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Logic Levels

The Arduino MEGA 2560 R3 runs on 5 volts and expects 5 volt compatible logic signals.

Specifically, on input the low logic level is -0.5 to 1.5V and the high logic level is 3.0 to 5.5V.

On output the low logic level is specified as max 0.9V@20mA drain and the high logic level as 4.2V@-20mA.

At 3.3V supply, most logic chips will fail to meet the Vih specification.

Two diodes' drop will shift the voltage levels to a working zone.

The AHC logic series have the ability to receive 5 volt signals while operating on 3.3 volts without damage. An option is to provide a separate trace for the power supply of all outgoing buffers, so the output will be 3.3V compatible if this is connected to 3.3V and 5V compatible if it is connected to 5V.