FPGA logic cells

0
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I have an small presentation about FPGA techonology. My questions is: If your FPGA has 85k logic cells, does this mean it can run 85k operations simultaneously?

What I am trying to achieve is to shock the audience with some crazy illustrated facts about FPGA technology or facts. The people who listens now very little about FPGA, so I want to impress them.

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logiccellfpgaxilinxzynq
If you want to go for shock, you can say it's a 85k-core processor! You should probably mention these cores run around 200MHz and can only perform basic binary operation though... Personally, I would go for the lower number of LUTs count, it's easier to explain what it does since it's actually physicial. Logic cells is a tentative marketing (thus falsified) attempt to compare to other technology. – Jonathan Drolet
I found this helpful - Logic cell concept in xilinx fpgas. Essentially logic cell is an abstraction depending on the architecture of a Configurable Logic Block, which is dependent on the device family. More information can be useful (add CLB Flip-Flops, CLB LUTs, max distributed RAM, Block RAM/FIFO, Total Block RAM, CMTs, I/O DLLs, HP/HR IOs, DSP Slices, Transceivers, interesting hard cells, ...) See Scary_Jeff's answer. – user1155120

3 Answers

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What's inside a 'cell' can vary per manufacturer, but the Xilinx definition (using this manufacturer as an example, as these are the devices that I'm familiar with) is one four-input look-up table, and one register. Xilinx devices are made up of a number of 'slices', and these contain a number of functional elements. These might include:

  • Look-up tables
  • Registers
  • Multiplexers
  • Logic for use in carry chains
  • etc

As an example, a Spartan6 LX4 has 600 slices, and the marketing material claims that this is equivalent to 3840 'logic cells'. You can look in the user guide for a device to determine exactly what is contained inside a slice.

In addition to this, there are other resources such as multipliers, memories, PLLs, etc.

I suppose you could say that one logic cell can perform one operation, but a single cell is only capable of very simple operations, for example an AND gate, 2:1 multiplexer, etc.

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I would say no, but it depends on what you mean by an operation. A logic cell has the capability to implement a number of logical functions (and/or/xor), and it has the ability to hold a state with storage elements. These two functions are how every digital system under the sun operates. Even addition and subtraction are higher level constructs built on top of logical functions. As in other answers, FPGA manufacturers publish guides on what is inside of their logic cell. It is this fundamental cell that is stamped repeatedly in the die to create this "array" as in Field Programmable Gate "Array".

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This yields a distinctly "more or less" answer. The logic blocks can be used in multiple modes, and you might even be able to pack more than one function in one (including with two independent outputs), but you must also be able to transport meaningful data to work on. It sounds like you have a 7z020 as an example. You may want to note that besides those logic cells, it also has 220 hardware multiply+add blocks. That amount is not random; the surrounding logic is enough to keep them fed in particular cases, every cycle. Looking in 7 Series FPGAs Configurable Logic Block User Guide (UG474), we find that the Logic Cells number given is an estimate of equivalent 4LUT+FF configurations. The reason this number is lower than the number of flipflops (106k) is that the input arguments for the two 5luts you can split a 6lut into must overlap.