VolvoRTI/Arduino/LIN_to_IR/README.md

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.