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.

ESP-AT uses the same gatts_data.csv format on all chips that support Bluetooth LE. The rules below describe which fields take effect on ESP32-C2.

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	02
2	16	0xC300	0x01	1	1	30
3	16	0x2901	0x11	1	1	30
…	…	…	…	…	…	…

Below are descriptions of the table above.

• perm describes attribute permissions. When a row’s perm takes effect (see the rules below), 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 the table is the service definition. Its attribute type is 0x2800 (primary service), and its value A002 is the 16-bit service UUID 0xA002.

• The second line is the characteristic declaration. UUID 0x2803 identifies this row as a characteristic declaration.

  • value: characteristic properties of the next characteristic value attribute (the following row). It is one byte (8 bits). Each bit indicates whether a property is supported (1) or not (0).

  On ESP32-C2, ESP-AT translates this property value internally; you do not need to handle the difference.

  For example, value 02 means the following characteristic has the READ property.

  Bit	Characteristic Property
  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 the characteristic value attribute of that characteristic. The uuid of this line is the characteristic UUID, and value is the characteristic’s initial value.

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

• The value field is optional. If it is left empty, ESP-AT fills the attribute with all zeros during initialization. For example, in the default table, the characteristic 0xC301 row leaves value empty, so the characteristic is initialized to all zeros.

Field Rules

• For the 0x2803 row, only value (properties of the next characteristic) takes effect. The perm of this row is ignored and does not take effect.

• For the characteristic value row, perm is ignored and does not take effect. If the optional 8th field described below is not provided, the characteristic capabilities come only from the value (properties) of the previous 0x2803 row.

• If the characteristic properties include NOTIFY or INDICATE, the CCCD (0x2902) is added automatically. Any 0x2902 row in gatts_data.csv is skipped and not processed (you may keep such rows for CSV compatibility; they simply do not take effect on ESP32-C2).

• Other descriptors (such as 0x2901) are still added. If the optional 8th field described below is not provided, perm of that descriptor row takes effect.

Optional 8th field

On ESP32-C2, characteristic value rows and descriptor rows may append an optional field after value. The 0x / 0X prefix is optional. If you add this field on other rows (such as the service definition 0x2800 or the characteristic declaration 0x2803), it does not take effect.

1. Characteristic value row: the 8th field is a 32-bit hexadecimal string that sets the full characteristic flag. If this field is present, the value (properties) of the previous 0x2803 row is ignored and does not take effect; this field takes precedence.

   For example, the original row:

   2,16,0xC300,0x01,1,1,30
   

   can also be written as:

   2,16,0xC300,0x01,1,1,30,0x00020000
   

   or:

   2,16,0xC300,0x01,1,1,30,00020000
   

   The field is defined as follows (bits can be combined with OR):

   #define BLE_GATT_CHR_F_BROADCAST                0x00000001
   #define BLE_GATT_CHR_F_READ                     0x00000002
   #define BLE_GATT_CHR_F_WRITE_NO_RSP             0x00000004
   #define BLE_GATT_CHR_F_WRITE                    0x00000008
   #define BLE_GATT_CHR_F_NOTIFY                   0x00000010
   #define BLE_GATT_CHR_F_INDICATE                 0x00000020
   #define BLE_GATT_CHR_F_AUTH_SIGN_WRITE          0x00000040
   #define BLE_GATT_CHR_F_RELIABLE_WRITE           0x00000080
   #define BLE_GATT_CHR_F_AUX_WRITE                0x00000100
   #define BLE_GATT_CHR_F_READ_ENC                 0x00000200
   #define BLE_GATT_CHR_F_READ_AUTHEN              0x00000400
   #define BLE_GATT_CHR_F_READ_AUTHOR              0x00000800
   #define BLE_GATT_CHR_F_WRITE_ENC                0x00001000
   #define BLE_GATT_CHR_F_WRITE_AUTHEN             0x00002000
   #define BLE_GATT_CHR_F_WRITE_AUTHOR             0x00004000
   #define BLE_GATT_CHR_F_NOTIFY_INDICATE_ENC      0x00008000
   #define BLE_GATT_CHR_F_NOTIFY_INDICATE_AUTHEN   0x00010000
   #define BLE_GATT_CHR_F_NOTIFY_INDICATE_AUTHOR   0x00020000
   

2. Descriptor row: the 8th field is an 8-bit hexadecimal string that sets the descriptor permission. If this field is present, perm of this row is ignored and does not take effect; this field takes precedence.

   For example, the original row:

   3,16,0x2901,0x11,1,1,30
   

   can also be written as:

   3,16,0x2901,0x11,1,1,30,0x80
   

   or:

   3,16,0x2901,0x11,1,1,30,80
   

   The field is defined as follows (bits can be combined with OR):

   #define BLE_ATT_F_READ             0x01
   #define BLE_ATT_F_WRITE            0x02
   #define BLE_ATT_F_READ_ENC         0x04
   #define BLE_ATT_F_READ_AUTHEN      0x08
   #define BLE_ATT_F_READ_AUTHOR      0x10
   #define BLE_ATT_F_WRITE_ENC        0x20
   #define BLE_ATT_F_WRITE_AUTHEN     0x40
   #define BLE_ATT_F_WRITE_AUTHOR     0x80
   

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.

   The declaration-row perm and the characteristic-value-row perm are ignored and do not take effect.

   index	uuid_len	uuid	perm	val_max_len	val_cur_len	value
   32	16	0x2803	0x01	1	1	0A
   33	16	0xC300	0x11	1	1	30

3. Add the characteristic descriptor (optional).

   The characteristic in step 2 uses properties 0A (READ | WRITE) and does not require a CCCD. The content below is a separate optional illustration for characteristics that support NOTIFY or INDICATE; it is not tied to the 0A example above. If you need NOTIFY, set the 0x2803 properties accordingly (for example 1A for READ | WRITE | NOTIFY).

   If properties include NOTIFY or INDICATE, CCCD is added automatically. You do not need to add a 0x2902 row; if the row exists in the CSV, it is skipped and does not take effect. Other descriptors (not 0x2902) are still added, and that row’s perm takes effect.

   After the above steps, the customized Bluetooth LE service can be defined as follows (the 0A READ | WRITE example; no 0x2902 row is needed):

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

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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