diff --git a/common/placer1.cc b/common/placer1.cc
index 01f822a5..0d7c0701 100644
--- a/common/placer1.cc
+++ b/common/placer1.cc
@@ -118,6 +118,12 @@ class SAPlacer
                               loc_name.c_str(), bel_type.c_str(ctx), cell->name.c_str(ctx), cell->type.c_str(ctx));
                 }
 
+                auto bound_cell = ctx->getBoundBelCell(bel);
+                if (bound_cell) {
+                    log_error("Cell \'%s\' cannot be bound to bel \'%s\' since it is already bound to cell \'%s\'\n",
+                              cell->name.c_str(ctx), loc_name.c_str(), bound_cell->name.c_str(ctx));
+                }
+
                 ctx->bindBel(bel, cell, STRENGTH_USER);
                 locked_bels.insert(bel);
                 placed_cells++;
diff --git a/gui/ecp5/mainwindow.h b/gui/ecp5/mainwindow.h
index f67f7a81..721c6c0b 100644
--- a/gui/ecp5/mainwindow.h
+++ b/gui/ecp5/mainwindow.h
@@ -43,7 +43,7 @@ class MainWindow : public BaseMainWindow
     void onProjectLoaded() override;
 
   protected Q_SLOTS:
-    virtual void new_proj();
+    void new_proj() override;
     void newContext(Context *ctx);
     void open_lpf();
     void open_base();
diff --git a/gui/generic/mainwindow.h b/gui/generic/mainwindow.h
index a4ce9958..bb6a4cf1 100644
--- a/gui/generic/mainwindow.h
+++ b/gui/generic/mainwindow.h
@@ -36,7 +36,7 @@ class MainWindow : public BaseMainWindow
     void createMenu();
 
   protected Q_SLOTS:
-    virtual void new_proj();
+    void new_proj() override;
     void newContext(Context *ctx);
 };
 
diff --git a/gui/ice40/mainwindow.h b/gui/ice40/mainwindow.h
index bb8ed75f..4a9a7d8e 100644
--- a/gui/ice40/mainwindow.h
+++ b/gui/ice40/mainwindow.h
@@ -44,7 +44,7 @@ class MainWindow : public BaseMainWindow
     void onProjectLoaded() override;
 
   protected Q_SLOTS:
-    virtual void new_proj();
+    void new_proj() override;
 
     void open_pcf();
     void save_asc();
diff --git a/ice40/cells.cc b/ice40/cells.cc
index 76e67ab7..fbb77b0c 100644
--- a/ice40/cells.cc
+++ b/ice40/cells.cc
@@ -383,6 +383,10 @@ bool is_clock_port(const BaseCtx *ctx, const PortRef &port)
                port.port == ctx->id("WCLKN");
     if (is_sb_mac16(ctx, port.cell) || port.cell->type == ctx->id("ICESTORM_DSP"))
         return port.port == ctx->id("CLK");
+    if (is_sb_spram(ctx, port.cell) || port.cell->type == ctx->id("ICESTORM_SPRAM"))
+        return port.port == id_CLOCK;
+    if (is_sb_io(ctx, port.cell))
+        return port.port == id_INPUT_CLK || port.port == id_OUTPUT_CLK;
     return false;
 }