Heart Rate Sensor
The heart_rate_sensor driver drives the TICKR heart rate monitor, pulling heart rate data from the TICKR.
GATT Device
The driver is primarily driven by a gatt.Device object which connects and reads data from the TICKR heart rate monitor through its characteristics.
| Service/Characteristic | UUID | Purpose |
|---|---|---|
heart_rate_service | ending-180d | Holds the heart_rate_measurement_characteristic characteristic |
heart_rate_measurement_characteristic | ending-2a37 | Readable characteristic which holds the measured heart rate data |
UUIDs can be matched to their characteristic/service using the following two Bluetooth SIG documents:
Extracting Heart Rate Data
Notification is enabled for the heart_rate_measurement_characteristic characteristic and so when it updates the characteristic_value_updated method is called with the new heart rate data.
The heart rate data is stored in a series of bytes with a preceding flag byte to indicate the format of the data and what data is present.
Flags
| Bit Position | Meaning |
|---|---|
| 0 | set if the heart rate is 16 bit (otherwise 8 bit) |
| 1 | set if contact is detected (only valid if bit 2 is also set) |
| 2 | set if contact status is reported |
| 3 | set if energy expenditure is reported |
| 4 | set if rr interval is reported |
| 5-7 | reserved for future use (ignored) |
Measurements
- Heart rate (beats per minute) [uint8, or uint16 if flags Bit 0 is set]
- Energy (kJ) [uint16, only present if flags Bit 3 is set]
- RR Interval (1/1024 seconds) [Remaining bytes until end of packet, only present if flags bit 4 is set]
MQTT Topics
Heart rate data is published to the following topic:
bike/{DEVICE_ID}/heartrate
This should be changed to conform to the MQTT Topics documents convention:
bike/{DEVICE_ID}/heartrate/report