
462
42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
27.12 Register Description
27.12.1 ADMUX – ADC Multiplexer Selection Register
Bit
7
6
5
4
3
2
1
0
NA ($7C)
REFS1
REFS0
ADLAR
MUX4
MUX3
MUX2
MUX1
MUX0
ADMUX
Read/Write
RW
Initial Value
0
Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in the following table.
Changes of these bits will take effect until a conversion start is requested by setting
ADSC. The internal voltage reference options may not be used if an external reference
voltage is being applied to the AREF pin.
Table 27-11. Reference Voltage Selections for ADC
REFS1
REFS0
Reference Voltage Selection
0
AREF, Internal VREF turned off
0
1
AVDD (1.8V)
1
0
Internal 1.5V Voltage Reference (no external capacitor at AREF pin)
1
Internal 1.6V Voltage Reference (no external capacitor at AREF pin)
Note:
1. ATmega128RFA1: Changes of the REFS bits will only take effect until the first
conversion start is requested. After this the ADC has to be disabled and enabled
again for new reference selections.
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the A/D 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 conversions. For a complete description of this bit, see
Bits 4:0 – MUX4:0: Analog Channel and Gain Selection Bits
The value of these bits selects which combination of analog inputs is connected to the
conversion, the change will not go in effect until this conversion is complete (ADIF in
ADCSRA is set). Note that the MUX5 bit is located in the ADCSRB register. A write
access to the MUX4:0 bits triggers the update of the internally buffered MUX5 bit, see
27.12.2 ADCSRB – ADC Control and Status Register B
Bit
7
6
5
4
3
2
1
0
NA ($7B)
AVDDOK
ACME
REFOK
ACCH
MUX5
ADTS2
ADTS1
ADTS0
ADCSRB
Read/Write
R
R/W
R
R/W
Initial Value
0
Bit 7 – AVDDOK: AVDD Supply Voltage OK
The analog functions of the ADC are powered from the AVDD domain. AVDD is
supplied from an internal voltage regulator. Setting the ADEN bit in register ADCSRA
will power-up the AVDD domain if not already requested by another functional group of
the device. The bit allows the user to monitor (poll) the status of the AVDD domain. A
status of 1 indicates that AVDD has been powered-up.