PIR Pin goes HIGH when receiving serial data

I have an Arduino Uno with two buttons, a stepper Motor, two HC-SR501 and a Raspberry attached over USB-Serial.
I can send and receive serial data without problems, but it took me now several hours to find out, that the Pin from the PIRs are going always up when Serial data are received.
The Pins are 6 and 7. The Pins from the Buttons (3/4) are uneffected from this. They will stay low during transmit.
Can anybody explain to me why this happens and how to avoid it?


Are you using pull down/ up resistors?

I think I have figured it out.
The power for the PIR was connected to 3.3 V.
Changed this to 5V and it works now.

But strange what this has to do with the Serial receiving.