ATtiny261A datasheet(152/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

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.

15.12 Temperature Measurement

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

single ended ADC11 channel. Selecting the ADC11 channel by writing the MUX5:0 bits in

ADMUX register to “111111” enables the temperature sensor. The internal 1.1V voltage refer-

ence 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 sin-

gle conversion mode to measure the voltage over the temperature sensor.

The measured voltage has a linear relationship to the temperature as described in Table 15-2

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 15-2 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 TOS 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.

ADC

VPOS VNEG

–

() 512

⋅

VREF

----------------------------------------------------- GAIN

⋅

Table 15-2.

Temperature vs. Sensor Output Voltage (Typical Case)

Temperature

-40

°C+25 °C+85 °C

ADC

230 LSB

300 LSB

370 LSB

ATtiny261A Datasheet(PDF) 152 Page - ATMEL Corporation

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