It's embarrassing, but here you go.
I'm trying to update my ATMEGA328 chip on my DIY Controller Board via ICSP (using USBASP v2.0 programmer and AVRFuses). So to do that, I had to hookup (6) wires between the USBASP and my Controller Board. Easy and simple.
I'm refining my program, edit, upload, edit, upload so due to my laziness.... I just left all the wires connected between the USBASP and my Controller Board. My Controller Board wouldn't run when powered on it's own supply, with the USBASP programmer board still connected to it, so I just disconnected the Vcc wire, leaving (5) wires still connected. If I need to re-program it, I just connect the 6th wire, the Vcc wire.
So everything seems dandy, the new firmware is working...
And that's when I noticed that an awful lot of RFI noise is happening... really loud. The preamp is only at 20dB gain and I can already hear it from my speaker monitor... and at 66dB gain (2000x voltage amplification), it's just awfully loud, really loud. The RFI noise is happening in coincidence with the GLCD updates.
I spent several hours trying to rewrite the firmware, change some logic to minimize noise, but then I realize that while I can minimize the occurrence of the noise, the problem is still there... and that's not good enough. I need to ELIMINATE this RFI noise. There should be no noise, even at 66dB gain (2000x voltage amplification). -- which means the problem is in the hardware?
Maybe I didn't do a good enough job of RFI filtering on the inputs, wrong cap values, etc?.... maybe my grounding scheme (ground fill - double sided, top and bottom) isn't good enough. Maybe I need to redesign the PCB and rethink/relayout my Digital Ground and Analog Ground.
The more I panic, the more I can't think straight. All I can think about is the back-to-square-one, and all the lost work and money I poured on this project.
I'm about to call it a day, and I saw another Controller Board prototype version I've made. It's basically the same as the newer Controller board, just minor revisions.
I hooked this old Controller Board to my Preamp Board and my preamp board was quiet! I'm a bit surprised. WAITTTTT...... that can't be! I compared the old Controller Board and the new Controller Board... what's the difference????? ---- you gotta be kidding me... tell me it's not true.... that stupid USBASP programmer board dangling with it's 5 wires connected to the ICSP port. Nooooooooo.... so I yanked the 5 wires off the board, powered it on, and ....... holy cow... sweet quietness even at max gain of 2000x amplification. So stupid stupid stupid of me.... why didn't I think of that 12 hours ago!
And that's it. Lesson for today is.... don't leave your USBASP programmer board connected to your project when you don't need it. -- I'm trying to save time from connecting these (6) ICSP wires, and I wasted 18+ hours of my life on a problem that wasn't there to begin with!