7220 datasheet(231/303 Pages) RENESAS

231 / 303 page

APPENDIX

6.4 Countermeasures against noise

7220 Group User’s Manual

6-12

(3)

Wiring to CNVss pin

Connect the CNVSS pin to the VSS pin with the

shortest possible wiring.

Reason

The processor mode of a microcomputer is

influenced by a potential at the CNVSS pin. If

a potential difference is caused by the noise

between pins CNVSS and VSS, the processor

mode may become unstable. This may cause

a microcomputer malfunction or a program

runaway.

(4)

Wiring to VPP pin of One Time PROM version

and EPROM version

When the VPP pin is also used as the CNVSS

pinV1

Connect an approximately 5 kW resistor to

the VPP pin the shortest possible in series and

also to the VSS pin. When not connecting the

resistor, make the length of wiring between

the VPP pin and the VSS pin the shortest possible

(refer to “countermeasure example 1 of

Figure 6.4.4”)

V1 When a microcomputer has the CNVSS pin,

the VPP pin is also used as the CNVSS pin.

Note: Even when a circuit which included an

approximately 5 kW resistor is used in

the Mask ROM version, the micro-

computer operates correctly.

Reason

The VPP pin of the One Time PROM and the

EPROM version is the power source input pin

for the built-in PROM. When programming in

the built-in PROM, the impedance of the VPP

pin is low to allow the electric current for writing

flow into the PROM. Because of this, noise

can enter easily. If noise enters the VPP pin,

abnormal instruction codes or data are read

from the built-in PROM, which may cause a

program runaway.

Fig.6.4.3 Wiring for CNVSS pin

Noise

CNVSS

VSS

CNVSS

VSS

N.G.

O.K.

Fig.6.4.4 Wiring for VPP pin of One Time

PROM and EPROM version

CNVSS/VPP

VSS

In the shortest

distance

Approximately

Ω

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