Enjoy Digital LiteX FPGA's

The LiteX framework provides a convenient and efficient infrastructure to create FPGA Cores/SoCs, to explore various digital design architectures and create full FPGA based systems.

Information specific to Litex and supported boards can be found on the project's homepage: https://github.com/enjoy-digital/litex

Nuttx has basic support for two softcores

• vexriscv: FPGA friendly RISC-V ISA CPU implementation
• vexriscv_smp: A more fully featured, Linux compatible core.

Currently, the only configured development board in the Arty A7 https://digilent.com/reference/programmable-logic/arty-a7/start. However, many Litex supported boards should work with either core, requiring minimal adjustment to the configuration.

Toolchain

Litex projects can be built with a generic RISC-V GCC toolchain. There are currently two options.

Prebuilt toolchain

A prebuilt RISC-V toolchain from SiFive can be used to build Litex projects:

# Download the prebuilt toolchain
 curl https://static.dev.sifive.com/dev-tools/riscv64-unknown-elf-gcc-8.3.0-2019.08.0-x86_64-linux-ubuntu14.tar.gz \ 
                  > riscv64-unknown-elf-gcc.tar.gz

# Unpack the archive
 tar -xf riscv64-unknown-elf-gcc.tar.gz 

# Add to path
 export PATH="HOME/path/to/riscv64-unknown-elf-gcc-8.3.0-2019.08.0-x86_64-linux-ubuntu14/bin:PATH

Custom built toolchain

The toolchain needs to be compiled locally in order to use a more modern version. At the time of writing, the source can be obtained from https://github.com/riscv-collab/riscv-gnu-toolchain and built with the following configuration:

 CFLAGS="-g0 -Os"
 CXXFLAGS="-g0 -Os"
 LDFLAGS="-s"

 ./configure \
  CFLAGS_FOR_TARGET='-O2 -mcmodel=medany' \
  CXXFLAGS_FOR_TARGET='-O2 -mcmodel=medany' \
  --prefix=path/to/install/to \
  --with-system-zlib \
  --with-arch=rv32ima \
  --with-abi=ilp32

  make

Important

The vexriscv_smp core requires [with-arch=rv32imac]{.title-ref}.

Check the linked github repository for other options, including building with multilib enabled.

Device tree support

Currently, the litex port requires that the memory mapped peripheral addresses and IRQ numbers match those generated by LiteX. Although, this approach is being phased-out in favour of using a flattened device tree (FDT) to dynamically instantiate drivers.

Generating and compiling the device tree:

 ./litex/tools/litex_json2dts_linux.py path/to/built/gateware/csr.json > board.dts
 dtc -@ -I dts -O dtb board.dts -o board.dtb

Ensure the board.dtb is placed in the NuttX root directory.

If a peripheral isn't working with the LiteX generated gateware, consider checking the addresses and IRQ numbers in

• arch/risc-v/src/litex/hardware/litex_memorymap.h
• arch/risc-v/include/litex/irq.h

Core specific information

cores//

Supported Boards

boards//