How to Customize Bluetooth® LE Services

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This document describes how to customize Bluetooth LE services on your ESP32 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.

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).

  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, both perm and value take effect: perm is the permission of the declaration attribute itself (usually 0x01, readable), and value is the properties of the next characteristic.

• For the characteristic value row, perm takes effect. It must not conflict with the properties in the previous 0x2803 row. Properties advertise which operations the characteristic supports; perm controls whether the stack actually allows those operations. If properties claim READ-only but the value-row perm is WRITE-only, clients will fail when they try to read.

  Typical mapping (encrypted or signed permission variants may be used when security is required):

  If properties include	Characteristic value perm should include at least
  READ	READ (0x01), or a read-related encrypted/authorized variant
  WRITE	WRITE (0x10), or a write-related encrypted/authorized/signed variant
  WRITE WITHOUT RESPONSE	WRITE (0x10), or a write-related encrypted/authorized/signed variant
  NOTIFY or INDICATE	Usually READ on the characteristic value; you must add the CCCD (0x2902) yourself. Its perm is typically READ | WRITE (0x11).

  You can combine properties. For example, 0A (READ | WRITE) should be paired with characteristic value perm 0x11 (READ | WRITE).

• If the characteristic supports NOTIFY or INDICATE, add a 0x2902 row in the table. Other descriptors (such as 0x2901) are also added as defined, and each descriptor row’s perm takes effect.

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 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 is 0x01. The characteristic-value-row perm is 0x11 (required to match READ | WRITE properties).

   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 the characteristic supports NOTIFY or INDICATE, add client characteristic configuration (0x2902) yourself. The example below sets value to 0000 (notifications and indications disabled).

   Example of a CCCD row:

   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 can be defined as follows. The table combines the service and characteristic from steps 1–2 with the optional CCCD illustration from step 3. For the 0A (READ | WRITE) characteristic alone, omit the 0x2902 row.

   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
   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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