# Architecture Decision Records A running log of the load-bearing technical decisions on this project. Each record states what was considered, what we chose, and what we accept as a consequence. Status moves from **Proposed** → **Accepted** → (eventually) **Superseded** when a later ADR replaces it. Format inspired by Michael Nygard's ADR template, kept short on purpose. **Related docs:** [`../README.md`](../README.md) — project overview, hardware table, macro plan. [`roadmap.md`](roadmap.md) — per-version scope (v0.1 → v1.x). [`v0.1-mvp-product.md`](v0.1-mvp-product.md) — what the v0.1 device must do. [`v0.1-mvp-technical.md`](v0.1-mvp-technical.md) — how v0.1 is built. [`qfd.md`](qfd.md) — Quality Function Deployment: requirements → functions → components, with the tradeoffs from this file ranked by user-facing weight. --- ## ADR-001: Language and runtime — Rust on `esp-idf-rs` (std) **Status:** Accepted — 2026-05-14 **Scope:** Whole project. ### Context The firmware needs: USB host, Wi-Fi + TLS, SPI peripherals, a SD filesystem, and a working git implementation that can push over HTTPS. All on an ESP32-S3 with 8 MB PSRAM. We also want the code to stay refactorable as features pile up across nine downstream releases. ### Options considered | Option | Pros | Cons | | ------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------- | | **C on ESP-IDF (no Arduino)** | Reference platform on the bare native SDK; every peripheral has a driver; smallest binary of the C-family options; no C++ runtime / exceptions / RTTI to reason about. | All memory safety on you; no RAII for resource cleanup; no generics so widget / state code gets repetitive; refactoring at scale is painful. | | **C++ on ESP-IDF** | Same peripheral coverage as C; RAII, templates, and `std::` containers ease widget / state code; mature in the ESP-IDF examples. | Exception / RTTI story on embedded is messy; ABI / linker surprises; memory safety still on you; binary larger than plain C. | | **Rust on `esp-idf-rs` (std)** | First-class Espressif-sponsored Rust support; `std` gives heap / threads / VFS / mbedtls; can use the broader Rust ecosystem (`gitoxide`, `ropey`, `embedded-graphics`). | Larger binary than `no_std`; longer build times; some `unsafe` at FFI seams. | | **Rust on `esp-hal` (no_std)** | Smallest binary, most "pure" embedded experience. | No `std` = no off-the-shelf git, no easy TLS, would re-implement a lot of plumbing. | | **Gleam + Shore on AtomVM** | Beautiful language, the user's stated preference. | BEAM on ESP32 is memory-hungry; no bindings for USB host, e-ink, SD, TLS, git in that ecosystem. Two research projects stacked. | | **MicroPython / CircuitPython** | Fastest to prototype. | Too slow for responsive editing at the latencies e-ink already imposes; GC pauses would surface as dropped keys. | | **TinyGo** | Modern, ergonomic. | ESP32-S3 support is thinner than Rust's; smaller ecosystem of embedded crates equivalents. | ### Decision **Rust on `esp-idf-rs` (std).** It's the sweet spot: keeps the door open to the entire Rust ecosystem we need (`gitoxide` especially), gets us threads and TLS without writing them, and has Espressif as an actual upstream. ### Consequences - Binary will be in the 1–2 MB range — comfortable in 16 MB flash. - Build times are real (clean build ~5–10 min). Acceptable. - Cross-compiling toolchain (`espup`) is one more thing to install. - We will not use `tokio` or async runtimes in v0.1 — see ADR-006. - Revisit if `esp-idf-rs` upstream stalls or if `gitoxide` doesn't compile cleanly against it (spike 7 is the kill-switch — see [v0.1 technical: hardware bring-up order](v0.1-mvp-technical.md#hardware-bring-up-order)). See also: [qfd.md §7](qfd.md#7-tradeoffs-and-their-why-linked-to-adrs) for the binary-size / build-time costs traded against ecosystem access. --- ## ADR-002: UI strategy — custom widgets on `embedded-graphics`, not Ratatui **Status:** Accepted — 2026-05-14 **Scope:** Whole project. ### Context We need a TUI-like editor (header, edit area, status, palettes later). The output medium is e-ink: pixel framebuffer with **partial-refresh windows** aligned to panel-internal regions, ~10× slower than an LCD per region. ### Options considered | Option | Pros | Cons | | --------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Ratatui** with a custom backend | Mature widget set, well-known API, lots of community examples. | Built for char-grid terminals over ANSI; per-cell diff fights e-ink's region-refresh model; backend would re-rasterise glyphs from cell-diffs; ~200 KB of binary and a leaky abstraction. | | **Raw `embedded-graphics` only** | Smallest footprint, full control. | Every screen built from primitives; no widget reuse; status line / palette would each be ad-hoc. | | **LVGL via Rust bindings** | Full GUI toolkit, themable. | Designed for actively-refreshing colour LCDs; e-ink integration is awkward; way more than we need. | | **Custom thin widget layer on `embedded-graphics`** | Borrow Ratatui's API ideas (`Layout`, `Block`, `Paragraph`) without its rendering model; dirty-rect tracking aligned to e-ink regions; ~500 LoC. | We own and maintain the layer. | ### Decision **Custom thin widget layer on `embedded-graphics`.