Logically Determined Design: Clockless System Design with NULL Convention Logic

CHAPTER 6

Memory Elements

The flow of a wavefront in a logically determined system cannot always be expressed directly in terms of physical path structure. The logical flow path of a wavefront can be directed temporarily through a physical wayside rest until its assigned rendezvous is imminent. Memory elements are these temporary wayside rests for wavefronts in their flow toward resolution.

6.1 THE RING REGISTER

The ring provides a restorative feedback mechanism for wavefront flow that can serve as a wavefront storage register. A wavefront can flow out of the ring and back into the ring, or a new wavefront can flow into the ring replacing the current wavefront. Construction of a ring register begins with the ring of at least three cycles, as shown in Figure 6.1a. There must be an output path from the ring, so another cycle is added that shares a path with one of the ring cycles. This adds the fan-out structure of Figure 6.1b to the ring. This fan-out is not conditional. The wavefront flows from registration stage O to both fan-out destinations. So registration stage O must be acknowledged by both destination registration stages. With this structure a DATA wavefront initialized in the ring will flow to registration stage S, and it will be blocked because registration stage O will ...