27 lines
2.4 KiB
Markdown
27 lines
2.4 KiB
Markdown
# IR Remote Emulation Test (ATtiny84 to Raspberry Pi)
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This folder contains a test environment to verify direct communication between an ATtiny84 and a Raspberry Pi running LineageOS (Android). The goal is to send remote control commands directly over a wire, bypassing the need for physical infrared (IR) LEDs and receivers.
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## Hardware Setup
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- **Microcontroller**: ATtiny84 running at 8MHz (3.3V)
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- **Target**: Raspberry Pi running LineageOS (3.3V GPIO)
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- **Connection**: A direct wire from ATtiny84 **PA0** (Physical Pin 13 / Arduino Digital 10) to Raspberry Pi **GPIO 24** (Physical Header Pin 18). Both devices share a common ground.
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- **Debug**: An LED on ATtiny digital pin 3 (PA6 / Physical Pin 7) flashes when a command is transmitted.
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## What We Learned
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During testing via ADB, we discovered several crucial details about how the Raspberry Pi handles IR signals in LineageOS:
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1. **Protocol Expectation**: The Raspberry Pi's `gpio_ir_recv` driver defaults to the **RC-6** protocol and uses the Microsoft MCE remote keymap (`rc-rc6-mce`). Sending standard NEC protocol will fail unless the kernel is manually reconfigured.
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2. **Signal Polarity**: The `gpio-ir-recv` device tree node uses the `GPIO_ACTIVE_LOW` flag. This means the Linux driver expects a signal identical to the output of a standard TSOP IR receiver:
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- **Idle (Space)** = `HIGH` (3.3V)
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- **Active Pulse (Mark)** = `LOW` (0V)
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3. **Encoding Rules**: RC-6 uses Manchester encoding. A Logic '1' is defined as a Mark followed by a Space (in our direct-wire context: `LOW` then `HIGH`), while a Logic '0' is a Space followed by a Mark (`HIGH` then `LOW`).
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## Implementation details
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The code in `IR_remote_test.cpp` implements a custom RC-6 Mode 6A (MCE) bit-banged sender. It bypasses 36kHz carrier modulation since the wire connects directly to the demodulated input GPIO on the Pi.
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It sends the specific 32-bit MCE scancode `0x800f0410` (Volume Up), including the proper RC-6 leader pulses, start bit, mode bits, and an alternating toggle bit.
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## Conclusion
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By properly matching the protocol (RC-6 MCE) and the TSOP hardware polarity (Mark=LOW, Space=HIGH), we were able to successfully inject remote control commands directly into the Android input subsystem without needing to modify any Android `.kl` (keylayout) files or custom kernel drivers. The Pi natively decodes the bit-banged signal as a standard `KEY_VOLUMEUP` hardware event.
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