ASICS

Application Specific Integrated Circuits or ASICs are, as the name indicates, non-standard integrated circuits that have been designed for a specific use or application. Generally an ASIC design will be undertaken for a product that will have a large production run, and the ASIC may contain a very large part of the electronics needed on a single integrated circuit. As may be imagined, the cost of an ASIC design is high, and therefore they tend to be reserved for high volume products.

Despite the cost of an ASIC design, ASICs can be very cost effective for many applications where volumes are high. It is possible to tailor the ASIC design to meet the exact requirement for the product and using an ASIC can mean that much of the overall design can be contained in one integrated circuit and the number of additional components can be significantly reduced. As a result they are widely used in high volume products like cell phones or other similar applications, often for consumer products where volumes are higher, or for business products that are widely used.

The first Application Specific Integrated Circuits (ASICs) traditionally addressed only logic functions. Now mixed signal ASIC designs can incorporate both analogue (including RF) and logic functions. These mixed signal ASICs are particularly useful in being able to make a complete system on chip, SoC. Here a complete system or product is integrated onto a chip and virtually no other components are required. This makes a mixed signal ASIC design a very attractive proposition for many applications.

ASIC beginnings

The beginnings of the ASIC can be traced back to the early 1980s. Around this time, ICs were beginning to make a major impact on the electronics industry. In view of the advantages that ICs provided, and the limited number that were available, some attempts were made to create a logic chips that could be easily focussed towards a specific application. One early initiative undertaken by Ferranti, a UK based company, used what was termed the uncommitted logic array (ULA). This scheme provided the customisation by varying the metal interconnect mask.

The first ULAs contained only a few thousand gates, but later versions had greater levels of flexibility and used different base dies customised by both metal and polysilicon layers. In some cases RAM elements were incorporated into the basic ULA.

From these early developments, a number of different types of ASIC have been developed. Now many ASICs are very complicated, and some are mixed signal ASICs that incorporate both analogue and digital circuitry.

ASIC basics

The development and manufacture of an ASIC design including the ASIC layout is a very expensive process. In order to reduce the costs, there are different levels of customisation that can be used. These can enable costs to be reduced for designs where large levels of customisation of the ASIC are not required. Essentially there are three levels of ASIC that can be used:

• Gate Array     This type of ASIC is the least customisable. Here the silicon layers are standard but the metallisation layers allowing the interconnections between different areas on the chip are customisable. This type of ASIC is ideal where a large number of standard functions are required which can be connected in a particular manner to meet the given requirement.
• Standard cell     For this type of ASIC, the mask is a custom design, but the silicon is made up from library components. This gives a high degree of flexibility, provided that standard functions are able to meet the requirements.
• Full custom design     This type of ASIC is the most flexible because it involves the design of the ASIC down to transistor level. The ASIC layout can be tailored to the exact requirements of the circuit. While it gives the highest degree of flexibility, the costs are very much higher and it takes much longer to develop. The risks are also higher as the whole design is untested and not built up from library elements that have been used before.

There are three different categories of ASICS:

• Full-Custom ASICS: These are custom-made from scratch for a specific application. Their ultimate purpose is decided by the designer. All the photolithographic layers of this integrated circuit are already fully defined, leaving no room for modification during manufacturing.
• Semi-Custom ASICs: These are partly customized to perform different functions within the field of their general area of application. These ASICS are designed to allow some modification during manufacturing, although the masks for the diffused layers are already fully defined.
• Platform ASICs: These are designed and produced from a defined set of methodologies, intellectual properties and a well-defined design of silicon that shortens the design cycle and minimizes development costs. Platform ASICs are made from predefined platform slices, where each slice is a premanufactured device, platform logic or entire system. The use of premanufactured materials reduces development costs for these circuits.