Developing a GATT server

When we want to share sensor data with client applications, we need more than a simple beacon. In this example, we will develop a Bluetooth temperature sensor that we can connect to and receive temperature readings by using a mobile application, such as nRF Connect. One interesting feature is that the sensor will be able to push data to the client application when a new reading is available.

The hardware preparation is easy. We will only use a DHT11 connected to GPIO17 of ESP32. After the hardware setup, we configure a new project:

1. Let's start with a new PlatformIO project with the following platformio.ini:

   [env:az-delivery-devkit-v4]
   platform = espressif32
   board = az-delivery-devkit-v4
   framework = espidf
   monitor_speed = 115200
   lib_extra_dirs =
       ../../common/esp-idf-lib/components

   esp-idf-lib contains the driver for DHT11. The source code is located here:

2. Then we need to add two files, app.h and app.c, into the project...