Architecture Overview

CrossInk is firmware for the Xteink X4 and X3, built with PlatformIO targeting the ESP32-C3 microcontroller.

At a high level, it is firmware that uses an activity-driven application architecture loop with persistent settings/state, SD-card-first caching, and a rendering pipeline optimized for e-ink constraints.

System at a glance

graph TD
    A[Hardware: ESP32-C3 + SD + E-ink + Buttons] --> B[open-x4-sdk]
    B --> C[lib/hal wrappers]
    C --> D[src/main.cpp runtime loop]
    D --> E[Activities layer]
    D --> F[State and settings]
    E --> G[Reader flows]
    E --> H[Home/Library/Settings flows]
    E --> I[Network/Web server flows]
    G --> J[lib/Epub parsing + layout + hyphenation]
    J --> K[SD cache in .crosspoint]
    E --> L[GfxRenderer]
    L --> M[E-ink display buffer]

Runtime lifecycle

Primary entry point is src/main.cpp.

flowchart TD
    A[Boot] --> B[Init GPIO and optional serial]
    B --> C[Init SD storage]
    C --> D[Load settings and app state]
    D --> E[Init display and fonts]
    E --> F{Resume reader?}
    F -->|No| G[Enter Home activity]
    F -->|Yes| H[Enter Reader activity]
    G --> I[Main loop]
    H --> I
    I --> J[Poll input and run current activity]
    J --> K{Sleep condition met?}
    K -->|No| I
    K -->|Yes| L[Persist state and enter deep sleep]

In each loop iteration, the firmware updates input, runs the active activity, handles auto-sleep/power behavior, and applies a short delay policy to balance responsiveness and power.

Activity model

Activities are screen-level controllers deriving from src/activities/Activity.h. Nested flows use the ActivityManager stack with startActivityForResult(), setResult(), and finish().

onEnter() and onExit() manage setup/teardown
loop() handles per-frame behavior
skipLoopDelay() and preventAutoSleep() are used by long-running flows (for example web server mode)

Top-level activity groups:

src/activities/home/: home and library navigation
src/activities/reader/: EPUB/XTC/TXT reading flows
src/activities/settings/: settings menus and configuration
src/activities/network/: Wi-Fi selection, AP/STA mode, file transfer server
src/activities/boot_sleep/: boot and sleep transitions

Reader and content pipeline

Reader orchestration starts in src/activities/reader/ReaderActivity.h and dispatches to format-specific readers. EPUB processing is implemented in lib/Epub/.

flowchart TD
    A[Select book] --> B[ReaderActivity]
    B --> C{Format}
    C -->|EPUB| D[EPUB loader]
    C -->|XTC| X[XTC reader]
    C -->|TXT| T[TXT reader]

    D --> E[Metadata and TOC]
    E --> F[CSS rules]
    F --> G[Section layout]
    G --> H[Page cache]
    H --> I[Page model]
    I --> J[GfxRenderer]
    J --> K[E-ink display]

    X --> I
    T --> I

For EPUBs, the metadata step reads or writes book.bin, the CSS step reads or writes css_rules.cache, and the section layout step writes sections/*.bin.

Why caching matters:

RAM is limited on ESP32-C3, so expensive parsed/layout data is persisted to SD
repeat opens/page navigation can reuse cached data instead of full reparsing

Reader internals call graph

This diagram zooms into the EPUB path to show the main control and data flow from activity entry to on-screen draw.

flowchart TD
    A[ReaderActivity onEnter] --> B{File type}
    B -->|EPUB| C[Create Epub object]
    B -->|XTC/TXT| Z[Use format-specific reader]

    C --> D[Epub load]
    D --> E[Locate container and OPF]
    E --> F[Build or load BookMetadataCache]
    F --> G[Load TOC and spine]
    G --> H[Load CSS cache or parse manifest/base-dir CSS]

    H --> I[EpubReaderActivity]
    I --> J{Section cache exists for current settings?}
    J -->|Yes| K[Read section bin from SD cache]
    J -->|No| L[Parse chapter HTML and layout text]
    L --> M[Apply typography settings and hyphenation]
    M --> N[Write section cache bin]

    K --> O[Build page model]
    N --> O
    O --> P[GfxRenderer draw calls]
    P --> Q[HAL display framebuffer update]
    Q --> R[E-ink refresh policy]

    S[SETTINGS singleton] -. influences .-> J
    S -. influences .-> M
    T[APP_STATE singleton] -. persists .-> U[Reading progress and resume context]
    U -. used by .-> I

Notes:

CSS files are collected from the OPF manifest and, when needed, discovered by streaming ZIP paths under the OPF content base directory; the firmware avoids preloading the full ZIP central directory for large books.
“section cache exists” depends on cache-busting parameters such as font, viewport size, paragraph alignment, forced paragraph indents, hyphenation, embedded CSS, image rendering, Bionic Reading, and Guide Dots settings
rendering favors reusing precomputed layout data to keep page turns responsive on constrained hardware
progress/session state is persisted so the reader can reopen at the last position after reboot/sleep

State and persistence

Two singletons are central:

src/CrossPointSettings.h (SETTINGS): user preferences and behavior flags
src/CrossPointState.h (APP_STATE): runtime/session state such as current book and sleep context

Most SD-card persistence lives under /.crosspoint/. See file inventory in docs/data-cache.md; for binary cache formats, see docs/file-formats.md.

Networking architecture

Network file transfer is controlled by src/activities/network/CrossPointWebServerActivity.h and served by src/network/CrossPointWebServer.h.

Modes:

STA: join existing Wi-Fi network
AP: create hotspot
Calibre Wireless: STA flow specialized for Calibre plugin uploads

Server behavior:

HTTP server on port 80
WebSocket upload server on port 81
WebDAV handler on the HTTP server
UDP discovery listener for upload clients
file operations backed by SD storage
browser APIs for file management, settings, fonts, OPDS servers, and saved Wi-Fi networks
activity requests faster loop responsiveness while server is running

Endpoint reference: docs/webserver-endpoints.md.

Build-time generated assets

Some sources are generated and should not be edited manually.

scripts/build_web.py generates src/network/html/*.generated.h from the web/ sources
scripts/gen_i18n.py generates lib/I18n/I18nKeys.h, I18nStrings.h, and I18nStrings.cpp
scripts/generate_hyphenation_trie.py generates hyphenation headers under lib/Epub/Epub/hyphenation/generated/

When editing related source assets, regenerate via normal build steps/scripts.

Key directories

src/: app orchestration, settings/state, and activity implementations
src/network/: web server and OTA/update networking
src/components/: theming and shared UI components
lib/hal/: hardware abstraction wrappers around open-x4-sdk
lib/Epub/: EPUB parser, layout, CSS handling, and hyphenation
lib/: supporting libraries (fonts, text, filesystem helpers, etc.)
open-x4-sdk/: hardware SDK submodule (display, input, storage, battery)
docs/: user and technical documentation

Embedded constraints that shape design

constrained RAM drives SD-first caching and careful allocations
e-ink refresh cost drives render/update batching choices
main loop responsiveness matters for input, power handling, and watchdog safety
background/network flows must cooperate with sleep and loop timing logic

Scope guardrails

Before implementing larger ideas, check: