Harucom-board

Harucom is about a $20 handmade single-board computer that demonstrates how a standalone Ruby computer can be built with mruby. It provides a complete programming environment with display output and keyboard input, all without a host PC.

Built around the RP2350A microcontroller, the board features DVI video output, USB host for keyboard input, stereo PWM audio, and Grove/SPI expansion connectors.

The name "Harucom" does not have one strictly defined meaning. For example, it stands for:

• Heuristically Accessible Ruby Computer
• Hakodate RubyKaigi Component
• Harukasan's Ruby Computer

Harucom aims to be an accessible computer where people can learn Ruby programming intuitively. By reducing the complexities of modern computing, I hope it allows users to dive into the essence of programming.

Features

• MCU: Raspberry Pi RP2350A
• Memory: 16 MB QSPI Flash + 8 MB QSPI PSRAM
• Video: DVI output via Type-A connector
• Audio: Stereo PWM audio via 3.5mm jack
• USB-C: Power supply & Data
• USB-A: Host port
• Input: 8 tactile buttons
• Expansion: 2x Grove (I2C/UART/ADC/GPIO), SPI, SWD

Block Diagram

---
config:
  flowchart:
    curve: linear
---
graph LR
    subgraph USB_PWR[USB & Power Supply]
        USBC([USB-C connector]) -- "5V" --> PWR["3.3V LDO"]
    end
    subgraph RP2354[RP2350A]
        MCU["RP2350A<br>Microcontroller"]
    end
    subgraph DVI_OUT[DVI Output]
        DVI(["HD Video<br>Type-A 19-pin connector"])
    end
    subgraph USB_HOST[USB Host]
        USBH([USB Type-A connector])
    end
    subgraph PWM_AUD[PWM Audio]
        AUD["Buffer"] --> JACK([3.5mm Audio Jack])
    end
    subgraph ADC_PAD[ADC Pad]
        ADC1["Resistor Ladder"] --> ADC_BTN1(["Buttons x 4"])
        ADC2["Resistor Ladder"] --> ADC_BTN2(["Buttons x 4"])
    end
    subgraph CONNECTORS[Connectors]
        direction TB
        CONN1(["Grove socket / 5V"])
        CONN2(["Grove socket / 3.3V"])
        CONN3(["1x06 socket / 3.3V"])
        CONN4(["1x05 + 1x02 sockets / 5V"])
    end

    USBC -- "USB" --> MCU
    PWR -- "3.3V" --> MCU
    MCU -- "HSTX" --> DVI
    MCU -- "PIO USB" --> USBH
    MCU -- "PWM" --> AUD
    MCU -- "ADC" --> ADC1
    MCU -- "ADC" --> ADC2
    MCU -- "I2C" --> CONN1
    MCU -- "UART/I2C/ADC" --> CONN2
    MCU -- "SPI" --> CONN3
    MCU -- "UART + SWD" --> CONN4

Loading

Directory Structure

harucom-board/
├── *.kicad_sch          # Schematic files
├── harucom-board.kicad_pcb  # PCB layout
├── harucom-board.kicad_pro  # KiCad project file
├── docs/                # Documentation
│   └── harucom-board-schematic.pdf
├── lib/                 # Custom symbol and footprint libraries
│   ├── Harucom_Board.kicad_sym
│   ├── Harucom_Board.pretty/
│   └── ...
└── jlcpcb/              # JLCPCB fabrication output
    └── production_files/

Documentation

• Schematic (PDF)

Tools

• KiCad 9.0

License

© 2026 Shunsuke Michii

This project is licensed under the CERN Open Hardware Licence Version 2 - Permissive (CERN-OHL-P v2).