Datasheet

301
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in
ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conversion and 13.5
ADC clock cycles after the start of an first conversion. When a conversion is complete, the result is written to the
ADC Data Registers, and ADIF is set. In Single Conversion mode, ADSC is cleared simultaneously. The
software may then set ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This assures a fixed delay
from the trigger event to the start of conversion. In this mode, the sample-and-hold takes place two ADC clock
cycles after the rising edge on the trigger source signal. Three additional CPU clock cycles are used for
synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conversion completes, while
ADSC remains high. For a summary of conversion times, see the table below.
Figure 24-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Figure 24-5. ADC Timing Diagram, Single Conversion
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
Sample & Hold
ADIF
ADCH
ADCL
Cycle Number
ADEN
1 212
13
14 15
16 17
18 19 20 21 22 23
24 25
1 2
First Conversion
Next
Conversion
3
MUX and REFS
Update
MUX
and REFS
Update
Conversion
Complete
1
2 3 4 5 6 7 8
9 10 11 12 13
Sign and MSB of Result
LSB of Result
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number
12
One Conversion Next Conversion
3
Sample & Hold
MUX and REFS
Update
Conversion
Complete
MUX and REFS
Update