feat: add comprehensive MicroPython implementation documentation for RP2040, ESP32, ESP32-S3, and ESP32-C3 boards
David Montero Crespo
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# MicroPython Implementation — Technical Summary
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> Issue #3 — Full MicroPython support for RP2040, ESP32, ESP32-S3, and ESP32-C3 boards.
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> Branch: `feature/micropython-rp2040`
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---
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## Overview
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Velxio supports MicroPython across **15 board variants** using two distinct execution strategies:
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| Strategy | Boards | Execution | Serial |
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|----------|--------|-----------|--------|
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| **Browser-side** (rp2040js) | Raspberry Pi Pico, Pico W | In-browser at 125 MHz | USBCDC |
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| **QEMU backend** (WebSocket) | ESP32, ESP32-S3, ESP32-C3 (13 variants) | Remote `qemu-system-xtensa` / `qemu-system-riscv32` | UART via WebSocket |
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### Supported Boards
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```
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RP2040 (browser): raspberry-pi-pico, pi-pico-w
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ESP32 Xtensa (QEMU): esp32, esp32-devkit-c-v4, esp32-cam, wemos-lolin32-lite
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ESP32-S3 Xtensa (QEMU): esp32-s3, xiao-esp32-s3, arduino-nano-esp32
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ESP32-C3 RISC-V (QEMU): esp32-c3, xiao-esp32-c3, aitewinrobot-esp32c3-supermini
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```
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---
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## Architecture
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```
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┌─────────────────────────────────────────────────────────────┐
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│ User clicks "Run" with MicroPython mode │
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│ │
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│ EditorToolbar.handleRun() │
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│ └── loadMicroPythonProgram(boardId, pyFiles) │
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│ ├── RP2040? → sim.loadMicroPython(files) │
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│ │ ├── getFirmware() [IndexedDB/remote/bundled]
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│ │ ├── loadUF2() [parse UF2 → flash]
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│ │ ├── loadUserFiles() [LittleFS → flash]
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│ │ └── USBCDC serial [REPL via USB CDC]
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│ │ │
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│ └── ESP32? → getEsp32Firmware() │
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│ ├── base64 encode │
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│ ├── bridge.loadFirmware(b64) │
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│ └── bridge.setPendingMicroPythonCode()
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│ └── on ">>>" detected: │
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│ _injectCodeViaRawPaste() │
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│ \x01 → \x05 → code → \x04
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└─────────────────────────────────────────────────────────────┘
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```
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---
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## Phase 1: RP2040 MicroPython (Browser-Side)
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### Files Created/Modified
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| File | Action |
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|------|--------|
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| `frontend/src/simulation/MicroPythonLoader.ts` | **Created** — UF2 parser, LittleFS builder, firmware cache |
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| `frontend/src/simulation/RP2040Simulator.ts` | Modified — `loadMicroPython()`, USBCDC serial, `micropythonMode` |
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| `frontend/src/types/board.ts` | Modified — `LanguageMode` type, `BOARD_SUPPORTS_MICROPYTHON` set |
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| `frontend/src/store/useSimulatorStore.ts` | Modified — `loadMicroPythonProgram`, `setBoardLanguageMode` |
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| `frontend/src/store/useEditorStore.ts` | Modified — `DEFAULT_MICROPYTHON_CONTENT`, `createFileGroup` overload |
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| `frontend/src/components/editor/EditorToolbar.tsx` | Modified — language selector, MicroPython compile/run flow |
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| `frontend/src/components/simulator/SerialMonitor.tsx` | Modified — Ctrl+C/D, REPL label |
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| `frontend/public/firmware/micropython-rp2040.uf2` | **Created** — bundled fallback (638 KB) |
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### UF2 Firmware Loading
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MicroPython for RP2040 uses UF2 format (USB Flashing Format). Each block is 512 bytes:
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```
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Offset Field
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0 Magic 0 (0x0a324655)
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4 Magic 1 (0x9e5d5157)
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12 Flash address (little-endian)
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32 Payload data (256 bytes)
