RISC-V on an FPGA, pt. 8

Some thoughts on SiFive Freedom U500 on the low-end Arty Board:

  • 256MB (249MB available), vs 128MB for the Nexys 4. However memory is used for the filesystem initramfs / tmpfs (see next point) so there is a trade-off between RAM and storage. Recall the Nexys 4 has a microSD card for storage.
  • The root filesystem is the initramfs, presumably loaded off the 16MB of SPI flash included with the board. So it’s more like an embedded target than something you could do any development on.
  • Slow – noticeably slower than lowRISC on the Nexys 4. However it’s also under half the price, and this is an FPGA design, and the performance of the real hardware will be completely different.
  • I was kind of expecting that ethernet would work, but in any case it doesn’t appear to work for me.
  • You can replace their Linux kernel and root image with your own — see section 5 in the documentation. However that step, as well as programming the board, requires the proprietary Vivado software. AFAICT there is no free software alternative.


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RISC-V on an FPGA, pt. 7

Today I’m going to try SiFive’s Freedom U500 64 bit RISC-V design on the very low-end $148 Arty Board. If you want more background into what SiFive are up to then I recommend watching this 15 minute video, but in brief they seem to be positioning themselves as a distributor and integrator of RISC-V.

The good thing is they compile everything you need including the Xilinx bitstream. The bad thing is that these are behind a registration wall with a poisonous license agreement (in particular, non-commercial use only). I hope this is only because of the bits of 3rd party proprietary IP they are using for ethernet, flash, UART, etc.

If you want to do this yourself, read the getting started guide here. Assuming you have Xilinx Vivado installed, following the instructions is completely straightforward except for one point: You need to do “Boot from Configuration Memory device” after programming. Anyway you will have a booting Linux/RISC-V in a few minutes.


After the cut, the boot output.
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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).

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.

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July 27, 2016 · 9:28 am

RISC-V on an FPGA, pt. 5

I’ve learned a few things about this process. These are just my random thoughts in no particular order:

  • You need the heavily proprietary Xilinx Vivado to compile the Verilog source code to a bitstream. However you can use free tools to write the bitstream to the FPGA (eg. xc3sprog). Edit: There is another option: You can write the bitstream onto the SD-card and switch the jumper JP1 to SD-card. On boot the FPGA will load the bitstream from the SD-card. This is actually much nicer because it means you don’t need to manually reprogram the FPGA every time you power it up.
  • The core lowRISC CPU is free, but that’s not everything that goes into the bitstream. Also in the bitstream are some peripherals, such as the UART, and those are Xilinx proprietary IP. So the bitstream isn’t fully free and more importantly isn’t redistributable. (Note, according to this talk there is a plan to fix this).
  • This is a nice summary of the v0.2 architecture, esp. page 4


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RISC-V on an FPGA, pt. 4

It boots!

lowRISC boot program
Load boot into memory
Load 11660 bytes to memory.
Read boot and load elf to DDR memory
Boot the loaded program...
[    0.000000] Linux version 3.14.41-g9a25e8d (rjones@moo.home.annexia.org) (gcc version 5.2.0 (GCC) ) #1 Mon Jul 25 19:07:50 BST 2016
[    0.000000] Available physical memory: 126MB
[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x00200000-0x07ffffff]
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x00200000-0x07ffffff]
[    0.000000] Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 31815
[    0.000000] Kernel command line: root=/dev/htifblk0
[    0.000000] PID hash table entries: 512 (order: 0, 4096 bytes)
[    0.000000] Dentry cache hash table entries: 16384 (order: 5, 131072 bytes)
[    0.000000] Inode-cache hash table entries: 8192 (order: 4, 65536 bytes)
[    0.000000] Sorting __ex_table...
[    0.000000] Memory: 124488K/129024K available (1725K kernel code, 120K rwdata, 356K rodata, 68K init, 211K bss, 4536K reserved)
[    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
[    0.000000] NR_IRQS:2
[    0.150000] Calibrating delay using timer specific routine.. 20.01 BogoMIPS (lpj=100097)
[    0.150000] pid_max: default: 32768 minimum: 301
[    0.150000] Mount-cache hash table entries: 512 (order: 0, 4096 bytes)
[    0.150000] Mountpoint-cache hash table entries: 512 (order: 0, 4096 bytes)
[    0.150000] devtmpfs: initialized
[    0.150000] NET: Registered protocol family 16
[    0.150000] bio: create slab  at 0
[    0.150000] Switched to clocksource riscv_clocksource
[    0.150000] NET: Registered protocol family 2
[    0.150000] TCP established hash table entries: 1024 (order: 1, 8192 bytes)
[    0.150000] TCP bind hash table entries: 1024 (order: 1, 8192 bytes)
[    0.150000] TCP: Hash tables configured (established 1024 bind 1024)
[    0.150000] TCP: reno registered
[    0.150000] UDP hash table entries: 256 (order: 1, 8192 bytes)
[    0.150000] UDP-Lite hash table entries: 256 (order: 1, 8192 bytes)
[    0.150000] NET: Registered protocol family 1
[    0.150000] futex hash table entries: 256 (order: 0, 6144 bytes)
[    0.150000] io scheduler noop registered
[    0.150000] io scheduler cfq registered (default)
[    0.180000] htifcon htif0: detected console
[    0.190000] console [htifcon0] enabled
[    0.190000] htifblk htif1: detected disk
[    0.190000] htifblk htif1: added htifblk0
[    0.190000] TCP: cubic registered
[    0.190000] VFS: Mounted root (ext2 filesystem) readonly on device 254:0.
[    0.190000] devtmpfs: mounted
[    0.190000] Freeing unused kernel memory: 68K (ffffffff80000000 - ffffffff80011000)
# uname -a
Linux ucbvax 3.14.41-g9a25e8d #1 Mon Jul 25 19:07:50 BST 2016 riscv GNU/Linux


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RISC-V on an FPGA, pt. 3

Compiling the lowRISC software, full GCC, Linux and busybox was straightforward. You have to follow the documentation here very carefully. The only small problem I had was their environment variable script uses $TOP which is also used by something else (bash? – not sure, anyway you just have to modify their script so it doesn’t use that name).

The above gets you enough to boot Linux on Spike, the RISC-V functional emulator. That’s not interesting to me, let’s see it running on my FPGA instead.

LowRISC again provide detailed instructions for compiling the FPGA which you have to follow carefully. I had to remove the -version flags in a script, but otherwise it went fine.

It wasn’t clear to me what you’re supposed to do with the final bitstream (./lowrisc-chip-imp/lowrisc-chip-imp.runs/impl_1/chip_top.new.bit) file, but in fact you use it in Vivado to program the device:


The simple hello world example was successful (output shown below is from /dev/ttyUSB1 connected to the dev board):


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RISC-V on an FPGA, pt. 2

The first step is to install the enormous, proprietary Xilinx Vivado software. (Yes, all FPGA stuff is proprietary and strange). You can follow the general instructions here. The install took a total of 41GB of disk space (no, that is not a mistake), and took a few hours, but is otherwise straightforward.

The difficult bit was getting the Vivado software to actually see the hardware. It turns out that Xilinx use a proprietary USB driver, and, well, long story short you have to run the install_drivers script buried deep in the Vivado directory tree. All it does is put a couple of files under /etc/udev/rules.d, but it didn’t do udevadm¬†control¬†--reload-rules so you have to do that as well, replug the cable, and it should be detectable in Vivado:


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