ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 251

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

24.4
9223B–AVR–09/11
Prescaling and Conversion Timing
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 logical one to the ADSC bit in ADCSRA. In this mode the ADC will perform suc-
cessive 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.
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.
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 contin-
uously 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.
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 ana-
log 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.
Atmel ATmega48PA/88PA/168PA [Preliminary]
ADEN
START
ADPS0
ADPS1
ADPS2
CK
Reset
7-BIT ADC PRESCALER
ADC CLOCK SOURCE
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