diff --git a/litex_boards/platforms/sipeed_tang_nano_4k.py b/litex_boards/platforms/sipeed_tang_nano_4k.py
index 61397ff..a80e632 100644
--- a/litex_boards/platforms/sipeed_tang_nano_4k.py
+++ b/litex_boards/platforms/sipeed_tang_nano_4k.py
@@ -91,4 +91,4 @@ class Platform(GowinPlatform):
 
     def do_finalize(self, fragment):
         GowinPlatform.do_finalize(self, fragment)
-        self.add_period_constraint(self.lookup_request("clk27", loose=True), 1e9/100e6)
+        self.add_period_constraint(self.lookup_request("clk27", loose=True), 1e9/27e6)