

Publishing Venue

IBM

Related People

Tsai, MY: AUTHOR

Abstract

A general design method for implementing an arbitrary Boolean logic function utilizing MOS pass transistors is described. Both static and dynamic pass transistor logic design methods are treated.

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PASS TRANSISTOR Realization of a General Boolean Function

A general design method for implementing an arbitrary Boolean logic function utilizing MOS pass transistors is described. Both static and dynamic pass transistor logic design methods are treated.

In VLSI design, it is generally realized that a true custom design has better performance and higher density. In certain chips, high performance and functionality are important; the advantage of a custom design would outweigh the cost related to its larger design effort. Pass transistor circuits have been used in a true custom fashion for its low power, high density and high performance [*]. Up to now, however, no general scheme to realize pass transistor circuits has been proposed.

In MOS circuitry, a logic "Hi" or "Lo" is generally represented by voltage levels at various nodes. The parasitic capacitance associated with the pass transistor circuits may induce unanticipated memory elements in a combinational circuit. An incorrect design of a pass

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wrong signal flow have to be eliminated in pass transistor circuit design. In this section a general scheme to implement a glitch-free pass transistor circuit will be discussed.

Implementation of an arbitrary Boolean function by static pass transistors can be achieved by a binary tree-type scheme. The true and complement of a variable are required to realize the function (Fig. l). As can be shown easily, there is one and only one path to Vdd or ground in this implementation; therefore, both the memory problem and the sneak path problem are eliminated. The use of true and complement prevents the signal from propagating backwards. The function can be programmed at each branch of the binary tree. If a "1", which is Vdd through a depletion load device, is connected to a branch, that minterm is in the Boolean function. Otherwise, the branch is not expressed in the Boolean function. Since this implementation is a universal logic function, any function can be realized. It is worth noting that every node in the circuit is either connected to the Vdd or ground. No ambiguity will exist.

The number (M) of devices needed is a function of the number (n) of variables:

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The minimiza...