
CHAPTER 13 A/D CONVERTER
User’s Manual U17790EJ2V0UD
492
(7) AVREF0 pin
(a) The AVREF0 pin is used as the power supply pin of the A/D converter and also supplies power to the
alternate-function ports. In an application where a backup power supply is used, be sure to supply the
same voltage as VDD to the AVREF0 pin as shown in Figure 13-15.
(b) The AVREF0 pin is also used as the reference voltage pin of the A/D converter. If the source supplying
power to the AVREF0 pin has a high impedance or if the power supply has a low current supply capability,
the reference voltage may fluctuate due to the current that flows during conversion (especially, immediately
after the conversion operation enable bit ADA0CE has been set to 1). As a result, the conversion accuracy
may drop. To avoid this, it is recommended to connect a capacitor across the AVREF0 and AVSS pins to
suppress the reference voltage fluctuation as shown in Figure 13-15.
(c) If the source supplying power to the AVREF0 pin has a high DC resistance (for example, because of
insertion of a diode), the voltage when conversion is enabled may be lower than the voltage when
conversion is stopped, because of a voltage drop caused by the A/D conversion current.
Figure 13-15. AVREF0 Pin Processing Example
AVREF0
Note
AVSS
Main power supply
Note Parasitic inductance
(8) Reading ADA0CRn register
When the ADA0M0 to ADA0M2, ADA0S, ADA0PFM, or ADA0PFT register is written, the contents of the
ADA0CRn register may be undefined. Read the conversion result after completion of conversion and before
writing to the ADA0M0 to ADA0M2, ADA0S, ADA0PFM, or ADA0PFT register. Also, when an external/timer
trigger is acknowledged, the contents of the ADA0CRn register may be undefined. Read the conversion result
after completion of conversion and before the next external/timer trigger is acknowledged. The correct
conversion result may not be read at a timing different from the above.
(9) Standby mode
Because the A/D converter stops operating in the STOP mode, conversion results are invalid, so power
consumption can be reduced.
Operations are resumed after the STOP mode is released, but the A/D
conversion results after the STOP mode is released are invalid. When using the A/D converter after the STOP
mode is released, before setting the STOP mode or releasing the STOP mode, clear the ADA0M0.ADA0CE bit
to 0 then set the ADA0CE bit to 1 after releasing the STOP mode.
In the IDLE1, IDLE2, or subclock operation mode, operation continues. To lower the power consumption,
therefore, clear the ADA0M0.ADA0CE bit to 0. In the IDLE1 and IDLE2 modes, since the analog input voltage
value cannot be retained, the A/D conversion results after the IDLE1 and IDLE2 modes are released are invalid.
The results of conversions before the IDLE1 and IDLE2 modes were set are valid.
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