12v to 3.3v logic lvl start oscillating

[bestrider14]

Hi,

Im new here but I need your help. I try to implement a circuit that will detect if my 12v as been switch on via a STM32 but via a logic gate to block a NFET to be activated at the same time. my circuit have 3 input that comings from my STM32 (on my image is done by a switch). So 1 input active the PFET to give me 12v. An other input activate an NFET to connect the circuit at the GND. The last input allow me to put a low current 3.3v (5v on my image) to test a fuse. So I made a logic circuit that if the 12v and the 3.3v is active at the same time I cant activate the NFET. Before 1 of my input of my XOR gate was at the input of my PFET but after I realise, I think, that it will be safer if a really check after the PFET and see the 12v. And now ho boy when I try to simulate this if I active my PFET and NFET to blow the fuse my circuit start oscillating and I dont know why. Im sorry if my schematic is not clean. I try so many thing and is getting worst every time I try something new. (my complet schematic is done on Kicad). So its where the node TES all my problem begin, I done 1000x test so my resistor value, diode might be wrong in this now. (I made it on 5v on my image cause the simulation need 5v too see an logic high but on my real schematic im on 3.3v with the right logic IC.)
 
thank you

[bestrider14]

I even try a simple voltage divider with a 100k, 36k resistor instead of the Zener and resistor. I still have the same result. I have a cap, it change nothing.

Edit: Ok I found what cause the oscillation. When the NFET is activated the line between the 2 FET go to 0v so it disactivate the NFET and now the loop begin. So now I need to isolate this node to stay at 3.3v when the FET become active.

[Terry Bites]

oscillating how fast?

[bestrider14]

On simulation I cant let it run cause the simulation crash. But I think if I could setup a low pass filter on the node TES it could work.

[bestrider14]

I almost get rid of my problem but I dont think this is the best solution. So in the 12v isolation I made a voltage divider to get my 3.3v after the Zener 6.2v. After in the pyro circuit (its 2x the same circuit) I add a resistor PYR_R15/R16 so it create a voltage drop there. so my logic see the voltage before the resistor so it keep the logic HIGH. In the screw terminal its where I put my igniter (1ohm and need 1.25A). The problem now its my resistor take a lot of power for almost nothing so that's why I think its not the best solution.

[bestrider14]

I put a flip flop problem solve.