Electronic – Connecting the RESET pin of AT8051

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.

A PNP transistor or P-channel MOSFET can be used as a switch to temporarily connect the RESET signal to Vcc.

^{simulate this circuit – Schematic created using CircuitLab}

Note that this arrangement inverts the operation of the RESET pin, so setting RESET# to LOW sets the RESET pin to HIGH. If you want to re-invert it again to keep the same polarity, you can do so with a simple NPN transistor.