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ASICs

Introduction

 Integrated Circuits (ICs) or "chips" are at the heart of almost all modern electronic products. Many are mass produced by a number of different manufactures and have commodity status. ASICs (Application Specific Integrated Circuits) are devices fabricated on a semiconductor chip using similar technology to normal "commodity" ICs such as microprocessors or amplifiers. Unlike commodity ICs, ASICs are designed for one particular task. The advantage of this is that a unique set of functions, that may be required by a product, can be included in one device. The device does not contain unnecessary or redundant features, so waste is eliminated. The disadvantage is that the cost is higher than for a standard device of similar complexity, due to lower production volumes.

 There are 4 basic reasons for deciding to develop an ASIC

ASIC Types

Full Custom - the supplier offers a manufacturing technology and the customer defines the circuit and the structure of transistors, diodes etc. These devices require the largest initial investment but give the lowest piece price in high volumes.

Standard Cell - the supplier offers a technology and a number of pre-defined blocks such as logic gates, operational amplifiers output drivers etc. These blocks can be placed on the chip as required. The customer defines the circuit. Standard cell ASIC technologies may offer pure digital, pure analogue or mixed elements. The time to production and initial investment is lower than that of full custom types.

Arrays - the supplier offers a range of chips with pre-defined sets of devices fabricated on them. The customer designs a schematic using these devices. The custom IC is produced by adding a connection layer or layers to the chip, connecting the pre-defined devices to form the required circuit. Arrays give the lowest initial charges and fastest time to production. Piece prices tend to be higher.

Embedded Controllers - these devices are based on existing microcontrollers. They include the microcontroller circuitry with some space for user definable circuitry, usually in standard cell or array configurations. They offer fast time to production when complex algorithms are required.

Common Mistakes When Specifying An ASIC

Put This Circuit Into An ASIC! - You have a working circuit with many discrete components but you need to reduce component count or size. You draw up the existing circuit and send it to an ASIC manufacturer and ask them to put it onto a chip. The result may be too expensive, may not work too well, may not be achievable and will not deliver additional free benefits.

The problem is that a discrete circuit is often not appropriate for fabrication on a chip. Reasons for this include the fact that the relative cost or size of different types of components vary enormously between integrated and discrete circuits and that the technology should be consistent between different circuit elements to make a cost effective IC.

Consider the case of a control circuit using capacitor-resistor timing elements with monostable circuits to provide timing functions. To fabricate this circuit on a chip would require relatively large analogue structures. The timing capacitors could not be fabricated on-chip and so each one would require an external pin. The cost would be high and component count reduction may not be too great. The same function could be achieved on a chip by using digital techniques. The result would be lower component count and better accuracy. It may be possible to include additional desirable features, that would be too complex in a discrete circuit, without increasing the cost.

Defining The Boundary - Which features should be included in ASICs? Should there be one ASIC or should the features be split between a number of devices? It is tempting to draw a line around most of your circuitry attempt to integrate the lot into one ASIC. The results can be high costs, high risk, technical problems and disastrous fault propagation (FMEA issues).

There are a number of reasons for this.

One is the issue of device technology. Some semiconductor manufacturing technologies are optimized for digital devices while others yield better analogue performance. A given technology will have an upper voltage limit. Some will permit the fabrication of power devices while others are limited to small signals. A few allow different areas of the device to have different properties. The choice of technology will determine the price of the ASIC and will be dictated by the types of circuitry included. Adding one extra feature may require the whole device to be fabricated with a more exotic and expensive technology.

Other problems result from the ASIC being a single device. It is possible that a fault on the ASIC could result in failure of all its functions. If the circuitry consisted of discrete components then there may not be a single failure that would result in such a loss of functionality. A technical problem with a large single device is the possibility of interference between circuits. This is a particular problem when sensitive low level circuits are placed on the same device as high power or very high speed circuits.

The more features that are placed on a single device, the greater the technical challenge and the more difficult it is to arrange a back-up strategy should the device not meet its deadlines. There is therefore a higher risk with more feature content.

How ASICs are specified

The way in which an ASIC design is specified to the supplier can vary from very high level to very low level definition. At the highest level the specification is a description of the required functionality. If this is in plain language (English for example) then it is very likely that there will be ambiguities and omissions in the specification. The lowest level of specification is one that defines the substrate and the pattern and parameters of each doping process that form the layers of the semiconductor structure. This is full custom design and requires highly specialized knowledge of various fields. Between these extremes lies a range of specification methods such as transistor level schematic, block (e.g. gate for digital, op-amp for analogue) level schematic or formal definition language such as VHDL.

If a device is specified at a high level then the supplier must undertake the low level design. This will incur a charge. This may by paid as part of a development charge at the beginning of the project, this fee will be in addition to the cost of production set-up activities such as lithographic masks. The design fee may be amortized over a certain number of devices and included in a raised purchase price.

Alternatives To ASICs

Due to the relatively set-up costs of an ASIC, it is unlikely to be cost effective at low volumes. ASICs are usually used at volumes in excess of 50k/annum. An ASIC may be appropriate at lower volumes if the technology was chosen for technical reasons.

For applications where an ASIC is not appropriate due to low volume, but some of the benefits of integration are desirable, there are other possibilities.

How KSB Electronics Can Help

Some useful links

Zetex Linear Arrays

Microchip's PICs