A daily tide chart on an Elecrow CrowPanel 5.79" E-Paper HMI (ESP32-S3, 792x272 dual-SSD1683 panel), built on plain ESP-IDF.

Once a day, just after midnight in the tide station's timezone, the device wakes from deep sleep and draws the new day's 24-hour chart (curve, high/low times and heights, gridlines), then goes back to sleep — the e-paper keeps the image with essentially no power. Tide predictions are deterministic, so a month of heights and extremes from the WorldTides API is cached in flash (surviving power loss) and the radio only comes up when the cached window runs low, roughly every three weeks. A failed refresh leaves the previous chart on screen and retries half an hour later.

The onboard controls navigate the cache without any network:

Control	Action
Wheel	Browse forward/backward a day at a time
EXIT	Return to today
MENU	Force a full refetch

Setup

1. Install ESP-IDF v5.5.x and activate it (. <idf-path>/export.sh).

2. Get a WorldTides API key and pick your station's coordinates from worldtides.com/tidestations.

3. Configure:

   cp main/config.example.h main/config.h
   # edit main/config.h: WiFi, timezone, API key, station

4. Build and flash:

   idf.py set-target esp32s3
   idf.py -p <port> flash monitor

Rendering on the host

The chart renderer compiles unchanged on a regular computer: tools/render links the real chart.c against stubbed framebuffer primitives and dumps a pixel-exact PBM of what the panel would show, using synthetic tides. Handy for previewing display changes without flashing:

The same render with a 12-hour clock, heights in feet, and m/d/y dates (TIDE_TIME_FMT, TIDE_UNITS, TIDE_DATE_FMT in config.h):

Notes

• Tide heights are requested relative to Chart Datum (datum=CD), the convention used on tide tables. The API bills 1 credit per 7 days each for heights and extremes, so the monthly cache costs ~8 credits per refetch (~12/month).
• The vertical scale fits each day's range; the horizontal scale is fixed, midnight to midnight in the station's timezone.
• The display driver lives in components/crowpanel579 and is reusable for other apps on this board. The dual-controller panel has several sharp edges (RAM bank swapping after every update, waveforms that can only clear or only draw, a seam byte shared by both controllers) — read components/crowpanel579/HARDWARE.md before touching it.