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```
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Constants:
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```typescript
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FLASH_START_ADDRESS = 0x10000000 // RP2040 XIP flash base
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MICROPYTHON_FS_FLASH_START = 0xa0000 // LittleFS starts at 640KB
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MICROPYTHON_FS_BLOCK_SIZE = 4096
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MICROPYTHON_FS_BLOCK_COUNT = 352 // 1.4 MB filesystem
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```
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### LittleFS Filesystem
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User files (e.g., `main.py`) are written into a LittleFS image in memory using the `littlefs` WASM package:
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1. Create backing buffer: `352 × 4096 = 1,441,792 bytes`
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2. Register 4 WASM callbacks via `addFunction()`:
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- `flashRead(cfg, block, off, buffer, size)` — reads from JS buffer
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- `flashProg(cfg, block, off, buffer, size)` — writes to JS buffer
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- `flashErase(cfg, block)` — no-op
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- `flashSync()` — no-op
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3. Format and mount: `_lfs_format()` → `_lfs_mount()`
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4. Write files: `cwrap('lfs_write_file')` for each user file
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5. Unmount and copy into flash at offset `0xa0000`
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### USBCDC Serial (not UART)
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MicroPython on RP2040 uses USB CDC for the REPL, not UART0:
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```typescript
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// In RP2040Simulator.loadMicroPython():
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this.usbCDC = new USBCDC(rp2040.usbCtrl);
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this.usbCDC.onDeviceConnected = () => this.usbCDC.sendSerialByte('\r'.charCodeAt(0));
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this.usbCDC.onSerialData = (value) => { /* forward to SerialMonitor */ };
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// Serial write routes through USBCDC when micropythonMode is true:
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serialWrite(text: string): void {
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if (this.micropythonMode && this.usbCDC) {
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for (const ch of text) this.usbCDC.sendSerialByte(ch.charCodeAt(0));
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} else {
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this.rp2040?.uart[0].feedByte(...);
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}
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}
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```
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### Firmware Caching Strategy
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All firmware uses a 3-tier loading strategy:
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1. **IndexedDB cache** — instant load via `idb-keyval`
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2. **Remote download** — from `micropython.org` with streaming progress
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3. **Bundled fallback** — from `/firmware/` (Vite public directory)
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Cache key: `micropython-rp2040-uf2-v1.20.0`
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---
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## Phase 2: ESP32/ESP32-S3 MicroPython (QEMU Backend)
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### Files Created/Modified
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| File | Action |
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|------|--------|
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| `frontend/src/simulation/Esp32MicroPythonLoader.ts` | **Created** — firmware download/cache for ESP32 variants |
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| `frontend/src/simulation/Esp32Bridge.ts` | Modified — `setPendingMicroPythonCode()`, raw-paste injection |
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| `frontend/src/store/useSimulatorStore.ts` | Modified — ESP32 path in `loadMicroPythonProgram` |
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| `frontend/src/store/useEditorStore.ts` | Modified — `DEFAULT_ESP32_MICROPYTHON_CONTENT` |
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| `frontend/src/components/simulator/SerialMonitor.tsx` | Modified — generic Ctrl+C/D for all boards |
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| `frontend/public/firmware/micropython-esp32.bin` | **Created** — bundled fallback (1.5 MB) |
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| `frontend/public/firmware/micropython-esp32s3.bin` | **Created** — bundled fallback (1.4 MB) |
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### Firmware Variants
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```typescript
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FIRMWARE_MAP = {
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'esp32': { remote: '.../ESP32_GENERIC-20230426-v1.20.0.bin', cacheKey: 'micropython-esp32-v1.20.0' },
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'esp32-s3': { remote: '.../ESP32_GENERIC_S3-20230426-v1.20.0.bin', cacheKey: 'micropython-esp32s3-v1.20.0' },
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'esp32-c3': { remote: '.../ESP32_GENERIC_C3-20230426-v1.20.0.bin', cacheKey: 'micropython-esp32c3-v1.20.0' },
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}
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```
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Board kind → firmware variant mapping:
