ESP32-C3: GAP Advertisement Issue With Connectable=False

by Alex Johnson 57 views

Introduction

In the realm of MicroPython and Bluetooth Low Energy (BLE), developers sometimes encounter unexpected behavior when configuring advertisement parameters. One such issue arises when attempting to disable connectability in GAP (Generic Access Profile) advertisements on the ESP32-C3. Specifically, setting connectable=False during advertisement configuration doesn't yield the expected outcome, leading to devices still being advertised as connectable. This article delves into the problem, its reproduction, observed behavior, and potential solutions, providing a comprehensive guide for developers facing this challenge.

Understanding the Problem: GAP Advertising and Connectability

At the core of BLE communication lies the concept of advertising. Devices broadcast advertisement packets to announce their presence and capabilities to nearby devices. These packets contain information such as the device name, supported services, and connection parameters. The connectable flag within the advertisement data dictates whether the advertising device allows connections from other devices. When connectable is set to False, the device should ideally only broadcast data without accepting connection requests. However, in certain scenarios, particularly with the ESP32-C3 and MicroPython, this setting may not function as expected. The device continues to advertise itself as connectable, despite the configuration.

Reproduction Steps: Demonstrating the Issue

To illustrate this problem, let's consider a practical example using the MicroPython environment on an ESP32-C3. The following steps outline how to reproduce the issue:

  1. Set up the Environment: Ensure you have the MicroPython firmware installed on your ESP32-C3. You'll also need a development environment capable of flashing and interacting with the device.
  2. Obtain the Code: Utilize the provided example code from the ubeacon repository, specifically the eddystone_uid.py script. This script is designed to broadcast Eddystone-UID frames, which are commonly used for beacon applications.
  3. Configure Advertisement Parameters: Within the script, locate the section responsible for setting up the GAP advertisement. Modify the parameters to include connectable=False. This should instruct the ESP32-C3 to advertise without allowing connections.
  4. Run the Code: Flash the modified script onto the ESP32-C3 and execute it.
  5. Observe the Behavior: Use a Bluetooth scanning tool on a smartphone or laptop to detect the advertising device. Despite setting connectable=False, the device will likely appear as connectable in the scan results.

This reproduction process highlights the discrepancy between the intended configuration and the actual behavior, confirming the issue with the connectable=False setting.

Observed Behavior: Advertised as Connectable

The most prominent observation is that the ESP32-C3 continues to advertise itself as connectable, even when explicitly configured not to. This behavior contradicts the expected outcome, where the device should only broadcast advertisement data without accepting connection requests. The practical implications of this issue are significant. In scenarios where a device is intended to operate solely as a broadcaster, such as a beacon, the ability to prevent connections is crucial for security and power efficiency. An unexpected connectable advertisement can expose the device to unwanted connections, potentially leading to security vulnerabilities or increased power consumption.

Potential Causes and Solutions

Several factors might contribute to this issue, and pinpointing the exact cause often requires a systematic approach. Here are some potential reasons and corresponding solutions:

Firmware or Library Bug

An underlying bug in the MicroPython firmware or the BLE library could be responsible for the incorrect behavior. In such cases, updating to the latest firmware version or library might resolve the issue. Firmware updates often include bug fixes and improvements that address such discrepancies.

Incorrect Configuration

While the code might explicitly set connectable=False, there could be conflicting configurations elsewhere in the application. Review the entire codebase to ensure there are no overriding settings that might be enabling connectability. Look for any other instances where advertising parameters are being modified.

Hardware Limitations

In rare cases, hardware limitations or specific characteristics of the ESP32-C3's Bluetooth module might influence the behavior. Consult the device's technical documentation and errata sheets to identify any known limitations related to advertising and connectability. If hardware limitations are the cause, alternative approaches or workarounds might be necessary.

Interference from Other Devices

In environments with high Bluetooth activity, interference from other devices might affect the advertising process. Try testing the device in a less congested environment to rule out interference as a factor.

