Electronic – AVR /RESET pin voltage

I discovered the solution to this problem!

I wrote a batch file to compile the code, convert to hex, and write to the microcontroller in one go. To avoid accidentally disabling the reset fuse, I wrote a second batch file.

At some point, I was experimenting with different optimization levels with avr-gcc. My command line parameters for avr-gcc included -O3 in both files. I discovered that this level of optimization was unreliable and caused some issues. I changed it back to -Os but only in the first batch file, not the second.

So, the code behavior change was a result of an incorrect optimization, which was used only in the script I used to also disable the reset fuse. It was finally caught by an astute coworker who noticed the difference in the two scripts.

I would guess, when you connect the +5V_SEC to +5V, there is a transient dip on +5V. That is perceived by the PIC as a brownout. The dip happens because the capacitors connected to +5V_SEC suddenly charge. You can probably see the dip with an oscilloscope.

[I've seen that sort of behavior before. I'll see if I can find the old oscilloscope screenshot.]

On a different note. Add a pull-down resistor from the gate of Q1 to ground. That will ensure that Q1 will not turn on RB7 is high impedance.