259
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
Example:
ADMUX = 0xED (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.
23.9
Register description
23.9.1
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 23-3. 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 differential channels are 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-
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.
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
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
ADMUX
Read/Write
R/W
Initial Value
00000000
Table 23-3.
Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
AREF, Internal Vref turned off
0
1
AVCC with external capacitor at AREF pin
1
0
Internal 1.1V Voltage Reference with external capacitor at AREF pin
1
Internal 2.56V Voltage Reference with external capacitor at AREF pin