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NEORV32

Overview

The NEORV32 is an open-source RISC-V compatible processor system intended as a ready-to-go auxiliary processor within larger SoC designs or as a stand-alone customizable microcontroller.

Fig. 205 NEORV32 (Credit: Stephan Nolting)

For more information about the NEORV32, see the following websites:

Supported Features

The neorv32 board configuration can be used a generic definition for NEORV32 based boards. Customisation to fit custom NEORV32 implementations can be done using devicetree overlays.

Zephyr currently supports the following hardware features of the NEORV32 Processor (SoC):

Interface	Controller	Driver/Component
INTC	on-chip	interrupt controller
MTIME	on-chip	system timer
GPIO	on-chip	gpio, non-interrupt
UART	on-chip	serial port-polling; serial port-interrupt
TRNG	on-chip	entropy

The default board configuration for the NEORV32 Processor (SoC) can be found in the defconfig file: boards/riscv/neorv32/neorv32_defconfig.

System Clock

The default board configuration assumes a system clock of 100 MHz. The clock frequency can be overridden by changing the clock-frequency property of the cpu0 devicetree node.

CPU

The default board configuration assumes the NEORV32 CPU implementation has the following RISC-V ISA extensions enabled:

C (Compresses Instructions)
M (Integer Multiplication and Division)
Zicsr (Control and Status Register (CSR) Instructions)

Internal Instruction Memory

The default board configuration assumes the NEORV32 SoC implementation has a 64k byte internal instruction memory (IMEM) for code execution. The size of the instruction memory can be overridden by changing the reg property of the imem devicetree node.

Internal Data Memory

The default board configuration assumes the NEORV32 SoC implementation has a 32k byte internal data memory (DMEM). The size of the data memory can be overridden by changing the reg property of the dmem devicetree node.

Serial Port

The default configuration assumes the NEORV32 SoC implements UART0 for use as system console.

Note

The default configuration uses a baud rate of 19200 to match that of the standard NEORV32 bootloader. The baudrate can be changed by modifying the current-speed property of the uart0 devicetree node.

True Random-Number Generator

The True Random-Number Generator (TRNG) of the NEORV32 is supported, but disabled by default. For NEORV32 SoC implementations supporting the TRNG, support can be enabled by setting the status property of the trng devicetree node to okay.

Programming and Debugging

First, configure the FPGA with the NEORV32 bitstream as described in the NEORV32 user guide.

Next, build and flash applications as usual (see Building an Application and Run an Application for more details).

Configuring a Console

Use the following settings with your serial terminal of choice (minicom, putty, etc.):

Speed: 19200
Data: 8 bits
Parity: None
Stop bits: 1

Flashing via JTAG

Here is an example for building and flashing the Hello World application for the NEORV32 via JTAG. Flashing via JTAG requires a NEORV32 SoC implementation with the On-Chip Debugger (OCD) and bootloader enabled.

Note

If the bootloader is not enabled, the internal instruction memory (IMEM) is configured as ROM which cannot be modified via JTAG.

# From the root of the zephyr repository
west build -b neorv32 samples/hello_world
west flash

The default board configuration uses an OpenOCD Debug Host Tools configuration similar to the example provided by the NEORV32 project. Other JTAGs can be used by providing further arguments when building. Here is an example for using the Flyswatter JTAG:

# From the root of the zephyr repository
west build -b neorv32 samples/hello_world -- -DBOARD_RUNNER_ARGS_openocd="--config;interface/ftdi/flyswatter.cfg;--config;neorv32.cfg;--cmd-pre-init;'adapter speed 2000'"
west flash

After flashing, you should see message similar to the following in the terminal:

*** Booting Zephyr OS build zephyr-vn.n.nn  ***
Hello World! neorv32

Note, however, that the application was not persisted in flash memory by the above steps. It was merely written to internal block RAM in the FPGA. It will revert to the application stored in the block RAM within the FPGA bitstream the next time the FPGA is configured.

The steps to persist the application within the FPGA bitstream are covered by the NEORV32 user guide. If the CONFIG_BUILD_OUTPUT_BIN is enabled and the NEORV32 image_gen binary is available, the build system will automatically generate a zephyr.vhd file suitable for initialising the internal instruction memory of the NEORV32.

In order for the build system to automatically detect the image_gen binary it needs to be in the PATH environment variable. If not, the path can be passed at build time:

# From the root of the zephyr repository
west build -b neorv32 samples/hello_world -- -DCMAKE_PROGRAM_PATH=<path/to/neorv32/sw/image_gen/>

Uploading via UART

If the CONFIG_BUILD_OUTPUT_BIN is enabled and the NEORV32 image_gen binary is available, the build system will automatically generate a zephyr_exe.bin file suitable for uploading to the NEORV32 via the built-in bootloader as described in the NEORV32 user guide.

Debugging via JTAG

Here is an example for the Hello World application.

# From the root of the zephyr repository
west build -b neorv32 samples/hello_world
west debug

Step through the application in your debugger, and you should see a message similar to the following in the terminal:

*** Booting Zephyr OS build zephyr-vn.n.nn  ***
Hello World! neorv32