ATmega324P datasheet(246/377 Pages) ATMEL

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7674F–AVR–09/09

ATmega164P/324P/644P

Figure 21-2. ADC Auto Trigger Logic

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, con-

stantly sampling and updating the ADC Data Register. The first conversion must be started by

writing a logical one to the ADSC bit in ADCSRA. 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 ADSC in ADCSRA to

one. ADSC can also be used to determine if a conversion is in progress. The ADSC bit will be

read as one during a conversion, independently of how the conversion was started.

21.5

Prescaling and Conversion Timing

Figure 21-3. ADC Prescaler

By default, the successive approximation circuitry requires an input clock frequency between 50

kHz and 200 kHz to get maximum resolution. If a lower resolution than 10 bits is needed, the

input clock frequency to the ADC can be higher than 200 kHz to get a higher sample rate.

The ADC module contains a prescaler, which generates an acceptable ADC clock frequency

from any CPU frequency above 100 kHz. 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

ADSC

ADIF

SOURCE 1

SOURCE n

ADTS[2:0]

CONVERSION

LOGIC

PRESCALER

START

CLK

ADC

EDGE

DETECTOR

ADATE

7-BIT ADC PRESCALER

ADC CLOCK SOURCE

ADPS0

ADPS1

ADPS2

Reset

ADEN

START

ATmega324P Datasheet(PDF) 246 Page - ATMEL Corporation

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