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.

Revisions of BLE_API.lib

Revision Date Message Actions
79:8b7c8c240540 18 months ago Update libraries. File  Diff  Annotate
77:a2ab41a7f067 23 months ago Update libraries to the latest revision. File  Diff  Annotate
76:8b36a7c45cd2 2015-11-26 updated underlying libraries. File  Diff  Annotate
74:c9d58e7847c4 2015-11-03 update to the latest of underlying ilbraries. File  Diff  Annotate
72:99c283dfe28d 2015-11-02 update to v2.0.0 of BLE_API. Add bleInitComplete() to main.c File  Diff  Annotate
71:469dbde1a238 2015-09-29 updating call to onDisconnection() due to updates to underlying libraries. File  Diff  Annotate
70:0a19690e8d23 2015-08-11 updating the underlying libraries. File  Diff  Annotate
69:7bc095e29592 2015-07-06 updating to the latest of the underlying libraries. File  Diff  Annotate
68:f6259731d1b6 2015-07-02 updating to the latest of the underlying libraries. File  Diff  Annotate
66:c09ddf226b9c 2015-06-20 switch to newer APIs; rename BLEDevice to BLE File  Diff  Annotate
65:cb76569f74f6 2015-06-20 make use of the gap() accessor for ble File  Diff  Annotate
63:4cda18716c67 2015-06-08 updating to the latest of the underlying libraries. File  Diff  Annotate
62:6573a90c88b1 2015-05-18 switch to the latest of the underlying libraries. File  Diff  Annotate
61:371d78b94ca8 2015-05-13 updating to S130 softdevice and the latest of BLE_API and nRF51822. File  Diff  Annotate
60:79da561d849b 2015-05-11 Updating to latest of the underlying libraries. File  Diff  Annotate
59:b32d42836762 2015-04-30 updating underlying libraries for v8 of the Nordic SDK. File  Diff  Annotate
58:ab9e41e1e9ed 2015-04-27 updating to the latest of the underlying APIs File  Diff  Annotate
57:d80d8dbaf86b 2015-04-15 updating to the latest from mbed SDK. includes update to Nordic SDKv7.1 File  Diff  Annotate
56:83623419d5e4 2015-03-24 updating underlying libraries File  Diff  Annotate
55:3a7d497a3e03 2015-02-09 removed unncessary Battery Service from HearRate demo code File  Diff  Annotate
54:54a3de277e6b 2014-12-09 updating underlying libraries. File  Diff  Annotate
53:06a74fd722b8 2014-11-28 Updating to 0.2.5 of BLE_API File  Diff  Annotate
52:6bbf62943106 2014-11-21 add some code to configure slower connection interval File  Diff  Annotate
49:5dfc445d89cd 2014-11-05 updating underlying libraries. File  Diff  Annotate
46:ee7c55907f36 2014-09-30 DFUService is now added automatically for DFU platforms. File  Diff  Annotate
42:06ebef2e0e44 2014-09-22 updating to 0.2.0 of the BLE_API File  Diff  Annotate
41:9cef0129da5f 2014-09-02 updated underlying libraries File  Diff  Annotate
39:6390604f904c 2014-08-22 Extract HeartRateService as a template to simplify the main application. File  Diff  Annotate
32:7761a83ba06d 2014-07-25 updating the underlying libraries. File  Diff  Annotate
31:380687727e63 2014-07-04 updating underlying libraries. File  Diff  Annotate
24:4dab884e2235 2014-06-13 updating underlying libraries. File  Diff  Annotate
21:1ea5a5ae55c9 2014-06-11 updating underlying libraries. File  Diff  Annotate
12:845161769325 2014-06-10 updating to the latest versions of the depending libraries. File  Diff  Annotate
7:daab8ba5139e 2014-06-10 Update to using the latest versions of the underlying APIs and waitForEvent() to reduce power consumption. File  Diff  Annotate
6:3a49b577f2ae 2014-06-05 updating to the latest version of the underlying libraries File  Diff  Annotate
2:bba892b1f4aa 2014-04-03 Updated to latest core libs File  Diff  Annotate
1:8b26777976b8 2014-04-01 Updated BLE_API location File  Diff  Annotate