Debugging Techniques

To further investigate the issue, employ debugging techniques such as:

  • Logging: Add logging statements to the code to track the values of relevant variables and the execution flow. This can help identify where the configuration is being set and whether it's being overwritten.
  • Packet Sniffing: Use a Bluetooth packet sniffer to capture and analyze the advertisement packets being transmitted by the ESP32-C3. This provides a low-level view of the data being broadcast and can reveal discrepancies in the connectability flag.
  • Testing with Different Devices: Test the advertisement behavior with multiple scanning devices to ensure the issue isn't specific to a particular device or platform.

Code Example and Explanation

To further illustrate the problem and potential solutions, let's examine a simplified code snippet based on the eddystone_uid.py example:

import bluetooth
import random
import struct
import time

# Bluetooth object
ble = bluetooth.BLE()
ble.active(True)

# GAP advertising data
adv_data = bytes([ 
    0x02, # Length
    0x01, # Flags
    0x06, # LE General Discoverable Mode + BR/EDR Not Supported
    0x03, # Length
    0x03, # Service UUID
    0xAA, # Eddystone Service UUID (16-bit)
    0xFE, # Eddystone Service UUID (16-bit)
    0x16, # Length
    0x16, # Service Data
    0xAA, # Eddystone Service UUID (16-bit)
    0xFE, # Eddystone Service UUID (16-bit)
    0x10, # Frame Type: UID Frame
    -59,  # TX Power (dBm at 0m)
    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, # Namespace ID
    0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, # Instance ID
    0x00, 0x00 # Reserved
])

# GAP advertising parameters
adv_interval_us = 250000 # 250ms

# Start advertising with connectable=False
ble.gap_advertise(adv_interval_us, adv_data, connectable=False) # connectable=False is set here

print("Advertising...")

# Run forever
while True:
    time.sleep_ms(1000)

In this code:

  • We initialize the Bluetooth module and set the advertising data (adv_data).
  • The crucial part is the ble.gap_advertise() function call, where we explicitly set connectable=False.
  • Despite this setting, the device may still be detected as connectable by scanning tools.

Troubleshooting Steps

  1. Verify Firmware Version: Ensure you are using the latest MicroPython firmware for ESP32-C3.
  2. Check for Conflicting Settings: Review your code for any other ble.gap_advertise() calls or configurations that might override the connectable setting.
  3. Test with Minimal Code: Create a minimal example that only sets up advertising with connectable=False to isolate the issue.
  4. Use Packet Sniffer: Capture Bluetooth packets to confirm the advertising data and check the connectability flag.

Alternative Approaches and Workarounds

If the connectable=False setting consistently fails, consider alternative approaches to achieve the desired behavior:

Filtering Connections

Instead of preventing connections at the advertising level, you can filter incoming connection requests. Implement logic within your application to reject connections based on certain criteria, such as device address or connection parameters. This approach allows the device to advertise as connectable but provides control over which connections are accepted.

Using a Non-Connectable Advertising Type

Explore different advertising types that inherently do not allow connections. For instance, the scan response advertising type is designed for providing additional information during active scanning and does not support connections. However, this might require adjustments to the scanning process on the receiving devices.

Conclusion

The issue of gap_advertise with connectable=False not working on ESP32-C3 highlights the complexities of BLE development and the importance of thorough testing and debugging. By understanding the problem, its reproduction, potential causes, and available solutions, developers can effectively address this challenge and ensure their devices function as intended. Remember to verify firmware versions, check for conflicting settings, employ debugging techniques, and consider alternative approaches when necessary.

By systematically addressing potential causes and employing debugging techniques, developers can overcome the connectable=False issue and ensure their ESP32-C3 devices advertise and behave as intended. The key lies in understanding the interplay between firmware, hardware, and software configurations, and adopting a methodical approach to troubleshooting.

For further information on Bluetooth Low Energy and MicroPython, refer to the official documentation and resources. You can also find helpful information and community support on websites such as the Bluetooth SIG.