7. How to Design Complex Digital Systems

Note links are at the bottom of the page

Now we take design one level higher. How do you approach designing a complex system that will consist of multiple modules? This module describes a structured approach to that exercise.

Many times the high level design process starts with a description of the high level algorithm written in C. This fact makes it very tempting to simply translate the C into Verilog and skip the design step. The purpose of this lesson is to illustrate some techniques to translate C into a hardware design. The reasons for this lesson are two-fold: (1) to show you how can “translate” if you like, if you still find hardware design daunting; and (2) drive home the point that an RTL design is very different from algorithmic C code. Note that while you can perform high level design like this, it does not guarantee an optimal or efficient design.

Objectives

• Understand how you approach a design so that control and datapath are clearly separated.
• Understand how C can be “translated” into Verilog
• Introduce influence of design on power consumption

Key Points

• By clearly separating control from datapath you can clearly track cause and effect. They communicate with each other via status lines and control lines.
• Controllers can be built from a combination of counters, FSMs and microcode.
• Steps in complex design are:

1. Determine the algorithm.
2. Explore the design. What is the critical path and how fast can it go? What resources are needed to achieve the performance targets?
3. Design the data path.
4. Identify the control and status lines.
5. Identify the control approach needed (counters, FSMs, or pipelined control).
6. Implement the controller
• Careful thought is required to maximize performance/area or performance/power.
• Power consumption is determined by the amount of node toggling. Wasted toggles (1à0 and 0 à 1 transitions) wastes power.

• C code mixes control and datapath into single statements. To convert C into RTL, you have to recognize which part of the statement is control and which part is datapath.

PDF files:

• http://www.ece.ncsu.edu/asic/2009/docs/complexity.pdf
• Note, these are 1-up. If you want to save paper, use the properties tab on the print command as you please (e.g. 2-up or 4-up).

PDF files are distributed in the links above. If you want the ppt version, please go to http://courses.ncsu.edu/ece520/lec/001/wrap/ . Note, this PPT version does NOT include answers to in-line questions.