ATMEGA640 datasheet(281/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Example:

ADMUX = 0xFB (ADC3 - ADC2, 10× gain, 2.56V reference, left adjusted result).

Voltage on ADC3 is 300mV, voltage on ADC2 is 500mV.

ADCR = 512 × 10 × (300 - 500) / 2560 = -400 = 0x270.

ADCL will thus read 0x00, and ADCH will read 0x9C. Writing zero to ADLAR right adjusts the result: ADCL = 0x70,

ADCH = 0x02.

26.8 Register Description

26.8.1

ADMUX – ADC Multiplexer Selection Register

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

These bits select the voltage reference for the ADC, as shown in Table 26-3. 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.

Note:

1. If 10x or 200x gain is selected, only 2.56V should be used as Internal Voltage Reference. For differential conver-

sion, only 1.1V cannot be used as internal voltage reference.

Table 26-2.

Correlation Between Input Voltage and Output Codes

VADCn

Read Code

Corresponding Decimal Value

VADCm + VREF / GAIN

0x1FF

511

VADCm + 0.999 VREF / GAIN

0x1FF

511

VADCm + 0.998 VREF / GAIN

0x1FE

510

...

VADCm + 0.001 VREF / GAIN

0x001

VADCm

0x000

VADCm - 0.001 VREF / GAIN

0x3FF

-1

...

VADCm - 0.999 VREF / GAIN

0x201

-511

VADCm - VREF / GAIN

0x200

-512

Bit

654

321

(0x7C)

REFS1

REFS0

ADLAR

MUX4

MUX3

MUX2

MUX1

MUX0

ADMUX

Read/Write

R/W

Initial Value

Table 26-3.

Voltage Reference Selections for ADC

REFS1

REFS0

Voltage Reference Selection(1)

AREF, Internal VREF turned off

AVCC with external capacitor at AREF pin

Internal 1.1V Voltage Reference with external capacitor at AREF pin

Internal 2.56V Voltage Reference with external capacitor at AREF pin

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