Predictive Thermometer Protocol

Combustion Developer Resources

© Combustion 2022 – DRAFT 0.1

This document describes how to interface with the Combustion Predictive Thermometer via Combustion’s open source Bluetooth communication protocol. The protocol supports accessing the follow thermometer features:

The Predictive Thermometer is turned on and off by removing or replacing it from the charging sleeve. When removed, the thermometer will continuously transmit advertising packets. Any number of devices can listen to these packets.

The thermometer also supports up to 3 concurrent BLE connections. When the probe has fewer than 3 active connections, it will advertised Connectable packets, otherwise it will advertise Unconnectable packets. Connections are used for requesting information not included in the advertising packets, such as device serial number or data records.

The thermometer’s advertising interval is dependent on its current mode of operation, with a default advertising interval of 250ms.

The format of the Advertising packet and scan response are shown in the following tables.

The probe's connection interval is dependent on its mode of operation. During normal operation the probe expects a connection interval between TBD and TBD. While in Instant Read mode, the probe updates its status more often and expects a connection interval between TBD and TBD.

The Predictive Probe implements the following GATT Services and Characteristics.

This standard BLE service provides static information about the Predictive Probe. The UUID for the Device Information Service is 0x180A.

Probe Status is a custom service that provides the current status of the Predictive Probe. The UUID for the Probe Status service is 00000100-CAAB-3792-3D44-97AE51C1407A.

This service has a single characteristic that supports BLE notifications. Each time a measurement is taken, the probe status is sent to each connected device that has subscribed to these notifications. The probe status includes the sequence number for first and last record on the probe and the current temperature from each sensor.

The UART service is a custom BLE service that emulates a UART. The UUID for the UART service is 6E400001-B5A3-F393-E0A9-E50E24DCCA9E. The RX characteristic is used to receive data and the TX characteristic is used to transmit data via BLE notifications. The format of the data sent and received over this service is described in the UART Messages section.

Each message will begin with the same 5 byte header, followed by the message payload.

Each response message will include a 7 byte header with the following format.

The following describes UART Messages relevant to the thermometer, other UART Messages for the Display & Range Extender are described here.

After receiving this message, the thermometer will update the thermometer ID in both its Advertising packet and its Status Characteristic.

Request Payload

Response Payload

The Set Thermometer ID response message has no payload.

After receiving this message, the thermometer will update the thermometer color in both its Advertising Packet and its Status Characteristic.

Request Payload

Response Payload

The Set Thermometer Color response message has no payload.

After receiving this message the thermometer will respond with a randomly generated session ID associated with currently logged data, and the interval between log entries.

Request Payload

Response Payload

The Read Session Information response message has no payload.

After receiving this message the thermometer will respond with a sequence of Read Log Response messages.

Request Payload

Response Payload

After receiving this message and successful response, the thermometer will enter Prediction Mode with the specified target temperature. The thermometer will update the fields in the Prediction Status of its Status Characteristic.

Request Payload

Response Payload

The Set Prediction Engine response message has no payload.

The Device Firmware Update (DFU) Service is a custom service provided by Nordic for updating the firmware on Combustion devices. Details TBD.

These are the definitions of the data formats common between advertising data and characteristic data used throughout this documentation.

The product type is an enumerated value in an 8-bit (1-byte) field:

0 Unknown

1 Predictive Thermometer

2 Display & Range Extender

Raw temperature data is expressed in a packed 104-bit (13-byte) field.

The allowable range for each sensor is -20°C - 369°C, and is quantized in steps of 0.05°C:

Temperature = (Raw Temperature Value * 0.05) - 20

Mode and ID data are expressed in a packed 8-bit (1-byte) field. The Mode field differentiates the current state of the probe and is used to adjust advertising and sensor sampling rates. Color ID and Probe ID provide a configurable thermometer identity (ID number, color, or both) that can be changed from time to time.

Battery Status is expressed in a packed 8-bit (1-byte) field along with the Virtual Sensors described below.

Virtual Sensors are expressed in the trailing 7-bits of an packed 8-bit field. The leading bit is reserved for battery status as described above.

Sensor nearest the geometric center of the food. This measurement will always be available, but defaults to the T1 sensor when the probe has not detected its insertion into food.

0 T1 sensor, located at tip of thermometer

1 T2 sensor

2 T3 sensor

3 T4 sensor, located tip-side of the minimum insertion line

4 T5 sensor, located handle-side of the minimum insertion line

5 T6 sensor

6 T7 sensor

7 T8 sensor, located in handle and thermal connected to heat-pipe mounting screw

Sensor that is both inside the food and nearest to the surface of the food. Because the thermometer must be inserted to the minimum insertion line above T4, the only sensors that can reasonably be near the surface of the food are T4 through T7. Available only when the thermometer has detected its insertion into food.

0 T4 sensor

1 T5 sensor

2 T6 sensor

3 T7 sensor

Sensor most representative of the prevailing ambient temperature near the surface of the food. Typically this will default to T8 located in the thermometer’s handle, but in certain situations the faster response time of sensors T5, T6, or T7 provide a more representative measurement. Because the thermometer must be inserted to the minimum insertion line above T4, the only sensors that can reasonably be outside of the food are T5 through T8. This measurement will always be available, but will default to T8 when the probe has not detected its insertion into food.

0 T5 sensor

1 T6 sensor

2 T7 sensor

3 T8 sensor

The Virtual Sensors and Prediction Engine state log are expressed as a 16-bit (2-byte) field.

TK

The prediction engine status is expressed in a packed 56-bit (7-byte) field.

The set prediction data is expressed in a packed 16-bit (2-byte) field.

Prediction State

The prediction state value is expressed in a 4-bit enumerated field.

0 Thermometer not inserted

1 Thermometer inserted

2 Heating

3 Predicting

4 Prediction Complete

5-15 Reserved states

The prediction mode expresses the input mode in a 2-bit enumerated field.

0 None

1 Time to removal

2 Removal and resting time

3 Reserved

Value describing the type of prediction being provided in the Prediction Value field. Enumerated in a 2-bit field.

0 None

1 Cooking

2 Resting

3 Reserved

User-configured input of target core doneness temperature. 10-bit value (0 to 1023) in units of 0.1 °C.

Prediction Target Temeprature = User-Supplied Raw Temperature Value * 0.1

The measured Virtual Core temperature at the detected start of heating. 10-bit value (0 to 1023) in units of 0.1 °C.

Heating Start Temperature = Raw Virtual Core Temperature Value * 0.1

Additionally, progress from start to done is calculated on a percentage basis:

Percentage Progress to Done = (Virtual Core Temperature - Heating Start Temperature) / (Prediction Target Temperature - Heating Start Temperature)

The current predicted time to the Target Temperature, in seconds, from now as a 17-bit value.

11-bit value. The estimated current core temperature from -200 to 1847 in units of 1/10 degree Celsius.

Estimated Core Temperature = (raw value * 0.1 C) - 20 C.

TBD