Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
241
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
24.4
Prescaling and Conversion Timing
Figure 24-3.
ADC Prescaler
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.
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 set by the ADPS bits in ADCSRA. The prescaler starts counting
from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler keeps running for
as long as the ADEN bit is set, and is continuously reset when ADEN is low.
frequency above 100kHz. The prescaling is set by the ADPS bits in ADCSRA. The prescaler starts counting
from the moment the ADC is switched on by setting the ADEN bit in ADCSRA. The prescaler keeps running for
as long as the ADEN bit is set, and is continuously reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the conversion starts at the
following rising edge of the ADC clock cycle.
following rising edge of the ADC clock cycle.
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.
ADCSRA is set) takes 25 ADC clock cycles in order to initialize the analog circuitry.
When the bandgap reference voltage is used as input to the ADC, it will take a certain time for the voltage to
stabilize. If not stabilized, the first value read after the first conversion may be wrong.
stabilize. If not stabilized, the first value read after the first conversion may be wrong.
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.
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.
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
ADSC remains high. For a summary of conversion times, see
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
START