ATmega88 datasheet(249/340 Pages) ATMEL

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249

7530H–AVR–02/09

ATmega48/88/168 Automotive

21.6

ADC Conversion Result

After the conversion is complete (ADIF is high), the conversion result can be found in the ADC

Result Registers (ADCL, ADCH).

For single ended conversion, the result is

where V

IN is the voltage on the selected input pin and VREF the selected voltage reference (see

Table 21-2 on page 249 and Table 21-3 on page 250). 0x000 represents analog ground, and

0x3FF represents the selected reference voltage minus one LSB.

21.6.1

ADC Multiplexer Selection Register – ADMUX

• Bit 7:6 – REFS1:0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 21-2. If these bits are

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

(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external

reference voltage is being applied to the AREF pin.

•

Bit 5 – ADLAR: ADC Left Adjust Result

The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.

Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the

ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-

sions. For a complete description of this bit, see “The ADC Data Register – ADCL and ADCH” on

page 251.

• Bit 4 – Res: Reserved Bit

This bit is an unused bit in the ATmega48/88/168, and will always read as zero.

• Bits 3:0 – MUX3:0: Analog Channel Selection Bits

ADC

VIN 1024

⋅

VREF

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

Bit

76543210

REFS1

REFS0

ADLAR

–

MUX3

MUX2

MUX1

MUX0

ADMUX

Read/Write

R/W

Initial Value

00000000

Table 21-2.

Voltage Reference Selections for ADC

REFS1

REFS0

Voltage Reference Selection

AREF, Internal Vref turned off

CC with external capacitor at AREF pin

Reserved

Internal 1.1V Voltage Reference with external capacitor at AREF pin

ATmega88 Datasheet(PDF) 249 Page - ATMEL Corporation

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