FFT: Valid Values of Number of Stages Using Block RAM

If you have used the FFT block in System Generator, you probably have seen the error message below.

You've got to love these "you-did-something-wrong-but-I-am-not-gonna-tell-you-how-to-fix-it" error messages. If you read through the datasheet, you will find it's not any better as it asks you to run Core Generator to get the valid values for the FFT transform length you chose. I've gone through this quite a few times, so I put together the table below showing valid values of "Number of Stages Using Block RAM" for all supported FFT sizes (data extracted from CoreGen 12.1) for me to use it as a quick reference. I hope this will also save other people a few minutes by not having to run CoreGen to get this simple info which should have been clearly stated in the error message.

FFT Size	Number of Stages Using Block RAM
8	0
16	0
32	0
64	0-1
128	0-2
256	0-3
512	0-4
1024	1-5
2048	2-6
4096	3-7
8192	4-8
16384	5-9
32768	6-10
65536	7-11