** Steal the widget _API shape_ from Ratatui (because it's a good shape) but render directly to a pixel framebuffer with our own dirty-rectangle tracking sized to the panel's refresh regions. ### Consequences - ~500 LoC of widget/layout code we maintain. Worth it. - We can tune refresh cadence (partial vs full) at the widget level. - If we later want to render to a terminal for desktop testing, we add a second backend; the widget API stays. Implementation: [v0.1 technical → render module](v0.1-mvp-technical.md#module-breakdown). Owns the two top-ranked functions (H1 latency, H2 region area) in [qfd.md §3](qfd.md#3-house-of-quality--whats--hows). --- ## ADR-003: Display — GDEY0579T93 + DESPI-c579 breakout **Status:** Accepted — 2026-05-14 **Scope:** v0.1 through v1.0. 10.3" upgrade remains on the v1.x table. ### Context The screen is the most user-facing hardware choice. It sets the aspect of the writing experience, the BOM cost, the GPIO budget, the framebuffer size, and the refresh feel. ### Options considered | Option | Size / Res | Aspect | Pros | Cons | | ------------------------------ | ----------------- | ----------- | ----------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------- | | **GDEY0579T93 + DESPI-c579** | 5.79" / 792×272 | 2.9:1 strip | SPI, partial refresh, small framebuffer (~27 KB), Freewrite-style narrow viewport, low power, low GPIO use. | Only ~11 visible lines of edit area; less context on screen. | | **Waveshare 7.5" V2** | 7.5" / 800×480 | 5:3 page | More lines visible, well-supported by `epd-waveshare` out of the box. | Bigger BOM, bigger framebuffer (~48 KB), more conventional / less typewriter-feeling. | | **Waveshare 10.3" + IT8951** | 10.3" / 1872×1404 | 4:3 | Real "page" experience; great for long-form. | +$80 BOM; parallel bus eats GPIO; IT8951 adds a controller board; overkill for v0.1. | | **2.9" / 4.2" smaller panels** | varied | varied | Cheap, common. | Too cramped for a typewriter; status bars eat the screen. | ### Decision **GDEY0579T93 driven over SPI via the DESPI-c579 breakout.** The strip aspect biases UX toward "current line + recent context" — the writing posture we actually want. Small framebuffer keeps PSRAM free for git pack data. The DESPI-c579 is a passive level-shifter / FPC adapter, not an active controller — same SPI driver model as any other e-paper. ### Consequences - Visible edit area is ~11 lines. UI design must embrace this (no multi-pane, no large headers). See [v0.1 product → screen layout](v0.1-mvp-product.md#screen-layout). - Driver: if `epd-waveshare` doesn't already support this panel's controller (SSD1683-class), we write ~300 LoC of `embedded-hal` SPI driver. Validated in spike 2 — see [v0.1 technical → hardware bring-up order](v0.1-mvp-technical.md#hardware-bring-up-order). - 10.3" upgrade path is preserved by keeping the renderer resolution-agnostic. See [roadmap → v1.x](roadmap.md#v1x--stretch--nice-to-have). --- ## ADR-004: Git implementation — `gitoxide` (`gix`) **Status:** Accepted — 2026-05-14 **Scope:** Whole project, all releases. ### Context The device must do `add`, `commit`, `push` over the network. Optionally later: `fetch`, `pull`, `branch`. The library must compile against `esp-idf-rs` (std, mbedtls available). ### Options considered | Option | Pros | Cons | | ------------------------------ | --------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------- | | **`libgit2-sys`** (C bindings) | Battle-tested, comprehensive, well-known. | C dependency complicates cross-compile to ESP32-S3; needs mbedtls glue; binary size; less Rust-idiomatic. | | **`gitoxide` (`gix`)** | Pure Rust, modular crates (we only depend on what we use), idiomatic API, active development. | Smart-HTTP push path is newer than libgit2's; PSRAM allocation patterns less battle-tested on embedded. | | **Hand-rolled HTTP + pack** | Smallest possible footprint. | Reinventing git internals; pack delta + ref discovery + index updates are not weekend work. | | **Shell out to `git` binary** | Trivial. | There is no `git` binary on the ESP32-S3. | ### Decision **`gitoxide`.