RISC-V on an FPGA, pt. 6

(Click for larger image)

The board on the left is the Digilent Nexys 4 DDR which I was using yesterday to boot Linux. It costs £261 (about $341) including tax and next day delivery. The board on the right is the cheaper Digilent Arty Board, which cost me $148 (two day delivery from the US) + £20 tax.

There are clear differences in the number of connectors, LEDs and buttons. The Nexys has VGA, ethernet, USB, 8 digit LED, many lights, a temperature sensor, and lots of buttons. The Arty has just the bare minimum as you’d expect given the price difference. Both boards use the same Xilinx Artix-7 family of FPGA, but they are not exactly the same. The expensive board on the left uses the XC7A100T which has 100K+ logic cells, the cheap board on the right uses the XC7A35T with 33K cells. What this means practically is that you are more limited in the size of designs which can be programmed on the smaller chip. Does this matter for RISC-V? Yes and no. The untether-v0.2 design I was using yesterday takes about 50K cells so it won’t fit on the smaller board. However SiFive apparently (I have not checked) have a reduced design that will fit. (Note that none of this affects the operating system or available RAM — those are separate issues. However a smaller design will have to cut a few corners, leave out parts of the chip that implement optimizations, etc and so will generally run slower).

Oddly the Arty Board (on the right) has more DDR3 RAM — 256MB vs 128MB. That is an improvement, since doing any real Linux work in 128MB of RAM is tough, but still not massive. The other significant difference is the Arty Board does not have a microSD-card socket. It has (just) 16MB of on-board flash instead. I’m not clear how you get Linux on there, but that’s what I’ll be exploring.

Finally it’s worth saying that both boards are incomplete, although in very minor ways. The Nexys 4 comes with an OTG USB cable, which is all you need to power the board, program it and use the serial port. However it omits a microSD-card which you will need to store Linux / other RISC-V software that you want to run. The Arty comes without any cables and thus requires that you supply an OTG USB cable. As mentioned above there seems to be no microSD-card option at all.