donglora/DESIGN.md

Design Principles

These are the non-negotiable first principles of this project. Every design decision, naming choice, module boundary, and abstraction is evaluated against them. When in doubt, choose simplicity and clarity over cleverness.

What This Is

A transparent LoRa radio over USB. Plug the dongle into your PC, talk to any LoRa radio within range. That's it.

What This Is Not

Not a mesh network node. Not a Meshtastic device. Not a protocol implementation. Not an application framework. This firmware has no opinion about what you do with your radio — it just gives you clean access to it.

Principles

1. Single purpose. Transparent LoRa radio over USB. No mesh, no routing, no application logic. Ever.

2. Protocol-agnostic. The firmware is a dumb pipe. It exposes clean LoRa parameters (frequency, bandwidth, spreading factor, coding rate, sync word, TX power) and nothing else. No raw register access, no chip-specific leakage.

3. Host-driven. The firmware makes zero autonomous decisions. The radio idles on boot until the host commands it. No persistence across reboots. The host is always in control.

4. Zero-config. Boots, enumerates USB, waits for commands. The host plugs in and goes.

5. Robustness over features. Tasks never panic. Errors are reported to the host. The radio self-recovers. Ship fewer features, but make them bulletproof.

6. Minimal footprint. Avoid heavy dependencies. Keep the binary small. Every dependency must justify its inclusion.

7. Auto-detect peripherals. If the board has a display, keyboard, or sensors, use them. No user configuration required.

8. World-class Rust. Fully leverage the type system for safety. Strict no_std. Always clippy-clean. Thoughtfully designed — everything has an obvious home. Anyone who reads this code should immediately think "this is nice."

9. Easy board support. Adding a new board means dropping a .rs file and adding a Cargo feature. Nothing else changes.

10. Support all common LoRa boards. This project supports every common LoRa board, and even some uncommon ones. The architecture is designed to accommodate them cleanly. If the code isn't structured for a new board, fix the code.

11. Minimal overhead. The dongle should add as little latency as possible between the radio and the host. No unnecessary copies, no blocking operations in the radio path, no computation that isn't strictly needed.

12. Standard toolchain. No custom Rust forks or special SDKs. Use stock rustup channels (stable, nightly). Xtensa targets use nightly + -Zbuild-std; Cortex-M and RISC-V targets use stable. Boards that need the esp toolchain (via espup) are supported but gracefully skipped when the toolchain isn't installed.

Architecture

Host PC
  │ USB CDC-ACM (COBS-framed fixed-size LE)
  ▼
usb_task ──Command──► radio_task ──► SX1262
         ◄──Response──     │
                      StatusWatch
                           ▼
                     display_task (optional) ──► OLED/TFT

Each task exclusively owns its peripheral. No shared state except typed channels and a watch signal. Pure actor model.

Peripheral Model

Boards can have optional peripherals beyond the radio and USB. The board's into_parts() returns Option<T> slots for each, and the corresponding task is only spawned when present.

Peripheral	Slot	Task	Examples
Display	Option<DisplayParts>	display_task	SSD1306 OLED, ST7789 TFT
Input	Option<InputParts>	input_task	T-Deck keyboard, buttons
Telemetry	Option<TelemetryParts>	telemetry_task	Battery voltage, temperature

Board files are always separate. A T-Deck is t_deck.rs, not a variant of Heltec V3. Each board owns its own init code and pin mappings. Shared logic between boards that use the same MCU can be factored into helper modules if it earns its keep, but never prematurely.

Technical Decisions

Decision	Choice	Why
USB framing	COBS + postcard	Compact, no_std native, battle-tested
Serialization	postcard + serde	Zero-alloc binary codec, perfect for embedded
Async runtime	Embassy	De facto standard for async embedded Rust
Display	Radio status dashboard	Freq, BW, SF, packet counts, RSSI/SNR
Boot behavior	Idle until host commands	Host-driven principle
Persistence	None	Always start fresh, true dumb pipe
Board codegen	Jinja2 via build.rs	Auto-discovers boards from filesystem
Rust toolchain	Stock rustup (stable/nightly)	No forks; espup for Xtensa boards only
Firmware output	firmware/donglora-{board}-{profile}.elf	Readable names, not buried in target/