** Modular means we pull only `gix-pack`, `gix-protocol`, `gix-transport`, etc. — not 200 KB of features we don't use. Pure Rust removes a class of cross-compile pain. The smart-HTTP path is validated in spike 7 _before_ we commit to integration; if it fails on the device, we fall back to `libgit2-sys` for v0.1 (documented as the kill-switch in the risk table). ### Consequences - We become an early-ish embedded user of `gitoxide`; bugs reported back upstream. - Auth via PAT in an Authorization header — no SSH (see ADR-005). - Performance on PSRAM during pack operations is a watched metric — top-3 priority in [qfd.md §6](qfd.md#6-critical-performance-budget). Implementation: [v0.1 technical → `git` module](v0.1-mvp-technical.md#module-breakdown) and [risks table](v0.1-mvp-technical.md#risks-and-how-well-know-they-bit-us). --- ## ADR-005: Auth — HTTPS + GitHub Personal Access Token **Status:** Accepted — 2026-05-14 **Scope:** v0.1 through at least v0.9. ### Context The device must authenticate to GitHub (or other git remotes) to push. Auth has to be: enterable on a tiny screen-less first-run flow, storable on-device, and reasonably secure for a personal appliance. ### Options considered | Option | Pros | Cons | | -------------------------------------- | -------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **HTTPS + PAT** | Trivial to implement; PAT is a string the user pastes during captive-portal setup; works with `gitoxide` smart-HTTP. | Long-lived secret on device; PAT rotation is manual. | | **HTTPS + OAuth device flow** | No secret typed by hand; user approves on github.com. | Adds an OAuth client app to maintain; token still has to live on device; more first-run UX work. | | **SSH** | No PAT; per-device deploy keys. | SSH on embedded is heavy (host-key handling, key generation); `gitoxide`'s SSH transport story is less mature than HTTPS; users would have to register the public key on GitHub anyway. | | **GitHub App with installation token** | Strongest model, rotating credentials. | Massive overhead for a single-user device. | ### Decision **HTTPS + PAT.** Stored in internal LittleFS, encrypted with a key derived from the chip's eFuse so a stolen SD card alone is not enough. Captive portal accepts the PAT during first-run setup. ### Consequences - The user must generate a PAT with `repo` scope. Documented in [v0.1 product → first-run flow](v0.1-mvp-product.md#first-run-provisioning-flow). - PAT is never logged. Validated in code review. - Rotation in v0.1 = wipe NVS and re-run setup. Proper rotation UI is v0.9 — see [roadmap → v0.9](roadmap.md#v09--robustness--). - Revisit if we ever want to support multiple remotes per device with different credentials. --- ## ADR-006: Concurrency — `std::thread` + channels, no async runtime **Status:** Accepted — 2026-05-14 **Scope:** v0.1 through at least v1.0. ### Context The firmware has several concurrent concerns: USB input, Wi-Fi maintenance, screen rendering, occasional git operations. None of them are I/O-bound at the scale where async wins. The number of "tasks" is bounded and small (≤ 8). ### Options considered | Option | Pros | Cons | | ------------------------------ | ----------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------- | | **`std::thread` + channels** | Boring, debuggable, stack traces work, no executor to tune; ESP-IDF FreeRTOS underneath is well-understood. | Each thread costs 8–32 KB stack depending on workload; not zero-cost like async. | | **`embassy` async** | Trendy, ergonomic, low memory per task. | `esp-idf-rs` and `embassy` don't mix cleanly; adopting embassy means dropping `std` and rewriting against `esp-hal` (ADR-001 reversed). | | **`tokio` on `esp-idf-rs`** | Familiar async. | Heavy executor, oversized for ≤ 8 tasks, mbedtls/`gitoxide` integration would need a lot of glue. | | **Single-threaded event loop** | Smallest memory. | Long-running ops (git push, full refresh) block input. | ### Decision **`std::thread` + `crossbeam-channel`.** Five tasks (`usb`, `wifi`, `ui`, `render`, `git`). Editor state behind a single `Mutex`. No `await`, no runtime to tune, no colour-of-functions problem. ### Consequences - ~76 KB of stack space across the five task stacks (8 + 8 + 16 + 12 + 32 KB — see [v0.1 technical → threads / tasks](v0.1-mvp-technical.md#threads--tasks) for the breakdown). Comfortable in the ESP32-S3's 512 KB internal SRAM. - Refresh / git / Wi-Fi each get their own thread, so a slow push doesn't freeze typing. - If task count balloons past ~10 (unlikely), revisit. --- ## ADR-007: Storage split — FAT-on-SD for working copy, LittleFS-on-flash for config **Status:** Accepted — 2026-05-14 **Scope:** Whole project. ### Context Two storage needs: a large, removable, growable area