There is a well known problem with rxd0 on the atmega 1284p that requires
a 100pf cap to ground. I,m not sure about the processor you are using. As others have suggested you may have an earthing problem but is more likely that your serial driver is not pulling the rxd low enough.
Try a 1k then a 100 - 330 pf cap. ( 2 separate tests). That will tell you what the issue is
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