CSS Mixed Signal ASIC Solutions

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Keith

ASICs vs. FPGAs

November 27th, 2010 by Keith

In considering how to integrate your electronic system, you could consider an ASIC (Application Specific Integrated Circuit) or a FPGA (Field Programmable Gate Array).   To make the best choice, the benefits and shortcomings of each technology need to be understood as they relate to your needs.

If you are the design engineer, the required circuit performance is a top concern, as well as the tools and skills you would need to successfully complete the design.   Costs are going to be a key consideration, especially for your company’s financial and program managers.  These costs would include both development (nonrecurring engineering (NRE)) and production (recurring unit cost).  Finally, time-to-market may be very important – to everyone, but probably especially so for marketing people.   So, to be successful, your approach needs to meet the needs of everyone involved.

First consider the question of costs.  The top level answer to that question is that ASICs will cost more to develop, but cost less to produce.  This usually means that FPGAs are more cost effective for lower volume applications and ASICs are better for higher volume applications.  To determine financially if an ASIC is best, you just need to determine at what volume the total cost is the same.  After that volume, you will begin to save money with the ASIC.

An example is useful.  Assume that for a large ASIC design at CSS, the NRE is $200,000, the FPGA and analog circuits it replaces cost $40 and the ASIC unit cost is projected to be $20 (you are saving $20/unit).  Further, your expected volume is 10,000 units annually for the next 5 years.  Referring to the chart below, the  break-even point (when your savings equals your NRE) occurs in one year after start of production and you will be saving $200,000 for each year to follow.  If you find that for your application, it takes too long to recover the ASIC NRE, an approach often taken is to implement first in a FPGA when volumes are expected to be low, and then convert to an ASIC when the application is successful and the volumes become larger.

Next consider the time-to-market.  An ASIC will usually take considerably longer since, in addition to the design, it needs to be fabricated from scratch and debugged.  This time can typically be 6 to 12 months from design to full production.  An FPGA typical development time can be shorter, since the devices do not need to be fabricated and last minute changes can be implemented in a shorter time.

The ASIC development time can be shortened if the ASIC design house has most of the basic cells needed available in a library (usually true for digital circuits and many of the analog circuits).  At CSS, we have a large library of both digital and analog cells, so often times the circuits just need to be configured to the application.  Many times all that is required is to optimize existing cells to meet specific requirements (power, speed, etc.).
FPGAs are sometimes used in an ASIC development as the first step of the ASIC design.  This is especially true if the digital design is complex or is not yet well defined.  By designing this digital part of the circuit in an FPGA, it can be fully evaluated, often in the intended application, before committing to the custom ASIC version.

Closely related to cost, is the degree to which your system can be integrated.  ASICs here have an advantage if you need analog circuits in your system.  Mixed-signal ASICs by definition have both analog and digital circuits available which can probably meet your design needs.  FPGAs with any significant analog circuitry are not available.  So an ASIC makes it possible to implement your design in a single chip, which is less expensive and takes up less board space.

Finally, consider circuit performance, design tools needed and the skills required.  Design tools for FPGAs are typically available at no cost from the FPGA manufacture, at least for the digital part of the circuit.   Although there is a learning curve, the skills needed to design the digital circuit on a FPGA can be learned by a competent designer.  The same designer may be able to find separate analog ICs to complete his circuit.   On the other hand, a mixed-signal ASIC design house will have all the tools and expertise to complete both the digital and analog designs in a single ASIC.  Often, the experience of the design house, along with their library of existing digital and analog cells, will considerably shorten the design time.  At CSS, we can provide a turn-key design service, relieving our customer of that significant responsibility – often in a shorter time than it would take to implement an FPGA design.

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