ATtiny25 datasheet(115/196 Pages) ATMEL

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115

7598H–AVR–07/09

ATtiny25/45/85

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

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 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 conver-

sion and 14.5 ADC clock cycles after the start of an 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 addi-

tional CPU clock cycles are used for synchronization logic.

In Free Running mode, a new conversion will be started immediately after the conversion com-

pletes, while ADSC remains high. For a summary of conversion times, see Table 18-1.

Figure 18-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)

Sign and MSB of Result

LSB of Result

ADC Clock

ADSC

Sample & Hold

ADIF

ADCH

ADCL

Cycle Number

ADEN

212

First Conversion

Conversion

MUX and REFS

Update

MUX and REFS

Update

Conversion

Complete

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