Subsystem design and layout of VLSI

Section 6.1

Some Architectural  Issues:

In all design process ,a logical and systematic approach is essential. This is particularly so in the case of the design of aVLSI system which could otherwise take so long as to render the whole system obsolete before it is off the drawing board.

As example, the case of a relatively straightforward MSI logic circuit comprising, say, 500 transistors. A reasonable time to allocate to the design and proving of such a circuit could be some two engineer-months. Consider, now the design of a 500,000 transistor VLSI system. Even if a linear relationship exists between complexity and design time, the required design time would be 2000  engineer-months or 170 engineer-years.In fact, the design time tends to rise exponentially with increased complexity. then we must adopt design methods which allow the handling  of complexity in reasonable periods of time and with reasonable amount of labour.

But now some sensible concepts applied in larger system design requirements.   

Guideline set as follows:

1. Define the requirements (properly & carefully).

2. Partition the overall architecture into appropriate subsystem.

3. Consider communication paths carefully in order to develop sensible interrelationships between subsystem.

4.  Draw a floor plan of how the system is to map onto the silicon (and alternate between 2,3 and 4 as necessary).

5. Aim for regular structures so that design is largely a matter of replication.

6. Draw suitable (stick or symbolic) diagrams of the leaf_cells of the subsystems.

7. Convert each cell to layout.