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I got into FPGA design last year for a project, and had some success with a Xilinx Spartan 6 dev board using ISE. I could do everything with this low cost board and ISE 14, which is free. I needed an embedded processor and used the excellent PicoBlaze - again thanks to great documentation this all worked very well and the project is currently in daily use.

Like many successful projects I can see things growing in future. I still have a fair amount of capacity on the FPGA, but the issue with PicoBlaze really is lack of a C compiler. I would like to find another processor that is very low cost or free, but for which I can build with gcc or whatever.

I had a look around opencores and some interesting things there - but a bit hard to tell what is actively maintained and has a user base. Can anyone recommend free/low cost embedded processor(s) that work well and are reasonably well documented?

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    \$\begingroup\$ I think ARM recently made some soft-cores free. \$\endgroup\$ – DKNguyen Oct 23 '19 at 21:34
  • \$\begingroup\$ @VoltageSpike not quite true: opencores.org/projects?expanded=Processor \$\endgroup\$ – JonRB Oct 23 '19 at 21:53
  • \$\begingroup\$ it looks like a comment is missing? VoltageSpike? \$\endgroup\$ – danmcb Oct 23 '19 at 22:30
  • \$\begingroup\$ I remember we were willing to pay $1m for the HC07 core until funding pulled the plug after 7 years. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 23 '19 at 23:09
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    \$\begingroup\$ or you could just use the microblaze and the gnu toolchain... \$\endgroup\$ – old_timer Oct 24 '19 at 2:21
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Besides the likes of RiscV, there is OpenRISC ( https://openrisc.io/ )

a free and open RISC instruction set architecture with DSP features

a set of free, open source implementations of the architecture

a complete set of free, open source software development tools, libraries, operating systems and applications

a variety of system-on-chip and system simulators

The or1200 implementation is done is verilog and is released under the LGPL license

https://github.com/openrisc/or1200

mor1kx is written in verilog and released under the OHLD license

https://github.com/openrisc/mor1kx

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I have heard a lot of good stuff about vexriscv. In general, there is a lot of work around the riscv instruction set, in terms of compilers, soft processors, and all sorts of ASIC implementations.

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The Hennessy & Patterson textbook now covers the RISCV architecture. Tons of academic papers on a wide range of RISCV implementations (including FPGA). Also there is a RISCV consortium consisting of multiple very large companies. So that open source architecture is well documented and likely to be well supported by software for a very long time.

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I have been using the Instant Soc RISC-V in a couple of projects now. It really works great. It build an optimized RISC-V core and all around that based on your C++ code. It uses gcc and a compiler that builds the HDL.

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