ATTINY25_07 Datasheet, PDF (123/201 Pages) ATMEL Corporation – 8-bit Microcontroller with 2/4/8K Bytes In-System Programmable Flash

ATtiny25/45/85 Auto
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 voltage 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:
a. When ADATE or ADEN is cleared.
b. During conversion, minimum one ADC clock cycle after the trigger event.
c. 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.
19.5.1
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.
19.5.2
ADC Voltage Reference
The voltage reference 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. V_REF can be selected as
either V_CC, or internal 1.1V / 2.56V voltage reference, or external AREF pin. The first ADC con-
version result after switching voltage reference source may be inaccurate, and the user is
advised to discard this result.
19.6
ADC Noise Canceler
The ADC features a noise canceler that enables conversion during sleep mode to reduce noise
induced from the CPU core and other I/O peripherals. The noise canceler can be used with ADC
Noise Reduction and Idle mode. To make use of this feature, the following procedure should be
used:
a. Make sure that the ADC is enabled and is not busy converting. Single Conversion
mode must be selected and the ADC conversion complete interrupt must be
enabled.
b. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion
once the CPU has been halted.
c. If no other interrupts occur before the ADC conversion completes, the ADC inter-
rupt will wake up the CPU and execute the ADC Conversion Complete interrupt
7598D–AVR–02/07
123