Datasheet

141
2586D–AVR–02/06
ATtiny25/45/85
20.8 Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC4 channel. Selecting the ADC4 channel by writing the MUX3..0 bits in ADMUX
register to “1111” 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 Table 20-6 on
page 141. The voltage sensitivity is approximately 1 mV/
°C and the accuracy of the temperature
measurement is +/-
10°C after bandgap calibration.
The values described in Table 20-6 on page 141 are typical values. However, due to the process
variation 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 appli-
cation 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 sofware cali-
bration can be done utilizing the formula:
T = { [ (ADCH << 8) | ADCL ] - TOS } / k
where ADCn are the ADC data registers, k is a fixed coefficient and T
OS
is the temperature sen-
sor offset value determined and stored into EEPROM as a part of the production test.
Table 20-6. Temperature vs. Sensor Output Voltage (Typical Case)
Temperature / °C-45 °C+25 °C +105 °C
Voltage / mV 242 mV 314 mv 403 mV