ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 281

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Operation
Starting a Conversion
2549K–AVR–01/07
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 1.1V or
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 is selected by writing to the MUX bits in ADMUX and ADC-
SRB. 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. A selection of ADC input pins can be
selected as positive and negative inputs to the differential amplifier.
If differential channels are selected, the voltage difference between the selected input
channel pair then becomes the analog input to the ADC. If single ended channels are
used, the amplifier is bypassed altogether.
The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage refer-
ence and input channel selections will not go into effect until ADEN is set. 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
completes before ADCH is read, neither register is updated and the result from the con-
version 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.
When ADC access to the Data Registers is prohibited between reading of ADCH and
ADCL, the interrupt will trigger even if the result is lost.
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 Trig-
gering 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 still is 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 trig-
ger a new conversion at the next interrupt event.
ATmega640/1280/1281/2560/2561
281