LAYOUT DESIGN:A GENERAL PROCEDURE TO FOLLOW

A GENERAL PROCEDURE TO FOLLOW

Figure 3.21 shows a general layout design flow that is applicable for all design types.

This procedure is straightforward and self-explanatory and could be applied to almost any engineering task. Of course we will concentrate on how it applies to layout design.

Step 1 will be covered in detail, as it is crucial for getting started on the right track. It is in this step that we collect and review our knowledge of layout design and apply it to the specific circuit design under consideration. The aim is to produce a strategy for attacking the design by documenting the general areas where all components and signals will go.

Step 2 is simply implementing the design: executing and possibly revising the floorplan based on the realities of implementation. One way to think of the design process is to “plan top down,” then “implement bottom up.” By this we mean that we first floorplan general areas and approaches with a top-level view. With this plan in place we implement the design by starting with the lowest level components first and fill the areas defined in the plan. The lower level design tasks are easier because the constraints imposed on them were defined in the top-level floorplan. As the general areas are completed, we adjust our plan for future work. With a sound floorplan, adjustments are minor and the completion of the design is easy.

Computer-based checks (which will be covered in Section 3.10) form the bulk of step 3. These checks should be done in a certain order as outlined in Section

3.10. On top of the computer-based checks a visual inspection is recommended, as the automatic computer checks are only as good as the rules that are coded into them. Make a plot of your design and look at it. Also, there are many aspects of most designs that cannot be checked by computer. An example of this is the degree of symmetry of a balanced layout. These visual checks should be part of the audit checklist as a reminder.

Step 4 is a final sanity and cross-check to confirm that all requirements have been met and none missed, along with a final extraction step.

One comment is necessary about the procedure and flowchart presented in Figure 3.21 and all procedures presented in this chapter. They all show that after each step in which the design is modified and evolved, it is necessary to go back to a previous step to readdress a requirement that may no longer be valid. Note that there are no shortcuts in the flow. All the arrows are going up only on the right side, and therefore all previous steps should be revisited after every modification.