Bluetooth Low Energy / Mbed 2 deprecated BLE_HeartRate Featured

Heart Rate Monitor example for the BLE API using nRF51822 native mode drivers

Dependencies:   BLE_API mbed nRF51822 X_NUCLEO_IDB0XA1

BLE_HeartRate implements the Heart Rate Service which enables a collector device (such as a smart phone) to connect and interact with a Heart Rate Sensor.

For the sake of simplicity and portability, the sensor in this case has been abstracted using a counter which counts up to a threshold and then recycles. The code can be easily extended to use the real heart rate sensor.

Apps on the collector device may expect auxiliary services to supplement the HRService. We've therefore also included the Device Information Service and the Battery Service.

BLE_API offers the building blocks to compose the needed GATT services out of Characteristics and Attributes, but that can be cumbersome. As a shortcut, it is possible to simply instantiate reference services offered by BLE_API, and we'll be taking that easier route. The user is encouraged to peek under the hood of these 'services' and be aware of the underlying mechanics. It is not necessary to use these ready-made services.

Like most non-trivial services, the heart-rate service is connection oriented. In the default state, the application configures the Bluetooth stack to advertise its presence and indicate connectability. A Central/Master device is expected to scan for advertisements from peripherals in the vicinity and then initiate a connection. Once connected, the peripheral stops advertising, and communicates periodically as a server using the Attribute Protocol.

Walkthrough of the code

Let's see how this magic is achieved. We'll be pulling out excerpts from main.cpp where most of the code resides.

You'll find that the entire system is event driven, with a single main thread idling most of its time in a while loop and being interrupted by events. An important startup activity for the application is to setup the event callback handlers appropriately.

The first thing to notice is the BLEDevice class, which encapsulates the Bluetooth low energy protocol stack.

BLEDevice

#include "BLEDevice.h"

BLEDevice  ble;

void disconnectionCallback(Gap::Handle_t handle, Gap::DisconnectionReason_t reason)
{
    ble.startAdvertising(); // restart advertising
}

int main(void)
{
    ble.init();
    ble.onDisconnection(disconnectionCallback);
 ...
    ble.startAdvertising();

    while (true) {
...
            ble.waitForEvent();
...
    }
}

There is an init() method that must be called before using the BLEDevice object. The startAdvertising() method is called to advertise the device's presence allowing other devices to connect to it.

onDisconnect() is a typical example of setting up of an event handler. With onDisconnect(), a callback function is setup to restart advertising when the connection is terminated.

The waitForEvent() method should be called whenever the main thread is 'done' doing any work; it hands the control over to the protocol and lets you save power. So when will waitForEvent() return? Basically whenever you have an application interrupt, and most typically that results in some event callback being invoked. In this example there is a Ticker object that is setup to call a function every second. Whenever the ticker 'ticks' the periodicCallback() is invoked, and then waitForEvent() returns, resuming the execution in main.

Interrupt to trigger periodic actions

void periodicCallback(void)
{
    led1 = !led1; /* Do blinky on LED1 while we're waiting for BLE events */

    /* Note that the periodicCallback() executes in interrupt context, so it is safer to do
     * heavy-weight sensor polling from the main thread. */
    triggerSensorPolling = true;
}

int main(void)
{
    led1 = 1;
    Ticker ticker;
    ticker.attach(periodicCallback, 1);
...

It is worth emphasizing that the periodicCallback() (or any other event handler) is called in interrupt context; and should not engage in any heavy-weight tasks to avoid the system from becoming unresponsive. A typical workaround is to mark some activity as pending to be handled in the main thread; as done through 'triggerSensorPolling'.

BLEDevice offers APIs to setup GAP (for connectability) and GATT (for services). As has been mentioned already, GATT services may be composed by defining Characteristics and Attributes separately (which is cumbersome), or in some cases by simply instantiating reference services offered by BLE_API. The following illustrates how straightforward this can be. You are encouraged to peek under the hood of these implementations and study the mechanics.

Service setup

    /* Setup primary service. */
    uint8_t hrmCounter = 100;
    HeartRateService hrService(ble, hrmCounter, HeartRateService::LOCATION_FINGER);

    /* Setup auxiliary services. */
    BatteryService           battery(ble);
    DeviceInformationService deviceInfo(ble, "ARM", "Model1", "SN1", "hw-rev1", "fw-rev1", "soft-rev1");

Setting up GAP mostly has to do with configuring connectability and the payload contained in the advertisement packets.

