diff --git a/litex/build/generic_toolchain.py b/litex/build/generic_toolchain.py
index f7b832ad8..8dae9d865 100644
--- a/litex/build/generic_toolchain.py
+++ b/litex/build/generic_toolchain.py
@@ -157,8 +157,9 @@ class GenericToolchain:
 
         return v_output.ns
 
-    def add_period_constraint(self, platform, clk, period):
-        clk.attr.add("keep")
+    def add_period_constraint(self, platform, clk, period, keep=True):
+        if keep:
+            clk.attr.add("keep")
         period = math.floor(period*1e3)/1e3 # Round to lowest picosecond.
         if clk in self.clocks:
             if period != self.clocks[clk]:
@@ -166,8 +167,9 @@ class GenericToolchain:
                     .format(self.clocks[clk], period))
         self.clocks[clk] = period
 
-    def add_false_path_constraint(self, platform, from_, to):
-        from_.attr.add("keep")
-        to.attr.add("keep")
+    def add_false_path_constraint(self, platform, from_, to, keep=True):
+        if keep:
+            from_.attr.add("keep")
+            to.attr.add("keep")
         if (to, from_) not in self.false_paths:
             self.false_paths.add((from_, to))