APPENDIX D LIST OF CAUTIONS
User’s Manual U16899EJ2V0UD
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(13/24)
Chapter
Cl
assi
fi
cati
on
Function Details
of
Function
Cautions Page
Noise
countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to the
AV
REF
pin and pins ANI0 to ANI7.
Because the effect increases in proportion to the output impedance of the analog
input source, it is recommended that a capacitor be connected externally, as
shown in Figure 12-19, to reduce noise.
p. 262
The analog input pins (ANI0 to ANI7) are also used as input port pins (P20 to
P27).
When A/D conversion is performed with any of ANI0 to ANI7 selected, do not
access port 2 while conversion is in progress; otherwise the conversion resolution
may be degraded.
p. 263
ANI0/P20 to
ANI7/P27
If a digital pulse is applied to the pins adjacent to the pins currently used for A/D
conversion, the expected value of the A/D conversion may not be obtained due to
coupling noise. Therefore, do not apply a pulse to the pins adjacent to the pin
undergoing A/D conversion.
p. 263
Input
impedance of
ANI0 to ANI7
pins
In this A/D converter, the internal sampling capacitor is charged and sampling is
performed for approx. one sixth of the conversion time.
Since only the leakage current flows other than during sampling and the current
for charging the capacitor also flows during sampling, the input impedance
fluctuates and has no meaning.
To perform sufficient sampling, however, it is recommended to make the output
impedance of the analog input source 10 k
Ω
or lower, or connect a capacitor of
around 100 pF to the ANI0 to ANI7 pins (see Figure 12-19).
p. 263
Hard
AV
REF
pin input
impedance
A series resistor string of several tens of k
Ω
is connected between the AV
REF
and
AV
SS
pins.
Therefore, if the output impedance of the reference voltage source is high, this
will result in a series connection to the series resistor string between the AV
REF
and AV
SS
pins, resulting in a large reference voltage error.
p. 263
Interrupt
request flag
(ADIF)
The interrupt request flag (ADIF) is not cleared even if the analog input channel
specification register (ADS) is changed.
Therefore, if an analog input pin is changed during A/D conversion, the A/D
conversion result and ADIF for the pre-change analog input may be set just
before the ADS rewrite. Caution is therefore required since, at this time, when
ADIF is read immediately after the ADS rewrite, ADIF is set despite the fact A/D
conversion for the post-change analog input has not ended.
When A/D conversion is stopped and then resumed, clear ADIF before the A/D
conversion operation is resumed.
p. 264
Conversion
result just after
A/D conversion
start
The A/D conversion value immediately after A/D conversion starts may not fall
within the rating range if the ADCS bit is set to 1 within 14
µ
s after the ADCE bit
was set to 1, or if the ADCS bit is set to 1 with the ADCE bit = 0. Take measures
such as polling the A/D conversion end interrupt request (INTAD) and removing
the first conversion result.
p. 264
Chapter 12
Soft
A/D
converter
A/D conversion
result register
(ADCR) read
operation
When a write operation is performed to the A/D converter mode register (ADM)
and analog input channel specification register (ADS), the contents of ADCR may
become undefined. Read the conversion result following conversion completion
before writing to ADM and ADS. Using a timing other than the above may cause
an incorrect conversion result to be read.
p. 264