Add car-off shutdown detection to LIN_to_IR firmware

Arduino/LIN_to_IR/README.md

@@ -27,9 +27,45 @@ The Steering Wheel Module (SWM) sends frames on LIN ID `0x20` with the navigatio
 | **RIGHT** | `d0 & 0x08` | `4` | `0x800f0421` | Navigate Right |
 | **ENTER** | `d1 & 0x08` | `5` | `0x800f0422` | Select / OK |
 | **BACK** | `d1 & 0x01` | `6` | `0x800f0423` | Back / Exit |
+| **F12 (Shutdown)** | LIN Silent > 5s | `7` | `0x800f046f` | Trigger Shutdown (via MacroDroid) |
 
 *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`.*
 
+## Car-Off (Shutdown) Detection
+
+To automatically power off the Raspberry Pi when the car is turned off:
+1. **Activity Monitor**: The ATtiny monitors LIN bus activity. The Central Electronic Module (CEM) continuously polls the LIN bus when the ignition is on.
+2. **Timeout**: If no LIN serial data is received for 5 seconds, the ATtiny assumes the car is off.
+3. **Shutdown Signal**: It transitions the car status to "off" and sends the MCE code `0x800f046f` (mapped to `KEY_F12`) 3 times to ensure transmission reliability.
+4. **State Protection**: It will not send the shutdown command again until the car is turned back on and LIN traffic resumes (which sets `is_car_on` back to `true`).
+
+## Raspberry Pi Configuration
+
+### 1. Keymap Setup
+Remap scancode `0x800f046f` to `KEY_F12` on the Raspberry Pi:
+- The custom TOML configuration is saved at `/data/rc-rc6-mce.toml` on the Pi.
+- To apply it dynamically:
+  ```bash
+  /vendor/bin/ir-keytable -w /data/rc-rc6-mce.toml
+  ```
+
+### 2. MacroDroid Persistence & Trigger
+Configure two macros in **MacroDroid** to manage keymap loading and shutdown:
+
+- **Macro 1: Load Keymap on Boot**
+  - **Trigger**: *Device Boot*
+  - **Action**: *Run Shell Command* (Check **Run as Root**)
+    ```bash
+    /vendor/bin/ir-keytable -w /data/rc-rc6-mce.toml
+    ```
+
+- **Macro 2: Shutdown on F12**
+  - **Trigger**: *Media / Key Pressed* -> Select **F12**
+  - **Action**: *Run Shell Command* (Check **Run as Root**)
+    ```bash
+    reboot -p
+    ```
+
 ## Protocol and Timing
 
 - **Protocol**: RC-6 Mode 6A (MCE).
@@ -46,3 +82,4 @@ The Steering Wheel Module (SWM) sends frames on LIN ID `0x20` with the navigatio
    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.
+