Game Boy Cell Phone Cartridge Project: Design Overview

Goal:
Build a fully self-contained cell phone inside a Game Boy cartridge that functions independently as a phone. When plugged into a Game Boy, the console acts as a dumb terminal/interface to manage calls, texts, and phone status.

Core Hardware Design

    Custom Cartridge:

        Houses all the phone electronics and power.

        Key components:

            Cellular module (e.g., SIM800L or SIM7000G for 2G/4G calling and SMS),

            Microcontroller (e.g., RP2040) to manage:

                Cellular control (AT commands),

                Communication with the Game Boy bus,

                Audio routing and logic.

            Battery (~1000mAh LiPo),

            Charge circuit and protection,

            Optional Bluetooth module (for wireless audio or headset support).

    Audio Path:

        Incoming and outgoing audio routed through:

            Game Boy speaker and a hacked-in microphone,

            Or via Bluetooth for higher-quality calls.

    Power:

        Fully self-powered by the cartridge battery.

        Game Boy functions only as a UI and does not power the phone.

    Custom Cartridge Shell:
 
	     Designed larger (thicker) than a standard cartridge (~20mm deep) to fit:

            Battery,

            Cellular hardware,

            Antennas,

            Ports (such as micro-USB for charging).

Software/Firmware Design:

    Cartridge Firmware (Embedded):

	     Written in C (RP2040 SDK).

        Responsibilities:

            Phone functionality (calls, texts, signal strength),

            Exposing status bytes to the Game Boy via specific memory addresses (e.g., addresses 0xA000 through 0xA010),

            Accepting commands (dial, answer, hang up) from the Game Boy.

    Game Boy UI (Terminal Program):

        Written in C using GBDK-2020 (Game Boy Development Kit).

        Displays:

            Signal strength bars,

            Phone status (Idle, Incoming Call, In Call, and so on),

            Caller ID information.

        Button mappings:

            Start: Answer call,

            Select: Hang up,

            A/B: Dial or back.

    Communication Protocol:

        The Game Boy reads and writes via the cartridge memory-mapped registers:
        Memory  Address Function
        0xA000	 Signal Strength (0-5 bars)
        0xA001	 Phone Status (Idle, Incoming Call, In Call, etc.)
        0xA002  Caller ID / Text Buffer
        0xA010	 Command Buffer (Dial, Hang Up)

    Mock Development Mode:

        While building the UI, simulated status data will be used initially (no hardware required).

        Allows UI development and testing to progress before the hardware is ready.

Next Steps:

    Set up GBDK and an emulator (mGBA),

    Build and test a mock UI app using simulated phone data,

    Begin prototyping the hardware (RP204 and cellular module),

    Design and 3D print a prototype jumbo cartridge shell.

Big Picture:
A completely custom retro-futuristic device that merges vintage hardware with modern cellular technology. The Game Boy-powered cell phone will operate with the cartridge functioning as the phone itself, and the console serving as the user interface.