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.


#include "BLEDevice.h"

BLEDevice  ble;

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

int main(void)

    while (true) {

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::COMPLETE_LOCAL_NAME, (uint8_t *)DEVICE_NAME, sizeof(DEVICE_NAME));
    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.


RevisionDateWhoCommit message
79:8b7c8c240540 2016-09-20 Vincent Coubard Update libraries. default tip
78:6f16dab7a60c 2016-08-05 vcoubard Update mbed lib to the latest revision.; This fix the build of this example which was somehow broken even if the example was not touched.
77:a2ab41a7f067 2016-04-11 vcoubard Update libraries to the latest revision.
76:8b36a7c45cd2 2015-11-26 rgrover1 updated underlying libraries.
75:8128a06c0a21 2015-11-03 vcoubard fix waiting condition for ble initialization process
74:c9d58e7847c4 2015-11-03 rgrover1 update to the latest of underlying ilbraries.
73:49b6090478e2 2015-11-02 rgrover1 make use of BLE::hasInitialized() before entering the main loop.
72:99c283dfe28d 2015-11-02 rgrover1 update to v2.0.0 of BLE_API. Add bleInitComplete() to main.c
71:469dbde1a238 2015-09-29 rgrover1 updating call to onDisconnection() due to updates to underlying libraries.
70:0a19690e8d23 2015-08-11 rgrover1 updating the underlying libraries.
69:7bc095e29592 2015-07-06 rgrover1 updating to the latest of the underlying libraries.
68:f6259731d1b6 2015-07-02 rgrover1 updating to the latest of the underlying libraries.
67:b2d2dee347c0 2015-07-01 rgrover1 update path for header includes.
66:c09ddf226b9c 2015-06-20 rgrover1 switch to newer APIs; rename BLEDevice to BLE
65:cb76569f74f6 2015-06-20 rgrover1 make use of the gap() accessor for ble
64:749d180e6a3d 2015-06-08 rgrover1 updating to the latest of the mbed sdk
63:4cda18716c67 2015-06-08 rgrover1 updating to the latest of the underlying libraries.
62:6573a90c88b1 2015-05-18 rgrover1 switch to the latest of the underlying libraries.
61:371d78b94ca8 2015-05-13 rgrover1 updating to S130 softdevice and the latest of BLE_API and nRF51822.
60:79da561d849b 2015-05-11 rgrover1 Updating to latest of the underlying libraries.
59:b32d42836762 2015-04-30 rgrover1 updating underlying libraries for v8 of the Nordic SDK.
58:ab9e41e1e9ed 2015-04-27 rgrover1 updating to the latest of the underlying APIs
57:d80d8dbaf86b 2015-04-15 rgrover1 updating to the latest from mbed SDK. includes update to Nordic SDKv7.1
56:83623419d5e4 2015-03-24 rgrover1 updating underlying libraries
55:3a7d497a3e03 2015-02-09 mbedAustin removed unncessary Battery Service from HearRate demo code
54:54a3de277e6b 2014-12-09 rgrover1 updating underlying libraries.
53:06a74fd722b8 2014-11-28 rgrover1 Updating to 0.2.5 of BLE_API
52:6bbf62943106 2014-11-21 rgrover1 add some code to configure slower connection interval
51:cba5deb3b404 2014-11-20 screamer Remove duplicate library
50:477004d54431 2014-11-17 rgrover1 add a check for gap-connectedness before polling for sensor data.
49:5dfc445d89cd 2014-11-05 rgrover1 updating underlying libraries.
48:8bca6365f8b0 2014-11-05 rgrover1 updating the underlying libraries.
47:430545f41113 2014-09-30 rgrover1 updating the underlying libraries.
46:ee7c55907f36 2014-09-30 rgrover1 DFUService is now added automatically for DFU platforms.
45:98c5a34b07a4 2014-09-29 rgrover1 move service initialization before starting advertisements.
44:f0df248cbe78 2014-09-23 rgrover1 updating to the latest of the underlying libraries.
43:dbb025ed4a55 2014-09-23 rgrover1 minor update to advertising payload; and fleshing out Device Information Service.
42:06ebef2e0e44 2014-09-22 rgrover1 updating to 0.2.0 of the BLE_API
41:9cef0129da5f 2014-09-02 rgrover1 updated underlying libraries
40:e73130c6f2bb 2014-08-22 rgrover1 white space diffs
39:6390604f904c 2014-08-22 rgrover1 Extract HeartRateService as a template to simplify the main application.
38:b2efa4f73d3a 2014-07-25 Rohit Grover merge
37:d310a72115c7 2014-07-11 Rohit Grover updated heart-rate demo to use the new connectionParams APIs
36:ea2a1b4f51c1 2014-07-11 Rohit Grover change hrm demo to do sensor polling from the main thread
35:ba3e3174331a 2014-07-11 Rohit Grover disconnectionCallback now takes a handle
34:44dc6efc0b50 2014-07-08 Rohit Grover use a declared constant for the Heart-Rate Location characteristic
33:e63df636d3b2 2014-07-08 Rohit Grover Remove a const attribute for a global variable used to initialize the location characteristic.
32:7761a83ba06d 2014-07-25 rgrover1 updating the underlying libraries.
31:380687727e63 2014-07-04 rgrover1 updating underlying libraries.
30:3dc9e6f2bc8c 2014-07-04 Rohit Grover remove the battery-level and device-information services
29:76d865c718a6 2014-07-04 Rohit Grover HRM demo now works with the nordic Android App (nRFToolBox)
28:bdfc8cc53f0b 2014-07-04 Rohit Grover add the battery level server to the HRM demo
27:97adf2b76b9c 2014-07-03 Rohit Grover add 16-bit UUIDs to the advertising payload; this is cosmetic
26:e6ad33b227c6 2014-07-03 Rohit Grover add deviceInformationService to the basic HRM demo
25:b3e91851d9e2 2014-06-26 rgrover1 updating to the latest of the underlying nRF51822 library.
24:4dab884e2235 2014-06-13 rgrover1 updating underlying libraries.
23:88a61b9d9814 2014-06-12 Rohit Grover merging updates from online IDE
22:299658c5fa3c 2014-06-12 Rohit Grover simplify demo to only contain the heart-rate service
21:1ea5a5ae55c9 2014-06-11 rgrover1 updating underlying libraries.
20:58bff62d0f7a 2014-06-11 Rohit Grover remove un-necessary callbacks from the demo