Advertiser setup

    ble.accumulateAdvertisingPayload(GapAdvertisingData::BREDR_NOT_SUPPORTED | GapAdvertisingData::LE_GENERAL_DISCOVERABLE);
    ble.accumulateAdvertisingPayload(GapAdvertisingData::COMPLETE_LIST_16BIT_SERVICE_IDS, (uint8_t *)uuid16_list, sizeof(uuid16_list));
    ble.accumulateAdvertisingPayload(GapAdvertisingData::GENERIC_HEART_RATE_SENSOR);
    ble.accumulateAdvertisingPayload(GapAdvertisingData::COMPLETE_LOCAL_NAME, (uint8_t *)DEVICE_NAME, sizeof(DEVICE_NAME));
    ble.setAdvertisingType(GapAdvertisingParams::ADV_CONNECTABLE_UNDIRECTED);
    ble.setAdvertisingInterval(1600); /* 1000ms; in multiples of 0.625ms. */

The first line (above) is mandatory for Bluetooth Smart, and says that this device only supports Bluetooth low energy. The 'general discoverable' is the typical value to set when you want your device to be seen by other devices on order to connect. Next comes the ID for the heart rate sensor service and the name of the device.

After the payload is set the code sets the advertising type and the advertising interval. In Bluetooth Smart timing values are typically multiples of 625 us.

If you are new to Bluetooth Smart there are probably a lot of terms that are new to you. There is a lot of information about this on the Internet.

main.cpp

Committer:
Rohit Grover
Date:
2014-06-10
Revision:
11:1d9aafee4984
Parent:
10:2436164b692e
Child:
13:3ca2045597e7

File content as of revision 11:1d9aafee4984:

/* mbed Microcontroller Library
 * Copyright (c) 2006-2013 ARM Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "mbed.h"
#include "BLEDevice.h"

BLEDevice  ble;

DigitalOut led1(LED1);

#define NEED_CONSOLE_OUTPUT 0 /* Set this if you need debug messages on the console;
                               * it will have an impact on code-size and power
                               * consumption. */

#if NEED_CONSOLE_OUTPUT
Serial  pc(USBTX, USBRX);
#define DEBUG(...) { pc.printf(__VA_ARGS__); }
#else
#define DEBUG(...) /* nothing */
#endif /* #if NEED_CONSOLE_OUTPUT */

/* Battery Level Service */
uint8_t            batt      = 72; /* Battery level */
uint8_t            read_batt = 0;  /* Variable to hold battery level reads */
GattService        battService (GattService::UUID_BATTERY_SERVICE);
GattCharacteristic battLevel   (GattCharacteristic::UUID_BATTERY_LEVEL_CHAR,
                                1, /* initialLen */
                                1, /* maxLen */
                                GattCharacteristic::BLE_GATT_CHAR_PROPERTIES_NOTIFY | GattCharacteristic::BLE_GATT_CHAR_PROPERTIES_READ);

/* Heart Rate Service */
/* Service:  https://developer.bluetooth.org/gatt/services/Pages/ServiceViewer.aspx?u=org.bluetooth.service.heart_rate.xml */
/* HRM Char: https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml */
/* Location: https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.body_sensor_location.xml */
GattService        hrmService    (GattService::UUID_HEART_RATE_SERVICE);
GattCharacteristic hrmRate       (GattCharacteristic::UUID_HEART_RATE_MEASUREMENT_CHAR,
                                  2, /* initialLen */
                                  3, /* maxLen */
                                  GattCharacteristic::BLE_GATT_CHAR_PROPERTIES_NOTIFY);
GattCharacteristic hrmLocation   (GattCharacteristic::UUID_BODY_SENSOR_LOCATION_CHAR,
                                  1, /* initialLen */
                                  1, /* maxLen */
                                  GattCharacteristic::BLE_GATT_CHAR_PROPERTIES_READ);

