144
8006K–AVR–10/10
ATtiny24/44/84
to perform a quick polarity check of the result, it is sufficient to read the MSB of the result (ADC9
in ADCH). If the bit is one, the result is negative, and if this bit is zero, the result is positive.
As default the ADC converter operates in the unipolar input mode, but the bipolar input mode
can be selected by writting the BIN bit in the ADCSRB to one. In the bipolar input mode two-
sided voltage differences are allowed and thus the voltage on the negative input pin can also be
larger than the voltage on the positive input pin.
16.12 Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC8 channel. Selecting the ADC8 channel by writing the MUX5:0 bits in ADMUX
register to “100010” enables the temperature sensor. The internal 1.1V reference must also be
selected for the ADC reference source in the temperature sensor measurement. When the tem-
perature 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 16-2The sensitivity is approximately 1 LSB /
°C and the accuracy depends on the method of user cal-
ibration. Typically, the measurement accuracy after a single temperature calibration is ±10
°C,
assuming calibration at room temperature. Better accuracies are achieved by using two
temperature points for calibration.
The values described in
Table 16-2 are typical values. However, due to 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 application soft-
ware. The sofware calibration can be done using the formula:
T = k * [(ADCH << 8) | ADCL] + T
OS
where ADCH and ADCL are the ADC data registers, k is the fixed slope coefficient and T
OS is the
temperature sensor offset. Typically, k is very close to 1.0 and in single-point calibration the
coefficient may be omitted. Where higher accuracy is required the slope coefficient should be
evaluated based on measurements at two temperatures.
Table 16-2.
Temperature vs. Sensor Output Voltage (Typical Case)
Temperature
-40
°C+25°C+85°C
ADC
230 LSB
300 LSB
370 LSB