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412
14.5
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.
14.6
Usage Notes
When using the A/D converter, note the following points:
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
≤
V
REF
.
2. Relationships of AV
CC
and AV
SS
to V
CC
and V
SS
AV
CC
, AV
SS
, V
CC
, and V
SS
should be related as follows: AV
SS
= V
SS
. AV
CC
and AV
SS
must not
be left open, even if the A/D converter is not used.
3. V
REF
Programming Range
The reference voltage input at the V
REF
pin should be in the range V
REF
≤
AV
CC
.
4. Note on Board Design
In board layout, separate the digital circuits from the analog circuits as much as possible.
Particularly avoid layouts in which the signal lines of digital circuits cross or closely approach
the signal lines of analog circuits. Induction and other effects may cause the analog circuits to
operate incorrectly, or may adversely affect the accuracy of A/D conversion.
The analog input signals (AN
0
to AN
7
), analog reference voltage (V
REF
), and analog supply
voltage (AV
CC
) must be separated from digital circuits 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.
5. Note on Noise
To prevent damage from surges and other abnormal voltages at the analog input pins (AN
0
to
AN
7
) and analog reference voltage pin (V
REF
), connect a protection circuit like the one in figure
14.7 between AV
CC
and AV
SS
. The bypass capacitors connected to AV
CC
and V
REF
and the filter
capacitors connected to AN
0
to AN
7
must be connected to AV
SS
. If filter capacitors like the
ones in figure 14.7 are connected, the voltage values input to the analog input pins (AN
0
to
AN
7
) will be smoothed, which may give rise to error. Error can also occur if A/D conversion is
frequently performed in scan mode so that the current that charges and discharges the capacitor
in the sample-and-hold circuit of the A/D converter becomes greater than that input to the
analog input pins via input impedance (Rin). The circuit constants should therefore be selected
carefully.
Summary of Contents for H8/3008
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