ESP32 ALDL Wireless Bridge

ESP32 native C firmware using ESP-IDF for reading GM 160-baud PWM ALDL data from a 1986–1988 Pontiac Fiero 2.8L V6 (1227170 ECM) and streaming it wirelessly over Bluetooth SPP.

This project turns an ESP32 into a reliable, low-latency wireless ALDL interface. It decodes the raw 160-baud PWM signal in real time and forwards clean 25-byte data frames over Bluetooth to the companion Android app (esp32-aldl-android).

It is implemented as a native ESP-IDF application in C, removing the overhead of the Arduino framework.

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Features

• Native C ESP-IDF Implementation — Built directly on ESP-IDF v5.2+ for maximum performance and low memory footprint
• Real-time 160-baud PWM ALDL decoding — Custom bit-banged decoder running in a high-priority FreeRTOS task (aldlDecodeTask)
• Robust pulse handling — Handles glitches, merged pulses, and idle gaps gracefully
• Hard frame synchronization — Prepends 0xAA 0x55 header for reliable packet alignment on the receiving side
• Bluedroid Bluetooth Classic SPP — Connects directly via Serial Port Profile (SPP) using "Just Works" Secure Simple Pairing (SSP) configuration
• Decoupled RTOS Architecture — ISR -> Ring Buffer -> Decoder task -> Bluetooth transmit queue -> BT TX task
• Periodic status reporting — Diagnostics reported over ESP-IDF log outputs (ESP_LOGI)

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Project Structure

esp32-aldl/
├── CMakeLists.txt         # Top-level CMake build configuration
├── sdkconfig.defaults     # Kconfig options (enabling BT, SPP, HZ=1000)
├── README.md              # Project documentation
└── main/
    ├── CMakeLists.txt     # Main component build script
    └── main.c             # C application logic (decoder and Bluetooth SPP)

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Supported Hardware

• ECM: GM 1227170 (1986–1988 Pontiac Fiero 2.8L V6)
• ALDL Mode: $24 / $24A mask (25-byte continuous broadcast frames)
• Microcontroller: ESP32 (Classic ESP32 with Classic Bluetooth capability)
• Bluetooth: Classic Bluetooth (BR/EDR)

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Pin Connections

Signal	ESP32 Pin	Notes
ALDL Data In	GPIO 4	Connect to ALDL pin M (data line)
GND	GND	Must share ground with the car/ECU
5V / 3.3V	—	Do not power the ESP32 from the ALDL line

Warning

The ALDL line is a 5V PWM signal. Since ESP32 GPIO pins are not 5V-tolerant, a simple voltage divider (e.g., 1kΩ + 2kΩ) or a bidirectional level shifter is required to step down the signal to 3.3V for long-term hardware reliability.

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Prerequisites & Installation

To build and compile the firmware, you need the official ESP-IDF v5.2.1 toolchain installed.

1. Clone ESP-IDF (v5.2.1 stable release):

   mkdir -p ~/esp
   git clone -b v5.2.1 --recursive https://github.com/espressif/esp-idf.git ~/esp/esp-idf
   

2. Run the toolchain installation:

   cd ~/esp/esp-idf
   ./install.sh esp32
   

3. Initialize the environment:

   source ~/esp/esp-idf/export.sh
   

4. (Optional) If cmake or ninja are missing in your environment, install them inside the active Python virtual environment:

   pip install cmake ninja
   

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Building and Flashing

Once the toolchain environment is loaded, execute the following commands in the esp32-aldl root workspace folder:

1. Set Build Target

idf.py set-target esp32

This command generates the build directories and loads settings from sdkconfig.defaults (enabling Bluetooth SPP and setting the FreeRTOS clock frequency to 1000 Hz).

2. Compile Project

idf.py build

This builds the bootloader, partition table, ESP-IDF drivers, Bluetooth stack, and the main application code, producing build/esp32-aldl.bin.

3. Flash & Monitor

Flash the firmware to the ESP32 and open the terminal console to view serial output (replace /dev/ttyUSB0 with your target serial port):

idf.py -p /dev/ttyUSB0 flash monitor

To exit the serial monitor, press Ctrl + ].

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How It Works

Signal Decoding

The firmware registers an Interrupt Service Routine (ISR) triggered on any edge (both rising and falling) of GPIO 4. Pulse widths are calculated in microseconds using esp_timer_get_time() and pushed to a volatile ring buffer.

The high-priority aldlDecodeTask pops pulses from the ring buffer:

• Glitches under 300us are discarded.
• Idle gaps over 13500us reset the decoder.
• Pulses are classified into Logical 0 (approx. 1.11ms), Logical 1 (approx. 4.16ms), or Merged pulses.
• Decoded bits are reconstructed into 25-byte frames.

Bluetooth Transmission

When a 25-byte frame is fully received, it is pushed to bt_queue. The btTransmitTask waits on the queue:

• Prepend the 2-byte hard-sync header (0xAA 0x55).
• Sends the packet (27 bytes total) over the Bluetooth Classic Serial Port Profile connection via the esp_spp_write() API.
• The device advertises itself under the classic Bluetooth name ESP32-ALDL.