Introduction

Key points of the article:

• A working example implementing the BLE central role and GATT client.
• A reference code of Android device as peripheral role and GATT server.

This article is for people who only have one Wireless Starter Kit (WSTK) and want to implement central role on it. If you have more than one WSTK and want to connect them, you could probably go through the SPP-over-BLE example which implements both the central and peripheral roles.

In case you only have one WSTK, we could use the smartphone as the peripheral and GATT server, then implement the central role and GATT client on the WSTK.

The typical work flow of the central device is shown as below:

1. Scan for nearby advertisements, you could add your own filter on the advertisements or scan responses to make decision to which device the central device should connect.
2. After filtered device is found, establish connection.
3. After connection established, discover services and characteristics.
4. Configure Client Characteristic Configuration(CCC) for characteristics if needed.

This is the typical behaviors central device does while connecting to a peripheral, but NOT all the processes are mandatory.

Features

To run this example, you need to have below devices.

• An EFR32BG or EFR32MG based board – this example is developed for the EFR32BG or EFR32MG based board and modules. It works well on the WSTK and take the assumption that the 2 buttons on the WSTK are connected to GPIO PF6 and PF7, and modifications (Probably on buttons mapping) may be needed to run on any customized boards.
• An Android smartphone – Make sure that this device supports BLE both on the hardware and software side. For hardware side, please check the device manual, and for the software side, Android has BLE software support from Android version 4.3.

GATT Server

A so-called “Demo service” is added to the server, and this service contains 2 characteristics “NOTIFY_CHAR” and “RW_CAHR”, this is done on the smartphone side.

Communication

This example demonstrates to use an EFR32 based board as a client and a smartphone as a server and send data back and forth between them. Below is a brief lists of features:

• Smartphone to send advertisements and EFR32 baord scan for it and connects to the desired one.
• EFR32 discover the gatt database on the smartphone.
• EFR32 writes/reads data to/from smartphone to demonstrate sending data from EFR32 to smartphone.
• Smartphone sends notification periodically to EFR32 to demonstrate sending data from smartphone to EFR32.

State Machine

There are 2 versions of the example code provided in the attachment, one is with a finite state machine implementation which gives more flexibility and expansibility, the other one is not, which is the lightest weight of the central role and GATT client implementation.

Import the example

To run this example, you will need to download Simplicity Studio and the attachment which includes 6 files, app.c, app.h, app_nofsm.c, fsm.c, fsm.h and the server.apk. Below is a brief guide on how to import them to your project.

1. Download the latest Bluetooth SDK via Simplicity Studio if you haven't done it.
2. Create a "soc-empty" project base on the board you are using as the starting point.
3. Drag the app.c(app_nofsm.c for no state machine version), app.h, fsm.c and fsm.h files to the project created just now, then compile and program to your board.
4. This is an optional step, include the Logging System to the project to get powerful logging like figure 1. By default, the code uses the simplest way to print data via VCOM.
5. Install the server.apk to your Android smartphone.

Run the example

After you have successfully import the example and done all the above steps, you can try below steps to verify the functionalities.

1. Open a serial terminal to connect to EFR32, which uses the USB-UART as the default interface.
2. Open the smartphone app, it will start advertising immediately. And EFR32 will also scan for it after it power up.
3. EFR32 device will autonomously connect to the smartphone if the advertisement payload matches.
4. Pressing PB0 on the WSTK will write 1 and 5 circularly to the RW_CHAR characteristic in the smartphone app gatt server database, pressing PB1 to read the current data back.
5. Smartphone will send notification with fixed payload "0x11 0x22" to the EFR32 device autonomously and periodically, the interval is determined by the value written in the RW_CHAR characteristic in last step in seconds.

Below are screenshots of the serial terminal and smartphone.

Figure 1. Terminal Output

Below is a brief description of the time point and the behaviors.

• At 1, system boot.
• From 1 to 2, EFR32 device is looking for the device which fit its filter and connects to it, then find services and characteristics, enable notification of the notify characteristic at last.
• At 3, smartphone app starts sending notification periodically.
• At 4, PB1 pressed to know what interval the smartphone is using to send notifications.
• At 5. PB0 pressed to change the interval from 5 seconds to 1 second.
• At 6, PB1 pressed again as point 4.
• At 7, Smartphone app was terminated intendedly, EFR32 goes back to disconnected state and tries to find devices again.

Figure 2. Smartphone App View