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Lifecycle: Beta. Last updated 2020-09-10.
The sensors periodically send their data through all channels they can: this is known as sensor heartbeat. The sensors try broadcast the data, if GATT is connected the sensor data is sent via GATT and NFC data field is updated to include the latest data.
On GATT the data is sent via Nordic UART Service TX Characteristic notifications. Bluetooth advertisements have the data encoded in Manufacturer Specific payload. NFC has the data as a binary in field utf-8 field with language "dt".
The heartbeat also feeds the watchdog of the application, if the data updates stop for any reason the application assumes that it has crashed and reboots.