The temperature measurement is based on an on-chip temperature sensor that is coupled to a single ended Temperature sensor channel. Selecting the Temperature sensor channel by writing ADMUX.MUX[3:0] to '1000' enables the temperature sensor. The internal 1.1V voltage reference must also be selected for the ADC voltage reference source in the temperature sensor measurement. When the temperature sensor is enabled, the ADC converter can be used in single conversion mode to measure the voltage over the temperature sensor.
The measured voltage has a linear relationship to the temperature as described in the following table. The voltage sensitivity is approximately 1mV/°C, the accuracy of the temperature measurement is ±10°C.
| Temperature | -45°C | +25°C | +85°C |
|---|---|---|---|
| Voltage | 242mV | 314mV | 380mV |
The values described in the table above are typical values. However, due to process variations the temperature sensor output voltage varies from one chip to another. To be capable of achieving more accurate results the temperature measurement can be calibrated in the application software. The software calibration requires that a calibration value is measured and stored in a register or EEPROM for each chip, as a part of the production test. The software calibration can be done utilizing the formula:
T = { [(ADCH << 8) | ADCL] - TOS} / k
where ADCH and ADCL are the ADC data registers, k is a fixed coefficient and TOS is the temperature sensor offset value determined and stored into EEPROM as a part of the production test.