issue #12: connect ground net to `0` if has different name

* If the ground net is called VSS or similar, then it needs to be
  connected to GND or 0

src/lctime/characterization/input_capacitance.py

@@ -73,7 +73,7 @@ def characterize_input_capacitances(
     setup_statements_string = "\n".join(setup_statements)
 
     # Add output load capacitance. Right now this is 1fF.
-    output_load_statements = "\n".join((f"Cload_{p} {p} GND 1f" for p in output_pins))
+    output_load_statements = "\n".join((f"Cload_{p} {p} {cell_conf.ground_net} 1f" for p in output_pins))
 
     # Choose a maximum time to run the simulation.
     time_max = cfg.time_step * 1e6
@@ -88,6 +88,18 @@ def characterize_input_capacitances(
 
     logger.debug("Measuring input capacitance.")
 
+    # If the ground net is not called GND, then it must be connected to GND or 0.
+    # This seems to be a must for SPICE simulators.
+    # See: https://codeberg.org/librecell/lctime/issues/12#issuecomment-2173526
+    if cell_conf.ground_net not in ["GND", "gnd", "0"]:
+        logger.debug(f"Connect ground net `{cell_conf.ground_net}` to simulators ground `0`.")
+        ground_connection = f"* Connect ground net to GND\n" \
+        f"vgnd {cell_conf.ground_net} 0 0"
+    else:
+        # Using standard name for ground net.
+        ground_connection = ""
+
+
     # Generate all possible input combinations for the static input pins.
     static_input_nets = [i for i in input_pins_non_inverted if i != active_pin]
     num_inputs = len(static_input_nets)
@@ -178,6 +190,8 @@ pre_set strict_errorhandling
 
 {setup_statements_string}
 
+{ground_connection}
+
 Xcircuit_under_test {" ".join(ports)} {cell_conf.cell_name}
 
 {output_load_statements}

src/lctime/characterization/ngspice_subprocess.py

@@ -100,6 +100,16 @@ def simulate_cell(
     # Load include files.
     setup_statements_string = "\n".join(setup_statements)
 
+    # If the ground net is not called GND, then it must be connected to GND or 0.
+    # This seems to be a must for SPICE simulators.
+    # See: https://codeberg.org/librecell/lctime/issues/12#issuecomment-2173526
+    if ground_net not in ["GND", "gnd", "0"]:
+        logger.debug(f"Connect ground net `{cell_conf.ground_net}` to simulators ground `0`.")
+        ground_connection = f"* Connect ground net to GND\n" \
+        f"vgnd {ground_net} 0 0"
+    else:
+        ground_connection = ""
+    
     input_voltages_static = dict()
     input_voltages_active = dict()
 
@@ -196,6 +206,8 @@ pre_set strict_errorhandling
 
 {setup_statements_string}
 
+{ground_connection}
+
 Xcircuit_under_test {" ".join(cell_ports)} {cell_name}
 
 * Output load capacitances.