Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 245
The ADC is enabled by setting the ADC Enable bit, ADEN in ADCSRA. Voltage reference 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
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. 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.
23.4 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 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 trigger a new conversion at the
next interrupt event.
Figure 23-2. ADC auto trigger logic
Using the ADC Interrupt Flag as a trigger source makes the ADC start a new conversion as soon as the ongoing
conversion has finished. The ADC then operates in Free Running mode, constantly sampling and updating the
ADC Data Register. The first conversion must be started by writing a logical one to the ADSC bit in ADCSRA. In
ADSC
ADIF
SOURCE 1
SOURCE n
ADTS[2:0]
CONVERSION
LOGIC
PRESCALER
START
CLK
ADC
.
.
.
.
EDGE
DETECTOR
ADATE