Datasheet

210
7734Q–AVR–02/12
AT90PWM81/161
• 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
17.5.2 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. V
REF
can be selected as
either AV
CC
, internal 2.56V reference, or external AREF pin.
AV
CC
is connected to the ADC through a passive switch. The internal 2.56V reference is gener-
ated from the internal bandgap reference (V
BG
) through an internal amplifier. If the external
AREF pin is connected to the ADC, 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.
The user may switch between AV
CC,
AREF pin and 2.56V as reference selection. The first ADC
conversion result after switching reference voltage source may be inaccurate, and the user is
advised to discard this result.
17.6 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 the ADNCDIS bit is reset
b. Make sure the ADATE bit is reset
c. Make sure that the ADC is enabled and is not busy converting (ADSC reset). Single
Conversion mode must be selected and the ADC conversion complete interrupt
must be enabled
d. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion
once the CPU has been halted
e. 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
routine. If another interrupt wakes up the CPU before the ADC conversion is com-
plete, that interrupt will be executed, and an ADC Conversion Complete interrupt
request will be generated when the ADC conversion completes. The CPU will
remain in active mode until a new sleep command is executed
Another possible procedure is possible for Auto trigger conversions:
a. Make sure the ADNCDIS bit is set
b. Make sure the ADATE bit is set
c. Enter ADC Noise Reduction mode (or Idle mode). The ADC will start a conversion
on the next triggering event
d. 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
routine. If another interrupt wakes up the CPU before the ADC conversion is com-
plete, that interrupt will be executed, and an ADC Conversion Complete interrupt