Designing C-elements for Testability

C-elements are used widely in asynchronous VLSI circuits. Fabrication faults in some CMOS C-elements can be undetectable by logic testing. Testable designs of static symmetric and asymmetric C-elements are given in this report which provide for the detection of single line stuck-at and stuck-open faults. We show that driving feedback transistors in the proposed testable static C-elements transforms their sequential functions into combinational AND and OR functions or into repeaters of one of their inputs depending on the driving logic value. This simplifies the testing of asynchronous circuits which incorporate a large number of state holding elements. The scan testable C-element described can be used in scan testing of the asynchronous circuit making the states of its memory elements controllable and observable.