ATmega640/1280/1281/2560/2561
Table 26-1. ADC Conversion Time
Sample & Hold (Cycles from
Start of Conversion)
Condition
Conversion Time (Cycles)
First conversion
13.5
1.5
25
13
Normal conversions, single ended
Auto Triggered conversions
Normal conversions, differential
2
13.5
13/14
1.5/2.5
26.4.1
Differential Channels
When using differential channels, certain aspects of the conversion need to be taken into
consideration.
Differential conversions are synchronized to the internal clock CKADC2 equal to half the ADC
clock. This synchronization is done automatically by the ADC interface in such a way that the
sample-and-hold occurs at a specific phase of CKADC2. A conversion initiated by the user (i.e., all
single conversions, and the first free running conversion) when CKADC2 is low will take the same
amount of time as a single ended conversion (13 ADC clock cycles from the next prescaled
clock cycle). A conversion initiated by the user when CKADC2 is high will take 14 ADC clock
cycles due to the synchronization mechanism. In Free Running mode, a new conversion is initi-
ated immediately after the previous conversion completes, and since CKADC2 is high at this time,
all automatically started (i.e., all but the first) Free Running conversions will take 14 ADC clock
cycles.
If differential channels are used and conversions are started by Auto Triggering, the ADC must
be switched off between conversions. When Auto Triggering is used, the ADC prescaler is reset
before the conversion is started. Since the stage is dependent of a stable ADC clock prior to the
conversion, this conversion will not be valid. By disabling and then re-enabling the ADC between
each conversion (writing ADEN in ADCSRA to “0” then to “1”), only extended conversions are
performed. The result from the extended conversions will be valid. See “Prescaling and Conver-
sion Timing” on page 278 for timing details.
281
2549L–AVR–08/07
ATMEL [ ATMEL ]
