@Tim Edwards regarding the ngspice (Xspice) implementation of

d_lut

and

d_genlut

there is a note in the ngspice manual that looks strange: "_Any unknown bit in the input vector will always produce an unknown output_" So it seems that a 2-input AND gate described as a

d_lut

with inputs '0' and 'X' will produce 'X' on the output which is clearly incorrect. How do you handle such situations?

I actually hadn't given that any thought. There should be a way to automatically determine which inputs in which combinations can be "don't care" conditions. The statement in the manual is correct, as that is the way I implemented it, even though the resulting behavior may not be logically correct.

@Tim Edwards the LUT defines output values for any 0/1 combination of the inputs. Anything else generates a 'X' on the output. I don't think there is a way to specify 'don't care' contitions on some inputs.