Reversible Logic Circuits Design Cosntraints

Reversible logic imposes many design constraints that need to be either ensured for optimized for implementing any particular Boolean functions.

Firstly, in reversible logic circuits the number of inputs must be equal to the outputs.

Secondly, for each input pattern there must be a unique output pattern.

Thirdly, each output will be used only once, that is no fan out is allowed.

Finally, the resulting circuits must be acyclic.

Reversible Logic Gates- An introduction

Reversible circuits or Gates can generates unique output vector form each input vector, and vice versa, i.e., there is a one to one correspondence between the input and output vectors. Thus, the number of outputs in a reversible gate or circuit has the same as the number of inputs, and commonly used traditional NOT gate is the only reversible gate. More formally, a reversible logic gate is a K-input, K-output (denoted K*K) device that maps each possible input pattern into a unique output pattern. While constructing reversible circuits with the help of reversible gates, some restrictions should be strictly maintained.

·        Fan-out is not permitted

·        Loops are not permitted

Motivation Behind Reversible Logic Gates

 High-performance chips releasing large amounts of heat impose practical limitation on how far can we improve the performance of the system. Reversible circuits that conserve information, by un-computing bits instead of throwing them away, will soon offer the only physically possible way to keep improving performance. Reversible computing will also lead to improvement in energy efficiency. Energy efficiency will fundamentally affect the speed of circuits such as nano-circuits and therefore the speed of most computing applications. To increase the portability of devices again reversible computing is required. It will let circuit element sizes to reduce to atomic size limits and hence devices will become more portable. Although the hardware design costs incurred in near future may be high but the power cost and performance being more dominant than logic hardware cost in today’s computing era, the need of reversible computing cannot be ignored.