ATA8742 ATMEL Corporation, ATA8742 Datasheet - Page 158

• Manufacturer Part Number: ATA8742
• Manufacturer: ATMEL Corporation

158
ATA8742
The prescaler starts counting from the moment the ADC is switched on by 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 conversion
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.
The actual sample-and-hold takes place 1.5 ADC clock cycles after the start of a normal conver-
sion and 14.5 ADC clock cycles after the start of an first conversion. When a conversion 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 addi-
tional CPU clock cycles are used for synchronization logic.
In Free Running mode, a new conversion will be started immediately after the conversion com-
pletes, while ADSC remains high. For a summary of conversion times, see
159.
Figure 25-4. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Figure 25-5. ADC Timing Diagram, Single Conversion
Cycle Number
ADC Clock
ADSC
ADIF
ADCH
ADCL
Cycle Number
ADC Clock
ADEN
ADSC
ADIF
ADCH
ADCL
1
1
2
MUX and REFS
Update
2
MUX and REFS
Update
12
3
13
Sample & Hold
4
14
5
15
6
Sample & Hold
16
First Conversion
17
7
One Conversion
18
8
19
9
20
10
Conversion
Complete
21
11
22
Conversion
Complete
23
12
24
13
25
Sign and MSB of Result
Sign and MSB of Result
LSB of Result
Table 25-1 on page
Next Conversion
1
Next
Conversion
1
LSB of Result
9151A–INDCO–07/09
2
2
MUX and REFS
Update
MUX and REFS
Update
3
3