You will always end up with an inaccurate estimate and one much worse than the physicist, because a digital circuit is so complicated.Īnyway, tools produce a lot of statistics. Or take physicists, who with some experience can pretty quickly do order-of-magnitude calculations. You may find a savant in some boiler room of a tech company for which that ain't true, but most engineers will correctly assume that it's not really a skill you should be wasting too much mental resources on and they will just let the tool show them. But nobody really does it beyond small scopes, because the complexity is so staggering. accountants will become very good at doing such a calculation. Kind of like how fast can you multiply 2 2-digit numbers. Of course the reasoning is only a "cache" that is filled from doing the actual measurements. How can i know how many LUT i need before i finish the whole program in fpga?įor small sections of your design you can manually reason about the LUT requirements.įor anything beyond simple building blocks you will have to synthesize the design and look at the statistics. Again though, getting an exact estimate is pretty much impossible with modern tools. Most designs (but certainly not all!) have around the same number of flops and LUTs (say within a factor of 1.5). If you can estimate Flip-Flop usage, you could take a guess based on that. The synthesis tools will do a lot of logical optimizations etc. Thus one of the most important phases of the FPGA CAD ow is the technology mapping step that maps an optimized circuit description into a LUT network present in the target FPGA architecture. They can be instantiated by hand if needed, but 99% of the time the synthesis/PAR tools will infer them for you.Įdit: As noted by others, guessing at LUT utilization is hard. number of LUTs needed to implement a given circuit de-termines the size and cost of the FPGA-based realization. That means any cloud of logic that can be represented by a truth table with 6 inputs and 2 outputs can be implemented in a single LUT. Different technologies will have different sizes, but a common size for Xilinx is 6 input / 2 output. The inputs to the cloud address into the memory and the value stored there is the desired output. Technology mapping is the last step of logic synthesis that performs the conversion from an AIG. Basically, they store the truth table for a cloud of combinational logic in a small memory. In FPGAs, the circuit is represented as a set of k-LUTs. They implement your combinational logic (AND, OR, NOT, XOR, etc.). LUTs (or Look Up Tables) are one of the primary primitives in most FPGAs.
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