Datasheet
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
212
21.4 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:
a. When ADATE or ADEN is cleared.
b. During conversion, minimum one ADC clock cycle after the trigger event.
c. 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.
21.4.1 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 conversion 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.
21.4.2 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 1.1V reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 1.1V 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 impedant voltmeter. Note that V
REF
is a high impedant 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 1.1V 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.