esp-idf/examples/bluetooth/bluedroid/classic_bt/bt_l2cap_client/tutorial/example_workflow.md

Description of Bluetooth L2CAP Connection Example Sequence Diagram

Introduction

This document describes the process of L2CAP initialization, connection, and data transfer established between Bluetooth L2CAP client and server provided in bt_l2cap_server example.

Initialization Process

The BT_Profile_StackA and BT_Profile_StackB in the above diagram represent the same Bluetooth protocol stacks (GAP, SDP, L2CAP in this example). StackA and StackB are to distinguish which device is interacting with the protocol stack. Both projects BT_L2CAP_Server(bt_l2cap_server) and BT_L2CAP_Client have the same initialization process but are independent of each other. The entry point to the example is the app_main() function, and the main function starts by initializing the non-volatile storage library. Then, the L2CAP initialization both the Server and Client can be divided into the following steps:

• Step1: esp_bt_controller_init()
• Step2: esp_bt_controller_enable()
• Step3: esp_bluedroid_init_with_cfg()
• Step4: esp_bluedroid_enable()
• Step5: esp_bt_gap_register_callback()

Connection Process

Step 6 is the setting phase of L2CAP. There are two alternative paths: one for the server's L2CAP setup and the other for the client's L2CAP setup.

• In the server path:

  • The L2CAP callback is registered by calling esp_bt_l2cap_register_callback().
  • The Server performs the L2CAP initialization by calling the esp_bt_l2cap_init() function, and an asynchronous callback event ESP_BT_L2CAP_INIT_EVT is returned to indicate the initialization completion.
  • Then the Server proceeds to register the virtual file system using esp_bt_l2cap_vfs_register() and starts the server using function esp_bt_l2cap_start_srv(). The asynchronous callback event ESP_BT_L2CAP_START_EVT is also returned to indicate the server's start completion.

• In the client path:

  • Compared with the server's path, the client path also calls functions esp_bt_l2cap_register_callback(), esp_bt_l2cap_init() and esp_bt_l2cap_vfs_register() to register L2CAP callback, initiate the L2CAP and register the the virtual file system. But the client does not need to start the service.

Step 7 in sequence diagram is the setting of SDP and process of service discovery. Similar to the L2CAP setup, there are separate paths for Server and Client.

• In the server path:
  • The SDP callback is registered by calling esp_sdp_register_callback().
  • The Server initiates SDP by calling esp_sdp_init(), and the asynchronous callback event ESP_SDP_INIT_EVT is returned to indicate the initialization completion.
  • Then the Server creates an SDP record by calling esp_sdp_create_record(), and an asynchronous callback event ESP_SDP_CREATE_RECORD_COMP_EVT is returned to indicate the completion of the record creation.
  • The Server can also set the device name by calling esp_bt_gap_set_device_name() and make itself connectable and discoverable by calling esp_bt_gap_set_scan_mode().
• In the client path:
  • The Client also calls functions esp_sdp_register_callback(), esp_sdp_init(), esp_sdp_create_record(), esp_bt_gap_set_device_name() and esp_bt_gap_set_scan_mode() to register SDP callback, initiate the SDP, create sdp record, set device name and set the scan mode.
  • Additionally, the Client calls esp_bt_gap_start_discovery() to start Inquiry. When the Inquiry process is completed, the asynchronous callback event ESP_BT_GAP_DISC_RES_EVT will be returned.

Once the event ESP_BT_GAP_DISC_RES_EVT is returned, the Client will try to make a L2CAP connection to the BD Address of Server by calling function esp_bt_connect() in step 8.

After the establishment of L2CAP connection, both the Server and Client will receive the asynchronous callback event ESP_BT_L2CAP_OPEN_EVT, and they can transfer data by calling the function l2cap_wr_task_start_up(). However, it is important to note that the data sender should call the function l2cap_wr_task_start_up() with handler l2cap_write_handle, while the data receiver should call the function with the handler l2cap_read_handle. Once the transmission of the data is completed, it's recommended to close the connection and deinitialize the bluetooth profile stack.