ATtiny88 datasheet(170/266 Pages) ATMEL

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8008C–AVR–03/09

ATtiny48/88

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 Table 17-2

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 17-2 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.

17.13 Register Description

17.13.1

ADMUX – ADC Multiplexer Selection Register

• Bits 7,4 – Res: Reserved Bits

These bits are reserved and will always read zero.

• Bit 6 – REFS0: Reference Selection Bits

This bit select the voltage reference for the ADC, as shown in Table 17-3. If this bit is changed

during a conversion, the change will not go in effect until this conversion is complete (ADIF in

ADCSRA is set).

Table 17-2.

Temperature vs. Sensor Output Voltage (Typical Case)

Temperature

-40

°C+25°C+85°C

ADC

230 LSB

300 LSB

370 LSB

Bit

765

4321

–

REFS0

ADLAR

–

MUX3

MUX2

MUX1

MUX0

ADMUX

Read/Write

R/W

Initial Value

000

0000

Table 17-3.

Voltage Reference Selections for ADC

REFS0

Voltage Reference Selection

Internal 1.1V Voltage Reference

CC Reference

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