Use of memories and programmable logic arrays for asynchronous sequential circuits

The design of asynchronous sequential circuits is commonly related to the problem of observing some specific timing constraints to avoid unreliable behaviour. State assignment and hazard-free construction of the combinational circuits for the state transition equations are the most essential topics to investigate. They contribute significantly to the design, particularly in comparison with that of synchronously operated systems. On the other hand, the application of digital circuitry for solving control tasks implies more and more asynchronous interaction between the controller and the controlled unit, and also the use of modern, highly-integrated modules. This paper investigates the possibility of applying arbitrarily chosen, but unique, codes for state assignments, using l.s.i. memories and programmable logic arrays for implementing more complex asynchronous sequential circuits than is possible with discrete or s.s.i/m.s.i. components. A basic model is derived to describe the time properties of such matrix arrays, and design rules are established to decide easily by some simple measurements, if a given module may be used in that application. The results of a broad investigation into available memories and field-programmable logic arrays show that reliable asynchronous sequential circuits may be implemented without having hazard problems and the need for special state-assignment procedures.