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ASIC basics tutorial

- an overview or tutorial of the basics of the ASIC, Application Specific Integrated Circuit, its advantages and disadvantages and the basics types available.


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.

Summary

ASIC designs offer a very attractive solution for many high volume applications. They enable significant amounts of circuitry to be incorporated onto a single chip. Had the circuits been assembled using proprietary chips, additional components, and hence board area would be needed. Manufacturing costs would be more. With sufficient volume, custom chips, in the form of ASICs offer a very attractive proposition. In addition to the cost aspects, ASICs may also be used sometimes because the enable circuits to be made that might not be technically viable using other technologies. They may offer speed, and performance that would not be possible is discrete components were used. When developing an ASIC, it is often necessary to employ another specialist company to provide the ASIC design service. By using their expertise the design can be undertaken more effectively - in terms of correct functionality, cost and timescale.

By Ian Poole

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