diff --git a/litex/build/generic_toolchain.py b/litex/build/generic_toolchain.py
index f7b832ad8..987268744 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]:
diff --git a/litex/build/xilinx/platform.py b/litex/build/xilinx/platform.py
index 14f6092bb..b3bfb1029 100644
--- a/litex/build/xilinx/platform.py
+++ b/litex/build/xilinx/platform.py
@@ -72,11 +72,11 @@ class XilinxPlatform(GenericPlatform):
     def build(self, *args, **kwargs):
         return self.toolchain.build(self, *args, **kwargs)
 
-    def add_period_constraint(self, clk, period):
+    def add_period_constraint(self, clk, period, keep=True):
         if clk is None: return
         if hasattr(clk, "p"):
             clk = clk.p
-        self.toolchain.add_period_constraint(self, clk, period)
+        self.toolchain.add_period_constraint(self, clk, period, keep=keep)
 
     def add_false_path_constraint(self, from_, to):
         if hasattr(from_, "p"):
diff --git a/litex/build/xilinx/vivado.py b/litex/build/xilinx/vivado.py
index 8535d2948..21400e7b4 100644
--- a/litex/build/xilinx/vivado.py
+++ b/litex/build/xilinx/vivado.py
@@ -384,7 +384,6 @@ class XilinxVivadoToolchain(GenericToolchain):
         if tools.subprocess_call_filtered(shell + [script], common.colors) != 0:
             raise OSError("Error occured during Vivado's script execution.")
 
-
 def vivado_build_args(parser):
     toolchain_group = parser.add_argument_group(title="Toolchain options")
     toolchain_group.add_argument("--synth-mode",                           default="vivado",  help="Synthesis mode (vivado or yosys).")