Rainbow Electronics ATtiny10 - Manual (Page 88)

88
8127B–AVR–08/09
ATtiny4/5/9/10
13.6 Changing Channel
The MUXn bits in the ADMUX Register are single buffered through a temporary register to which
the CPU has random access. This ensures that the channel selection only takes place at a safe
point during the conversion. The channel is continuously updated until a conversion is started.
Once the conversion starts, the channel selection is locked to ensure a sufficient sampling time
for the ADC. Continuous 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 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.
13.6.1 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
channel 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
channel 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.
13.6.2 ADC Voltage Reference
The reference voltage of the ADC determines the conversion range, which in this case is limited
to 0V (V
GND
) and V
REF
= V
cc
. Channels that exceed V
REF
will result in codes saturated at 0xFF.
13.7 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: