The Role of Custom Design in ASIC Chips
Abstract
The performance of an ASIC can be greatly improved without increasing design time by judiciously employing a number of custom design techniques, including floorplanning, prerouting critical signals, tiling datapaths, and generating crafted cells. These techniques all structure the design by routing the critical wires first and then placing the devices. This key-wires-first approach exploits the structure of the logic to reduce wire loads, provide early visibility of the timing and power dissipation of a design, and gives the designer control of the key wiring. These techniques can all be applied within a standard ASIC CAD flow alongside less critical blocks that are implemented without structure. There is a continuum of design styles between full custom, where every wire is hand placed, to fully automated, where none are. The key to applying custom design techniques to an ASIC is to limit customization by structuring the few key wires that give the most leverage and leaving the rest to automated tools. Examples of critical wires that are easy to structure are global signals and buses, datapath bit and word lines, and signals internal to crafted cells. Circuits play an important, but subordinate, role in custom design. Structured wiring is the key enabler for fast circuits such as domino logic and low-swing signaling. These circuits require a well-controlled load and noise environment that cannot be guaranteed with automated wiring. When the wiring is well controlled the use of fast circuits gives an additional performance boost, but one that is less than a factor of two. Traditionally, custom design has been restricted to high-performance components like microprocessors. However, as geometries continue to shrink, both the importance of wire delay and the increase in ASIC complexity will necessitate a significant expansion in the use of custom techniques as automated flows increasingly fail to meet performance, power, and area goals. (1 table, 4 figures, 6 refs.)7
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1998
- Accession Number
- ADA419646
Entities
People
- Andrew L Chang
- Bill Dally
Organizations
- Stanford University