/* Device Information service */
static const uint8_t deviceName[] = {'m', 'b', 'e', 'd'};
GattService        deviceInformationService (GattService::UUID_DEVICE_INFORMATION_SERVICE);
GattCharacteristic deviceManufacturer (
    GattCharacteristic::UUID_MANUFACTURER_NAME_STRING_CHAR,
    sizeof(deviceName), /* initialLen */
    sizeof(deviceName), /* maxLen */
    GattCharacteristic::BLE_GATT_CHAR_PROPERTIES_READ);

static const uint16_t uuid16_list[] = {
    GattService::UUID_BATTERY_SERVICE,
    GattService::UUID_DEVICE_INFORMATION_SERVICE,
    GattService::UUID_HEART_RATE_SERVICE
};

void timeoutCallback(void)
{
    DEBUG("Advertising Timeout!\n\r");
    // Restart the advertising process with a much slower interval,
    // only start advertising again after a button press, etc.
}

void connectionCallback(void)
{
    DEBUG("Connected!\n\r");
}

void disconnectionCallback(void)
{
    DEBUG("Disconnected!\n\r");
    DEBUG("Restarting the advertising process\n\r");
    ble.startAdvertising();
}

void updatesEnabledCallback(uint16_t charHandle)
{
    if (charHandle == hrmRate.getHandle()) {
        DEBUG("Heart rate notify enabled\n\r");
    }
}

void updatesDisabledCallback(uint16_t charHandle)
{
    if (charHandle == hrmRate.getHandle()) {
        DEBUG("Heart rate notify disabled\n\r");
    }
}

void periodicCallback(void)
{
    led1 = !led1; /* Do blinky on LED1 while we're waiting for BLE events */

    if (ble.getGapState().connected) {
        /* Update battery level */
        batt++;
        if (batt > 100) {
            batt = 72;
        }
        ble.updateCharacteristicValue(battLevel.getHandle(), (uint8_t *)&batt, sizeof(batt));

        /* Update the HRM measurement */
        /* First byte = 8-bit values, no extra info, Second byte = uint8_t HRM value */
        /* See --> https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml */
        static uint8_t hrmCounter = 100;
        hrmCounter++;
        if (hrmCounter == 175) {
            hrmCounter = 100;
        }
        uint8_t bpm[2] = {0x00, hrmCounter};
        ble.updateCharacteristicValue(hrmRate.getHandle(), bpm, sizeof(bpm));
    }
}

/**************************************************************************/
/*!
    @brief  Program entry point
*/
/**************************************************************************/
int main(void)
{
    led1 = 1;
    Ticker ticker;
    ticker.attach(periodicCallback, 1);

    /* Setup the local GAP/GATT event handlers */
    ble.onTimeout(timeoutCallback);
    ble.onConnection(connectionCallback);
    ble.onDisconnection(disconnectionCallback);
    ble.onUpdatesEnabled(updatesEnabledCallback);
    ble.onUpdatesDisabled(updatesDisabledCallback);

    /* Initialise the nRF51822 */
    DEBUG("Initialising the nRF51822\n\r");
    ble.init();

    ble.accumulateAdvertisingPayload(GapAdvertisingData::BREDR_NOT_SUPPORTED);
    ble.accumulateAdvertisingPayload(GapAdvertisingData::COMPLETE_LIST_16BIT_SERVICE_IDS, (uint8_t *)uuid16_list, sizeof(uuid16_list));
    ble.accumulateAdvertisingPayload(GapAdvertisingData::HEART_RATE_SENSOR_HEART_RATE_BELT);

    ble.setAdvertisingType(GapAdvertisingParams::ADV_CONNECTABLE_UNDIRECTED);
    ble.setAdvertisingInterval(160); /* 100ms; in multiples of 0.625ms. */
    ble.startAdvertising();

    /* Add the Device Information service */
    deviceInformationService.addCharacteristic(deviceManufacturer);
    ble.addService(deviceInformationService);
    ble.updateCharacteristicValue(deviceManufacturer.getHandle(), deviceName, sizeof(deviceName));

    /* Add the Battery Level service */
    battService.addCharacteristic(battLevel);
    ble.addService(battService);
    ble.updateCharacteristicValue(battLevel.getHandle(), (uint8_t *)&batt, sizeof(batt));

    /* Add the Heart Rate service */
    hrmService.addCharacteristic(hrmRate);
    hrmService.addCharacteristic(hrmLocation);
    ble.addService(hrmService);
    /* Set the heart rate monitor location (one time only) */
    /* See --> https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.body_sensor_location.xml */
    uint8_t location = 0x03; /* Finger */
    ble.updateCharacteristicValue(hrmLocation.getHandle(), (uint8_t *)&location, sizeof(location));

    while (true) {
        ble.waitForEvent();
    }
}