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# OOK Transition-Count Decode Plan
## Goal
Decode an OOK carrier by counting `T0` pin transitions in fixed time windows, then reconstruct framed packet bits with error checking.
## Sampling Strategy
- Use Timer0 as an external event counter on `PB0/T0`.
- Use Timer1 CTC interrupt at about 1 ms to snapshot and clear `TCNT0`.
- Treat each 1 ms transition count as one energy sample.
## Envelope Detection (Hysteresis)
- Convert transition count to an ON/OFF envelope state.
- Use separate thresholds:
- `on_threshold`: count high enough to declare carrier ON.
- `off_threshold`: count low enough to declare carrier OFF.
- Keep `on_threshold > off_threshold` to avoid chatter.
- Adapt thresholds from observed low/high sample levels over time.
## Symbol Timing and Bit Decision
- Group multiple 1 ms samples into one bit period (`samples_per_bit`).
- Decide each bit by majority ON samples inside the bit period.
- Apply simple timing recovery:
- When envelope transitions happen very early/late in a bit window, nudge the phase toward bit boundaries.
## Decoder State Machine
1. `SEARCH_PREAMBLE`
- Wait for `N` consecutive ON bits.
2. `SYNC`
- Require one OFF delimiter bit.
3. `SYNC_WORD`
- Require `0xAA` (`10101010`) before payload decode.
4. `FEC`
- Shift in two Hamming(8,4) codewords and decode one payload byte.
- Correct single-bit errors and reject detected multi-bit errors.
## Packet Format (Current Implementation)
- Preamble: `4` ON bits (about `16 ms` at 4 samples/bit and ~1 ms/sample).
- Sync delimiter: `1` OFF bit.
- Sync word: `0xAA` (`8` bits, msb-first).
- Hamming codeword for upper nibble (`8` bits).
- Hamming codeword for lower nibble (`8` bits).
- Payload: `1` byte total (`16` coded bits, after delimiter + sync word).
## Runtime Behavior
- RX runs when TX is idle.
- TX control (`PB3` low edge) triggers one OOK frame transmit.
- RX resumes automatically after TX frame completion.
## Outputs
- `rx_packet_data`: last valid decoded payload byte.
- `rx_packet_ready`: set when a valid packet is decoded.
- `rx_good_packets`, `rx_bad_packets`: decode statistics.
- `rx_corrected_codewords`: count of Hamming codewords corrected by FEC.
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