The marketing specsmanship here is based on reasoning such as: “We have a carry line that our competitor does not, and we estimate that in 1.9% of circumstances it will allow us to implement a function in 4 logic cells rather than 5, so we are going to claim a. However, their Logic Cell and Logic Element counts are based on their estimate of the equivalent numbers of 4-input LUTS that would be required to implement the same logic as the number of actual, physical logic elements on their FPGAs. Both companies long ago abandoned the 4-input LUT for more efficient, wider, LUT-like structures. For Xilinx and Altera, at least, the number of “Logic Cells” and “Logic Elements,” respectively, do not correspond to any structures one would find on the chip.
Fpga lut to asic gates free#
Note that this metric is not by any means free from the influence of marketing’s crafty hand, either. Luckily (meaning much to the chagrin of those of us who make some of our living poking fun at FPGA marketing), the industry quietly moved to a more realistic estimate system based loosely around the concept of 4-input LUT equivalents. So, an FPGA with a million System Gates might be equivalent to an ASIC with 100K logic gates – or less. The best estimates in the hallway at the time were somewhere around 10:1. Therefore, the number of useful, equivalent gates on an FPGA compared with a typical ASIC was dramatically lower than the System Gate count. Furthermore, the logic on an FPGA was far less efficient than that on an ASIC, and most FPGAs could not handle anything near 100% utilization. The problem with this approach, of course, is that a very small percentage of those transistors were actually used to create something that could reasonably be called a “gate.” The majority of transistors on an FPGA were used for interconnect fabric which, according to ASIC designers at the time, had a “gate” value of zero.
More realistically, we think they might have taken the total number of transistors on the chip, divided by the number of transistors that were thought to be needed to construct a typical logic gate, and using that as the System Gate total. The best we could determine at the time, System Gates were determined by taking the competitor’s similar device, looking at the datasheet, multiplying their System Gate total by 1.15, and issuing a press release claiming that their device was 15% larger. Or, at least nobody who remembers is admitting it. What is a System Gate? (We hear you ask…)
Once Xilinx and Altera had gone after each other with the System Gate ruse, the other challengers really had no alternative but to fall in line. In an effort to make their devices seem useful and, more importantly, bigger than the other guys’, device densities were given in terms of spectacularly optimistic “System Gates.” Just about the whole industry was complicit in this facade. 0 for combinational circuit and 1 for sequential circuit.In the early days of FPGAs, marketing was all bluster. Here, we need to choose whether we want our output through combinational circuit or sequential circuit. And then output of LUT goes through two paths. And while we give input to FPGA it goes through LUT via address bar and access specific memory address in LUT.
By using program we store output of truth table for say AND gate in LUT. AND, OR, NOT or any other complex logic gate. LUT is used to configure any type of logic gates. How can I get two output from LUT? What I am doing wrong?įrom study what I understood about FPGA is, This image represent building block of FPGA which is CLB. For this I need to design CLB, more specifically LUT for half adder. I want to implement half adder using FPGA.
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