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Confused about FPGA devices recommended by Xilinx for my FFT project

Hi everyone.

I have selected spartan 3E family for an FFT project due to its low cost. The FFT core (v 4.0) specs say that for spartan3E devices, the footprint is as follows (see table) and they "recommend" the device in the table.

FFT =EF=BB=BFLength Slices Block RAMs 18x18 Mults Device

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256 485 2 2 xc3s500e 1024 517 3 2 xc3s1200e 8192 578 18 2 xc3s1600e

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However, based on device datasheets, the devices recommended are way overkill. For example, lets take FFT length of 8192.

We need 578 slices, 18 BRAM, 2 mults. Lets look at the family Spartan 3E:

Device System Gates Slices BRAM 18x18mult

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XC3S100E 100K             960	  72kbit         4
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Reply to
Telenochek
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Hi Telenochek, acording to the numbers you gave the advice from Xilinx was correct. In your case it's not the number of LUTs that defines the border, but the BRAMS.

You need 18 BRAMs, each has a size of 18KBit in S3E devices. Datasheet Page 4 says: ? Block RAM provides data storage in the form of

18-Kbit dual-port blocks.

so 18 x 18 x 1024 = 331776 Bits

and so the closest fitting device is the S3E500 with 360 KBits.

And that's what is said in the 4.1 Datasheet table 28 and 30. In table 24 and 26 you find the s3E1600 Device, but the cores listed in there need more BRAMS, and Multiplyers, so they don't fit in the S3E500 anymore. also, there are max Clock frequencies given in these tables. Maybe these can't be acheived with the smaller devices due to mapping or routing problems.

But anyway, if you are still unsure about the device you should choose just make a test-synthesis and implementation. If there's something wrong with the size of the chosen device you will know instantly, after the error messages and warnings are printed out. Takes just a few minutes.

Have a nice synthesis

Eilert

Telenochek schrieb:

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