mruby-denko-milkv-duo

June 13, 2025 ยท View on GitHub

mruby-denko-milkv-duo Demo

The Milk-V Duo is a tiny (same footprint as a Raspberry Pi Pico) single board computer, but with a 1 GHz RISC-V CPU running Buildroot Linux. This project makes it simple to do electronics programming on it in mruby. It implements Denko::Board compatible with the CRuby gem, so all those peripheral drivers/classes can be used as-is, and are included in the the mruby build. About 50 are implemented so far.

Supported Milk-V Boards

  • Milk-V Duo 64M
  • Milk-V Duo 256M
  • Milk-V Duo S

Installation

  • Download the appropriate Buildroot Linux image for your board, from the official repo.
  • Using balenaEtcher or a similar tool, flash the image to a micro SD card.
  • Insert the SD card into your board, and connect it to your computer.
  • Download and unzip the mruby binaries for your board, from the releases section of this repo.
  • The Milk-V Duo creates a new network interface on your comptuer, so you can SSH/SCP into it.
  • Its IP address is 192.168.42.1, default username is root, and password is milkv
  • Copy the binaries onto the board: 
    # Some older macs may require you omit the -O flag
scp -O UNZIPPED_BINARY_FOLDER/* root@192.168.42.1:/usr/local/bin
  • SSH into the board: ssh root@192.168.42.1
  • Try the mirb shell, or copy over examples from this folder, and try them with mruby FILENAME.rb
  • If you have issues with PWM / I2C / SPI, see the Pinmux section below.

Pinmux

Some pins on the Duo can perform multiple functions, but only one at a time (multiplexing).

Default (at Boot) Multiplexing Setup

Milk-V Duo Default Pinout

Notes:

  • The Linux boot console uses UART4 by default.
  • Hardware SPI2 (or SPI3 on the Duo S) is the only usable SPI device available. It becomes /dev/spidev0 in both cases. When initializing Denko::SPI::Bus give no index at all (default 0), or index: 0.

Remultiplexing

  • View the current multiplexing setup at any time with duo-pinmux -l or by calling Board#map from mruby.
  • Use duo-pinmux to remux pins as needed. See official documentation here.

Notes:

  • Unlike most Linux systems, remuxing does not require a reboot to take effect.
  • However, changes made by duo-pinmux are not saved across reboots.
  • GP26 and GP27 must be muxed to their default GPIO functions for the SARADC (analog input) to work.

Known Issues

  • Sensor::QMP6988 works only on I2C::BitBang buses, not on hardware I2C.
  • Sensor::RCWL9620 does not work at all.

Limitations (relative to CRuby Denko)

  • mruby does not have Thread. Methods like #poll and #blink will not work, since they cannot start their own threads in the background to update peripheral state, like they do on CRuby.
  • Board#digital_listen is implemented however, but in a C thread that collects pin change events.
    • It does not use interrupts, but polls, aiming for ~10 kHz.
    • Peripherals like RotaryEncoder will not be as accurate as they are on denko-piboard.
    • It still cannot update DigitalIO instances and run callbacks in the background.
    • Do this by calling Board#handle_listeners periodically, in your application loop.
  • Board#analog_listen is not implemented at all, as it would be too slow.
  • Board#spi_listen is not implemented. SPI::InputRegister#listen will not work.
  • Milk-V Duo has no on-board digital-to-analog converters (DACs), but the AnalogIO::Output class is included. It should work with external DACs, when support for these is added.
  • Milk-V Duo has no on-board EEPROM. EEPROM::Board is automatically included, but will not work.
  • UART hardware is available, but mruby support is not implemented yet.
    • The UART classes are automatically included in the build, but will not work yet.
    • Consequently, Sensor::JSNSR04T also will not work.
  • /dev/i2c-0 is fixed at 400 kHz and /dev/i2c-1 is fixed at 100 kHz. These are reconfigurable, if you build your own Linux image from here.

Build Instructions

  • On Ubuntu 24.04: sudo apt install wget git make gcc
  • Install Ruby 3.3 or later, from apt, rbenv or elsewhere.
  • Clone mruby at this commit or later.
  • Clone the Milk-V Duo SDK, so duo-sdk and mruby are in the same directory.
  • Copy the build config from this repo into mruby/build_config
  • Edit MILKV_DUO_VARIANT in the copied file to match your board: 64M, 256M or S.
  • Modify the build config as neeeded.
  • In mruby root: rake MRUBY_CONFIG=build_config/denko_milkv_duo.rb
  • When completed, the cross-compiled binaries will be in mruby/build/#{MILKV_DUO_VARIANT}/bin
  • Folow the install instructions above.