ATmega32(L)
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. See “Differential Gain
Channels” on page 207 for details on differential conversion timing.
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
conversion and 13.5 ADC clock cycles after the start of a 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 2 ADC clock cycles after the rising edge on the trigger
source signal. Three additional CPU clock cycles are used for synchronization logic.
When using Differential mode, along with Auto Trigging from a source other than the
ADC Conversion Complete, each conversion will require 25 ADC clocks. This is
because the ADC must be disabled and re-enabled after every conversion.
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 81.
Figure 101. ADC Timing Diagram, First Conversion (Single Conversion Mode)
Next
First Conversion
Conversion
Cycle Number
1
2
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1
2
3
ADC Clock
ADEN
ADSC
ADIF
MSB of Result
LSB of Result
ADCH
ADCL
MUX and REFS
Update
Conversion
Complete
Sample & Hold
MUX and REFS
Update
205
2503J–AVR–10/06
ATMEL [ ATMEL ]
