atmega32c1 ATMEL Corporation, atmega32c1 Datasheet - Page 233

• Manufacturer Part Number: atmega32c1
• Description: Atmega32m1 Automotive 8-bit Avr Microcontroller With 32k/64k Bytes In-system Programmable Flash
• Manufacturer: ATMEL Corporation

18.5.1
18.5.2
7647A–AVR–02/08
ADC Input Channels
ADC Voltage Reference
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 updating ADMUX in one of these conditions, the new settings will affect the next ADC
conversion.
When changing channel selections, the user should observe the following guidelines to ensure
that the correct channel is selected:
•
•
•
The reference voltage for the ADC (V
ended channels that exceed V
either AV
AV
ated from the internal bandgap reference (V
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
also be measured at the AREF pin with a high impedant voltmeter. Note that V
impedant source, and only a capacitive load should be connected in a system.
If the user has a fixed voltage source connected to the AREF pin, the user may not use the other
reference voltage options in the application, as they will be shorted to the external voltage. If no
external voltage is applied to the AREF pin, the user may switch between AV
reference selection. The first ADC conversion result after switching reference voltage source
may be inaccurate, and the user is advised to discard this result.
AREF pin is alternate function with ISRC Current Source output. When current source is
selected, the AREF pin is not connected to the internal reference voltage network. See AREFEN
and ISRCEN bits in Section “ADC Control and Status Register B– ADCSRB”, page 244.
CC
1. When ADATE or ADEN is cleared.
2. During conversion, with taking care of the trigger source event, when it is possible.
3. After a conversion, before the interrupt flag used as trigger source is cleared.
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.
In Free Running mode, because the amplifier clear the ADSC bit at the end of an amplified
conversion, it is not possible to use the free running mode, unless ADSC bit is set again by
soft at the end of each conversion.
is connected to the ADC through a passive switch. The internal 2.56V reference is gener-
CC
, internal 2.56V reference, or external AREF pin.
REF
will result in codes close to 0x3FF. V
REF
) indicates the conversion range for the ADC. Single
BG
) through an internal amplifier. In either case, the
ATmega32/64/M1/C1
REF
can be selected as
CC
and 2.56V as
REF
is a high
REF
233
can