TAILIEUCHUNG - Designing with FPGAs and CPLDs- P3

Designing with FPGAs and CPLDs- P3: Designations used by companies to distinguish their products are often claimed as trademarks. In all instances where CMP Books is aware of a trademark claim, the product name appears in initial capital letters, in all capital letters, or in accordance with the vendor’s capitalization preference. Readers should contact the appropriate companies for more complete information on trademarks and trademark registrations. All trademarks and registered trademarks in this book are the property of their respective holders | 44 Chapter 3 Field Programmable Gate Arrays FPGAs There are two main types of antifuses in production today. In one type conductor 1 is polysilicon and conductor 2 is n diffused silicon. The insulator is oxide-nitride-oxide ONO and the link is composed of silicon. In the second type of antifuse both conductors are metal the insulator is amorphous silicon and the link is composed of titanium or tungsten silicide. SRAM-based FPGAs have the advantage of being reprogrammable. Especially as FPGAs become larger and therefore more expensive it is a nice feature during debugging to be able to reprogram them rather than toss out a bad design. SRAM-based FPGAs can also be reprogrammed while in the system which makes in-field upgrading very easy. Programmers can alter a communication protocol or add a feature to the FPGA by a simple software change. SRAM-based FPGAs allow you to include small memories like FIFOs in your design though large memories inside an FPGA are not cost effective. Also SRAM-based FPGAs can be used for reconfigurable computing a concept whereby computers contain FPGAs and algorithms can be compiled to run in the FPGAs. A disadvantage of SRAM-based FPGAs is that they re reprogrammable. Some applications particularly military ones often require that a device be nonvolatile and not susceptible to changes from radiation or power glitches. Antifuse FPGAs fit these criteria. In theory antifuse FPGAs are much faster than SRAM FPGAs. This is because antifuse FPGAs have a real connection between conductors for routing traces as opposed to the logic or transistors used in SRAM-based FPGAs. Although the antifuse connections have a high resistance and therefore some RC delay associated with them this delay should be much lower than the delay in SRAM-based FPGAs. You ll notice that I use some wishy-washy terms here. The reason is that in practice antifuse FPGAs are not significantly faster than SRAM-based FPGAs despite the theory. That s because every semiconductor .

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