xschem lvs export options to fit klayout

[Scafir]

Is your feature request related to a problem? Please describe.
Currently, if one wants to export a netlist from xschem for use in klayout LVS, one needs to:

• Enable "LVS netlist + Top level is a .subckt"
• Disable "Use 'spiceprefix' attribute"

As the options are only kept during runtime, it can make first contacts with LVS quite confusing.

Describe the solution you'd like
The proper options can be enabled if the following is added to the xschemrc file

###########################################################################
#### IHP-Open-PDK klayout LVS export settings
###########################################################################
#### Set "LVS netlist + Top level is a .subckt".
set lvs_netlist 1
#### Do not "Use 'spiceprefix' attribute"
set spiceprefix 0

This could be added as a patch on top of /foss/pdks/ihp-sg13g2/libs.tech/xschem/xschemrc. It might also make sense to add it instead to /headless/.xschem/xschemrc before the import if it would also be useful for sky130 or GF180.

Describe alternatives you've considered

I considered submitting a PR to upstream IHP-Open-PDK, but I think that these options might affect LVS export for other tools like netgen, which is not necessarily a clear win. Maybe you could confirm this is the case. There is more information about the spiceprefix in IHP-GmbH/IHP-Open-PDK#607.
However, I think it might make sense for a container geared towards open technologies and teaching to support open software first and proprietary, out-of-container second.

[hpretl]

@Scafir For the next (and following) image builds, we are now using our fork of the IHP PDK (so that we have our changes in earlier). Please make a PR against https://github.com/iic-jku/IHP-Open-PDK (dev branch), and we will merge it. So we have it in until it is also merged into the IHP repo.

[hpretl]

@Scafir We can also add it to the xschemrc in headless. If so, please provide a PR by adapting ./images/iic-osic-tools/skel/headless/.xschem/xschemrc.

[rjordans]

IIRC the spiceprefix is required for simulation, so while this fix makes it easier to do LVS, it breaks simulations (which is probably a more common usecase within xschem)!

[Scafir]

That's a very good point @rjordans , which I had overlooked. Thank you!
With a quick test, both "Use 'spiceprefix' attribute" and "LVS netlist + Top level is a .subckt" must be set differently in ngspice simulation and klayout LVS.

That's too bad, there's thus no clear solution to fit both LVS and simulation. The "spiceprefix" difference could eventually be solved on the IHP-Open-PDK klayout side, but there are no clear solution that I see for the "LVS netlist [...]".

Maybe a feature request could be opened on xschem side to have a menu item specifically for LVS netlist export?

[rjordans]

The IHP menu is also part of the PDK files, it lives in libs.tech/xschem/xschem-menu

With some TCL scripting in that file you can probably make an entry that sets the spiceprefix and lvs netlist style, exports a netlist, and sets the attributes back again. Then users can use that button if they want to export the lvs netlist and use the simulation netlist without worrying about getting the settings right

[rjordans]

That would make the change something like this:

diff --git a/ihp-sg13g2/libs.tech/xschem/xschem-menu b/ihp-sg13g2/libs.tech/xschem/xschem-menu
index e4e0ae7..cba7357 100755
--- a/ihp-sg13g2/libs.tech/xschem/xschem-menu
+++ b/ihp-sg13g2/libs.tech/xschem/xschem-menu
@@ -222,9 +222,53 @@ proc display_bip_params {instname} {
   return $txt
 }
 
+# Add Klayout LVS netlist menu item
+# 
+# Needs lvs_netlist=1 and spiceprefix=0
+#
+proc klayout_lvs_netlist {} {
+  global lvs_netlist spiceprefix top_is_subckt
+
+  set old_lvs_netlist $::lvs_netlist
+  set old_spiceprefix $::spiceprefix
+  set old_top_is_subckt $::top_is_subckt
+
+  set lvs_netlist 1
+  set spiceprefix 0
+  set top_is_subckt 1
+
+  xschem netlist
+
+  set spiceprefix $old_spiceprefix
+  set lvs_netlist $old_lvs_netlist
+  set top_is_subckt $old_top_is_subckt
+}
+
+
+# Add Magic/netgen LVS netlist menu item
+# 
+# Needs lvs_netlist=0 and spiceprefix=1
+#
+proc netgen_lvs_netlist {} {
+  global lvs_netlist spiceprefix top_is_subckt
+
+  set old_lvs_netlist $::lvs_netlist
+  set old_spiceprefix $::spiceprefix
+  set old_top_is_subckt $::top_is_subckt
+
+  set lvs_netlist 0
+  set spiceprefix 1
+  set top_is_subckt 1
+
+  xschem netlist
+
+  set spiceprefix $old_spiceprefix
+  set lvs_netlist $old_lvs_netlist
+  set top_is_subckt $old_top_is_subckt
+}
 
 # these commands are executed when xschem has completed initialization.
-# add a SKY130 menu entry
+# add a PDK specific menu entry
 proc menupdk {} {
   global has_x netlist_dir
   if { [info exists has_x] } {
@@ -299,6 +343,12 @@ value="tcleval(
       }
       get_sel_inst_name
     }
+
+    ## Create one entry
+    $topwin.menubar.ihp add command -label {Create Klayout LVS netlist} -command klayout_lvs_netlist
+
+    ## Create one entry
+    $topwin.menubar.ihp add command -label {Create Magic/netgen LVS netlist} -command netgen_lvs_netlist
   }
 }

[rjordans]

And with a bit more improvement (adding the option to set another directory for the LVS netlist than which is used for simulation)

xschem-menu.patch

[StefanSchippers]

It is easy to add a simple procedure that does all the housekeeping tasks to generate a netlist for LVS.

The following small procedure:

proc save_netlist {{dir {}} {filename {}} {init_path {.}}} {
  if {$dir eq {}} {
    set dir [tk_chooseDirectory -initialdir $init_path]
  }
  if {$dir ne {}} {
    if {$filename eq {}} {
      set filename [file rootname [xschem get current_name]].spice
    }
    xschem netlist [list $dir/$filename]
  }
}

allows you to save the netlist in a custom directory and also with a custom name (if you want to change it).
You can also change the global flags:
lvs_netlist spiceprefix top_is_subckt
in the above procedure (declare them as globals in the procedure!) .