When it comes to electronic control devices for new products, there is a long standing question: FPGA or ASIC? Carl Hudson of SWINDON Silicon gives advice
A FPGA is a user configurable device that employs a binary image file to implement the required functionality. Although some FPGAs include analogue blocks, they are mainly used for digital centric designs. An ASIC, on the other hand, is a custom designed integrated circuit that is dedicated for one particular application and can handle both digital and analogue functions. It is up to the designer to specify which type of device is most appropriate to his particular product, but the wrong decision can make the difference between success and failure in the market place. Unfortunately there is no easy answer.
In the early stages of product’s life cycle, FPGA and ASIC devices will look very similar: system and block design, partitioning and system modelling are common activities. It is only as the design progresses that there is a divergence. In particular, analogue blocks need careful consideration because they may not be available or have the desired performance in a FPGA. Another consideration is cost. It is a popular misconception that ASICs are expensive but, in many instances, they can be cost effective. Take, for example, the classic System on a Chip (SoC) design that requires a microprocessor together with standard interfaces and logic blocks. At first sight, a designer may think this would be best suited to an FPGA, as it will give quick access to a platform that the software developer can use, and FPGA’s allow for a shorter development time. They may also think that such a design in an ASIC would require latest geometries and be very expensive, but many SoC designs are implemented on lower cost geometries such 350nm and 180nm. Software, in high level C, can be developed in parallel with the hardware.
When a product comes to market customer feedback can often mean that some revision of its features is needed and, again, the designer might think that may a FPGA gives more flexibility. However, the ASIC development path does allow for devices to be held at various stages of production so that modifications can be performed with a quick turn-around. In fact even if the ASIC is in production, most changes can be made at a relatively low cost.
The unit cost of a FPGA is constant whilst that of an ASIC reduces as the volume off-take increases so, depending on the anticipated product sales volume, there will be a point at which the ASIC becomes attractive.
The life cycle of a product is determined by competition or replacement, but the longer it is the more risk there is of components becoming obsolete. The supply of a FPGA is determined by the supplier and is based on multiple customers for that component. If the market declines, the supplier will stop manufacturing, leaving the user to support the product. The equivalent in ASIC terms is a process becoming obsolete. In this situation there are options available to the user such as process transfer and wafer storage. If the product has gone through a certification programme then replacing a component can lead to expensive re-qualification, so it is important to take this into account when considering the choice.
Because an ASIC is designed for a specific application, its performance will always be optimised, whereas the performance of a FPGA will be a compromise. The recent advances in ASIC technology means that things like low power and high speed have been improved so, if performance is a key product discriminator, then an ASIC may well be the only viable option. And quite often size really is everything. An ASIC is a customised device so will always have the minimum size and, if this is critical for the design it may well override other decision factors and point the way to an ASIC.
As engineers we like definitive answers. For some products it will be obvious which is the best solution given the product, market and predicted volumes. However there are many grey areas where it is not obvious whether a FPGA or ASIC is the best option. ASICs can often be disregarded on the grounds of perceived cost when they will give a much better solution, so the designer has to avoid common prejudices and generalisations and thoroughly investigate both device types. Solution providers will be happy to provide information and have these discussions: they will even have tools and documents that will assist the process. Only when all the latest facts are available can the designer make the best selection for the product – and it may not be the one originally anticipated.
For more information, please visit http://www.swindonsilicon.co.uk/asic-applications/industrial/
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