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CHAPTER 11 A/D CONVERTER
User
’
s Manual U15017EJ2V0UD
Figure 11-8. Processing of Analog Input Pin
(5) ANI0 through ANI11
The analog input pins (ANI0 to ANI11) are multiplexed with port pins (P00 to P03 and P11 to P17).
When one of ANI0 to ANI11 is selected for A/D conversion, do not execute an instruction that inputs data to
the port during the conversion. If such an instruction is executed, the conversion resolution may drop.
When a digital pulse is applied to a pin adjacent to the analog input pins during A/D conversion, the expected
A/D conversion value may not be obtained because of coupling noise. Therefore, do not apply a pulse to the
pins adjacent to the analog input pins during A/D conversion.
(6) Input impedance of AV
DD
pin
The reference voltage source pin is also used as the AV
DD
pin. A series resistor string with a resistance of about
21.4 k
is connected between the AV
DD
and AV
SS
pins.
If the output impedance of the reference voltage source is high, therefore, the impedance is virtually connected
in series with the resistor string between the AV
DD
and AV
SS
pins, increasing the error of the reference voltage.
(7) Interrupt request flag (ADIF)
The interrupt request flag (ADIF) is not cleared even if the contents of the A/D converter input select register
(ADIS) are changed.
If the analog input pin is changed during A/D conversion, therefore, the A/D conversion result of the old analog
input may be written to ADIS immediately before ADIS is rewritten, and consequently, the conversion end
interrupt flag may be set. If the ADIF is read immediately after ADIS has been rewritten, ADIF may be set
despite that the A/D conversion of the new analog input has not been completed.
Before resuming A/D conversion that has been stopped, clear ADIF.
Reference voltage input
C = 100 to 1,000 pF
Clamp with diode with low V
F
(0.3 V or less) if there is possibility
that noise greater than AV
DD
and less than AV
SS
is superimposed.
V
DD1
AV
SS
V
SS1
AV
DD