350
15.5
Interrupts
The A/D converter generates an interrupt (ADI) at the end of A/D conversion. The ADI interrupt
request can be enabled or disabled by the ADIE bit in ADCSR.
15.6
Application Notes
When using the A/D converter, note the following points.
Setting Ranges of Analog Power Supply and Other Pins:
1. Analog Input Voltage Range
During A/D conversion, the voltages input to the analog input pins AN
n
should be in the range
AV
SS
≤
AN
n
≤
AV
ref
. (n = 0 to 7)
2. AV
CC
and AV
SS
Input Voltages
AV
SS
should be equal to V
SS
. If the A/D converter is not used, the values should be AV
CC
= V
CC
and AV
SS
= V
SS
3. AV
ref
Input Range
The analog reference voltage input at the AV
ref
pin should be in the range AV
ref
≤
AV
CC
. If the
A/D converter is not used, the value should be AV
ref
= V
CC
.
Notes on Board Design:
In board design, digital circuitry and analog circuitry should be as
mutually isolated as possible, and layout in which digital circuit signal lines and analog circuit
signal lines cross or are in close proximity should be avoided as far as possible. Failure to do so
may result in incorrect operation of the analog circuitry due to inductance, adversely affecting A/D
conversion values.
Also, digital circuitry must be isolated from the analog input signals (AN
0
to AN
7
), analog
reference voltage (AV
ref
), and analog power supply (AV
CC
) by the analog ground (AV
SS
). The
analog ground (AV
SS
) should be connected to a stable digital ground (V
SS
) at one point on the
board.
Notes on Noise Countermeasures:
A protection circuit connected to prevent damage due to an
abnormal voltage such as an excessive surge at the analog input pins (AN
0
to AN
7
) or analog
reference power supply pin (AV
ref
) should be connected between AV
CC
and AV
SS
as shown in
figure 15.7.
Also, the bypass capacitors connected to AV
CC
, AV
ref
and the filter capacitor connected to AN
0
to
AN
7
must be connected to AV
SS
.
If a filter capacitor is connected as shown in figure 15.7, the input currents at the analog input pins
(AN
0
to AN
7
) are averaged, and so an error may arise. Also, when A/D conversion is performed
frequently, as in scan mode, if the current charged and discharged by the capacitance of the
sample-and-hold circuit in the A/D converter exceeds the current input via the input impedance