18.2 Operation
The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The minimum value
represents GND and the maximum value represents the voltage on the AREF pin minus 1 LSB. Optionally, AVCC or an
internal 2.56V reference voltage may be connected to the AREF pin by writing to the REFSn bits in the ADMUX Register.
The internal voltage reference may thus be decoupled by an external capacitor at the AREF pin to improve noise immunity.
The analog input channel are selected by writing to the MUX bits in ADMUX. Any of the ADC input pins, as well as GND and
a fixed bandgap voltage reference, can be selected as single ended inputs to the ADC.
The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference is set by the REFS1 and REFS0
bits in ADMUX register, whatever the ADC is enabled or not. The ADC does not consume power when ADEN is cleared, so
it is recommended to switch off the ADC before entering power saving sleep modes.
The ADC generates a 10-bit result which is presented in the ADC Data Registers, ADCH and ADCL. By default, the result is
presented right adjusted, but can optionally be presented left adjusted by setting the ADLAR bit in ADMUX.
If the result is left adjusted and no more than 8-bit precision is required, it is sufficient to read ADCH. Otherwise, ADCL must
be read first, then ADCH, to ensure that the content of the Data Registers belongs to the same conversion. Once ADCL is
read, ADC access to Data Registers is blocked. This means that if ADCL has been read, and a conversion completed before
ADCH is read, neither register is updated and the result from the conversion is lost. When ADCH is read, ADC access to the
ADCH and ADCL Registers is re-enabled.
The ADC has its own interrupt which can be triggered when a conversion completes. The ADC access to the Data Registers
is prohibited between reading of ADCH and ADCL, the interrupt will trigger even if the result is lost.
18.3 Starting a Conversion
A single conversion is started by writing a logical one to the ADC start conversion bit, ADSC. This bit stays high as long as
the conversion is in progress and will be cleared by hardware when the conversion is completed. If a different data channel
is selected while a conversion is in progress, the ADC will finish the current conversion before performing the channel
change.
Alternatively, a conversion can be triggered automatically by various sources. Auto triggering is enabled by setting the ADC
Auto trigger enable bit, ADATE in ADCSRA. The trigger source is selected by setting the ADC trigger select bits, ADTS in
ADCSRB (See description of the ADTS bits for a list of the trigger sources). When a positive edge occurs on the selected
trigger signal, the ADC prescaler is reset and a conversion is started. This provides a method of starting conversions at fixed
intervals. If the trigger signal is still set when the conversion completes, a new conversion will not be started. If another
positive edge occurs on the trigger signal during conversion, the edge will be ignored. Note that an interrupt flag will be set
even if the specific interrupt is disabled or the Global Interrupt Enable bit in SREG is cleared. A conversion can thus be
triggered without causing an interrupt. However, the interrupt flag must be cleared in order to trigger a new conversion at the
next interrupt event.
Figure 18-2. ADC Auto Trigger Logic
ADTS[2:0]
ADC Prescaler
START
CLKADC
ADIF
ADATE
SOURCE 1
.
.
.
.
Conversion
Logic
Edge
Detector
SOURCE n
ADSC
ATmega16/32/64/M1/C1 [DATASHEET]
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MICROCHIP [ MICROCHIP ]
