Quickly Add an Accelerometer with SPI


IOT (Internet of Things) solutions often use accelerometer sensors, such as an IMU (Inertial Measurement Unit). The EFR32BG22 Thunderboard (kit SLTB010A) has an on board 6-axis ICM-20648 sensor from vendor TDK InvenSense.

This lab shows how to quickly and efficiently add the accelerometer sensor.

Topic Covered

Getting Started

Ensure that you have the correct hardware and software prepared to successfully complete the lab.

Hardware Requirements

Software Requirements

Install Tools

Download and install Simplicity Studio v5 if not already installed. Ensure that you have GSDK 3.1.x and Bluetooth Stack installed.

Connect your Hardware

Attach the EFR32BG22 Thunderboard kit to the PC with Simplicity Studio installed by using a USB cable (not a charging cable). Connecting between the PC host USB port and the J-Link USB port (USB micro) on the kit.

USB J-Link

Hardware and Software Introduction

Connection between IMU Sensor and EFR32BG22

See the EFR32BG22 Thunderboard schematic for detailed hardware information (schematic for EFR32BG22 Thunderboard in PDF format).

Hardware Connection

The following are the EFR32BG22 pins used to connect ICM-20648:

EFR32BG22 USART(SPI) Peripheral

EFR32BG22 has 2 USART peripheral instances. The USART peripheral is a flexible I/O module, which supports the following modes:

See the EFR32xG22 Wireless Gecko Reference Manual Section 21 on how to use this peripheral. The ICM-20648 sensor on EFR32BG22 Thunderboard uses the SPI interface. The sensor also supports I2C Fast Mode.

Accelerometer Sensor ICM-20648

This sensor provides the following:

Note: The sensor supports features such as calibration, auxiliary I2C interface, and others. In this lab, features like calibration were not included.

Software Architecture

Below is an overview of the software architecture:

Software Architecture

The figure shows the following:


Creating the Project

  1. If the EFR32BG22 Thunderboard isn't plugged into the PC using the USB cable, do so now.
  2. In the Launcher->Debug Adapters window, click on the Thunderboard EFR32BG22 (ID:xxxxxxxxx). The kit (end with SLTB010A), board (end with BRDxxxxx Rev Axx) and device (EFR32BG22CxxxFxxxxxxx) debug information should be displayed in the Launcher->Debug Adapters window.

Debug Adapters Window

  1. Information about the target hardware and software will appear in Launcher->Overview tab (together with the Adapter FW and Secure FW version). If this does not appear, click on the Launcher button in the top right corner.

Note: If the Secure FW shows as Unknown, click on Read FW Version on the right side of it to get the version. You may also upgrade the Adapter FW to the latest version.

  1. Select the Preferred SDK to the latest version. For this lab, the latest version of Gecko SDK Suite v3.1.1 is used.
  2. Click on Create New Project in the upper right-hand corner. A New Project Wizard window should appear.

Note: If you already have projects in the workspace, you may not see the Create New Project button. You can try other options to create a new project, e.g., File->New->Silicon Labs New Project Wizard... or 'Examples Projects & Demos' tab.

Launcher Overview

  1. For this lab, the Bluetooth - SoC Empty project is used as the starter project. Scroll and select Bluetooth - SoC Empty.

Note: To filter the projects, Select/Checked the Bluetooth for the Technology Type and input empty for Filter on keywords.

Project Filter

  1. Click Next to move on.
  2. Rename the project under Project name. For this lab, name the project soc_spi_acc.
  3. Select (check) Copy contents under With project files to copy the project files into your project. This makes version control easier to manage and future updates to the Simplicity Studio libraries will not impact the copied files in this project.
  4. Check Use default location (workspace).
  5. Click Finish to generate the project.

Project Rename

  1. The IDE perspective launched automatically.
  2. You can now see gatt_configuration.btconf, soc_spi_acc.slcp and readme.

IDE Window

Summary of Previous Steps

Congratulations! The SoC Empty is successfully created. Additionally, the Bluetooth - SoC Empty project will pre-install some software components. To see what was installed, check the Installed Components under Software Components.

Pre-Installed Components

You will see that the following components were installed.

If you don't see these components, make sure that you followed the procedure described above.

Installing the IMU Sensor Software Components

  1. Select the Software Components tab on the top.
  2. Scroll down to the different sections (such as Platform). Note all of the components that you can easily install for your application.
  3. Install the following components using the Install button, as shown in the image. The process is repeated for all components that need to be added.

For example, take a look at the information shown on the right side of the view for the IMU - Inertial Measurement Unit component. Usually information such as Description, Quality level, and Dependencies of the component is provided. The view also provides a good overview of all functions that are defined/added for this component.

If you click the View Dependencies button, you can see that one of dependency components is Platform->Board Drivers->ICM20648 - Motion Sensor. Installing the component IMU - Inertial Measurement Unit will also automatically install Platform->Board Drivers->ICM20648 - Motion Sensor.

