Datasheet

2. Next, you can configure the Heart Rate Measurement characteristic (hrmc above). The values that you set for this

will depend on the characteristic definition, but for convenience sake we've documented the key information in the

comments in the code above.

' hrmc.setProperties(CHR_PROPS_NOTIFY); ' - This sets the PROPERTIES value for the characteristic, which

determines how the characteristic can be accessed. In this case, the Bluetooth SIG has defined the characteristic

as Notify, which means that the peripheral will receive a request ('notification') from the Central when the Central

wants to receive data using this characteristic.

` hrmc.setPermission(SECMODE_OPEN, SECMODE_NO_ACCESS); ` - This sets the security for the characteristic, and

should normally be set to the values used in this example.

` hrmc.setFixedLen(2); ` - This tells the Bluetooth stack how many bytes the characteristic contains (normally a value

between 1 and 20). In this case, we will use a fixed size of two bytes, so we call .setFixedLen. If the characteristic

has a variable length, you would need to set the max size via .setMaxLen.

' hrmc.setCccdWriteCallback(cccd_callback); ' - This optional code sets the callback that will be fired when the CCCD

record is updated by the central. This is relevant because the characteristic is setup with the NOTIFY property.

When the Central sets to 'Notify' bit, it will write to the CCCD record, and you can capture this write even in the

CCCD callback and turn the sensor on, for example, allowing you to save power by only turning the sensor on

(and back off) when it is or isn't actually being used. For the implementation of the CCCD callback handler, see

the full sample code at the bottom of this page.

' hrmc.begin(); ' Once all of the properties have been set, you must call .begin() which will add the characteristic

definition to the last BLEService that was '.begin()ed'.

3. Optionally set an initial value for the characteristic(s), such as the following code that populates 'hrmc' with a correct

values, indicating that we are providing 8-bit heart rate monitor values, that the Body Sensor Location characteristic is

present, and setting the first heart rate value to 0x04:

The CCCD callback handler has the following signature:

Note that we use .notify() in the example above instead of .write(), since this characteristic is setup with the

NOTIFY property which needs to be handled in a slightly different manner than other characteristics.

// Set the characteristic to use 8-bit values, with the sensor connected and detected

uint8_t hrmdata[2] = { 0b00000110, 0x40 };

// Use .notify instead of .write!

hrmc.notify(hrmdata, 2);