Device Temperature Measurement

Principle

The device has an integrated temperature sensor which is part of the Supply Controller (SUPC). The analog signal of that sensor can be converted into a digital value by the ADC. The digital value can be converted into a temperature in °C by following the steps in this section.

Configuration and Conditions

In order to conduct temperature measurements, configure the device according to these steps.

1.Configure the clocks and device frequencies according to the Electrical Characteristics.
2.Configure the Voltage References System of the Supply Controller (SUPC):
1. 2.1.Enable the temperature sensor by writing a '1' to the Temperature Sensor Enable bit in the VREF Control register (SUPC.VREF.TSEN).
2. 2.2.Select the required voltage for the internal voltage reference INTREF by writing to the Voltage Reference Selection bits (SUPC.VREF.SEL). The required value can be found in the Electrical Characteristics.
3. 2.3.Enable routing INTREF to the ADC by writing a '1' to the Voltage Reference Output Enable bit (SUPC.VREF.VREFOE).
3.Configure the ADC:
1. 3.1.Select the internal voltage reference INTREF as ADC reference voltage by writing to the Reference Control register (ADC.REFCTRL.REFSEL).
2. 3.2.Select the temperature sensor vs. internal GND as input by writing TEMP and GND to the positive and negative MUX Input Selection bit fields (ADC.INPUTCTRL.MUXNEG and .MUXPOS, respectively).
3. 3.3.Configure the remaining ADC parameters according to the Electrical Characteristics.
4. 3.4.Enable the ADC and acquire a value, ADC_m.

Calculation Parameter Values

The temperature sensor behavior is linear, but it is sensitive to several parameters such as the internal voltage reference - which itself depends on the temperature. To take this into account, each device contains a Temperature Log row with individual calibration data measured and written during the production tests. These calibration values are read by software to infer the most accurate temperature readings possible.

The Temperature Log Row basically contains the following parameter set for two different temperatures ("ROOM" and "HOT"):

Calibration temperatures in °C. One at room temperature temp_R, one at a higher temperature temp_H:
- ROOM_TEMP_VAL_INT and ROOM_TEMP_VAL_DEC contain the measured temperature at room insertion, temp_R, in °C, separated in integer and decimal value.
  Example: For ROOM_TEMP_VAL_INT=0x19=25 and ROOM_TEMP_VAL_DEC=2, the measured temperature at room insertion is 25.2°C.
- HOT_TEMP_VAL_INT and HOT_TEMP_VAL_DEC contain the measured temperature at hot insertion, temp_H, in °C. The integer and decimal value are also separated.
For each temperature, the corresponding sensor value at the ADC in 12-bit, ADC_R and ADC_H:
- ROOM_ADC_VAL contains the 12-bit ADC value, ADC_R, corresponding to temp_R. Its conversion to Volt is denoted V_ADCR.
- HOT_ADC_VAL contains the 12-bit ADC value, ADC_H, corresponding to temp_H. Its conversion to Volt is denoted V_ADCH.
Actual reference voltages at each calibration temperature in Volt, INT1V_R and INT1V_H, respectively:
- ROOM_INT1V_VAL is the 2’s complement of the internal 1V reference value at temp_R: INT1V_R.
- HOT_INT1V_VAL is the 2’s complement of the internal 1V reference value at temp_H: INT1V_H.
- Both ROOM_INT1V_VAL and HOT_INT1V_VAL values are centered around 1V with a 0.001V step. In other words, the range of values [0,127] corresponds to [1V, 0.873V] and the range of values [-1, -127] corresponds to [1.001V, 1.127V]. INT1V == 1 - (VAL/1000) is valid for both ranges.

Calculating the Temperature by Linear Interpolation

Using the data pairs (temp_R, V_ADCR) and (temp_H, V_ADCH) for a linear interpolation, we have the following equation:

(VADC⁢−VADCRtemp−⁢tempR)=(VADCH⁢−VADCRtempH−⁢tempR)

The voltages V_x are acquired as 12-bit ADC values ADC_x, with respect to an internal reference voltage INT1V_x:

[Equation 1]

VADCx⁢= ADCx⁢⋅INT1Vx212−1

For the measured value of the temperature sensor, ADC_m, the reference voltage is assumed to be perfect, i.e., INT1V_m=INT1V_c=1V. These substitutions yield a coarse value of the measured temperature temp_C:

[Equation 2]

tempC=tempR+[{(ADCm⋅INT1Vc(212-1))−(ADCR⋅INT1VR(212-1))}⋅(tempH−tempR)(ADCH⋅INT1VH(212-1))-(ADCR⋅INT1VR(212-1))]

Or, after eliminating the 12-bit scaling factor (2¹²-1):

[Equation 3]

tempC=tempR+[{ADCm⋅INT1Vc−(ADCR⋅INT1VR)}⋅(tempH−tempR){(ADCH⋅INT1VH)-(ADCR⋅INT1VR)}]

Equations 3 is a coarse value, because we assumed that INT1V_c=1V. To achieve a more accurate result, we replace INT1V_c with an interpolated value INT1V_m. We use the two data pairs (temp_R, INT1V_R) and (temp_H, INT1V_H) and yield:

(INT1Vm⁢−INT1VRtempm−⁢tempR)=(INT1VH⁢−INT1VVRtempH−⁢tempR)

Using the coarse temperature value temp_c, we can infer a more precise INT1V_m value during the ADC conversion as:

[Equation 4]

INT1Vm=INT1VR+((INT1VH−INT1VR)⋅(tempC−tempR)(tempH−tempR))

Back to Equation 3, we replace the simple INT1V_c=1V by the more precise INT1V_m of Equation 4, and find a more accurate temperature value temp_f:

[Equation 5]

tempf=tempR+[{ADCm⋅INT1Vm−(ADCR⋅INT1VR)}⋅(tempH−tempR){(ADCH⋅INT1VH)-(ADCR⋅INT1VR)}]