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.