Datasheet

196
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
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.
23.5 Prescaling and Conversion Timing
Figure 23-3. 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 fre-
quency 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 follow-
ing rising edge of the ADC clock cycle. See “Differential Gain Channels” on page 214 for details on differential
conversion timing.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is switched on (ADEN in ADC-
SRA 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 a 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 2 ADC clock cycles after
the rising edge on the trigger source signal. Three additional CPU clock cycles are used for synchronization logic.
When using Differential mode, along with Auto Trigging from a source other than the ADC Conversion Complete,
each conversion will require 25 ADC clocks. This is because the ADC must be disabled and re-enabled after every
conversion.
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