Since my last board flopped, I looked at it again and noticed a ground loop (because the DB9 casing completed the loop).
Now I adjusted my board so that there is no loop of any kind. Instead, I'm running a VCC track next to a GND plane with 1mm clearance right underneath the 40-pin DIP AT89S52 microcontroller (on opposite side of course since I'm doing single-sided PCB). I'm also planning to add a couple of 0.047uF decoupling capacitors.
For clarity, I added a picture of part of my circuit. I highlighted the ground wire in red and will convert it to a plane after so I don't waste etchant.
The circled green items are the 0.047uF ceramic decoupling capacitors.
They say ground loops are bad, but is vcc next to ground this close bad as well? and would modifying my clearance between the two planes affect microcontroller operation? and no I won't use 0 clearance or I'll blow the batteries up.