# Volvo LIN-to-IR Controller (ATtiny84)

This project implements the production firmware for the ATtiny84 to interface a Volvo V50 Steering Wheel Module (SWM) LIN bus with a Raspberry Pi running LineageOS (Android). 

It decodes steering wheel navigation buttons from the vehicle's LIN bus and translates them into bit-banged RC-6 Mode 6A (MCE) remote control commands sent over a direct wire connection.

## Pin Configurations (ATtiny84)

| ATtiny84 Pin | Digital Pin (Arduino) | Function | Description |
| :--- | :--- | :--- | :--- |
| **PA0** | `10` | IR Output | Direct-wired to Raspberry Pi GPIO 24 |
| **PA5** | `8` | LIN RX | SoftwareSerial RX from MCP2004 RXD |
| **PA1** | `9` | LIN FAULT/TXE | MCP2004 Fault Detect / Transmit Enable |
| **PA2** | `11` | LIN CS | MCP2004 Chip Select (Active High) |
| **PA6** | `3` | Debug LED | Flash on command transmission |

## Key Mapping

The Steering Wheel Module (SWM) sends frames on LIN ID `0x20` with the navigation key statuses. The firmware decodes these and maps them to the following Microsoft MCE remote scancodes:

| Button | LIN Frame Trigger | Active Button Code | MCE Scancode | Action on LineageOS |
| :--- | :--- | :--- | :--- | :--- |
| **UP** | `d0 & 0x01` | `1` | `0x800f041e` | Navigate Up |
| **DOWN** | `d0 & 0x02` | `2` | `0x800f041f` | Navigate Down |
| **LEFT** | `d0 & 0x04` | `3` | `0x800f0420` | Navigate Left |
| **RIGHT** | `d0 & 0x08` | `4` | `0x800f0421` | Navigate Right |
| **ENTER** | `d1 & 0x08` | `5` | `0x800f0422` | Select / OK |
| **BACK** | `d1 & 0x01` | `6` | `0x800f0423` | Back / Exit |

*Note: In previous revisions, `ENTER` and `BACK` were reversed. This has been corrected so that pressing `ENTER` maps to `0x800f0422` and `BACK` maps to `0x800f0423`.*

## Protocol and Timing

- **Protocol**: RC-6 Mode 6A (MCE).
- **Time Unit (1T)**: `444us`.
- **Modulation**: None. Timing is bit-banged directly since the output pin is wired directly to the Pi's IR receiver GPIO (which expects demodulated active-low signals).
- **Idle (Space)**: `HIGH` (3.3V).
- **Active Pulse (Mark)**: `LOW` (0V).
- **Toggle Bit**: Alternates state on each new button press, but remains constant for repeated holds.
- **Repeat Interval**: Holds trigger repeat IR commands sent every `250ms`.

## Software Architecture

1. **SoftwareSerial Interrupt Isolation**: 
   Since SoftwareSerial RX interrupts consume about 1ms per byte, they will disrupt the precise microsecond-level timing of bit-banged IR frames. To avoid this, interrupts are disabled (`noInterrupts()`) during the 37ms window of IR transmission and re-enabled (`interrupts()`) immediately afterward.
2. **Release detection**:
   The SWM continuously transmits frames on the LIN bus. If no button frames are detected or a frame with all 0s is received, the current active button resets to idle. A timeout of `200ms` ensures that if the LIN bus goes quiet, button repeats cease immediately.