Architecture Overview #

CrossPoint is firmware for the Xteink X4 (unaffiliated with Xteink), 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[freeink-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. Some flows use src/activities/ActivityWithSubactivity.h to host nested activities.

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 LR
    A[Select book] --> B[ReaderActivity]
    B --> C{Format}
    C -->|EPUB| D[lib/Epub/Epub]
    C -->|XTC| E[lib/Xtc reader]
    C -->|TXT| F[lib/Txt reader]
    D --> G[Parse OPF/TOC and collect CSS refs]
    G --> H[Build/load book.bin and css_rules.cache]
    H --> I[Layout pages/sections]
    I --> J[Write section cache]
    J --> K[Render current page via GfxRenderer]

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, hyphenation, embedded CSS, image rendering, and Focus Reading 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

Typical persisted areas on SD:

/.crosspoint/
  epub_<hash>/
    book.bin
    css_rules.cache
    progress.bin
    cover.bmp
    sections/*.bin
    img_* cache files
  settings.json
  state.json

sections/*.bin contains rendered pages plus anchor, paragraph, list-item, and page-start visible-text-offset lookup tables. The offset table makes reading positions content-based: KOReader XPaths resolve to an exact chapter offset, and the current layout derives the corresponding page. 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_html.py generates src/network/html/*.generated.h from HTML files
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 freeink-sdk
lib/Epub/: EPUB parser, layout, CSS handling, and hyphenation
lib/: supporting libraries (fonts, text, filesystem helpers, etc.)
freeink-sdk/: hardware SDK submodule (display, input, storage, battery). Docs: https://freeink.org/docs
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: