7540 datasheet(316/366 Pages) RENESAS

316 / 366 page

3-94

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

APPENDIX

7540 Group

3.4 Countermeasures against noise

(5) Wiring to VPP pin of One Time PROM version

Connect an approximately 5 k

Ω 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.

Note: Even when a circuit which included an approximately 5 k

Ω resistor is used in the Mask ROM

version, the microcomputer operates correctly.

q Reason

The VPP pin of the One Time PROM 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. 3.4.5 Wiring for the VPP pin of the One Time PROM

3.4.2 Connection of bypass capacitor across VSS line and VCC line

Connect an approximately 0.1

µF bypass capacitor across the VSS line and the VCC line as follows:

• Connect a bypass capacitor across the VSS pin and the VCC pin at equal length.

• Connect a bypass capacitor across the VSS pin and the VCC pin with the shortest possible wiring.

• Use lines with a larger diameter than other signal lines for VSS line and VCC line.

• Connect the power source wiring via a bypass capacitor to the VSS pin and the VCC pin.

Fig. 3.4.6 Bypass capacitor across the VSS line and the VCC line

CNVSS/VPP

VSS

In the shortest

distance

Approximately

Ω

VSS

VCC

VSS

VCC

N.G.

O.K.

7540 Datasheet(PDF) 316 Page - Renesas Technology Corp

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