Dependency Component

Note: You can input a keyword on the upper right side of Search keywords, component's name box to filter the component. For example, the keyword inertial will cause only the last three components above to be visible.

I/O Stream

IMU - Inertial Measurement Unit

GATT Service

Note: Although IMU - Inertial Measurement Unit is classified as a Board Driver, you can still install it even when using a custom board. However, you must configure the dependency component Platform->Board Drivers->ICM20648 - Motion Sensor to match pinouts of your custom board. Otherwise, the build will report errors, as will be shown in the code explanation section.

Note: Installing the Inertial Measurement Unit sensor adds sl_sensor_imu.c/h files. These files are dependent on the Board Control. In the future, you will be able to add support for a custom board by adding a Board Control file to the same location where you located the option for the Thunderboard BG22.

Board Control

Note: Inertial Measurement Unit GATT Service configuration button should link to the btconf->gatt_service_imu.xml. In the future, you will be able to add support for a custom board.

IMU GATT Service Configuration

Summary of Previous Steps

IMU - Inertial Measurement Unit Component

After you add/install the IMU - Inertial Measurement Unit, you will see that files, such as sl_icm20648.c and sl_icm20648.h were added.


This component also adds the driver files sl_imu_dcm/sl_imu_math.c/sl_imu_fuse.c/sl_imu.c, which are also prepared by Silicon Labs for ICM-20648.

IMU Driver 1 IMU Driver 2

Inertial Measurement Unit sensor Component

After you add/install the Inertial Measurement Unit sensor, you will see that additional files were added.

IMU Driver 3

Inertial Measurement Unit GATT Service Component

After you add/install the Inertial Measurement Unit GATT Service, you will see that additional files were added.

GATT Service

Note: This will add Acceleration and Orientation service.

In the file sl_event_handler.c, you can see the API sl_gatt_service_imu_step was added into the routine sl_internal_app_process_action. This will transmit Acceleration and Orientation data to a client when the client subscribe the service.

    void sl_internal_app_process_action(void)

GATT Configuration

  1. Go back to the gatt_configuration.btconf file (tab). If you have closed this tab, you can re-open it either via Project Explorer or Software Components->Advanced Configurators->Bluetooth GATT Configurator.

GATT Configuration 1 GATT Configuration 2

  1. You can rename the device name via Custom BLE GATT->Generic Access->Device Name->Value settings->Initial value. Here, it is renamed to spi_acc.

Device Name

  1. Later in the lab, EFR Connect mobile app will display the Acceleration and Orientation values under their UUIDs. You can write them down to verify which one you are viewing.

Note: You can simplify by recording the first 5 unique characters of the UUID. Accel = c4c1f6; Orient = b7c4b6

GATT Acceleration

Adding the Project Source Files

  1. Copy the provided app.c file to the top level of the project. The source files and code details are found at the Code Explanation section. app.c will overwrite the existing file to add the new application. The source files can be dragged and dropped into Simplicity Studio or placed in this file path: C:\Users\user_account\SimplicityStudio\v5_workshop\soc_spi_acc

Note: Where user_account is the default workspace and Simplicity Studio installation path. You can also edit the app.c file manually.

Build and Flash the Project

  1. Build the project by clicking on the hammer icon in the top left corner of the Simplicity Studio IDE perspective.

Build with the Hammer Icon

  1. Right-click on the hex file (under GNU ARM xxx - Debug or Binaries) and select Flash to Device... to make the Flash Programmer window appear.

Flash to Device

Note: hex image is recommended here rather than bin image. If you choose 'bin', you need explicitly give the base address in the Flash Programmer. Flash Programmer Note: If a Device Selection window appears, select the correct device.

  1. Click Program to flash the device.

Note: The EFR32BG22 has additional security features and in some cases (i.e., when the board is first plugged in), the tools will prompt to query the Debug Challenge Interface (DCI). Select the connected device and then the link for Click to Query Lock Status. The device target to program text will no longer be grayed out. Then, select OK.

Flash Programmer


Connecting with EFR Connect App

  1. Open the mobile app EFR Connect and select the Develop->Browser.

EFR Connect Develop

  1. With the EFR Connect App, Connect to the device.

EFR Connect Browser

Note: If multiple Bluetooth devices are visible:

Simplicity Commander

RSSI Strength

Note: If the board is not found, press the reset button on the kit or click Stop Scanning then Start Scanning in the app.

  1. Click the Service->Characteristic->Notify button. Here, the Service and Characteristic are shown as Unknown because they are not standard. Click the 'Notify' bell at the bottom of the Characteristic to let the app (as client) subscribe the service.

Click Notify

Note: You should already have the UUID in step 19.

  1. You should see the sensor data get updated regularly. You can change the orientation of the Thunderboard or shake it (change acceleration) to see this change.

IMU Data Update

Note: observe “Average” of value in one position. Then, change orientation and you should notice a change in the average value.

Code Explanation

The following sections explain critical lines of code pertinent to this lab. The code can be found in different files (driver).

