October 23, 1997

The blind men and the elephant

Wolfram Blume

The user of tool-centric EDA tools is put in the position of being each of the blind men: First, a circuit is a schematic; then, a circuit is behavior; then, it is a layout; and so on. How can EDA do better?

The four blind men who encountered an elephant reported it to be

1. Like a rope (by the one who grabbed the tail),

2. Like a wall (by the one who encountered the side),

3. Like a pillar (by the one who wrapped his hands around a leg), and

4. Like a snake (by the one who felt the trunk).

Each of the four was correct but woefully incomplete.

And so it goes with circuit design. You can view a circuit as a schematic, as a layout, as a behavior (for example, simulation waveforms), as a mechanical shape that must fit into an assigned space, as a source of EMI, as a consumer of power, or as any number of other things.

Today's EDA products are still "tool-centric," meaning that most manufacturers optimize products to deal with one function. For example, pc-board layout editors are heavily optimized to satisfy the needs of layout designers, whose primary job is to lay out boards day in and day out. It is obvious that a layout designer views the circuit as a layout; how others view the circuit is of little concern to him. What is less obvious is that the layout editor forces the view "a circuit is a layout" on all its users in many ways, some of them quite subtle.

This comment applies not just to layout editors, but to all EDA tools that focus on getting one job done; that is, tools that are tool-centric. The user of such a collection of EDA tools is put in the position of being each of the blind men: First, a circuit is a schematic; then, a circuit is behavior; then, it is a layout; and so on. How can EDA do better? The answer lies in the story of the blind men and the elephant.

What does it take to see the entire elephant? Sight. Sight is better than touch to understand the elephant because sight gives simultaneous access to all of the elephant, whereas touch gives only sequential access to each of its aspects.

Carrying this idea over to EDA, we arrive at the conclusion that to provide a designer with "sight," the EDA product must provide simultaneous access to all aspects of the circuit. How the product accomplishes this task is not critical. It might be as straightforward as having multiple windows on a screen, each of which shows one aspect. But simultaneous access has several implications, including the fact that changes made to one aspect must be visible in the other aspects. For example, a layout change should update the simulation waveforms if the layout change affects the circuit's behavior.

Another implication is that the process of designing a circuit should not be defined by a sequence of tool usage. Instead of progressing from schematic entry to simulation to layout, for example, it should be possible to begin with a kernel of the circuit done in all its aspects and then progressively add the pieces around the kernel. The circuit is designed from the center outward. Finally, it should be possible, even easy, to make trade-offs between different aspects at any time. In a mixed-signal circuit, for example, it can often help the layout to put some of the analog functions into digital, or vice versa.

In general, if you want to work with a circuit as a single entity, then you must be able to see it as such. Today, many EDA products consign the designer to "feeling" his way through the circuit design. The future lies with providing sight.

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