7721 datasheet(527/577 Pages) RENESAS

527 / 577 page

7721 Group User’s Manual

17–64

APPENDIX

Appendix 8. Countermeasure against noise

5. Setup for I/O ports

Setup I/O ports by hardware and software as follows:

q Connect a resistor of 100

Ω or more to an I/O port in series.

q Read the data of an input port several times to confirm that input levels are equal.

q Since the output data may reverse because of noise, rewrite data to the output port’s Pi register

periodically.

q Rewrite data to port Pi direction registers periodically.

6. Reinforcement of the power source line

q For the Vss and Vcc lines, use thicker wiring than that of other signal lines.

q When using a multilayer printed circuit board, the Vss and Vcc patterns must each be one of the middle

layers.

q The following is necessary for double-sided printed circuit boards:

•On one side, the microcomputer is installed at the center, and the Vss line is looped or meshed around

it. The vacant area is filled with the Vss line.

•On the opposite side, the Vcc line is wired the same as the Vss line.

•The power source lines of external devices which are connected by bus to the microcomputer must be

connected to the microcomputer's power source lines with the shortest possible wiring.

Reasons: With external devices connected to the microcomputer, the levels of many of the signal lines

(total external address buses: 24 bits) may change simultaneously, causing noise on the power

source line.

Noise

Direction register

Port latch

Data bus

Port

Fig. 12 Setup for I/O ports

7721 Datasheet(PDF) 527 Page - Renesas Technology Corp