kmod's blog

5Sep/134

Nexys 4

I've blogged in the past about my Nexys 3, though I haven't used it very much lately (other than leaving it in bitcoin-mining-mode, where it's earned me about ten cents in the past week).

I was browsing the Digilent website for some an ARM-based Raspberry-Pi equivalent (I already forget why), and I checked out their new products page and saw that they've just released the Nexys 4.  I think I'll get one of these eventually, since looking at the product page there are a lot of improvements to areas I was hoping to improve:

  • The biggest change is an upgrade to Xilinx's new 7-series fpga, the Artix-7.  There are some weird economics around the Artix-7, which I've been meaning to blog about, but the key point is that the XC7A100T-CS324 part they include -- I assume the full part number is  XC7A100T-1CSG324C -- starts at around $130 on Digikey, which makes the Nexys 4 look like a pretty good deal (for comparison, the Spartan 6 LX15 on the Nexys 3 starts around $28).  This Artix part is quite big, weighing in at 100k cells -- Xilinx originally planned on offering smaller sizes, but currently there are only the 100k and 200k variants.  100k cells is about 7 times the capacity of the Nexys 3 board; the 7-series includes a process and architectural upgrade as well, which presumably give power and speed improvements in addition to the capacity increase.
  • Less groundbreaking, but still nice, is that the peripherals are improved.  I'm probably the most excited about the cheapest ones they added: they increased the number of slide switches and leds from 8 to 16, and put two 4-digit seven segment displays on the board.  There are a bunch of other cool things like an audio jack, accelerometer, and temperature sensor as well; you can see the full list on their product page.

One thing to keep in mind is that the Xilinx software is quite expensive, and at least for my purposes I'd like to stay with chips their WebPack license supports; it took me a while to find, but here's the doc explaining compatibility.  For the Spartan 6 line, the WebPack license only goes up to the LX75, keeping the largest few chips reserved for paid usage.  For Artix, presumably because it's their low-cost chip and they only offer two variants, both the 100T and the 200T versions are supported in WebPack, offering quite a bit larger fpga capacities in Xilinx's non-hobbyist software tier.

So overall I'm very excited about the upgrades they made and it definitely looks like the Nexys 4 is much better than its predecessor, though personally I feel like I'm more at the point that I'd rather learn how to design my own FPGA board than pay another $300 for another dev board

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  1. Have you played with this board yet?
    I am trying to find a reference design that shows how to make the Xilinx EMC
    (external memory controller) work with the Nexys4 board to control the PSRAM on the board.
    Haven’t been able to locate any examples yet.

  2. Would it be feasible to make custom Virtex-7 board? Chips look quite expensive – BGAs. I wonder if its possible to make breadboard version like XuLa with all pins broken out and small footprint.

    • Taking a quick look at digikey, it looks like the cheapest Virtex-7’s start off at around $2.5k. If you break them out onto through-hole pins I imagine you lose most of the very-high-speed capabilities of the chip due to signal integrity issues, and you might as well go with a cheaper chip: the Artix-7 is 60% of the size at 10% of the cost, though you lose some speed. I guess theoretically it’s possible, but I’m not sure why you’d want to do it?

      And yes unfortunately all high-end FPGAs are BGAs now; I guess the people who are willing to spend a lot on them all want to use lots of IO.


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