Realization of BCD adder using Reversible Logic

This paper proposes novel carry select and carry look-ahead BCD adders using reversible logic. Reversible logic gates are widely known to be compatible with future computing technologies which virtually dissipate zero heat. Adders are fundamental building blocks in many computational units. For this reason, we have simulated several adder circuits using the reversible gates. Among all the other adders, the main virtue of BCD adders is that it allows easy conversion to decimal digits for printing or display and faster decimal calculations. These reversible BCD circuits are basis of the decimal ALU of primitive Quantum CPU. The proposed BCD adders have been simulated in VLSI and static timing report was analyzed. Everyday new technology which is faster, smaller and more complex than its predecessor is being developed. The increase in clock frequency to achieve greater speed and increase in number of transistors packed onto a chip to achieve complexity of a conventional system results in increased power consumption. Almost all the millions of gates used to perform logical operations in a conventional computer are irreversible. That is, every time a logical operation is performed some information about the input is erased or lost and is dissipated as heat. As per Landauer (1), for irreversible logic, each bit of information lost generates kTln2 Joules of heat energy, where k is Boltzmann's constant and T is absolute temperature at which the computation is performed. For room temperature T, the amount of heat dissipated for one bit is small i.e. 2.9×10-21 J (2). The current processors, first of all dissipate 500 times this amount of heat every time a bit is lost. Secondly, assuming every transistor out of more than 4×107 dissipates heat at the processor frequency of 2GHz, the figure becomes 4×1019*kTln2 J/sec. Although, it is around 0.1W at 400 degree Kelvin, Moore's law predicts exponential growth of heat generated due to information loss which will be an intolerable amount in the next decade. This heat dissipation dramatically reduces the performance and lifetime of the circuits. The solution is to use revolutionary technology which enables extremely low power consumption and heat dissipation in computing. Reversible logic, which is an actively researched area, is considered as an apt alternative. The employment of