First of all the

tcleval(...)

is xschem specific, so do not look into other manuals. in the example above

value

is assigned to something that contains a tcl variable

SKYWATER_MODELS

. The double backslash escapes

$

so it will not have any special meaning to xschem. This is however not needed anymore and a

value=.lib $SKYWATER_MODELS/sky130.lib.spice tt

will work fine too. The

@

character replaces

value

with the instance attribute:

.lib $SKYWATER_MODELS/sky130.lib.spice tt

, however this resulting string is then passed (due to the

tcleval(....)

wrapper) to tcl so the variable will be substituted. At the end you get something like:

.lib /usr/local/share/pdk/sky130A/libs.tech/ngspice/sky130.lib.spice tt

This is necessary since ngspice does not understand environment variables in netlists. This works because in sky130 there is a tcl variable set in the

xschemrc

file:

set SKYWATER_MODELS ${PDK_ROOT}/${PDK}/libs.tech/ngspice

where

PDK_ROOT

is set to

/usr/local/share/pdk

and

PDK

is set to

sky130A

(or sky130B) The whole point here is to avoid using absolute paths in xschem since if this is done porting a schematic on a differet computer will likely not work as installation paths for pdks and libraries are system specific.