ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 254

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

24.5
24.5.1
24.5.2
254
Changing Channel or Reference Selection
Atmel ATmega48PA/88PA/168PA [Preliminary]
ADC Input Channels
ADC Voltage Reference
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:
When updating ADMUX in one of these conditions, the new settings will affect the next ADC
conversion.
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 chan-
nel 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.
The reference voltage for the ADC (V
ended channels that exceed V
either AV
AV
ated from the internal bandgap reference (V
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
is a high impedance source, and only a capacitive load should be connected in a system.
REF
CC
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.
can also be measured at the AREF pin with a high impedance voltmeter. Note that V
is connected to the ADC through a passive switch. The internal 1.1V reference is gener-
CC
, internal 1.1V reference, or external AREF pin.
REF
will result in codes close to 0x3FF. V
REF
) indicates the conversion range for the ADC. Single
BG
) through an internal amplifier. In either case,
REF
can be selected as
9223B–AVR–09/11
REF