37.2 Temperature Calculation

This section describes the steps involved in calculating the die temperature, TMEAS:
  1. Obtain the ADC count value of the measured analog voltage: The analog output voltage, VMEAS, is converted to a digital count value by the Analog-to-Digital Converter (ADC) and is referred to as ADCMEAS.
  2. Obtain the Gain value from the DIA table. This parameter is TSLR1 for the low range setting or TSHR1 for the high range setting of the temperature indicator module. Refer to the DIA table in the “Memory Organization” chapter for more details.
  3. Obtain the Offset value from the DIA table. This parameter is TSLR3 for the low range setting or TSHR3 for the high range setting of the temperature indicator module. Refer to the DIA table in the “Memory Organization” chapter for more details.
Important: The Gain and Offset values obtained from the DIA table are 14-bit signed values that require sign extension to be correctly interpreted as 16-bit signed integers.

The following equation provides an estimate for the die temperature based on the above parameters:

Equation 37-1. Sensor Temperature (in ℃)
T M E A S = ( A D C M E A S × G a i n ) 256 + O f f s e t 10
Where:

ADCMEAS = ADC reading at temperature being estimated

Gain = Gain value stored in the DIA table (must be converted into 16-bit signed integer)

Offset = Offset value stored in the DIA table (must be converted into 16-bit signed integer)

Note: It is recommended to take the average of ten measurements of ADCMEAS to reduce noise and improve accuracy.

Temperature Calculation (C)

// offset and gain must be converted to int16_t data type
// ADC_MEAS is uint16_t data type
// Temp_in_C is int24_t data type

int16_t gain;
int16_t offset;
gain ^= (1 << 14);
gain -= (1 << 14);                        // Convert to 16-bit signed int
offset ^= (1 << 14);
offset -= (1 << 14);                      // Convert to 16-bit signed int
ADC_MEAS = ((ADRESH << 8) + ADRESL);      // Store the ADC Result
Temp_in_C = (int24_t)(ADC_MEAS) * gain;   // Multiply the ADC Result by
                                          // Gain and store the result
                                          // in a signed variable
Temp_in_C = Temp_in_C / 256;              // Divide (ADC Result * Gain) by 256
Temp_in_C = Temp_in_C + offset;           // Add (Offset) to the result
Temp_in_C = Temp_in_C / 10;               // Divide the result by 10 and store
                                          // the calculated temperature
Important: If the application requires more precise temperature measurement, additional calibrations steps will be necessary. For these applications, two-point or three-point calibration is recommended. For additional information on two-point calibration method, refer to the following Microchip application note, available at the corporate website (www.microchip.com):
  • AN2798, “Using the PIC16F/PIC18F Ground Referenced Temperature Indicator Module”