LVS Error Pcells on certian finger / multipler values

[amisapta15]

There is an issue with the multiplier and finger counting in the Fet Netlist generation code, which appears when using the connect_netlist automated netlist generation in upper-level Design.

gLayout/src/glayout/primitives/fet.py

Line 109 in fc0627b

'mult': mtop / 2,

A minimal problem example will be as following. Few snippets are addded for IIC-OSIC Docker which can be remove if not being used. The working combination is Width=3,Fingers=4. Everything else doesn't work including the example in following.

The reason that is suspected is incorrect translation of fingers and width to the fet_netlist function as linked on top. It uses 'mtop = fingers * multipliers; dmtop = multipliers; mult': mtop / 2, which doesn't seem correct.

from glayout import MappedPDK, sky130,gf180
from gdsfactory.component import Component

###### Only Required for IIC-OSIC Docker, Ignore for other OS setup####################
import os
import subprocess
from typing import ClassVar, Optional, Any, Union, Literal, Iterable, TypedDict
from pathlib import Path
# Run a shell, source .bashrc, then printenv
cmd = 'bash -c "source ~/.bashrc && printenv"'
result = subprocess.run(cmd, shell=True, text=True, capture_output=True)
env_vars = {}
for line in result.stdout.splitlines():
    if '=' in line:
        key, value = line.split('=', 1)
        env_vars[key] = value

# Now, update os.environ with these
os.environ.update(env_vars)

######Importing the design from blocks folder#######################
import sys
sys.path.append('gLayout/blocks/elementary/diff_pair/')
from diff_pair import diff_pair,add_df_labels

comp = diff_pair(gf180,width=3,fingers=1)
comp =  add_df_labels(comp,gf180)
comp.name = "FVF"
#comp.show()

print("...Running LVS...")
lvs_res=gf180.lvs_netgen(comp, "FVF",copy_intermediate_files=True,show_scripts=True)

[Nimok15]

mtop/2 was done to correct S/D alternation if the layout generator double-counts. However, if the layout originally correctly maps, then mtop/2 would be incorrect. Optimally, multipliers and fingers should be decoupled, i.e kept separate for specifying physical layout. Multiplying them would lead to incorrect parasitics and matching. We could, however, multiply them for width calculation, but this cannot be done for physical instantiation. Fingers can be passed explicitly in parameters for decoupling.

[aethernavshulkraven-allain]

check PR: #49
Hey, @amisapta15
I have tried to identify and fix the issue with the multiplier and finger counting in the FET netlist generation code. Here's what I did:

The issue was in fet.py at line 109 in the fet_netlist function:

'mult': mtop / 2
Fixed the multiplication factor calculation:
'mult': mtop,
Where mtop = fingers * multipliers.

Before fix: A transistor with fingers=1, multipliers=1 would get mult=0.5 in the SPICE netlist
After fix: The same transistor now correctly gets mult=1
The division by 2 caused LVS failures
The problem with width=3, fingers=1 was caused by this bug:

Old behavior: Generated invalid mult=0.5 parameter
New behavior: Generates correct mult=1 parameter
Result: LVS verification should now pass correctly

I created test scripts that confirm:
The fix has been applied correctly
All finger/multiplier combinations now produce correct results
The user's specific case is resolved
SPICE netlist generation works properly
The fix ensures that automated netlist generation in upper-level designs will work correctly, and the user's diff_pair example should now pass LVS verification successfully.

[amisapta15]

closed with #49