Datasheet

V850ES/JG3 CHAPTER 13 A/D CONVERTER
R01UH0015EJ0300 Rev.3.00 Page 436 of 870
Sep 30, 2010
(7) AV
REF0 pin
(a) The AV
REF0 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
V
DD to the AVREF0 pin as shown in Figure 13-15.
(b) The AV
REF0 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 AV
REF0 and AVSS pins to suppress the
reference voltage fluctuation as shown in Figure 13-15.
(c) If the source supplying power to the AV
REF0 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. AV
REF0 Pin Processing Example
AV REF0
Note
AV
SS
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.