Datasheet

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.
26.4. Prescaling and Conversion Timing
Figure 26-3. ADC Prescaler
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
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 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. See Differential Gain Channels 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 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 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 two
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 that 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.
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 table ADC Conversion Time at the
end of this section.
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
261