Datasheet

Figure 25-2. ADC Auto Trigger Logic
ADSC
ADIF
SOURCE 1
SOURCE n
ADTS[2:0]
CONVERSION
LOGIC
PRESCALER
START
CLK
ADC
.
.
.
.
EDGE
DETECTOR
ADATE
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the
ongoing conversion has finished. The ADC then operates in Free Running mode, constantly sampling
and updating the ADC Data Register. The first conversion must be started by writing a '1' to
ADCSRA.ADSC. In this mode the ADC will perform successive conversions independently of whether the
ADC Interrupt Flag (ADIF) is cleared or not.
If Auto Triggering is enabled, single conversions can be started by writing ADCSRA.ADSC to '1'. ADSC
can also be used to determine if a conversion is in progress. The ADSC bit will be read as '1' during a
conversion, independently of how the conversion was started.
25.4. Prescaling and Conversion Timing
Figure 25-3. ADC Prescaler
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
CK
ADPS0
ADPS1
ADPS2
CK/128
CK/2 CK/4 CK/8
CK/16
CK/32
CK/64
Reset
ADEN
START
By default, the successive approximation circuitry requires an input clock frequency between 50kHz and
200kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the input clock frequency
to the ADC can be higher than 200kHz to get a higher sample rate.
The ADC module contains a prescaler, which generates an acceptable ADC clock frequency from any
CPU frequency above 100kHz. The prescaling is selected by the ADC Prescaler Select bits in the ADC
Control and Status Register A (ADCSRA.ADPS). The prescaler starts counting from the moment the ADC
Atmel ATmega644A [DATASHEET]
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