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```
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esp32, esp32-devkit-c-v4, esp32-cam, wemos-lolin32-lite → 'esp32'
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esp32-s3, xiao-esp32-s3, arduino-nano-esp32 → 'esp32-s3'
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esp32-c3, xiao-esp32-c3, aitewinrobot-esp32c3-supermini → 'esp32-c3'
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```
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### Raw-Paste REPL Protocol
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Unlike RP2040 (which loads files into LittleFS), ESP32 boards inject user code into the MicroPython REPL after boot using the **raw-paste protocol**:
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```
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Step 1: Wait for ">>>" in serial output (REPL ready)
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Step 2: Send \x01 (Ctrl+A) — enter raw REPL mode
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Step 3: Send \x05 (Ctrl+E) — enter raw-paste mode
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Step 4: Send code bytes in 256-byte chunks (10ms between chunks)
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Step 5: Send \x04 (Ctrl+D) — execute the code
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```
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Implementation in `Esp32Bridge`:
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```typescript
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// State fields
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private _pendingMicroPythonCode: string | null = null;
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private _serialBuffer = '';
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micropythonMode = false;
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// In serial_output handler — detect REPL prompt
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if (this._pendingMicroPythonCode && this._serialBuffer.includes('>>>')) {
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this._injectCodeViaRawPaste(this._pendingMicroPythonCode);
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this._pendingMicroPythonCode = null;
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}
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// Timing: 500ms initial delay → 100ms after Ctrl+A → 100ms after Ctrl+E
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// → 10ms between 256-byte chunks → 50ms before Ctrl+D
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```
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### WebSocket Protocol Flow
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```
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Frontend Backend (QEMU)
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│ │
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│ start_esp32 { firmware_b64, board }
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│──────────────────────────────────→│
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│ │ QEMU boots MicroPython
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│ │
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│ serial_output { data: ">>>" } │
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│←──────────────────────────────────│
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│ │
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│ esp32_serial_input { bytes: [0x01] } ← Ctrl+A (raw REPL)
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│──────────────────────────────────→│
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│ │
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│ esp32_serial_input { bytes: [0x05] } ← Ctrl+E (raw-paste)
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│──────────────────────────────────→│
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│ │
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│ esp32_serial_input { bytes: [...code...] } ← code chunks
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│──────────────────────────────────→│
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│ │
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│ esp32_serial_input { bytes: [0x04] } ← Ctrl+D (execute)
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│──────────────────────────────────→│
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│ │
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│ serial_output { data: "...output..." }
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│←──────────────────────────────────│
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```
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---
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## Phase 3: ESP32-C3 MicroPython
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### Files Modified
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| File | Action |
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|------|--------|
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| `frontend/src/types/board.ts` | Modified — added C3 boards to `BOARD_SUPPORTS_MICROPYTHON`, fixed comments |
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| `frontend/src/simulation/Esp32MicroPythonLoader.ts` | Modified — added `esp32-c3` firmware variant |
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| `frontend/public/firmware/micropython-esp32c3.bin` | **Created** — bundled fallback (1.4 MB) |
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### Key Finding
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ESP32-C3 boards were documented as "browser emulation (Esp32C3Simulator)" in the type definitions, but the actual runtime code routes them through `Esp32Bridge` (QEMU backend) — the `Esp32C3Simulator` is dead code. This was corrected.
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The `isEsp32Kind()` function in `useSimulatorStore.ts` already included C3 boards, so the QEMU bridge infrastructure was already wired up. Only the MicroPython-specific additions were needed.
