How to Customize Bluetooth® LE Services

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This document describes how to customize Bluetooth LE services on your ESP32-C2 with the Bluetooth LE service source file provided by ESP-AT.

The Bluetooth LE services are defined as a multivariate array of GATT structures, and the array contains at least one primary service whose attribute type is defined as 0x2800. Each service always consists of a service definition and several characteristics. Each characteristic always consists of a value and optional descriptors. Please refer to Part Generic Attribute Profile (GATT) of Bluetooth Core Specification for more information.

Bluetooth LE Service Source File

The ESP-AT project creates Bluetooth LE services based on its Bluetooth LE service source file. It is located in customized_partitions/raw_data/ble_data/gatts_data.csv. The table below shows the default source file.

index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
0	16	0x2800	0x01	2	2	A002
1	16	0x2803	0x01	1	1	2
2	16	0xC300	0x01	1	1	30
3	16	0x2901	0x11	1	1	30
…	…	…	…	…	…	…

Below are descriptions of the table above.

• perm field describes the permission. Its definition in the ESP-AT project is as follows:

  /* relate to BTA_GATT_PERM_xxx in bta/bta_gatt_api.h */
  /**
  * @brief Attribute permissions
  */
  #define    ESP_GATT_PERM_READ                  (1 << 0)   /* bit 0 -  0x0001 */    /* relate to BTA_GATT_PERM_READ in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_READ_ENCRYPTED        (1 << 1)   /* bit 1 -  0x0002 */    /* relate to BTA_GATT_PERM_READ_ENCRYPTED in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_READ_ENC_MITM         (1 << 2)   /* bit 2 -  0x0004 */    /* relate to BTA_GATT_PERM_READ_ENC_MITM in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_WRITE                 (1 << 4)   /* bit 4 -  0x0010 */    /* relate to BTA_GATT_PERM_WRITE in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_WRITE_ENCRYPTED       (1 << 5)   /* bit 5 -  0x0020 */    /* relate to BTA_GATT_PERM_WRITE_ENCRYPTED in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_WRITE_ENC_MITM        (1 << 6)   /* bit 6 -  0x0040 */    /* relate to BTA_GATT_PERM_WRITE_ENC_MITM in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_WRITE_SIGNED          (1 << 7)   /* bit 7 -  0x0080 */    /* relate to BTA_GATT_PERM_WRITE_SIGNED in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_WRITE_SIGNED_MITM     (1 << 8)   /* bit 8 -  0x0100 */    /* relate to BTA_GATT_PERM_WRITE_SIGNED_MITM in bta/bta_gatt_api.h */
  #define    ESP_GATT_PERM_READ_AUTHORIZATION    (1 << 9)   /* bit 9 -  0x0200 */
  #define    ESP_GATT_PERM_WRITE_AUTHORIZATION   (1 << 10)  /* bit 10 - 0x0400 */
  

• The first line of table is the service definition with a UUID of 0xA002.

• The second line is the declaration of a characteristic. UUID 0x2803 means the characteristic declaration. The value 2 sets the permission. The length of permission is 8 bits, and each bit represents permission for an operation. 1 indicates that the operation is supported, and 0 indicates not supported.

  Bit	Permission
  0	BROADCAST
  1	READ
  2	WRITE WITHOUT RESPONSE
  3	WRITE
  4	NOTIFY
  5	INDICATE
  6	AUTHENTICATION SIGNED WRITES
  7	EXTENDED PROPERTIES

• The third line defines a characteristic of the service. UUID of this line is the characteristic’s UUID, and value is the characteristic’s value.

• The fourth line defines a descriptor of the characteristic (optional).

For more information about UUID, please refer to Bluetooth Special Interest Group (SIG) Assigned Numbers.

If you use the default source file on your ESP32-C2 without any modification and establish a Bluetooth LE connection, you will get the following result after querying the server service on the client side.

Customize Bluetooth LE Services during Compilation

If you want to customize the Bluetooth LE services, follow the steps below.

Modify the Bluetooth LE Service Source File

You can define more than one service. For example, if you want to define three services (Server_A, Server_B and Server_C), these three services need to be arranged in order. Since the definition of each service is similar, here we define one service as an example, and then you can define others one by one accordingly.

1. Add the service definition.

   In this example, we define a primary service with a value of 0xFF01.

   index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
   31	16	0x2800	0x01	2	2	FF01

2. Add the characteristic declaration and characteristic value.

   In this example, we define a readable and writable characteristic with UUID 0xC300, and set its value to 0x30.

   index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
   32	16	0x2803	0x11	1	1	0A
   33	16	0xC300	0x11	1	1	30

3. Add the characteristic descriptor (optional).

   In this example, we add client characteristic configuration. Its value 0x0000 represents notifications and indications are disabled.

   index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
   34	16	0x2902	0x11	2	2	0000

After the above steps, the customized Bluetooth LE service has been defined as follows.

index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
31	16	0x2800	0x01	2	2	FF01
32	16	0x2803	0x11	1	1	0A
33	16	0xC300	0x11	1	1	30
34	16	0x2902	0x11	2	2	0000

Please modify the GATTS configurations according to your own needs and generate mfg_nvs.bin file.

Generate mfg_nvs.bin

Please refer to Generate mfg_nvs.bin document to generate the mfg_nvs.bin file with the Low Energy Bluetooth services.

Download mfg_nvs.bin

Please refer to Download mfg_nvs.bin document.

After the download is complete, re-establish the Bluetooth LE connection. Query the server service on the client side as follows:

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