38C3 datasheet(129/224 Pages) RENESAS

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2-64

38C3 Group User’s Manual

APPLICATION

2.5 A-D converter

2.5.4 Notes on A-D converter

(1)

Analog input pin

s Make the signal source impedance for analog input low, or equip an analog input pin with an

external capacitor of 0.01

µF to 1 µF. Further, be sure to verify the operation of application

products on the user side.

q Reason

An analog input pin includes the capacitor for analog voltage comparison. Accordingly, when

signals from signal source with high impedance are input to an analog input pin, charge and

discharge noise generates. This may cause the A-D conversion precision to be worse.

(2)

A-D converter power source pin

The AVSS pin is A-D converter power source pin. Regardless of using the A-D conversion function

or not, connect it as following :

• AVSS : Connect to the VSS line.

q Reason

If the AVSS pin is opened, the microcomputer may have a failure because of noise or others.

(3)

Clock frequency during A-D conversion

The comparator consists of a capacity coupling, and a charge of the capacity will be lost if the clock

frequency is too low. Thus, make sure the following during an A-D conversion.

• f(XIN) is 500 kHz or more.

• Use clock divided by main clock (f(XIN)) as internal system clock.

• Do not execute the STP instruction and WIT instruction.

38C3 Datasheet(PDF) 129 Page - Renesas Technology Corp