Datasheet

where V

is the voltage on the selected input pin, and V

REF

the selected voltage reference (see also

descriptions of ADMUX.REFSn and ADMUX.MUX). 0x000 represents analog ground, and 0x3FF

represents the selected reference voltage minus one LSB.

28.8 Temperature Measurement

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 1 LSB/°C, the accuracy of the temperature measurement is

±10°C assuming calibration at room temperature. Better accuracies are achieved by using two

temperature points for calibration.

Table 28-2. Temperature vs. Sensor Output Voltage (Typical Case)

Temperature -40°C 25°C +105°C

ADC 205 LSB 270 LSB 350 LSB

The values described in the table above 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 software. The

software calibration can be done using the formula:

T = { [(ADCH << 8) | ADCL] - T

} / k

where ADCH and ADCL are the ADC data registers, k is a fixed coefficient and T

is the temperature

sensor offset. Typically, k is very close to 1.0 and in single-point calibration the coefficient may be omitted.

Gain and offset varies from device to device, so calibration has to be done for each device. Refer to

AVR122: Calibration of the AVR's Internal Temperature Reference for the detail.

28.9 Register Description

ATmega48PA/88PA/168PA

Analog-to-Digital Converter (ADC)

Datasheet Complete

DS40002011A-page 325