
549
19.4.2
Scan Mode (SCAN = 1)
Scan mode is useful for monitoring analog inputs in a group of one or more channels. When the
ADST bit is set to 1 by software, or by timer or external trigger input, A/D conversion starts on
the first channel in the group (AN0 when CH2 = 0; AN4 when CH2 = 1). When two or more
channels are selected, after conversion of the first channel ends, conversion of the second
channel (AN1 or AN5) starts immediately. A/D conversion continues cyclically on the selected
channels until the ADST bit is cleared to 0. The conversion results are transferred for storage
into the ADDR registers corresponding to the channels.
When the operating mode or analog input channel must be changed during analog conversion, to
prevent incorrect operation, first clear the ADST bit to 0 in ADCSR to halt A/D conversion.
After making the necessary changes, set the ADST bit to 1 to start A/D conversion again. The
ADST bit can be set at the same time as the operating mode or input channel is changed.
Typical operations when three channels (AN0 to AN2) are selected in scan mode are described
next. Figure 19.4 shows a timing diagram for this example.
1. Scan mode is selected (SCAN = 1), scan group 0 is selected (CH2 = 0), analog input
channels AN0 to AN2 are selected (CH1 = 1, CH0 = 0), and A/D conversion is started
(ADST = 1)
2. When A/D conversion of the first channel (AN0) is completed, the result is transferred to
ADDRA. Next, conversion of the second channel (AN1) starts automatically.
3. Conversion proceeds in the same way through the third channel (AN2).
4. When conversion of all the selected channels (AN0 to AN2) is completed, the ADF flag is
set to 1 and conversion of the first channel (AN0) starts again. If the ADIE bit is set to 1 at
this time, an ADI interrupt is requested after A/D conversion ends.
5. Steps 2 to 4 are repeated as long as the ADST bit remains set to 1. When the ADST bit is
cleared to 0, A/D conversion stops. After that, if the ADST bit is set to 1, A/D conversion
starts again from the first channel (AN0).