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Why the LEDs connected to ports light upon reset? How can I prevent it?

Latest Updated:12/01/2011

Question:

The LEDs connected to the ports light upon reset. What is the cause and how can I prevent it?

Answer:

At reset, all the ports enter the input mode (high-impedance status).
When TTL is used for the external buffer, there is a high likelihood that the high-impedance status will be judged as a high level.
If the circuit is designed so that LEDs light when the port outputs are high level, the LEDs will light as a result.

The handling to prevent this is to make the LEDs light when a low level is output.

Suitable Products
RL78 Family
78K Family