====== nRF52840 Zephyr BLE client ======
Usually, the BLE client role plays some user device eg. mobile phone, tablet or computer. In IoT systems sometimes BLE is used for M2M (machine-to-machine) data transmission. In such a situation an automatic BLE client must be implemented to connect to the peripherals and read the data eg. coming from BLE-capable sensors.
BLE client must be configured to connect to the peripheral which exposes some specific service and characteristic. To do the lab exercise there is a need to create the peripheral device first.
===== BLE Health Temperature peripheral =====
The default Health Temperature example is sufficient to make the laboratory. How to import the project, flash, and execute is shown in {{https://dokuwiki.zmitac.aei.polsl.pl/en:edu:zmitac:s2:aih:lab:nrf52zephyrbleperipheral|peripheral example}}.
===== BLE central ====
Among the examples provided with Zephyr OS, there is also one complementary to the Health Temperature peripheral. It is named central_ht ((..\.platformio\packages\framework-zephyr\samples\bluetooth\central_ht)). The program is quite complex, here we will explain the most important elements and the process of establishing the connection.\\
The process of establishing and configuring a connection has a few steps:
  - scanning the neighbourhood to find connectable devices
  - comparing the advertising data to look for information on the required service
  - establishing the connection to peripheral
  - retrieving information on services and characteristics
  - subscribing to the interesting data with a notification or indication mechanism (optional)
  - reading data (by reading the characteristic value or automatically with notify/indicate)

==== Scanning the neighbourhood ====
In the main function, after the successful initialisation of the Bluetooth subsystem, scanning for nearby devices is started.
<code>
err = bt_enable(NULL);
...
err = scan_start();
...
</code>
The "scan_start()" function calls "bt_le_scan_start(&scan_param, device_found)" function. The second parameter is the callback function which processes scan results. 
==== Searching advertising data for info on service ====
The "device_found()" function parses received advertising data with the "bt_data_parse()" function to the function "eir_found()". The latter function searches advertising data for the UUID of the Health Thermometer (0x1809). 
==== Establishing the connection ====
After a successful search establishes a connection with the remote device by calling the function "bt_conn_le_create()".\\
Further processing is done by callback functions registered by the macro:
<code>
BT_CONN_CB_DEFINE(conn_callbacks) = {
	.connected = connected,
	.disconnected = disconnected,
};
</code>
Zephyr OS keeps the "bt_conn_cb" structure that allows the programmer to attach callback functions to chosen events. "BT_CONN_CB_DEFINE" macro attaches "connected" and "disconnected" functions to appropriate events.
The "connected()" function is executed after successfully establishing the connection to the peripheral device. 
==== Discovering the service ====
This function registers the callback function "discover_func()", and starts the discovery process by searching for the Health Thermometer service (UUID = 0x1809). 
==== Searching for the characteristic ====
After finding this service discovery_func() is executed which performs further searching. First, it looks for the Temperature Measurement characteristic (UUID = 0x2a1c). 
==== Subscribing to the data with indication ====
This characteristic should have the descriptor Client Characteristic Configuration (UUID = 0x1902) which makes it possible to enable notifications or indications. The indication enabling and registering the "notify_func()" as the callback for incoming indications finishes the establishing connection process.\\
==== Ready! ====
Notice that after every successful discovery the function "discovery_func()" is called as the callback.\\
While the peripheral device updates the thermometer sensor reading, our central device receives this data and calls the "notify_func()" callback which prints the data using the UART port.
===== Tasks to do during the laboratory =====
  - Create the Health Thermometer peripheral device and test if it operates properly
  - Import central device program "central_ht" (main.c and prj.conf files must be imported)
  - Compile and execute it observing its behaviour with and without peripheral running
  - Modify the peripheral code and central code to enable LED control on the central board with a button press on the peripheral board

<note>
If you find problems with undefined BT_ATT_FIRST_ATTRIBUTE_HANDLE and BT_ATT_LAST_ATTRIBUTE_HANDLE define them as in the following code.
</note>
<code>
#define BT_ATT_FIRST_ATTRIBUTE_HANDLE 0x0001
#define BT_ATT_LAST_ATTRIBUTE_HANDLE 0xFFFF
</code>
<note>
To send simple binary data you can use a binary sensor service (UUID = 0x183B) with a binary sensor response characteristic (UUID = 0x2B2C).
</note>