RISC-V Playground

If you want to dive into the RISC-V architecture, have a look at the RISC-V Playground.

This projects contains a beginner friendly RISC-V ‘fantasy microcontroller’ for the FPGA featuring a RV32IMC processor and a selection of peripherals:

• Textmode LCD driver with 7-Bit ASCII font
• Random number generator
• GPIO registers for PMOD pin access
• Timer tick interrupt
• LEDs
• UART terminal, 115200 Baud 8N1
• 1 kb initialised RAM for bootloader
• 128 kb RAM initialised using file read interface over SPI

Detailed descriptions, memory map and register set are described in the README file.

Docs on RISC-V itself

• Instruction set quick reference, recommended: http://www.riscvbook.com/greencard-20181213.pdf

• Complete specifications: https://riscv.org/technical/specifications/ See Volume 1, Unprivileged Spec for instruction set, and Volume 2, Privileged Spec for interrupt infrastructure.

Quickstart

Clone both the bitstream tools repo

git clone https://github.com/badgeteam/mch2022-tools/

and the FPGA repo

git clone --recursive https://github.com/badgeteam/mch2022-firmware-ice40/

go to the

mch2022-firmware-ice40/projects/RISCV-Playground/

folder and load both the bitstream for the FPGA and a RISC-V binary:

webusb_fpga.py riscv-playground.bin 0xdabbad00:fw/tinyblinky/tinyblinky.bin

Connect to the serial terminal using your favourite terminal emulator with 115200 baud 8N1 LF on ttyACM1.

Get RISC-V assembler

The GNU binutils for RISC-V include the assembler.

Unlike as for the FPGA tools that change rapidly, you can just have a look for binary packages in your distribution.

For Debian 11 Stable “Bullseye”, one gets using

apt-cache search binutils | grep riscv

binutils-riscv64-linux-gnu - GNU binary utilities, for riscv64-linux-gnu target
binutils-riscv64-linux-gnu-dbg - GNU binary utilities, for riscv64-linux-gnu target (debug symbols)
binutils-riscv64-unknown-elf - GNU assembler, linker and binary utilities for RISC-V processors

Both binutils-riscv64-linux-gnu and binutils-riscv64-unknown-elf are fine, but you might have to adjust the actual invocations to the tools depending on which package(s) you actually installed.

Despite the names, these also support 32 bit RISC-V targets.

Example firmware

Bootloader

This one is included into the bitstream for default. It initialises the LCD display and initialises the 128 kb RAM from file “0xdabbad00” using the file read interface over SPI provided by the ESP32 firmware.

Tinyblinky

A little blinky in RISC-V assembler. A nice “hello world” project.

Interrupt

An example on how to use interrupts on RISC-V, including notes on compressed opcodes and and small tools for printing hex numbers.

Mandelbrot

Explore the Mandelbrot and Tricorn fractals in ASCII art. This example shows how to use the LCD and buttons in assembler.

Hello GCC

A small project in C featuring serial terminal, buttons, LED and LCD.

Forth

This is a port of Mecrisp-Quintus, a 32 Bit Forth implementation, available under GPL3.

For more info, get the full release of Mecrisp-Quintus here:

http://mecrisp.sourceforge.net/

Useful for debugging, and maybe for you, too.

If you have not used Forth before, better start with this implementation of Forth that comes with much more badge support code.