Datasheet

156
8197C–AVR–05/11
ATtiny261A/461A/861A
• Bits 7:6 – REFS[1:0]: Voltage Reference Selection Bits
These bits together with the REFS2 bit from the ADC Control and Status Register B (ADCSRB)
select the voltage reference for the ADC, as shown in Table 15-4.
If these bits are changed during a conversion, the change will not go in effect until this conver-
sion is complete (ADIF in ADCSR is set). Also note, that when these bits are changed, the next
conversion will take 25 ADC clock cycles.
Special care should be taken when changing differential channels. Once a differential channel
has been selected the input stage may take a while to stabilize. It is therefore recommended to
force the ADC to perform a long conversion when changing multiplexer or voltage reference set-
tings. This can be done by first turning off the ADC, then changing reference settings and then
turn on the ADC. Alternatively, the first conversion results after changing reference settings
should be discarded.
It is not recommended to use an external AREF higher than (V
CC
- 1V) for channels with differ-
ential gain, as this will affect ADC accuracy.
Internal voltage reference options may not be used if an external 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 comple te description of this bit, see “ADCL and ADCH – The ADC Data Register”
on page 155.
• Bits 4:0 – MUX[4:0]: Analog Channel and Gain Selection Bits
These bits and the MUX5 bit from the ADC Control and Status Register B (ADCSRB) select
which combination of analog inputs are connected to the ADC. In case of differential input, gain
selection is also made with these bits. Selecting the same pin as both inputs to the differential
Table 15-4. Voltage Reference Selections for ADC
REFS2 REFS1 REFS0 Voltage Reference Selection
X00
V
CC
used as voltage reference,
disconnected from AREF
X01
External voltage reference at AREF pin,
internal voltage reference turned off
0 1 0 Internal 1.1V voltage reference
011Reserved
110
Internal 2.56V voltage reference (V
CC
> 3.0V),
without external bypass capacitor,
disconnected from AREF
111
Internal 2.56V voltage reference (
V
CC
> 3.0V),
with external bypass capacitor at AREF pin. Note:
external voltages may not be applied to the pin!