## Summary
Reduces active-reading power draw ~3.2x, measured end-to-end on an X3
with a [Nordic
PPK2](https://www.nordicsemi.com/Products/Development-hardware/Power-Profiler-Kit-2)
at the battery terminals (slim build, USB cable removed, 3.8v). Three
independent changes:
1. **Light-sleep between input polls when idle** (the big one): after 1
s of inactivity the main loop replaces its busy `delay(50)` with 50 ms
timer-paced `esp_light_sleep_start()` slices. Poll cadence and input
latency are unchanged — the CPU sleeps between polls instead of spinning
at 10 MHz. Guards: never sleeps while a `HalPowerManager::Lock` is held
(render in flight), WiFi is up, or USB is connected (light sleep kills
the CDC link).
2. **Two-stage idle backoff**: downclock at 500 ms, light sleep at 1 s,
replacing the flat 3 s full-speed window. Renders re-raise the clock via
`Lock` the moment they start, so the full-speed tail only ever served
loop bookkeeping. (The 3 s value in #852 was a placeholder;
responsiveness verified on device against stock.)
3. **Refresh BUSY-wait downclock**: drops the CPU to 10 MHz while
`pollBusy()` polls the BUSY pin (0.3–2 s per refresh), via new optional
hooks in the SDK — **depends on Free-Ink/freeink-sdk/pull/8**. Plus a 1
Hz throttle on the X3's I2C-based USB detection (was polled every loop).
(1+2) reduced average static-page-current from 9.68mA to 3.45mA.
Reducing the battery-monitor polling frequency further reduced it to
2.78mA.
Before merging, we'll need to update the submodule reference.
## Measured results (PPK2, X3)
| Metric | Before | After |
|---|---|---|
| Idle on a static page | 9.68 mA | **2.78 mA** |
| Fast page turn (incl. post-turn behavior) | ~100 mC | **28.98 mC** |
| Post-turn 160 MHz tail | 21.2 mA x 3 s | eliminated |
| Session average @ 30 s/page | ~11.6 mA | **~3.6 mA (~3.2x reading
time)** |
| Deep sleep | 12.8 µA | unchanged |
## Typical-day profiles
Assuming someone spends 30s per page.
- **Light**: 1 h reading, 10 lock/wake cycles, remaining ~23 h deep
sleep
- **Heavy**: 2 h reading, 20 lock/wake cycles, remaining ~22 h deep
sleep
- Per-day cost components: reading 11.6 → 3.6 mA (session avg),
lock+wake cycle
0.113 → 0.099 mAh each (sleep-screen mode), deep sleep 12.8 µA
(unchanged,
~0.29 mAh/day)
- Excluded: WiFi sessions, cold chapter indexing, self-discharge
### Days per charge (650mAh)
| Profile | Before | After | Ratio |
|---|---|---|---|
| Light (1 h/day, 10 locks) | 13.0 mAh/day → ~50 days | 4.9 mAh/day →
~133 days | 2.7× |
| Heavy (2 h/day, 20 locks) | 25.7 mAh/day → ~25 days | 9.5 mAh/day →
~68 days | 2.7× |
| Pure active reading | 11.6 mA → ~56 h | 3.6 mA → ~180 h | 3.2× |
### Caveats
- "After" saturates in reality: Li-ion self-discharge (~2–3%/month) is
not modeled
and becomes the limiter once firmware draw is this low.
- Faster page-turners trend toward ~2.8×, slower readers slightly above
3.2×.
- Frequent locking dilutes the ratio (wake = full cold boot, barely
touched by the
power branch): at 10–20 locks/day, cycles are ~20% of the "after"
budget.
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**YES**_
---------
## Summary
* **What is the goal of this PR?**
* Prevent reset-only watchdog events from opening the crash-report flow
with an empty panic reason.
* **What changes are included?**
* Record a marker in RTC memory only when panic diagnostics are
captured.
* Continue reporting explicit panics and CPU lockups.
* Require that marker before treating watchdog resets as crashes, and
clear it with the existing panic state.
## Scope Check
CrossPoint is intentionally narrow. See
[SCOPE.md](../blob/master/SCOPE.md) and
[ROADMAP.md](../blob/master/ROADMAP.md).
Please confirm:
- [x] I have read SCOPE.md and ROADMAP.md.
- [x] This PR is **not** a new built-in theme (themes are temporarily
closed pending the move to SD-loaded themes).
- [x] This PR is **not** a new external network connector (sync engine,
cloud storage, remote file access, etc.).
- [x] This PR is **not** an interactive app, writing tool,
RSS/news/browser, media playback, or PDF feature.
- [x] The stock firmware does not already handle this well, **and** no
other popular CrossPoint fork already does
(or, if one does, I explain why CrossPoint still needs it below).
- [x] If this PR touches `freeink-sdk/`, `lib/hal/`, the bootloader,
OTA, or recovery code, I have coordinated with
the relevant maintainer.
**If this PR was opened against the previous (broader) scope and was
already in flight under Phase 0, link the
relevant Discussion or issue so reviewers can see the history.**
## Additional Context
* This is a focused port of CrossInk commit `c96d4275`.
* The change adds 4 bytes of RTC no-init state; it does not affect
storage formats, normal boot routing, or real panic reporting.
* Hardware follow-up: on an X3/X4, confirm a reset-only watchdog boots
normally without an empty crash report, while a genuine panic still
reaches the crash-report screen.
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**<YES>**_
Ties bookmarks to the actual text on the page rather than to a page number. Page numbers shift whenever the book gets re-laid-out — changing font size, margins, line spacing, or rotating the screen — which could previously land you on the wrong spot.
This is a followup of #2805, making bookmarks use the same mechanism. Several minor improvments ride along.
