CHAPTER 13 A/D CONVERTER
Page 320 of 920
(5) Analog input (ANIn) pins
<1> The analog input pins are also used as input port pins.
When A/D conversion is performed with any of the ANI0 to ANI2, ANI13, ANI14, and ANI19 pins selected,
do not change to output value P20 to P22, P120, P155, and P156 while conversion is in progress;
otherwise the conversion resolution may be degraded.
<2> If a pin adjacent to a pin that is being A/D converted is used as a digital I/O port pin, the A/D conversion
result might differ from the expected value due to a coupling noise. Be sure to prevent such a pulse from
being input or output.
(6) Input impedance of analog input (ANIn) pins
This A/D converter charges a sampling capacitor for sampling during sampling time.
Therefore, only a leakage current flows when sampling is not in progress, and a current that charges the
capacitor flows during sampling. Consequently, the input impedance fluctuates depending on whether sampling
is in progress, and on the other states.
To make sure that sampling is effective, however, we recommend using the converter with analog input sources
that have output impedances no greater than 1 k
Ω
. If a source has a higher output impedance, lengthen the
sampling time or connect a larger capacitor (with a value of about 0.1
μ
F) to the pin from among ANI0 to ANI2,
ANI13, ANI14, and ANI19 to which the source is connected (see
). The sampling capacitor may be
being charged while the setting of the ADCS bit is 0 and immediately after sampling is restarted and so is not
defined at these times. Accordingly, the state of conversion is undefined after charging starts in the next round of
conversion after the value of the ADCS bit has been 1 or when conversion is repeated. Thus, to secure full
charging regardless of the size of fluctuations in the analog signal, ensure that the output impedances of the
sources of analog inputs are low or secure sufficient sampling time for the completion of conversion.
(7) 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 flag for
the pre-change analog input may be set just before the ADS register rewrite. Caution is therefore required since,
at this time, when ADIF flag is read immediately after the ADS register rewrite, ADIF flag 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 flag before the A/D conversion operation is
resumed.
Summary of Contents for RL78/G1H
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