NEC mPD789101 User Manual page 146

CHAPTER 11 8-BIT A/D CONVERTER ( µ PD789104, 789124 SUBSERIES)
(6) Noise prevention
To maintain a resolution of 8 bits, watch for noise to the AV
impedance of the analog input source is, the larger the effect by noise. To reduce noise, attach an external
capacitor to the relevant pins as shown in Figure 11-8.
C = 100 to 1000 pF
(7) ANI0 to ANI3
The analog input pins (ANI0 to ANI3) are alternate-function pins. They are used also as port pins (P60 to
P63).
If any of ANI0 to ANI3 has been selected for A/D conversion, do not execute input instructions for the ports;
otherwise the conversion resolution may become lower.
If a digital pulse is applied to a pin adjacent to the analog input pins during A/D conversion, coupling noise
may occur which prevents an A/D conversion result from being attained as expected. Avoid applying a digital
pulse to pins adjacent to the analog input pins during A/D conversion.
(8) Interrupt request flag (ADIF0)
Changing the content of A/D converter mode register 0 (ADM0) does not clear the interrupt request flag
(ADIF0).
If the analog input pins are changed during A/D conversion, therefore, the conversion result and the conversion
end interrupt request flag may reflect the previous analog input immediately before writing to ADM0 occurs.
In this case, ADIF0 may appear to be set if it is read-accessed immediately after ADM0 is write-accessed,
even when A/D conversion has not been completed for the new analog input.
In addition, when A/D conversion is restarted, ADIF0 must be cleared beforehand.
146
Figure 11-10. Analog Input Pin Treatment
If noise not lower than AV
AV
SS
the voltage at the pin by attaching a diode with a
small V
V
DD
AV
DD
AV
SS
V
SS
User's Manual U13045EJ2V0UM00
and ANI0 to ANI3 pins. The higher the output
DD
or not higher than
DD
is likely to come to the AV pin, clamp
DD
(0.3 V or lower).
F