Implementing Multiple GAP Roles on the Kinetis KW40Z Wireless Microcontroller

[#1]

Overview

The Bluetooth specification defines 4 Generic Access Profile (GAP) roles for devices operating over a Low Energy physical transport [1]:

• Peripheral
• Central
• Broadcaster
• Observer

The Bluetooth Low Energy Host Stack implementation on the Kinetis KW40Z offers devices the possibility to change between any of the 4 roles at run time. This article will present the interaction with the Bluetooth Low Energy Host API needed to implement a GAP multiple role device.

General Procedure instructions

Running the GAP roles requires the application to go through the following 3 steps:

• Configuration - Stack configuration for the desired GAP role

The application needs to configure the stack parameters, e.g. advertising parameters, advertising data, scan parameters, callbacks. Note that configuration of the advertising parameters or scanning response and advertising data can be done only once if the values don’t change at runtime. The configuration is always made in the Link Layer Standby state.

• Start - Running the role

The application needs to start advertising, scanning or initiate connection.

• Stop - Return to Standby state

When changing between roles, the Link layer must always go through the Link Layer Standby state.

Running as a GAP Broadcaster or GAP Peripheral

The GAP Broadcaster or Peripheral sends advertising events. Additionally, the GAP Peripheral will accept the establishment of a LE link. This is why the GAP Observer will only support the Non Connectable Advertising mode (gAdvNonConnectable_c).

Both roles requires configuration of advertising data, advertising parameters. The configuration (gAppAdvertisingData, gAppScanRspData and gAdvParams) usually resides in app_config.c. The confirmation events for setting these parameters is received in BleApp_GenericCallback. The confirmation event for the changing state of advertising is received in BleApp_AdvertisingCallback.

Configuration

/* Setup Advertising and scanning data */
Gap_SetAdvertisingData(&gAppAdvertisingData, &gAppScanRspData);
/* Setting only for GAP Broadcaster role */
gAdvParams. advertisingType = gAdvNonConnectable_c;
/* Set advertising parameters*/
Gap_SetAdvertisingParameters(&gAdvParams);

Start

App_StartAdvertising(BleApp_AdvertisingCallback, BleApp_ConnectionCallback);

Stop

Gap_StopAdvertising();

Running as a GAP Observer

The GAP Observer receives advertising events. Unlike the GAP Peripheral or Broadcaster, it does not need to set scanning parameters separately. It passes the configuration with the start procedure. The configuration (gAppScanParams) usually resides in app_config.c. The confirmation event for the changing state of scanning is received in BleApp_ScanningCallback.

Configuration and Start

App_StartScanning(&gAppScanParams, BleApp_ScanningCallback);

Stop

Gap_StopScanning ();

Running as a GAP Central

The GAP Central initiates the establishment of the LE link. Like the GAP Observer, it passes the configuration with the start procedure. The configuration (gConnReqParams) usually resides in app_config.c. The confirmation event for the changing state of link is received in BleApp_ConnectionCallback.

Configuration and Start

Gap_Connect(&gConnReqParams, BleApp_ConnectionCallback);

Stop

Gap_Disconnect(deviceId);

Example

An out-of-the box example for multiple role is attached. The application named blood_pressure_multi_role implements a Blood Pressure GATT client and server and can switch between the following GAP roles: Peripheral, Observer and Central. The contents of the archive needs to be copied to the following location:

<Installer Path>\KW40Z_Connectivity_Software_1.0.1\ConnSw\examples\bluetooth\

The application can be found at:

<Install Path specified>\KW40Z_Connectivity_Software_1.0.1\ConnSw\examples\bluetooth\blood_pressure_multi_role\frdmkw40z\bare_metal\build\iar\blood_pressure_multi_role.eww

Running as GAP Peripheral

Press SW4. LED1 will start flashing and the console will show that the Link Layer enters Advertising. If the Link Layer was in a previous state, it will go through Standby.

static void BleApp_Advertise(void)
{
    /* Ensure Link Layer is in Standby */
    BleApp_GoToStandby();
    shell_write(" GAP Role: Peripheral\n\r");
    mGapRole = gGapPeripheral_c;
    /* Start GAP Peripheral */
    App_StartAdvertising(BleApp_AdvertisingCallback, BleApp_ConnectionCallback);
}

Running as GAP Observer

Press SW3. A chasing LED pattern will start and the console will show that the Link Layer enters Scanning. If the Link Layer was in a previous state, it will go through Standby.

static void BleApp_Scan(void)
{
    /* Ensure Link Layer is in Standby */
    BleApp_GoToStandby();
    shell_write(" GAP Role: Observer\n\r");
    mGapRole = gGapObserver_c;
    /* Start GAP Observer */
    App_StartScanning(&gAppScanParams, BleApp_ScanningCallback);
}

Running as GAP Central

If the Link Layer is in scanning and finds a Blood Pressure Sensor, it will go through Standby and initiate connection.

static void BleApp_Connect(void)
{
    /* Ensure Link Layer is in Standby */
    BleApp_GoToStandby();
    shell_write(" GAP Role: Central\n\r");
    mGapRole = gGapCentral_c;
    /* Start GAP Central */
    Gap_Connect(&gConnReqParams, BleApp_ConnectionCallback);
}

Returning to Standby

Pressing SW3 for more than 2 seconds, brings the Link Layer back in Standby.

static void BleApp_GoToStandby(void)
{
    /* Check if connection is on */
    if (mPeerInformation.deviceId != gInvalidDeviceId_c)
    {
        /* Stop GAP Central or Peripheral */
        Gap_Disconnect(mPeerInformation.deviceId);
    }
    if (mAdvOn)
    {
        /* Stop GAP Peripheral or Bradcaster */
        Gap_StopAdvertising();
    }
    if (mScanningOn)
    {
        /* Stop GAP Observer */
        Gap_StopScanning();
    }
}

References

[1] BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part C], 2.2 PROFILE ROLES