Adiabatic circuit

Adiabatic circuits are low power circuits which use "reversible logic" to conserve energy.

Unlike traditional CMOS circuits, which dissipate energy during switching, adiabatic circuits reduce dissipation by following two key rules:

While this is an area of active research, current techniques rely heavily on transmission gates and trapezoidal clocks to achieve these goals.

The basic concepts of Adiabatic logic will be introduced.

"Adiabatic" is a term of Greek origin that has spent most of its history associated with classical thermodynamics. It refers to a system in which a transition occurs without energy (usually in the form of heat) being either lost to or gained from the system. In the context of electronic systems, rather than heat, electronic charge is preserved. Thus, an ideal adiabatic circuit would operate without the loss or gain of electronic charge. The first usage of the term "Adiabatic" in this context appears to be traceable back to a paper presented in 1992 at the Second Workshop on Physics and Computation. Although an earlier suggestion of the possibility of energy recovery was made by Bennett where in relation to the energy used to perform computation, he stated "This energy could in principle be saved and reused".

Etymology of the term "adiabatic logic". Because of the Second Law of Thermodynamics, it is not possible to completely convert energy into useful work. However, the term "Adiabatic Logic" is used to describe logic families that could theoretically operate without losses. The term "Quasi-Adiabatic Logic" is used to describe logic that operates with a lower power than static CMOS logic, but which still has some theoretical non-adiabatic losses. In both cases, the nomenclature is used to indicate that these systems are capable of operating with substantially less power dissipation than traditional static CMOS circuits.

There are several important principles that are shared by all of these low-power adiabatic systems. These include only turning switches on when there is no potential difference across them, only turning switches off when no current is flowing through them, and using a power supply that is capable of recovering or recycling energy in the form of electric charge. To achieve this, in general, the power supplies of adiabatic logic circuits have used constant current charging (or an approximation thereto), in contrast to more traditional non-adiabatic systems that have generally used constant voltage charging from a fixed-voltage power supply.

The power supplies of adiabatic logic circuits have also used circuit elements capable of storing energy. This is often done using inductors, which store the energy by converting it to magnetic flux. There are a number of synonyms that have been used by other authors to refer to adiabatic logic type systems, these include: "Charge recovery logic", "Charge recycling logic", "Clock-powered logic", "Energy recovery logic" and "Energy recycling logic". Because of the reversibility requirements for a system to be fully adiabatic, most of these synonyms actually refer to, and can be used inter-changeably, to describe quasi-adiabatic systems. These terms are succinct and self-explanatory, so the only term that warrants further explanation is "Clock-Powered Logic". This has been used because many adiabatic circuits use a combined power supply and clock, or a "power-clock". This a variable, usually multi-phase, power-supply which controls the operation of the logic by supplying energy to it, and subsequently recovering energy from it.

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