Reversible Computing: Adiabatic Capacitive Logic

The performance of modern computers is limited due to their heat dissipation. Even though, following Moore’s Law, the number of transistors in microprocessors keeps increasing, their speeds have been capped around 4 GHz for almost two decades to limit heat generation. Such speeds are well below the RC time constant limit of the circuits, sacrificing speed to prevent the chips from melting since traditional CMOS circuits dissipate power on every switching in the form of heat. Reversible computing is a viable alternative to traditional circuit implementations since it reduces heat generation by avoiding unnecessary dissipation, therefore introducing a trade-off between speed and power. By using reversible logic and switching the circuits slowly (adiabatically), relative to their RC time constants, power can be dramatically reduced.

The most developed approach to reversible computing is adiabatic CMOS which changes the CMOS power supplies to ramping clocks, and operates them adiabatically, but its lowest energy dissipation is still limited by passive power, the energy wasted due to leakage current caused simply by applying a voltage to the circuit. A new approach, Adiabatic Capacitive Logic (ACL), implements reversible computing by using variable capacitors as pull-up and pull-down networks. ACL eliminates leakage current and therefore is not limited by passive power. ACL variable capacitors can be implemented using microelectromechanical machines (MEMS). This work presents the design and fabrication of adiabatic capacitive logic MEMS devices that pave the way to implement adiabatic reversible computing not limited by leakage.

A radio frequency (RF) reflectometry system is used to measure the capacitance change in ACL MEMS devices. The RF reflectometry system includes a custom-made RF micromanipulator probe with an on-board matching network to match the impedance of the MEMS devices. A low-frequency measurement technique is also used to measure the variable capacitance. This work presents the first experimental implementation of gap-closing MEMS for adiabatic reversible computing.

Adiabatic reversible logic devices require ramping clocks to implement reversible computing. Therefore, a complete adiabatic reversible computing system needs to consider the implementation of adiabatic ramping clocks. This work presents MEMS piezoelectric resonators as an efficient approach to implement ramping clocks for reversible computing that not only supply charge to the logic but can also recycle the energy used by the logic. The two different MEMS devices presented in this work pave the way for a complete system for reversible computing that includes both the adiabatic reversible logic and the required ramping clocks.