Datasheet

ATtiny828 [DATASHEET]

8371A–AVR–08/12

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

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.

15.5 Prescaling and Conversion Timing

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. It is not recommended to use a higher input clock frequency than 1 MHz.

Figure 55. ADC Prescaler

The ADC module contains a prescaler, as illustrated in Figure 55 on page 140, 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 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.

A normal conversion takes 15 ADC clock cycles, as summarised in Table 49 on page 143. The first conversion after the

ADC is switched on (ADEN in ADCSRA is set) takes 26 ADC clock cycles in order to initialize the analog circuitry, as

shown in Figure 56 below.

7-BIT ADC PRESCALER

ADC CLOCK SOURCE

ADPS0

ADPS1

ADPS2

CK/128

CK/2

CK/4

CK/8

CK/16

CK/32

CK/64

Reset

ADEN

START