49 lines
2.9 KiB
Markdown
49 lines
2.9 KiB
Markdown
# Volvo LIN-to-IR Controller (ATtiny84)
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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).
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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.
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## Pin Configurations (ATtiny84)
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| ATtiny84 Pin | Digital Pin (Arduino) | Function | Description |
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| :--- | :--- | :--- | :--- |
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| **PA0** | `10` | IR Output | Direct-wired to Raspberry Pi GPIO 24 |
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| **PB2** | `2` | LIN RX | SoftwareSerial RX from MCP2004 RXD |
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| **PB0** | `0` | LIN TX | SoftwareSerial TX (Unused dummy) |
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| **PB1** | `1` | LIN FAULT/TXE | MCP2004 Fault Detect / Transmit Enable |
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| **PB3** | `11` | LIN CS | MCP2004 Chip Select (Active High) |
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| **PA7** | `3` | Debug LED | Flash on command transmission |
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## Key Mapping
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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:
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| Button | LIN Frame Trigger | Active Button Code | MCE Scancode | Action on LineageOS |
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| :--- | :--- | :--- | :--- | :--- |
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| **UP** | `d0 & 0x01` | `1` | `0x800f041e` | Navigate Up |
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| **DOWN** | `d0 & 0x02` | `2` | `0x800f041f` | Navigate Down |
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| **LEFT** | `d0 & 0x04` | `3` | `0x800f0420` | Navigate Left |
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| **RIGHT** | `d0 & 0x08` | `4` | `0x800f0421` | Navigate Right |
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| **ENTER** | `d1 & 0x08` | `5` | `0x800f0422` | Select / OK |
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| **BACK** | `d1 & 0x01` | `6` | `0x800f0423` | Back / Exit |
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*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`.*
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## Protocol and Timing
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- **Protocol**: RC-6 Mode 6A (MCE).
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- **Time Unit (1T)**: `444us`.
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- **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).
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- **Idle (Space)**: `HIGH` (3.3V).
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- **Active Pulse (Mark)**: `LOW` (0V).
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- **Toggle Bit**: Alternates state on each new button press, but remains constant for repeated holds.
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- **Repeat Interval**: Holds trigger repeat IR commands sent every `250ms`.
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## Software Architecture
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1. **SoftwareSerial Interrupt Isolation**:
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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.
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2. **Release detection**:
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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.
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