ATmega1284P datasheet(255/380 Pages) ATMEL

255 / 380 page

255

8059D–AVR–11/09

ATmega1284P

Example:

ADMUX = 0xED (ADC3 - ADC2, 10x gain, 2.56V reference, left adjusted result)

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

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.

21.9

21.9.1

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 21-3. 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:

If 10x og 200x gain is selected, only 2.56V should be used as Internal Voltage Reference.

• 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 ”ADCL and ADCH – The ADC Data Register” on

page 258.

• Bits 4:0 – MUX4:0: Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC.

These bits also select the gain for the differential channels. See Table 21-4 on page 256 for

details. 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).

Bit

76543210

(0x7C)

REFS1

REFS0

ADLAR

MUX4

MUX3

MUX2

MUX1

MUX0

ADMUX

Read/Write

R/W

Initial Value

00000000

Table 21-3.

Voltage Reference Selections for ADC

REFS1

REFS0

Voltage Reference Selection

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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