ATtiny44 datasheet(144/236 Pages) ATMEL

144 / 236 page

144

8006H–AVR–10/09

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

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 16-2

The 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

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

ATtiny44 Datasheet(PDF) 144 Page - ATMEL Corporation