The **CMD-R-REFILL** (Command-R Refill) is typically associated with the open-source hardware community, specifically within the "Cyberdeck" or portable terminal subculture. It refers to a specific PCB (Printed Circuit Board) design intended to replace or upgrade the internal components of vintage 1980s calculators (like the Texas Instruments TI-59 or similar form factors) to turn them into modern Linux terminals or macro pads. Below is an explanation of the core electronic parts involved in such a build. --- ## 1. Core Logic & Control The brain of the device is usually a microcontroller or a Single Board Computer (SBC) that fits within the slim profile of the calculator shell. | Component | Function | Common Choice | | :--- | :--- | :--- | | **Microcontroller** | Handles keyboard scanning and USB communication. | RP2040 (Raspberry Pi Pico) or ATMega32U4. | | **SBC Host** | Provides the operating system (Linux). | Raspberry Pi Zero 2W or Radxa Zero. | | **Logic Level Shifter** | Converts signals between different voltages (e.g., 5V to 3.3V). | TXS0108E or simple MOSFET circuits. | --- ## 2. Display Interface Modern refills replace the old LED/VFD bubble displays with high-resolution LCDs or OLEDs. * **OLED Modules:** Usually 128x32 or 128x64 pixels using the **SSD1306** driver. They are preferred for their thin profile. * **TFT LCDs:** 1.3" to 2.0" screens using the **ST7789** driver for color terminal output. * **FPC Connector:** Flexible Printed Circuit connectors are used to link the screen to the PCB to save vertical space. --- ## 3. Keyboard Matrix The CMD-R-REFILL utilizes the original physical buttons of the calculator but replaces the underlying circuitry. * **Tactile Switches:** 6mm x 6mm low-profile switches are often soldered to the new PCB to align with the original plastic keys. * **Diodes:** (1N4148 or similar) arranged in a matrix to prevent "ghosting" when multiple keys are pressed simultaneously. * **Shift Registers:** If the microcontroller has limited I/O pins, chips like the **74HC165** are used to read all buttons. --- ## 4. Power Management Since these devices are meant to be portable, they require a modern power delivery system. ### Key Components: 1. **LiPo Battery:** Typically a flat 500mAh to 1000mAh cell. 2. **Charging IC:** Usually a **TP4056** or **MCP73831** for safe USB charging. 3. **Boost Converter:** (e.g., MT3608) to step up the battery voltage (3.7V) to a stable 5V for the Raspberry Pi or Display. 4. **USB-C Port:** Modern standard for both charging and data/programming. --- ## 5. Storage and Connectivity * **MicroSD Slot:** Required if using a Raspberry Pi Zero to host the OS. * **Expansion Headers:** Broken-out GPIO pins for connecting external sensors or modules (I2C, SPI, UART). --- ### Basic Block Diagram ```mermaid graph LR A[USB-C Power] --> B[TP4056 Charger] B --> C[LiPo Battery] C --> D[5V Boost Converter] D --> E[Main Controller/Pi Zero] E --> F[Display] E --> G[Key Matrix] ```