ATmega88 datasheet(243/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

21.4

Changing Channel or Reference Selection

The MUXn and REFS1:0 bits in the ADMUX Register are single buffered through a temporary

selection only takes place at a safe point during the conversion. The channel and reference

selection is continuously updated until a conversion is started. Once the conversion starts, the

channel and reference selection is locked to ensure a sufficient sampling time for the ADC. Con-

tinuous updating resumes in the last ADC clock cycle before the conversion completes (ADIF in

ADCSRA is set). Note that the conversion starts on the following rising ADC clock edge after

ADSC is written. The user is thus advised not to write new channel or reference selection values

to ADMUX until one ADC clock cycle after ADSC is written.

If Auto Triggering is used, the exact time of the triggering event can be indeterministic. Special

care must be taken when updating the ADMUX Register, in order to control which conversion

will be affected by the new settings.

If both ADATE and ADEN is written to one, an interrupt event can occur at any time. If the

ADMUX Register is changed in this period, the user cannot tell if the next conversion is based

on the old or the new settings. ADMUX can be safely updated in the following ways:

When ADATE or ADEN is cleared.

During conversion, minimum one ADC clock cycle after the trigger event.

After a conversion, before the Interrupt Flag used as trigger source is cleared.

When updating ADMUX in one of these conditions, the new settings will affect the next ADC

conversion.

21.4.1

ADC Input Channels

When changing channel selections, the user should observe the following guidelines to ensure

that the correct channel is selected:

In Single Conversion mode, always select the channel before starting the conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the conversion to complete before changing the channel selection.

In Free Running mode, always select the channel before starting the first conversion. The chan-

nel selection may be changed one ADC clock cycle after writing one to ADSC. However, the

simplest method is to wait for the first conversion to complete, and then change the channel

selection. Since the next conversion has already started automatically, the next result will reflect

the previous channel selection. Subsequent conversions will reflect the new channel selection.

21.4.2

ADC Voltage Reference

The reference voltage for the ADC (V

REF) indicates the conversion range for the ADC. Single

ended channels that exceed V

REF will result in codes close to 0x3FF. VREF can be selected as

either AV

CC, internal 1.1V reference, or external AREF pin.

CC is connected to the ADC through a passive switch. The internal 1.1V reference is gener-

ated from the internal bandgap reference (V

BG) through an internal amplifier. In either case, the

external AREF pin is directly connected to the ADC, and the reference voltage can be made

more immune to noise by connecting a capacitor between the AREF pin and ground. V

REF can

also be measured at the AREF pin with a high impedant voltmeter. Note that V

REF is a high

impedant source, and only a capacitive load should be connected in a system.

ATmega88 Datasheet(PDF) 243 Page - ATMEL Corporation

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