38D2 datasheet(125/136 Pages) RENESAS

125 / 136 page

Rev.3.02

Apr 10, 2008

Page 125 of 131

REJ03B0177-0302

38D2 Group

Notes on A/D Conversion

1. Analog Input Pin

Set 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.

In addition, operations of application products should be verified

thoroughly on the user side.

An analog input pin has a built-in capacitor for analog voltage

comparison. Thus if a signal from the high impedance signal

source is input to the analog input pin, charge and discharge

noise w i ll b e g e n e r a ted. This may cause th e A/D

conversion/comparison accuracy to drop.

2. Clock Frequency during A/D Conversion

The comparator input consists of a capacity coupling. If the

conversion rate is too low, the A/D conversion accuracy may

deteriorate due to a charge lost, so set f(XIN) 500 kHz or more for

A/D conversion in XIN mode. Also, do not execute the STP or

WIT instruction during A/D conversion.

In low-speed mode (when on-chip oscillator is selected), as A/D

conversion is performed using the internal on-chip oscillator,

there is no limit on the minimum frequency for f(XIN).

3. ADKEY Function

When the ADKEY enable bit is set to “1”, the analog input pin

selection bits are disabled. Do not execute the A/D conversion by

a program while ADKEY is enabled. Enabling ADKEY does not

change bits 0 to 2 of ADCON.

4. A/D Conversion Immediately After ADKEY Function

Started

In the ADKEY function, A/D conversion is not performed to the

analog input voltage immediately after starting the function. This

causes the A/D conversion result immediately after starting the

function to be undefined. If the A/D conversion result of the

analog input voltage applied to the ADKEY pin is required,

select the analog input pin corresponding to ADKEY before

performing A/D conversion.

5. Input Voltage Applied to ADKEY Pin

Set the input to the ADKEY pin into a steep falling waveform

and stabilize the input voltage within eight cycles (1

μs when

f(XIN) = 8 MHz) from the moment the input voltage reaches VIL

or lower.

The actual threshold voltage for the ADKEY pin is between VIH

and VIL.

To prevent unnecessary ADKEY operation due to noise or other

factors, set the ADKEY pin voltage to VIH (0.9 VCC) or more

while the input is waited.

6. Register Operation during A/D Conversion

The A/D conversion operation is not guaranteed if the following

are preformed:

• The CPU mode register is operated during A/D conversion

operation

• The AD control register is operated during A/D conversion

operation

• The STP or WIT instruction is executed during A/D

conversion operation

7. A/D Converter Power Source Pin

Connect to the A/D converter power source pin to AVSS or VSS

whether the A/D conversion function is used or not.

If the AVSS pin is left open, the MCU may operate incorrectly

because the pin will be affected by noise or other factors.

38D2 Datasheet(PDF) 125 Page - Renesas Technology Corp

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