How to select a FPGA

by trial and error..

Just place it and see.. start with a 150 and then go up or down as required.

selecting a FPGA?

FPGA?

Do you mean you're almost finish writing the HDL? If so the easiest thing to do is send it through your synthesizer and see what logic usage it reports.

If you haven't written the HDL, the next question is whether you think you have enough time to write the HDL before having to make a decission on a FPGA. The HDL doesn't have to be functionally correct, so you don't have to waste time debugging it. You just have to make sure the synthesizer isn't optimizing logic away due to a coding error. You still have to make a ballpark guess so you can do a sanity check on what the synthesizer reports.

I'm not sure if the following situation applies to you but here goes:

If the PCB folks need a decission sooner, then you gotta make an educated guess and tack on some safety margin. If you're lucky, there is a previous design which is very similar to yours, that you can use. If there's a lot of reptitive logic, that can make it easier since you just figure out the size of one block and multiply by how often it's instantiated. If your design is datapath heavy, then concentrate more on the datapath logic and make a rough guess for your control logic.

Also remember that a lot of FPGAs come in different sizes (logic wise) but have the same packaging/PCB-footprint. So you can give the PCB folks a package that they can start working with, while you have some freedom in picking a size. If your design turns out to be larger than you guess, you might be able to choose a larger size FPGA in the same package. You have to make sure such a larger part exists, what the lead time is, and the price. Be sure to ask your chip vendor for that information on all the sizes in a given package. Also remember that when it comes to choosing a package, the main thing is to make sure it has enough I/O for your needs, so usually you worry about I/O first, then logic.

--Vinh

Peter Alfke wrote: : Here is a beginning. : Look at your design: : How many flip-flops, how many BlockRAMs, how many multipliers (if any), : how many different clocks, and how many I/O? : It should only take minutes to answer these questions. Then you look at : the overview in the data sheet and select an FPGA that has at least what : you need.

Also have a look at the package option and the availability too...

Followup to: By author: In newsgroup: comp.arch.fpga

FPGA?

If you already have the RTL, the easiest is to let the synthesis tools tell you ... both the Xilinx and Altera tools have teh option of "pick a device". If the resulting device is very full, you may want to pick one size larger just in case you need to change your design in the future. This applies to some applications, but not to all.

-hpa