FreeRTOS on NEORV32

• Requirements
• How To Run
• Porting Details

This repository provides a full-featured port of FreeRTOS for the NEORV32 RISC-V Processor. It implements a simple demo derived from the FreeRTOS "blinky" demo applications that is extended with some processor-specific features.

Requirements

Tools and Framework

• NEORV32 submodule
• FreeRTOS kernel-only submodule
• Prebuilt RISC-V GCC toolchain
• GHDL for simulating the processor, optional

Minimal Processor Configuration

• CPU architecture and ISA extensions: rv32i_zicsr_zifencei
• Memory
  • IMEM (instruction memory): 16kB
  • DMEM (data memory): 8kB
• Peripherals
  • RISC-V machine timer (MTIME)
  • general-purpose timer (GPTMR), optional
  • UART0 as interface console
  • general purpose IO controller (GPIO); high-active LED connected to bits 1:0, LED at pin 0 is used as "heart beat"

How To Run

1. Clone this repository recursively (to include the submodules):

$ git clone --recurse-submodules https://github.com/stnolting/neorv32-freertos.git

2. Install a RISC-V GCC toolchain that is able to emit code for a 32-bit architecture. Make sure that (at least) the required ISA extensions are supported. An exemplary prebuilt toolchain for x86 Linux can be download from:

github.com/stnolting/riscv-gcc-prebuilt

3. Navigate to the demo folder and compile the application:

neorv32-freertos/demo$ make clean_all exe

Tip

You can check the RISC-V GCC installation by running make check.

4. Upload the generated neorv32_exe.bin file via the NEORV32 bootloader:

<< NEORV32 Bootloader >>

BLDV: Jul 28 2023
HWV:  0x01090003
CLK:  0x05f5e100
MISA: 0x40901105
XISA: 0xc0000fbb
SOC:  0xfffff06f
IMEM: 0x00008000
DMEM: 0x00002000

Autoboot in 8s. Press any key to abort.
Aborted.

Available CMDs:
 h: Help
 r: Restart
 u: Upload
 s: Store to flash
 l: Load from flash
 x: Boot from flash (XIP)
 e: Execute
CMD:> u
Awaiting neorv32_exe.bin... OK
CMD:> e
Booting from 0x00000000...

<<< NEORV32 running FreeRTOS V10.4.4+ >>>

GPTMR IRQ Tick
GPTMR IRQ Tick
GPTMR IRQ Tick

Tip

Alternatively, you can also use the processor's on-chip debugger to upload the application via the generated main.elf file.

5. If you have GHDL installed you can also run the demo in simulation using the processor's default testbench / simulation mode:

neorv32-freertos/demo$ sh sim.sh

Porting Details

The processor-specific FreeRTOS parts are configured by two files:

• FreeRTOSConfig.h (customize according to your needs)
• freertos_risc_v_chip_specific_extensions.h (do not change!)

The NEORV32-specific parts are configured right inside the main.c file and the according makefile. The hardware abstraction layer (HAL) is provided by the NEORV32 software framework, which also provides the start-up code and linker script.

As the linker script is also responsible for configuring application- and setup-specific memory layout the actual configuration has to be overridden according to the application setup. For example the heap size is configured by configTOTAL_HEAP_SIZE in FreeRTOSConfig.h. This size also needs to be configured for the linker script, which is done inside the makefile:

override USER_FLAGS += "-Wl,--defsym,__neorv32_heap_size=3500"

Tip

More information regarding the NEORV32 software framework can be found in the online data sheet.

The NEORV32 supports all RISC-V exceptions and interrupts plus additional platform-specific interrupts. As FreeRTOS only supports the MTIME timer interrupt and the "environment call" exception two additional functions are provided to handle platform-specific exceptions and interrupts:

void freertos_risc_v_application_interrupt_handler(void);
void freertos_risc_v_application_exception_handler(void);

These functions are populated in the main.c file to showcase how to attach handlers for these traps.