I have a question about the

.GLOBAL GND

.GLOBAL VDD

added automatically by xschem to my spice netlist before simulation. I just discovered this by being suprised that my circuit worked even without power

It's never a good idea to use globals in SPICE netlists for an ASIC design, because it overrides all the hierarchy and connectivity. There should be a way to get xschem not to output

.GLOBAL

statements; I'm not sure what it is.

I think you could make a 0 V voltage source between the VDD and VDD_OSC and measure its current.

@Matt Venn The symbols for vdd and vss in the xschem library have the global property. For this reason, many of the designs I see have avoided using the vdd and vss symbols in the schematic. When I created the schematics for the i/o cells, I copied these symbols to a local library and deleted the global property. This allows intuitive schematics without the global definition.

thanks all

I use only the GND symbol (at least once) to setup a global reference ground. Otherwise I agree: No global nets in IC design, ever. This is a disastrous design fail just waiting to happen.

Likewise, I use GND in the testbench which establishes a relationship to the SPICE global node

0

which is always global (at least ngspice defines it this way); there should be an absolute ground reference somewhere, but only in a testbench and never inside of a circuit that is part of an ASIC.