7546 datasheet(91/95 Pages) RENESAS

91 / 95 page

Rev.1.21

Nov 15, 2006

page 91 of 93

REJ03B0160-0121

7546 Group

5. A/D conversion accuracy

As for AD translation accuracy, on the following operating condi-

tions, accuracy may become low.

(1) Since the analog circuit inside a microcomputer becomes sen-

sitive to noise when VREF voltage is set up lower than Vcc

voltage, accuracy may become low rather than the case

where VREF voltage and Vcc voltage are set up to the same

value..

(2) When VREF voltage is lower than [ 3.0 V ], the accuracy at the

low temperature may become extremely low compared with

that at room temperature. When the system would be used at

low temperature, the use at VREF=3.0 V or more is recom-

mended.

Notes on Watchdog Timer

1. The watchdog timer is operating during the wait mode. Write

data to the watchdog timer control register to prevent timer un-

derflow.

2. The watchdog timer stops during the stop mode. However, the

watchdog timer is running during the oscillation stabilizing time

after the STP instruction is released. In order to avoid the un-

derflow of the watchdog timer, the watchdog timer count source

selection bit (bit 7 of watchdog timer control register (address

3916)) before executing the STP instruction.

3. The STP instruction function selection bit (bit 6 of watchdog

timer control register (address 3916)) can be rewritten only once

after releasing reset. After rewriting it is disable to write any

data to this bit.

Notes on RESET pin

1. Connecting capacitor

In case where the RESET signal rise time is long, connect a ce-

ramic capacitor or others across the RESET pin and the Vss pin.

And use a 1000 pF or more capacitor for high frequency use.

When connecting the capacitor, note the following :

• Make the length of the wiring which is connected to a capacitor

as short as possible.

• Be sure to verify the operation of application products on the

user side.

If the several nanosecond or several ten nanosecond impulse

noise enters the RESET pin, it may cause a microcomputer fail-

ure.

Notes on A/D conversion

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 1µF.

Further, be sure to verify the operation of application products on

the user side.

An analog input pin includes the capacitor for analog voltage com-

parison. 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/comparison

precision to be worse.

2. 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. This may

cause the A/D conversion precision to be worse. Accordingly, set

f(XIN) in order that the A/D conversion clock is 250 kHz or over

during A/D conversion.

3. A/D conversion clock selection

Select f(XIN)/2 as an A/D conversion clock by setting the A/D con-

version clock selection bit (bit 3 of A/D control register (address

3416)) when RC oscillation is used.

The f(XIN) can be also used as an A/D conversion clock only when

ceramic oscillation or on-chip oscillator is used.

4. Read A/D conversion register

• 8-bit read

Read only the A/D conversion low-order register (address 3516).

•10-bit read

Read the A/D conversion high-ordrer register (address 3616) first,

and then, read the A/D conversion low-order register (address

3516).

In this case, the high-order 6 bits of address 3616 returns “0”

when read.

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