question about LVS export

[olisnr]

i do an .cdl export for LVS, for a MOSFET with w=16µm and ng=9, and that gives an effective w of 15.975µm in the layout. this produces an LVS error. if i change the value in Xschem the netlist-export rounds it to 15.97µm, and this gives me still an error :)

the grid of the iHP-openPDK is 5nm. could You export that precision?

[StefanSchippers]

Please check the netlist. If I set w=8.123456 on a transistor in xschem the netlist has w=8.123456. Probably some other tool in the middle is rounding these values.

[olisnr]

yes, the tool is TCL :) w="tcleval([string map {{$} {}} [ev {16*$nw}]])"... may be its better to try to get some tolerance into the LVS script.

[StefanSchippers]

Ah ok, then replace ev with expr. ev rounds the result to 4 significant digits, it is normally used for printing final results. For data processing better to avoid any rounding.

[StefanSchippers]

Latest commit adds procedure ev7 (same as ev, 7 significant digits). Rounding to a reasonable number of digits is useful to clean up floating rounding errors like 0.99999999999898 (I prefer 0.1)

[olisnr]

super. i tried also to do an new function simply without the rounding:

## NEWNEWNEW evaluate expression extended precision. if expression has errors or does not evaluate return expression as is
proc eve {args} {
  set i [join $args]
  if {![catch {uplevel #0 expr $i} res]} {
    return $res
  } else {
    # puts stderr "proc ev: $res"
    return $args
  }
}

7 digits gives me 1mm big MOSFETs.

thanks a lot!

[olisnr]

what do You think, would it make sense to write LVS-netlists with .cdl extension?

[StefanSchippers]

Changing this will probably break existing flows, anyway you can do that very easily. Before generating the netlist: xschem set netlist_name [file rootname [xschem get current_name]].cdl xschem netlist

[olisnr]

yes, i did such a thing via an command launcher.sym:

descr="write LVS netlist"
tclcommand="
    xschem set netlist_type spice
    set lvs_ignore 1
    set lvs_netlist 1
    set spiceprefix 0

    xschem netlist [xschem get current_dirname]/[xschem get current_name].cdl
"

i think, its strange that the file it generates is in the symulation directory. should the current_dirname not be the dirname, where the schematics sit?

[StefanSchippers]

The command above will write a file xxxx.sch.cdl in the same place where xxxx.sch is present. the same command is: xschem netlist [xschem get schname].cdl

The cdl file in simulation directory is probably an older one?

[olisnr]

not exactly. it goes to the simulation folder:

https://github.com/user-attachments/assets/83879b8a-54d7-47a8-b785-2a474b7bd931

may be the netlist function channges first the directory and then reads the current_dirname?

[StefanSchippers]

The problem is that you probably have this option set:

#### if this is set to '1' netlists and simulations will go into a simulation/ folder
#### inside the directory containing the top level schematic.
#### if this is set to '2' netlists and simulations will go into a simulation/[schname]/ folder
#### inside the directory containing the top level schematic.
#### Default: not set (0).
set local_netlist_dir 1

If this is the case the path you specify will be overridden and be dir_of_schematic/simulation so add this before netlisting: set local_netlist_dir 0

[olisnr]

thanks, then i can do it like this (without specify a full paht):

tclcommand="
    xschem set netlist_type spice
    set lvs_ignore 1
    set lvs_netlist 1
    set spiceprefix 0
    set last_local_netlist_dir $local_netlist_dir
    set local_netlist_dir 0
    xschem netlist [xschem get current_name].cdl
    set local_netlist_dir $last_local_netlist_dir
"

[StefanSchippers]

If you don't set a path (something with '/' characters) xschem assumes you want to keep the netlisting directory as is and only change the netlist filename. However you can change the xschem netlist command to: xschem netlist ./[xschem get current_name].cdl or xschem netlist [pwd]/[xschem get current_name].cdl if you want to generate the cdl netlist in current directory.

The apparently meaningless splitting of netlist directory and netlist filename has been done because in some configurations xschem generates one file per hierarchical block, in this case you can specify a netlist directory but obviously not a filename (in this configuration filename is obtained from the schematic name).