Datasheet

274
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
26.5 Changing Channel or Reference Selection
The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary register to which the
CPU has random access. This ensures that the channels and reference selection only takes place at a safe point
during the conversion. The channel and reference selection is continuously updated until a conversion is started.
Once the conversion starts, the channel and reference selection is locked to ensure a sufficient sampling time for
the ADC. Continuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in
ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after ADSC is written. The
user is thus advised not to write new channel or reference selection values to ADMUX until one ADC clock cycle
after ADSC is written.
If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special care must be taken
when updating the ADMUX Register, in order to control which conversion will be affected by the new settings.
If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the ADMUX Register is
changed in this period, the user cannot tell if the next conversion is based on the old or the new settings. ADMUX
can be safely updated in the following ways:
1. When ADATE or ADEN is cleared.
2. During conversion, minimum one ADC clock cycle after the trigger event.
3. After a conversion, before the Interrupt Flag used as trigger source is cleared.
When updating ADMUX in one of these conditions, the new settings will affect the next ADC conversion.
Special care should be taken when changing differential channels. Once a differential channel has been selected,
the stage may take as much as 125µs to stabilize to the new value. Thus conversions should not be started within
the first 125µs after selecting a new differential channel. Alternatively, conversion results obtained within this
period should be discarded.
The same settling time should be observed for the first differential conversion after changing ADC reference (by
changing the REFS1:0 bits in ADMUX).
26.5.1 ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the correct
channel is selected:
In Single Conversion mode, always select the channel before starting the conversion. The channel selection may
be changed one ADC clock cycle after writing one to ADSC. However, the simplest method is to wait for the con-
version to complete before changing the channel selection.
In Free Running mode, always select the channel before starting the first conversion. The channel selection may
be changed one ADC clock cycle after writing one to ADSC. However, the simplest 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.
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.
26.5.2 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 AVCC, internal 1.1V reference,
internal 2.56V reference or external AREF pin.