I have a sky130 ngspice netlist with the MOSFETs defined as "X0 Y A GND SUB nmos_3p3 w=37 l=10" . Unfortunately the characterization engine does not support transistors as X yet, so I tried to rename it to "M0 Y A GND SUB nmos_3p3 w=37 l=10" which seems to work. What is the difference between X and M? Should it simulate correctly that way? Why is Sky130 using X instead of M?

"M" means use one of the built-in MOS models. "X" means that there is a subcircuit defined somewhere (probably including various components including one or more "M"s). You need to include a file that defined the "nmos_3p3" subcircuit

I have included the necessary include filesz that's not the problem. What I am wondering is whether simulating with M will give the same results as with X for nmos_3p3

It is interesting that the model and the subcircuit have exactly the same name, though I suppose it isn't confusing for Spice. In theory the subcircuit simulation should be better since it adds components like parasitic capacitors and resistors to the model

Ok, so I should get roughly similar results, the difference is just the capacitors/resistors? That sounds good.

Yes, just the model should be faster

That's even better, then I can provide a faster variant too. Thanks a lot for the explanation!

You can find the subcircuit in the included file (a .lib?) and see what you will be missing by not using it

Is there a way to flatten a spice netlist?

Try using the

listing

command inside your spice. It will output the flattened netlist of everything