Datasheet
Figure 27-5. ADC Timing Diagram, Free Running Conversion
11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number
1
2
One Conversion Next Conversion
3 4
Conversion
Complete
Sample and Hold
MUX and REFS
Update
Table 27-1. ADC Conversion Time
Condition Sample & Hold
(Cycles from Start of Conversion)
Conversion Time
(Cycles)
Extended conversion 13.5 25
Normal conversions, single ended 1.5 13
27.5 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 both ADFR 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 ADFR 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.
27.5.1 ADC Input Channels
When changing channel selections, the user should observe the following guidelines to ensure that the
correct channel is selected:
AVR 8-Bit Microcontroller
ADC - Analog to Digital Converter
© 2017 Microchip Technology Inc.
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