for the git working copy and notes; and a small, durable, never-removed area for device config (Wi-Fi credentials, PAT, remote URL). ### Options considered | Option | Pros | Cons | | -------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------ | | **SD (FAT) for working copy + LittleFS (internal) for config** | Plays to each medium's strengths; user can pop the SD to read on desktop; config can't be lost by yanking the card. | Two filesystems to manage. | | **All on SD** | One filesystem. | Config disappears if SD is removed; PAT on FAT is harder to protect than on encrypted NVS. | | **All in internal flash** | Single medium; encrypted. | 16 MB flash limits notes growth; no desktop-side access; SD slot becomes pointless. | | **SPIFFS for everything** | Single FS, well-known on ESP32. | SPIFFS isn't great with large files; no removability. | ### Decision **FAT on SD for `/sd/repo/` and `/sd/local/`. LittleFS on internal flash for `/nvs/config.toml`.** PAT inside config is encrypted with an eFuse- derived key. ### Consequences - User can plug the SD into a laptop and read/edit files there. Discouraged but possible. - Config survives SD reformatting. - Power-loss safety on FAT is weaker than LittleFS — we mitigate with atomic-rename writes (see [v0.1 technical → `persistence`](v0.1-mvp-technical.md#module-breakdown) and [file layout](v0.1-mvp-technical.md#file-layout)). --- ## ADR-008: MVP power — wall-powered, battery deferred to v0.8 **Status:** Accepted — 2026-05-14 **Scope:** v0.1 only. Revisited in ADR-future at v0.8. ### Context "DIY typewriter" suggests portability, which suggests battery. But battery adds: charging circuit, BMS, thermal margin, soft power switch, lid-close detection, sleep states. Each of those has its own bring-up cost. ### Options considered - **USB-C wall power, no battery.** Simple, safe, lets us measure real draw before sizing a cell. - **18650 + IP5306 from day one.** Pretty close to a known-good pattern; IP5306 handles charge + 5 V boost. - **LiPo + dedicated charger IC + buck/boost.** More control, more parts. ### Decision **Wall power only for v0.1.** Battery is its own phase (v0.8) once the power profile of "boot + type + idle + push" is measured on real hardware. Sizing a battery before measuring is guessing. ### Consequences - v0.1 device is tethered. Not the final aesthetic, but the right MVP — scope is in [v0.1 product → out of scope](v0.1-mvp-product.md#out-of-scope-for-v01). - We can decide cell capacity from real numbers in v0.8, not specs sheets. - Lid-close detection / deep sleep slips to v0.8 with the battery — see [roadmap → v0.8](roadmap.md#v08--power-battery--sleep--). --- ## ADR-009: Keyboard transport — USB host (TinyUSB) **Status:** Accepted — 2026-05-14 **Scope:** v0.1 through at least v1.0. ### Context The Nuphy keyboard speaks both wired USB-C (HID) and Bluetooth LE (HID). The ESP32-S3 has USB OTG (host capable) and BLE 5. Either transport works. ### Options considered | Option | Pros | Cons | | -------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------- | | **USB host (TinyUSB)** | Keyboard draws no battery of its own; ESP32-S3 powers it through the host port; standard boot-protocol HID is well-supported; no radio contention with Wi-Fi during push. | One more USB connector on the enclosure; cable between device and keyboard (or shared chassis). | | **BLE-HID** | No cable; keyboard can be slightly remote from the device. | Keyboard has its own battery to manage; BLE shares the 2.4 GHz radio with Wi-Fi, so a `Ctrl-G` push contends with input; pairing UX is more first-run work. | | **UART receiver (custom keyboard firmware)** | Lowest latency, simplest stack. | Requires reflashing the Nuphy or building a passthrough; not viable as a product choice. | ### Decision **USB host (TinyUSB) for v0.1.** BLE-HID is kept as a documented fallback if TinyUSB host turns out unstable ([spike 4](v0.1-mvp-technical.md#hardware-bring-up-order) is the gate). ### Consequences - Enclosure design must include a USB-A or USB-C port for the keyboard. - The Nuphy's own battery is irrelevant when wired — saves the user a charging surface. - Wi-Fi and keyboard input do not contend for radio time. - If we ever want a fully wireless build, we revisit with a BLE-HID ADR. --- ## How to add a new ADR 1. Append a new `## ADR-NNN: ` section to this file. 2. Status starts as **Proposed**, with today's date. 3. Once merged + agreed, flip to **Accepted**. 4. When superseded, leave the old ADR in place and add **Superseded by ADR-MMM** to its status line. Never delete. 5. Cross-reference from the relevant section of the README or design docs if the decision is load-bearing for code review.