A Study on self-timed asynchronous subthreshold logic

This paper investigates self-timed asynchronous design techniques for subthreshold digital circuits. In this voltage range extremely high voltage-dependent delay uncertainties arise which make the use of synchronous circuits rather inefficient or their reliability doubtful. Delay-line controlled circuits face these difficulties with self-timed operation with the disadvantage of necessary timing margins for proper operation. In this paper we discuss these necessary timing overheads and present our approach to their analysis and reduction to a minimum value by the use of circuit techniques allowing completion detection. Transistor-level simulation results for an entirely delay-adaptable counter under variable supply down to 200 mV are presented. Additionally an analytical comparison and simulation of timing and energy consumption of more complex subthreshold asynchronous circuits is shown. The outcome is that a combination of delay-line based circuits with circuits using completion detection is promising for applications where the supply voltages are at extremely low levels.