M38261ECFS datasheet(82/91 Pages) RENESAS

82 / 91 page

Rev.1.00

Sep 06, 2006

page 82 of 88

REJ03B0181-0100

3826 Group (One Time PROM version)

3.3.9 Notes on A/D converter

(1)

Analog input pin

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.

● 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)

Analog power source input pin AVss

The AVSS pin is an analog power source input pin. Regardless of using the A/D conversion

function or not, connect it as following :

• AVSS : Connect to the VSS line

● Reason

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

(3)

Reference voltage input pin VREF

Connect an approximately 1000 pF capacitor across the AVss pin and the VREF pin. Besides,

connect the capacitor across the VREF pin and the AVss pin at equal length as close as possible.

(4)

Clock frequency during A/D conversion

Use the A/D converter in the following conditions:

• Select XIN-XOUT as system clock φ by the system clock selection bit (bit 7 of CPU mode register

(address 3B16)). When selecting XCIN-XCOUT as system clock φ, the A/D conversion function cannot

be used.

• f(XIN) is 500 kHz or more.

• Do not execute the STP or WIT instruction during A/D conversion.

● Reason

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

clock frequency is too low. This may cause the A/D conversion precision to be worse.

(5)

When the falling edge is input to the ADT pin during A/D conversion at the time of A/D external

trigger effective, the conversion processing is interrupted and the A/D conversion starts again. In

addition, even if “0” is set to the AD conversion completion bit by the program during A/D conversion,

re-conversion is not performed but the original conversion is continued.

(6)

The A/D converter will not operate normally if one of the following operation is applied during the

A/D conversion:

•Writing to CPU mode register

•Writing to AD control register

•Executing the STP instruction and WIT instruction

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