LiteX VexRiscv

LiteX VexRiscv is an example of a system on a chip (SoC) that consists of a VexRiscv processor and additional peripherals. This setup has been generated using LiteX SoC Builder and can be used on various FPGA chips. The bitstream (FPGA configuration file) can be obtained using both vendor-specific tools and open-source tools, including the SymbiFlow toolchain.

The litex_vexriscv board configuration in Zephyr is meant for the LiteX VexRiscv SoC implementation generated for the Digilent Arty A7-35T Development Board.

LiteX is based on Migen/MiSoC SoC builder and provides ready-made system components such as buses, streams, interconnects, common cores, and CPU wrappers to create SoCs easily. The tool contains mechanisms for integrating, simulating, and building various designs that target multiple chips of different vendors. More information about the LiteX project can be found on LiteX’s website.

VexRiscv is a 32-bit implementation of the RISC-V CPU architecture written in the SpinalHDL. The processor supports M, C, and A RISC-V instruction set extensions, with numerous optimizations that include multistage pipelines and data caching. The project provides many optional extensions that can be used to customize the design (JTAG, MMU, MUL/DIV extensions). The implementation is optimized for FPGA chips. More information about the project can be found on VexRiscv’s website.

LiteX VexRiscv with SymbiFlow

To run the ZephyrOS on the VexRiscv CPU, it is necessary to prepare the bitstream for the FPGA on a Digilent Arty A7-35 Board. This can be achieved using the LiteX SoC Builder together with the SymbiFlow toolchain.

SymbiFlow is an Open Source Verilog-to-Bitstream FPGA synthesis flow, targeting FPGAs of multiple vendors. Currently, it targets the Xilinx 7-Series, Lattice iCE40, Lattice ECP5 FPGAs, QuickLogic EOS S3 and is gradually being expanded to provide a comprehensive end-to-end FPGA synthesis flow. More information about the project can be found on Symbiflow’s website.

Bitstream Generation

In order to generate the bitstream for the Digilent Arty A7-35 Board, proceed with the following instruction:

1. Install the SymbiFlow toolchain using instruction available in the symbiflow-examples repository.

2. Install the RISC-V toolchain:

   wget https://static.dev.sifive.com/dev-tools/riscv64-unknown-elf-gcc-8.1.0-2019.01.0-x86_64-linux-ubuntu14.tar.gz
   tar -xf riscv64-unknown-elf-gcc-8.1.0-2019.01.0-x86_64-linux-ubuntu14.tar.gz
   export PATH=$PATH:$PWD/riscv64-unknown-elf-gcc-8.1.0-2019.01.0-x86_64-linux-ubuntu14/bin/
   

3. Download LiteX:

   wget https://raw.githubusercontent.com/enjoy-digital/litex/master/litex_setup.py
   chmod +x litex_setup.py
   ./litex_setup.py init
   ./litex_setup.py install
   

4. Generate the bitstream:

   cd litex/litex/boards/targets && ./arty.py --toolchain symbiflow --cpu-type vexriscv --sys-clk-freq 80e6 --build
   

Programming and debugging

Building

Applications for the litex_vexriscv board configuration can be built as usual (see Building an Application). In order to build the application for litex_vexriscv, set the BOARD variable to litex_vexriscv.

Booting

You can boot from a serial port using flterm:, e.g.:

flterm --port /dev/ttyUSB0 --kernel <path_to_zephyr.bin> --kernel-adr 0x40000000