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Honored Contributor
Honored ContributorFFT Megacore resource usage different between software and user guide
Dear all,
I post a message because I have some trouble with the resource usage of the FFT MegaCore function. I follow the procedure described in user guide (p 2-1 -> 2-7) to evaluate resource usage of the MegaCore function, and I find values differents from the user guide. Below, you have the details with two examples. The version of the software is the latest, 9.0. The target device is Stratix III. The transform length is 4096 points and the data & twiddle precision is 16 bits as specified in the user guide. I tried with the streaming architecture, using the 4 mults/2 adders complex multipliers structure as specified in the user guide. It is not specified in the user guide, but I let the implementation of multiplier by default (i.e. DSP Blocks / Logic Cells, anyway it is the one that consume the less ALUTs). With this parameter I obtain the following resource usage : ALUTs ___________________ 6077 Memory Bits ____________ 622592 M9K RAM Blocks _____________ 76 DSP Block 18-bit elements ____ 48 Whereas the user guide give these values (page 1-8) : ALUTs ___________________ 3732 Memory Bits ____________ 622848 M9K RAM Blocks _____________ 76 18 * 18 mults _______________ 24 The memory usage is approximately the same, but the number of ALUT is almost divided by two, and the number of multiplier is half the number of DSP block whereas each DSP block provides eight 18 x 18 multipliers (source : http://www.altera.com/products/devices/stratix-fpgas/stratix-iii/overview/architecture/st3-dsp.html) (http://www.altera.com/products/devices/stratix-fpgas/stratix-iii/overview/architecture/st3-dsp.html%29). I tried with other architectures, and it is the same. Here an example with burst data flow architecture, quad output engine architecture and one engine. The result is the same and the Block Throughput Cycles is also different. Me : ALUTs ___________________ 3504 Memory Bits ____________ 229376 M9K RAM Blocks _____________ 28 DSP Block 18-bit elements ____ 24 Block Throughput Cycles ___ 13421 User guide (page 1-10/11) : ALUTs ___________________ 1926 Memory Bits ____________ 229632 M9K RAM Blocks _____________ 28 DSP Block 18-bit elements ____ 12 Block Throughput Cycles ____ 6157 Can anybody tell me why I obtain values differents from the user guide ? Many thanks. Jérôme
