Add noninverting logic function to custom decoder cells.

compiler/custom/nand2_dec.py

@@ -70,3 +70,7 @@ class nand2_dec(design.design):
         """
         self.add_graph_edges(graph, port_nets)
 
+    def is_non_inverting(self):
+        """Return input to output polarity for module"""
+
+        return False

compiler/custom/nand3_dec.py

@@ -70,3 +70,7 @@ class nand3_dec(design.design):
         """
         self.add_graph_edges(graph, port_nets)
 
+    def is_non_inverting(self):
+        """Return input to output polarity for module"""
+
+        return False

compiler/custom/nand4_dec.py

@@ -70,3 +70,7 @@ class nand4_dec(design.design):
         """
         self.add_graph_edges(graph, port_nets)
 
+    def is_non_inverting(self):
+        """Return input to output polarity for module"""
+
+        return False