How do you go about troubleshooting a misbehaving microcontroller circuit? A few months ago I manufactured a new batch of Floppy Emu disk emulators. A number of them failed QA at the factory, with a set of symptoms that I’d never seen before in all my years of developing this device: Most of them simply wouldn’t boot up at all, despite verifying that power was good and the mcu was correctly programmed. Some exhibited “haunted” behavior, seemingly jumping to random sections of the mcu program code, outputting messages on the display that made no sense given the context. One of them appeared to work in slow motion, with LED blinking and display updates noticeably more sluggish than normal. This was odd, to say the least. I have a lot of experience with the ATMEGA1284 microcontroller and the Floppy Emu circuitry that surrounds it, and I’ve become an expert at guessing what’s wrong based on the symptoms of misbehaving boards. These were all new and bizarre symptoms to me.…