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Resolving FPGA board design challenges

20 Mar 2014  | Michael Dunn

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If you have limited or zero board design experience with FPGAs, the idea of putting one in your next project can be overwhelming – especially if it's a 1,000-pin monster. Continue reading to get a feel for the selection and design process, and for lots of gotchas you'll need to avoid.

Pick a vendor, any vendor
Your first issue is of course vendor and device selection. Often, the vendor decision comes down to whichever you've had the best experience with previously – not applicable if you're an FPGA virgin. Or perhaps the decision has already been made by the person who will be designing the internal logic (which may be you), based perhaps on available vendor or 3rd-party IP and its cost. Local FAE support should be considered too.

The vendor's software tools can also factor into your decision. Download and play with them – no hardware required. You can still take a design all the way to the simulation stage. This is also a way to determine how big an FPGA you'll need, assuming your internal logic design is mostly done.

For a deep dive into FPGA waters, head to the various vendors' sites. Just make sure you have a day free if you hope to grok the vast amount of information presented (and not always as clearly as might be wished for). Altera and Xilinx are frontrunners, both in market share and leading-edge technology. Their parts use internal configuration RAM, so an external ROM with the configuration data is required to "boot" the parts (though both companies also have small, non-volatile, CPLD-like parts). Other vendors to consider are Microsemi/Actel, Lattice, and Cypress. Some features you'll find in their parts include very low quiescent power, ROM-based configuration for "instant-on" start-up, and analogue peripherals.

OK, you've settled on a vendor. Next, pick an FPGA family and size. Vendors tend to break up their offerings into several families, typically differentiated by some vague notions of low-, middle-, and high-end performance (and size range). How much on-chip RAM is needed? How many DSP/multiplier blocks, or gigabit transceivers? You'll probably need to scour the datasheets to figure out things like maximum clock frequencies and I/O delays to help you decide which family is right for you. Again, having your HDL code available is a big help, because the design software will let you know which parts it will fit into, and if they can meet your performance requirements.

Your application may also benefit from the ability to upgrade devices without changing the PCB. Some FPGA families contain subsets of pin-compatible devices, letting you switch to a larger (or cheaper and smaller) device as your needs change. Just be sure to design for the lowest common denominator pinout.

Don't forget to consider details like how I/O banks will be partitioned for different supply voltages and I/O standards, your PLL requirements, and DDR interfaces.

Scotty, we need more power!
It's usually not hard to calculate the maximum current required for a board. FPGA power supply design can be trickier though. The current required will largely depend on the logic design and clock frequencies. The same part could easily draw 0.5W with one design load, and 5W with another.

Development tools (or a separate program or spreadsheet) should provide power estimates for a given design, but they may require lots of additional information from you, some of which may be only educated guesses. If an FPGA devboard is available, it should have a way to measure the various supply currents. Some even have a built-in ammeter display! Just be sure to add lots of headroom to account for design changes and process/temperature corners.

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