
CHAPTER 9 A/D CONVERTER
119
(4) Noise measures
To maintain the 8-bit resolution, care must be exercised that no noise is superimposed on the AV
DD
and ANI0
through ANI7 pins. The higher the output impedance of the analog input source, the heavier the influence of
noise. To suppress noise, connecting external C as shown in Figure 9-8 is recommended.
Figure 9-8. Processing of Analog Input Pin
(5) ANI0 through ANI7
The analog input pins (ANI0 through ANI7) are multiplexed with port pins (P10 through P17).
When one of ANI0 through ANI7 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
A series resistor string with a resistance of about 24.7 k
W
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 parallel 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 analog input channel specification
register (ADS0) 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 ADS0 immediately before ADS0 is rewritten, and consequently, the conversion end
interrupt flag may be set. If ADIF is read immediately after ADS0 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 = 100-1000 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