HOME       UP       PREV       NEXT (Semi-custom (cell-based) Design)  

The figure presents a taxonomy of chip design approaches. The top-level division is between standard parts, ASICs and field-programmable parts. Where a standard part is not suitable the choice between full-custom and semi-custom and field-programmable approaches has to be made, depending on performance, production volume and cost requirements.

Standard Parts

A standard part is essentially any chip that a chip manufacturer is prepared to sell to someone else along with a datasheet and EDA (electronic design automation) models. The design may actually previously have been an ASIC for a specific customer that is now on general release. However, most standard parts are general-purpose logic, memory and microprocessor devices. These are frequently full-custom designs designed in-house by the chip manufacturer to make the most of in-house fabrication line, perhaps using optimisations not made available to others who use the line as a foundry. Other standard parts include graphics controllers, LAN controllers, bus interface devices, and miscellaneous useful chips.

Masked ASICs.

A masked ASIC (application specific integrated circuit) is a device manufactured for a customer involving a set of masks where at least some of the masks are used only for that device. These devices include full-custom and semi-custom ASICs and masked ROMs.

A full-custom chip (or part of a chip) has had detailed, manual design effort expended on its circuits and the position of each transistor and section of interconnect. This allows an optimum of speed and density and power consumption.

Full-custom design is used for devices which will be produced in very large quantities: e.g. millions of parts where the design cost is justified. Full-custom design is also used when required for performance reasons. Microprocessors, memories and digital signal processing devices are primary users of full-custom design.

In semi-custom design, each cell has a fixed design and is repeated each time it is used, both within a chip and across many devices which have used the library. This simplifies design, but drive power of the cell is not optimised for each instance.

Semi-custom is achieved using a library of logic cells and is used for general-purpose VLSI design.

14: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.