• • • CHAPTER 8 RT LEVEL DESIGN

WITH CONVENTIONAL SWITCHING CIRCUIT COMPONENTS

Throughout this book we have illustrated register transfer level digital design using RTM's as a standard set of building blocks. We did this partly for uniformity and partly because the RTM notation lends itself to quickly understandable system diagrams. However, the concepts of register transfer level design are for the most part applicable for any well-chosen set of building blocks. We illustrate this by presenting here three of the designs that were done using RTM's earlier in the book, and implementing them using ad hoc techniques with conventional 'switching circuit components. We make no claim that all types of register transfer level design are represented in these problems. One was done by one of the authors and the other two by another of the authors, so only two of the many possible alternative register transfer level design styles are illustrated.

We are attempting to show that the principles illustrated in this book should be of use in all system designs, regardless of the set of primitives used. However, the choice of the set of primitives can have a considerable affect on the ease of design. Since no standard design approach or set of register transfer level, components is used in this chapter, design decisions are based on the characteristics of the problem at hand. Thus, the reader will see varying sizes and types of building blocks, varying notations, and varying synthesis approaches in the three problems. (The building blocks are about the same size as those currently available from integrated circuit manufacturers.) This flexibility allows the designer to optimize solutions locally in a way that is not possible with fixed sets of modules, such as RTM's. On the other hand, the price paid for this flexibility is the loss of standardization, with all of its attendant advantages.

APPLICATIONS OF THE RTM CONTROL STRUCTURE TO CONVENTIONAL RT LEVEL DESIGN

It should be noted that while we have warned the reader that the methods of control used in this book are highly specific to RTM's, the RTM control structure also is easily used with conventional switching circuit design. In Chapter 7, where the internal operations of the various modules are described, it can be observed that certain common switching circuit components are used to implement each of the control operations. That is: Kevoke consists of a two state device utilizing two flip-flops; Kbranch requires a flip flop to sample the incoming Boolean variable, and two AND gates to route the output control signals; Ksubroutine is actually just a flip-flop to mark the state of control temporarily for one of several possible control return points; K(serial-merge) and K(parallel- merge) are just the negative logic OR and AND gates, respectively.

In introductory logical design texts on sequential circuit design, assumptions can often be made to simplify the control part, because only clocked, synchronous logic is used. On the other hand RTM's operate asynchronously, in that a single operation on the Bus can take an arbitrary amount of time. This requirement adds to the control complexity. If one were to relax the requirements to only permit synchronous clocked logic, then it would be possible to use a single flip-flop for Kevoke, and no flip-flop would be required for the Kbranch, thus simplifying the modules.

With this view of the control elements, problems 3 and 4 of Chapter 7 can be considered using the control scheme of this book together with available

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