Different Value Types of Characteristics#
Introduction#
Bluetooth LE (BLE) is used for wireless communication, which is achieved by operations on characteristics.
Characteristics#
Characteristics are a GATT Profile concept, which defines Client and Server roles. Server holds the state or data and client can request the state or data. Below is the definition of server and client from the Bluetooth SPEC.
Client is the device that initiates commands and requests towards the server and can receive responses, indications and notifications sent by the server.
Server is the device that accepts incoming commands and requests from the client and sends responses, indications and notifications to a client.
The GATT Profile specifies the structure in which profile data is exchanged. This structure defines basic elements, such as services and characteristics, which are used in a profile.
Profile#
A profile is the top level of the hierarchy, which is composed of one or more services necessary to fulfill a use case. A service is composed of characteristics or references to other services. Each characteristic contains a value and may contain optional information about the value. The service and characteristic and the components of the characteristic (i.e., value and descriptors) contain the profile data and are stored in Attributes on the server.
Service#
A service is a collection of data and associated behaviors to accomplish a particular function or feature of a device or portions of a device. A service may include other primary or secondary services and/or a set of characteristics that make up the service. GATT also specifies the format of data contained on the GATT server. Attributes, as transported by the Attribute protocol, are formatted as Services and Characteristics. Services may contain a collection of characteristics. Characteristics contain a single value and any number of descriptors describing the characteristic value.
Characteristic#
A characteristic is a value used in a service along with properties and configuration information about how the value is accessed and information about how the value is displayed or represented. A characteristic definition contains a characteristic declaration, characteristic properties, and a value. It may also contain descriptors that describe the value or permit configuration of the server with respect to the characteristic value.
Characteristic Value Types#
The above section introduces how to achieve communication among BLE devices. This section describes the differences among the types of a characteristic value. In the UG188: Blue Gecko Bluetooth Profile Toolkit Developer's Guide, three types of values can be used: hex, utf-8 and user. See chapter 2.5.3 for more information.
Available Types:
hex: The characteristic value is a hexadecimal value. In the next figure, the initialization value has the format of 0xXXXX, the characteristic value has the length of 2 bytes, and is initialized with the value 0x1122.
utf-8: The characteristic is a string and it takes a string as its initialization value. In this example, the characteristic has 13 bytes length and is initialized with the string value "Empty Example".
user: When the characteristic type is marked as type="user", the application has to initialize the characteristic value and also provide it when a read operation occurs, for example. The Bluetooth stack does not initialize the value nor automatically provide the value when it's being read. When this is set, the Bluetooth stack generates gatt_server_user_read_request or gatt_server_user_write_request, which must be handled by the application.
Default: utf-8