Datasheet

method is to wait for the first conversion to complete, and then change the channel selection. Since
the next conversion has already started automatically, the next result will reflect the previous
channel selection. Subsequent conversions will reflect the new channel selection.
• In Free Running mode, because the amplifier clear the ADSC bit at the end of an amplified
conversion, it is not possible to use the free running mode, unless ADSC bit is set again by soft at
the end of each conversion.
The user is advised not to write new channel or reference selection values during Free Running mode.
When switching to a differential gain channel, the first conversion result may have a poor accuracy due to
the required settling time for the automatic offset cancellation circuitry. The user should preferably
disregard the first conversion result.
22.6.2. ADC Voltage Reference
The reference voltage for the ADC (V
REF
) indicates the conversion range for the ADC. Single ended
channels that exceed V
REF
will result in codes close to 0x3FF. V
REF
can be selected as either AV
CC
,
internal 2.56V reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 2.56V reference is generated from
the internal bandgap reference (V
BG
) through an internal amplifier. In either case, the external AREF pin
is directly connected to the ADC, and the reference voltage can be made more immune to noise by
connecting a capacitor between the AREF pin and ground. V
REF
can also be measured at the AREF pin
with a high impedance voltmeter. Note that V
REF
is a high impedance source, and only a capacitive load
should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no external
voltage is applied to the AREF pin, the user may switch between AV
CC
and 2.56V as reference selection.
The first ADC conversion result after switching reference voltage source may be inaccurate, and the user
is advised to discard this result.
AREF pin is alternate function with ISRC Current Source output. When current source is selected, the
AREF pin is not connected to the internal reference voltage network. See AREFEN and ISRCEN bits in
ADCSRB – ADC Control and Status Register B.
If differential channels are used, the selected reference should not be closer to AV
CC
than indicated in
ADC Characteristics of Electrical Characteristics chapter.
22.7. ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise induced
from the CPU core and other I/O peripherals. The noise canceler can be used with ADC Noise Reduction
and Idle mode. To make use of this feature, the following procedure should be used:
1. Make sure that the ADC is enabled and is not busy converting. Single Conversion mode must be
selected and the ADC conversion complete interrupt must be enabled.
2. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion once the CPU
has been halted.
3. If no other interrupts occur before the ADC conversion completes, the ADC interrupt will wake up
the CPU and execute the ADC Conversion Complete interrupt routine. If another interrupt wakes up
the CPU before the ADC conversion is complete, that interrupt will be executed, and an ADC
Conversion Complete interrupt request will be generated when the ADC conversion completes. The
CPU will remain in active mode until a new sleep command is executed.
Atmel ATmega16M1/32M1/64M1 [DATASHEET]
Atmel-8209F-ATmega16M1/32M1/64M1_Datasheet_Complete-10/2016
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