# Flipdot display: notes for Claude

A PlatformIO project with two firmware targets for an 84×19 Brose flipdot clock.
See README.md for the overview.

## Layout and build

- `platformio.ini` has the environments `nano`, `esp32` (OTA to 10.1.1.198) and
  `esp32_usb` (COM11). The sources are split with `build_src_filter` (`src/nano/`,
  `src/esp32/`).
- `lib/BROSE9323/` is the shared driver. Its behaviour depends on build flags set in
  `platformio.ini`, not in the header:
  - `ESPSERIAL` (ESP32): draw calls become serial commands.
  - `FLIPDOT_PLCC_ADAPTER` (Nano, optional): alternative pinout.
- PlatformIO CLI is not on PATH. Run `~/.platformio/penv/Scripts/pio.exe run` (Git Bash)
  or `$env:USERPROFILE\.platformio\penv\Scripts\pio.exe run` (PowerShell).
- If an ESP32 build fails with `No module named 'intelhex'`, run
  `~/.platformio/penv/Scripts/python.exe -m pip install intelhex`.
- After any change, build both targets: `pio run -e nano -e esp32`.

## Constraints

- **Nano RAM is tight** (ATmega328P, 2 KB). The frame buffers are allocated with
  `calloc`. Avoid `String`, large stack buffers and new globals in Nano code. Put constant
  data in `PROGMEM`.
- The Nano and the ESP32 share one UART. On the ESP32, `Serial` is the display link, so
  **don't print debug output on the ESP32's `Serial`**: the Nano would read it as
  commands. The Nano ignores unknown command bytes, but text that starts with
  s/S/F/D/P/B/R/T will be run as a command.
- `src/esp32/secrets.h` holds the real credentials and is gitignored. Never commit it.
  Add new secrets to `secrets.example.h` as placeholders.
- The relay (ESP32 GPIO 13, `LOW` = on) powers both the panels and the Nano. Each time it
  switches on, the Nano cold-boots and then sends `READY`. The ESP's `relayOn()` waits for
  that message, at most `NANO_BOOT_TIMEOUT`. The timeout also covers the case where the
  Nano's TX isn't wired to the ESP's RX.
- `relayOff()` ends `Serial` and sets the TX pin to input. Otherwise the idle-high TX line
  powers the Nano through its RX pin. Never send display commands while `relay_on` is
  false.
- The Nano keeps no state over a power cycle except the flip time in EEPROM. The driver
  forces a full redraw on the first `display()` after boot (`_force_next`).
- Hardware is not available here. Builds can be checked, but runtime behaviour can't.

## Nano serial parser (`src/nano/main.cpp`)

- Line based and non-blocking: bytes are collected into a 64-character buffer and a
  command runs when `\n` arrives. Bitmap (`B`) pixel data is the exception. It is drawn
  while it arrives, so it can be longer than the buffer. Keep `loop()` free of busy-waits.
- A full redraw (`display(true)`) blocks for about 0.5 s, which overflows the 64-byte
  serial receive buffer. Only run it when no data is expected: right after a `D`, or after
  2 s with no serial data. The ESP sends one frame per second, ending with `D`.
- Flip time is stored in EEPROM as a magic word at address 0 and a `uint16_t` at
  address 2. Values outside 50–2000 µs are rejected.
- The Nano only replies `READY` (at boot) and `ERR: ...`. There is no per-frame
  acknowledgement. Whether the Nano's TX is wired to the ESP's RX is unconfirmed, so every
  wait for a reply needs a timeout.
- To test the parser without hardware, compile `main.cpp` with g++ (Strawberry Perl
  provides it at `C:\Strawberry\c\bin`) and mock headers for `Arduino.h`, `EEPROM.h` and
  `BROSE9323.h`.

## Git

- Remote: https://mangavalk.com/mangavalk/Flipdot-display.git, branch `main`.