ATtiny261-ESSZ datasheet(155/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

19.8.3

Bipolar Differential Conversion

As default the ADC converter operates in the unipolar input mode, but the bipolar input mode

can be selected by writing 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. If differential channels and a bipolar input mode are

used, the result is

where VPOS is the voltage on the positive input pin, VNEG the voltage on the negative input pin,

and VREF the selected voltage reference. The result is presented in two’s complement form, from

0x200 (-512d) through 0x000 (+0d) to 0x1FF (+511d). The GAIN is either 1x, 8x, 20x or 32x.

However, if the signal is not bipolar by nature (9 bits + sign as the 10th bit), this scheme loses

one bit of the converter dynamic range. Then, if the user wants to perform the conversion with

the maximum dynamic range, the user can perform a quick polarity check of the result and use

the unipolar differential conversion with selectable differential input pair. When the polarity check

is performed, 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.

19.9

Temperature Measurement

The temperature measurement is based on an on-chip temperature sensor that is coupled to a

single ended ADC input. MUX[4..0] bits in ADMUX register 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 con-

verter 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 19-2.

The voltage sensitivity is approximately 1LSB/°C and the accuracy of the temperature measure-

ment is ±10°C using manufacturing calibration values (TS_GAIN, TS_OFFSET).

The values described in Table 19-2 are typical values. However, due to the process variation the

temperature sensor output varies from one chip to another.

ADC

VPOS VNEG

–

() 512

⋅

VREF

-------------------------------------------------------

GAIN

⋅

Table 19-2.

Temperature vs. Sensor Output Voltage (Typical Case): Example ADC Values

Temperature / °C

-40°C

+25 °C

+125 °C

0x00F6

0x0144

0c01B8

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