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---
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## Store Integration
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### `loadMicroPythonProgram(boardId, files)`
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```typescript
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// In useSimulatorStore.ts
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async loadMicroPythonProgram(boardId, files) {
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if (isEsp32Kind(board.boardKind)) {
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// ESP32 path: firmware → QEMU bridge → raw-paste injection
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const firmware = await getEsp32Firmware(board.boardKind);
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const b64 = uint8ArrayToBase64(firmware);
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esp32Bridge.loadFirmware(b64);
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esp32Bridge.setPendingMicroPythonCode(mainFile.content);
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} else {
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// RP2040 path: firmware + LittleFS in browser
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await sim.loadMicroPython(files);
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}
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// Mark as loaded
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board.compiledProgram = 'micropython-loaded';
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}
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```
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### `setBoardLanguageMode(boardId, mode)`
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When toggling between Arduino and MicroPython:
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1. Stops any running simulation
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2. Clears `compiledProgram`
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3. Deletes old file group
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4. Creates new file group with appropriate defaults:
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- RP2040 MicroPython → `main.py` with `Pin(25)` blink
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- ESP32 MicroPython → `main.py` with `Pin(2)` blink
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- Arduino → `sketch.ino` with default Arduino code
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---
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## UI Integration
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### Language Mode Selector
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Appears in `EditorToolbar` next to the board name pill when `BOARD_SUPPORTS_MICROPYTHON.has(boardKind)`:
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```
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[Arduino C++ ▼] ↔ [MicroPython ▼]
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```
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Changing the selector:
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- Switches `languageMode` on the `BoardInstance`
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- Replaces editor files with language-appropriate defaults
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- Updates compile button text: "Compile (Ctrl+B)" → "Load MicroPython"
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### Serial Monitor
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- **REPL label**: Shows "MicroPython REPL" in magenta when in MicroPython mode
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- **Baud rate**: Hidden in MicroPython mode (REPL has no baud rate)
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- **Ctrl+C**: Sends `\x03` (keyboard interrupt) — works for both RP2040 and ESP32
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- **Ctrl+D**: Sends `\x04` (soft reset) — works for both RP2040 and ESP32
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- **Placeholder**: "Type Python expression... (Ctrl+C to interrupt)"
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### Run Button Behavior
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For MicroPython boards, the Run button:
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1. If firmware not yet loaded → auto-loads firmware first, then starts
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2. If already loaded → starts simulation directly
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3. Enabled even without `compiledProgram` (unlike Arduino mode)
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---
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## Firmware Files
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| File | Size | Board | Version |
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|------|------|-------|---------|
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| `micropython-rp2040.uf2` | 638 KB | RP2040 | v1.20.0 (2023-04-26) |
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| `micropython-esp32.bin` | 1.5 MB | ESP32 Xtensa | v1.20.0 (2023-04-26) |
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| `micropython-esp32s3.bin` | 1.4 MB | ESP32-S3 Xtensa | v1.20.0 (2023-04-26) |
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| `micropython-esp32c3.bin` | 1.4 MB | ESP32-C3 RISC-V | v1.20.0 (2023-04-26) |
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All firmware uses MicroPython **v1.20.0** (stable release, 2023-04-26).
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Firmware source: `https://micropython.org/resources/firmware/`
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---
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## Dependencies Added
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| Package | Version | Purpose |
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|---------|---------|---------|
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| `littlefs` | 0.1.0 | LittleFS WASM for creating filesystem images (RP2040) |
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| `idb-keyval` | 6.x | IndexedDB key-value wrapper for firmware caching |
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---
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## Git History
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```
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990ae4b feat: add MicroPython support for RP2040 boards (Pico / Pico W)
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7c9fd0b feat: add MicroPython support for ESP32/ESP32-S3 boards via QEMU bridge
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0bc4c03 feat: add MicroPython support for ESP32-C3 (RISC-V) boards
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```
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||||||
|
| Commit | Files Changed | Lines Added | Lines Removed |
|
||||||
|
|--------|:------------:|:-----------:|:------------:|
|
||||||
|
| `990ae4b` (RP2040) | 12 | +587 | -38 |
|
||||||
|
| `7c9fd0b` (ESP32/S3) | 8 | +296 | -33 |
|
||||||
|
| `0bc4c03` (ESP32-C3) | 3 | +16 | -4 |
|
||||||
|
| **Total** | **23** | **+899** | **-75** |
|
||||||
Loading…
Reference in New Issue