Embedded SoPC Design with Nios II Processor and Verilog Examples

CHAPTER 8

SELECTED TOPICS OF VERILOG

Since the main focus of this book is on digital design, we just introduce the minimal subset of Verilog and rely on some simple guidelines and templates. In this chapter, we examine several selected Verilog topics in more detail. Except for the last section, which provides an overview of simulation-related constructs, these topics are related to synthesis and help us to develop more sophisticated codes. This chapter can be skipped without affecting the remaining chapters.

8.1 BLOCKING VERSUS NONBLOCKING ASSIGNMENT

There are two kinds of assignments that can be used in an always block: blocking assignment and nonblocking assignment. Three simple guidelines were given in the earlier chapters:

• Separate the circuit into registers and combinational circuits.
• Select a proper template for the registers, which use nonblocking assignments inside.
• Use blocking assignments to describe the combinational circuits.

We examine the two kinds of assignments and explain the rationale behind the guidelines in this section, and introduce an alternative coding style in the next section.

8.1.1 Overview

Blocking assignment The basic syntax of a blocking assignment is

When the assignment is executed, the right-hand-side expression is evaluated and assigned to the left-hand-side variable without interruption from any other statements. Thus, it “blocks” the other assignments ...