LAYOUT DESIGN FLOWS:MICROPROCESSOR DESIGN FLOW

MICROPROCESSOR DESIGN FLOW

The most famous microprocessors are the Intel Pentium series, Digital’s Alpha, and the workstations’ RISC chips. Otherwise, small microprocessors are widely used as control devices for household appliances, business machines, cars, toys, etc.

An MPU, or microprocessor unit, is an extremely complicated integrated circuit that accepts coded instructions, executes the instructions received, and delivers the requested result.

Microprocessors are the most complicated chips, so in general every step is very well defined and checked at high levels of abstraction. Only after the concepts are proved are they entered in a normal full-custom design flow. The following is a very simplified example of a development flow for a microprocessor (Figure 4.1).

1. Chip functional specification: The first step is to define from marketing information the kind of functionality that is required for the chip. Based on the blocks and possible applications, the project leader decides to pursue a known standard architecture or develop a new one.

The chip must comply with standards for bus interfaces, timing specifications, and manufacturing packages; therefore, the project leader has to decide the order of importance and priorities for each of them. Generally,

not every possible requirement can be met, because it would take too much time to address them all. After all, a chip that is perfect but does not come out in time for the market does not bring revenue to the company.

2. Chip architecture: The architecture of a chip is essentially the way the chip is broken down into functional blocks and how the blocks are connected together. In defining the architecture it is the choice and definition of different blocks that is optimized to meet the desired performance goals.

In a layout sense, the architecture denotes where and how big each block has to be, where it makes sense to place the blocks, what the logical relationship is between them, and what the important signals are, among many others. At this level the chip looks like a chip with numerous empty blocks inside surrounded by a frame of pads.

3. Digital simulations: Digital or functional simulations are performed as part of validating a behavioral model of the intended design. The simulation verifies that the chip architecture is feasible and will perform the desired operation.

4. Schematic design: At this point the different blocks are refined as schematics that represent the design to the transistor level. At this level the designer has

to deal with real-world phenomena, such as dissipated heat, power consumption, resistance, and capacitance of devices and lines. Functionality and size of the design are also verified.

5. Layout: Layout is the design step when a simulated schematic is delivered to the layout designers to generate the polygon-based representation of the circuit.

6. Tape-out to manufacturing: Once the layout of a full chip is finished and checked against process requirements and against the final schematic, it is the time to prepare the data for manufacturing.