Accelerometer (ICM-20648) Driver


This is a header file generated automatically by the Simplicity Studio Pintool/Software Component. You may need to change the pin map based on your hardware. Use the Software Components->Platform->Board drivers->ICM20648 - Motion Sensor->Configure to change it.

Driver Pintool

The pin map for ICM-20648 is here: Driver Pinmap Note: It matches the schematic mentioned earlier in the document, section Connection between IMU sensor and EFR32BG22.


This file is located in folder, as follows:


This is the driver file for ICM-20648 sensor prepared by Silicon Labs.

Driver File Path 1

If you use a sensor from another vendor, you may need to consider implementing the similar driver for it. Consider contacting the vendor for help to implement the driver.


These files are re-packed of the API provided in the driver sl_icm20648.c. The high-level code (app.c and other) calls APIs, such as sl_sensor_imu_init, sl_sensor_imu_get, and others provided in the file sl_sensor_imu.c/h to initialize, enable/disable the IMU sensor, and read sensor data. These files are in the following folders:

C:\SiliconLabs\SimplicityStudio\v5\developer\sdks\gecko_sdk_suite\v3.1\hardware\driver\imu\src C:\SiliconLabs\SimplicityStudio\v5\developer\sdks\gecko_sdk_suite\v3.1\app\bluetooth\common\sensor_imu

Driver File Path 2 Driver File Path 3


Application (app.c)

The SoC Empty project generates a default app.c source file with a skeleton Bluetooth event handler. The app.c file provided for this lab adds code to handle the BLE connection and notifications.

Connection Opened

The IMU sensor is initialized and enabled when the event sl_bt_evt_connection_opened_id is received. The IMU sampling does not start until a connection has been made and the user has enabled GATT Acceleration Notification (or Orientation Notification) characteristics.

    /* place 1, code added for accelerometer workshop */
    static void sensor_init(void)

    // -------------------------------
    // This event indicates that a new connection was opened.
    case sl_bt_evt_connection_opened_id:
      /* place 4, code added for accelerometer workshop */

Connection Closed

When the connection is closed, the sl_bt_evt_connection_closed_id event is triggered. To save power when no devices are connected, the sensor was disabled via sensor_deinit function.

    /* place 2, code added for accelerometer workshop */
    static void sensor_deinit(void)

    // -------------------------------
    // This event indicates that a connection was closed.
    case sl_bt_evt_connection_closed_id:
      // Restart advertising after client has disconnected.
      sc = sl_bt_advertiser_start(
      sl_app_assert(sc == SL_STATUS_OK,
        "[E: 0x%04x] Failed to start advertising\n",
      /* place 5, code added for accelerometer workshop */

Sensor Data Read

The code below placed in app.c will periodically read sensor data.

    /* place 3, code added for accelerometer workshop */
    sl_status_t sl_gatt_service_imu_get(int16_t ovec[3], int16_t avec[3])
      return sl_sensor_imu_get(ovec, avec);

This was called by sl_internal_app_process_action->sl_gatt_service_imu_step. It overrides the weak routine sl_gatt_service_imu_get in the sl_gatt_service_imu.c.

SL_WEAK sl_status_t sl_gatt_service_imu_get(int16_t ovec[3], int16_t avec[3])
  return SL_STATUS_FAIL;

Pay special attention to the variable imu_state in the file sl_gatt_service_imu.c. event sl_bt_evt_connection_closed_id and sl_bt_evt_gatt_server_characteristic_status_id will change its value.

App Code Call Hierarchy

App Code Call Hierarchy

BLE Notification

After the user has enabled GATT notifications to the characteristic, the sl_bt_evt_gatt_server_characteristic_status_id event is triggered. In this event, the device will periodically update the characteristic value until the device disconnects.


This file was automatically generated and is in the following folder (workspace):


    void sl_internal_app_process_action(void)


This file is in the following folder:


    void sl_gatt_service_imu_step(void)
      if (imu_state) {
        if (SL_STATUS_OK == sl_gatt_service_imu_get(imu_ovec, imu_avec)) {
          if (imu_acceleration_notification) {
          if (imu_orientation_notification) {

This routine calls imu_orientation_notify and imu_acceleration_notify to send the sensor data to the client.

BLE Notification Call Hierarchy

BLE Notification Call Hierarchy



Porting Considerations

Other Sensors

EFR32BG22 Thunderboard also integrates other sensors:

If your solution needs these sensors, you may use a similar procedure to add them.


The application generated via Bluetooth SoC Empty project doesn't include the bootloader. Program the bootloader to the device first. In some cases, the bootloader may be missing from the device if it has been completely erased. If that happens, do the followingS:

IMU Sensor Data Interpretation

The EFR Connect shows the sensor raw data. To interpret data, see the sensor and driver data sheets. To show a meaningful sensor data, look at the source code of the Thunderboard app.


Peripheral Examples Gecko Platform Documentation Simplicity Studio v5 User's Guide