ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 283

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

2549K–AVR–01/07
setting the ADEN bit in ADCSRA. The prescaler keeps running for as long as the ADEN
bit is set, and is continuously reset when ADEN is low.
When initiating a single ended conversion by setting the ADSC bit in ADCSRA, the con-
version starts at the following rising edge of the ADC clock cycle.
A normal conversion takes 13 ADC clock cycles. The first conversion after the ADC is
switched on (ADEN in ADCSRA is set) takes 25 ADC clock cycles in order to initialize
the analog circuitry.
When the bandgap reference voltage is used as input to the ADC, it will take a certain
time for the voltage to stabilize. If not stabilized, the first value read after the first conver-
sion may be wrong.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal
conversion and 13.5 ADC clock cycles after the start of an first conversion. When a con-
version is complete, the result is written to the ADC Data Registers, and ADIF is set. In
Single Conversion mode, ADSC is cleared simultaneously. The software may then set
ADSC again, and a new conversion will be initiated on the first rising ADC clock edge.
When Auto Triggering is used, the prescaler is reset when the trigger event occurs. This
assures a fixed delay from the trigger event to the start of conversion. In this mode, the
sample-and-hold takes place two ADC clock cycles after the rising edge on the trigger
source signal. Three additional CPU clock cycles are used for synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conver-
sion completes, while ADSC remains high. For a summary of conversion times, see
Table 126.
Figure 116. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Cycle Number
ADC Clock
ADEN
ADSC
ADIF
ADCH
ADCL
1
2
MUX and REFS
Update
12
ATmega640/1280/1281/2560/2561
13
14
15
Sample & Hold
16
First Conversion
17
18
19
20
21
22
Conversion
Complete
23
24
25
Sign and MSB of Result
Next
Conversion
1
LSB of Result
2
MUX and REFS
Update
3
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