diff --git a/xc7/arch.cc b/xc7/arch.cc
index 042fb689..cc755254 100644
--- a/xc7/arch.cc
+++ b/xc7/arch.cc
@@ -929,14 +929,19 @@ TimingPortClass Arch::getPortTimingClass(const CellInfo *cell, IdString port, Id
             return TMG_CLOCK_INPUT;
         if (port == id_CIN)
             return TMG_COMB_INPUT;
-        if (port == id_COUT || port == id_O)
+        if (port == id_COUT)
             return TMG_COMB_OUTPUT;
+        if (port == id_O) {
+            // LCs with no inputs are constant drivers
+            if (cell->lcInfo.inputCount == 0)
+                return TMG_IGNORE;
+            return TMG_COMB_OUTPUT;
+        }
         if (cell->lcInfo.dffEnable) {
             clockPort = id_CLK;
             if (port == id_OQ)
                 return TMG_REGISTER_OUTPUT;
-            else
-                return TMG_REGISTER_INPUT;
+            return TMG_REGISTER_INPUT;
         } else {
             return TMG_COMB_INPUT;
         }
@@ -996,15 +1001,19 @@ void Arch::assignCellInfo(CellInfo *cell)
         cell->lcInfo.clk = get_net_or_empty(cell, id_CLK);
         cell->lcInfo.cen = get_net_or_empty(cell, id_CEN);
         cell->lcInfo.sr = get_net_or_empty(cell, id_SR);
-//        cell->lcInfo.inputCount = 0;
-//        if (get_net_or_empty(cell, id_I0))
-//            cell->lcInfo.inputCount++;
-//        if (get_net_or_empty(cell, id_I1))
-//            cell->lcInfo.inputCount++;
-//        if (get_net_or_empty(cell, id_I2))
-//            cell->lcInfo.inputCount++;
-//        if (get_net_or_empty(cell, id_I3))
-//            cell->lcInfo.inputCount++;
+        cell->lcInfo.inputCount = 0;
+        if (get_net_or_empty(cell, id_I1))
+            cell->lcInfo.inputCount++;
+        if (get_net_or_empty(cell, id_I2))
+            cell->lcInfo.inputCount++;
+        if (get_net_or_empty(cell, id_I3))
+            cell->lcInfo.inputCount++;
+        if (get_net_or_empty(cell, id_I4))
+            cell->lcInfo.inputCount++;
+        if (get_net_or_empty(cell, id_I5))
+            cell->lcInfo.inputCount++;
+        if (get_net_or_empty(cell, id_I6))
+            cell->lcInfo.inputCount++;
     }
 }