![]() Here is an example in Verilog for 6-input LUTs on Xilinx FPGAs. The x axis is the suite of benchmark designs, ordered from the lowest to highest 6-input LUT/ALM ratio. The y axis shows the ratio of 6-input LUTs to ALMs required to implement each design. You can also instantiate an LUT primitive and store the data you want. Figure 3 shows the device utilization compar ison between the 6-input LUT and ALM architectures across the benchmark suite defined above. But given 2-inputs, there’s lots of possible output combinations, which all must be possible to satisfy given a 2-input LUT. Each LUT on Xilinx and Intel (formerly Altera) FPGAs mostly has one output, hence a 4-input LUT stores 16-bit data. The above examples show a 2-input LUT that has been configured to be an AND gate and an OR gate. If it's for a single prototype, just use the biggest FPGA you can afford. A Look-Up Table (LUT) is how any arbitrary Boolean logic gets implemented inside your FPGA. Bob, the equations you present are precisely what I am using in my hand-written LUT62-based implementation. I suggest you choose your vendor, port enough of your design to get an idea of how big a FPGA you need and choose a FPGA with an upgrade path (if you want to market). The same design on two different foundries should have similar system gates number, as waste is not really an issue for ASIC. Keep in mind one LUT has only one output. So using one LUT6 we can realise any 6-input or less 1 output boolean functions. How do we instantiate LUTs directly in verilog scripts. System gates is a common measure of ASIC design complexity. Fig: LUT-6 from two LUT-3, Altera FPGA Architecture White Paper. Same with fast-carry logic, I don't know if they count that in equivalent gate number, but be advised that number is inflated. A Xilinx FPGA should fit 1.5 times the logic of an Altera FPGA, since it's LUT have 6 instead of 4, right? Well, it largely depends on the design, if the design can't use 6-inputs much, the unused ones are wasted. They are aggregated in logic blocks which has other features like fast-carry chain, registers and distributed memory.Ĭonverting to system gates is useful, but don't forget it's also a marketing war. In their most recent architecture, Xilinx use 6-input LUT and altera 4-input LUT. Xilinx use LUT, Altera LE, microsemi/lattice possibly something else. LUT, Logic Cell and Logic Element are all the same to me: the most basic FPGA general logic primitive.
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