diff --git a/generic/viaduct_helpers.cc b/generic/viaduct_helpers.cc
index 36bdd6be..e9f7fa60 100644
--- a/generic/viaduct_helpers.cc
+++ b/generic/viaduct_helpers.cc
@@ -82,7 +82,7 @@ void ViaductHelpers::remove_nextpnr_iobs(const pool<CellTypePort> &top_ports)
 }
 
 int ViaductHelpers::constrain_cell_pairs(const pool<CellTypePort> &src_ports, const pool<CellTypePort> &sink_ports,
-                                         int delta_z)
+                                         int delta_z, bool allow_fanout)
 {
     int constrained = 0;
     for (auto &cell : ctx->cells) {
@@ -96,6 +96,8 @@ int ViaductHelpers::constrain_cell_pairs(const pool<CellTypePort> &src_ports, co
                 continue;
             if (!src_ports.count(CellTypePort(ci.type, port.first)))
                 continue;
+            if (!allow_fanout && port.second.net->users.size() > 1)
+                continue;
             for (auto &usr : port.second.net->users) {
                 if (!sink_ports.count(CellTypePort(usr)))
                     continue;
diff --git a/generic/viaduct_helpers.h b/generic/viaduct_helpers.h
index 8cba8411..cac22304 100644
--- a/generic/viaduct_helpers.h
+++ b/generic/viaduct_helpers.h
@@ -70,7 +70,8 @@ struct ViaductHelpers
     // expects a set of top-level port types
     void remove_nextpnr_iobs(const pool<CellTypePort> &top_ports);
     // Constrain cells with certain port connection patterns together with a fixed z-offset
-    int constrain_cell_pairs(const pool<CellTypePort> &src_ports, const pool<CellTypePort> &sink_ports, int delta_z);
+    int constrain_cell_pairs(const pool<CellTypePort> &src_ports, const pool<CellTypePort> &sink_ports, int delta_z,
+                             bool allow_fanout = true);
     // Replace constants with given driving cells
     void replace_constants(CellTypePort vcc_driver, CellTypePort gnd_driver,
                            const dict<IdString, Property> &vcc_params = {},