Breaking The Three Laws


Part Deux: How many ASIC Gates does it take to fill an FPGA?

Last week’s blog How many ASIC Gates does it take to fill an FPGA? definitely stirred the pot. Part Deux (two?) goes back to basics filling in some gaps and follows up with data supplied by my good friends over at Xilinx.

So first for clarification, apparently not everyone who reads my blog understands what a Xilinx Logic Cell, LC, is or what a Look Up Table, LUT, is. I was going to create a nice set of slides explaining but thanks again to Xilinx, here is one they prepared earlier.  It should be noted that Xilinx provided me with written permission to re-use these as part of my blog. The full (PUBLIC) Xilinx presentation can be found here:

The Xilinx Logic Cell basics, a cell including combinatorial logic, arithmetic logic and a register. Your RTL source code is “mapped” into these Logic Cells.

XIlinx Logic Cell

A Logic Cell is the basic building block within the Xilinx devices and there is a *lot* of these per device, 4.4 Million in the new Xilinx Virtex UltraScale VU440. Yes that’s right, they are very, very, very small.

Part of the Logic Cell is a Look Up Table which is used to implement combinatorial logic.

Xilinx LUT

My contact over at Xilinx also provided me the Xilinx used standard calculation of equivalent ASIC gates of the Xilinx devices.

–//– From Xilinx

Here are the basic principles that we’ve tried to stick to when generating ASIC gate count numbers:

  • –       6 to 24 gates per LUT (depending on the number of inputs used)
  • –       RAM bits are equivalent
  • –       Up to 100 ASIC gates per I/O;
  • –       7 gates per register

So for the VU440 here’s how this shakes down:


LUTs :

  • Minimum    6 x 2,532,960 LUTs = 15,197,760
  • Maximum 24 x 2,532,960 LUTs =  60, 791, 040 ASIC gates

IOs :  100 x 1456 = 145600 ASIC gates

Registers : 7 x 5.065,920 = 35,461,440 ASIC gates

So by this math we could have claimed anywhere from 50,804,800 to 96,398,080 ASIC gate equivalent in the VU440.

–//– End From Xilinx

So great, this passes what I call the basic “Stupid” test, as in there is sound logic and defendable data behind the calculation. It also highlights the large variance in “gate counts” which is significantly influenced by the number of inputs used as part of the look up table calculation. It’s exactly as I stated in last week’s blog, the conversion from ASIC gates to FPGA ASIC gate equivalent is design specific. Some designs will map well, some not so much.

In the material that Xilinx supplied I also spotted this slide which reinforces this point.

ASIC to FPGA mapping considerations

Specifically for Prototypers, the conclusion is very important. The Prototypers goal is to NOT modify the golden RTL source for prototyping otherwise you will not be validating a true representation of the design. Yes, some modifications are needed such as RAM’s, gated clock conversion but these can be verified as identical and are an acceptable trade-off. Outside of this the RTL is not customized for FPGA meaning that many of the dedicated resources cannot be directly mapped to. This leads to inefficient mapping of the RTL code to FPGA and is why a “fudge” factor is required in the ASIC gate equivalent calculations. The restriction is typically the Look Up Tables for combinatorial logic mapping, you run out of those before anything else. (not all the time but most of the time). By being conservative in the ASIC gate count capacity claims the vendor can ensure that expectations are met for a majority of designs

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