diff --git a/artiq/coredevice/core.py b/artiq/coredevice/core.py
index 2f51fd8e6..e4e227d45 100644
--- a/artiq/coredevice/core.py
+++ b/artiq/coredevice/core.py
@@ -57,7 +57,7 @@ class Core(AutoContext):
             self.ref_period = self.runtime_env.internal_ref_period
             self.comm.switch_clock(False)
         else:
-            self.ref_period = self.external_clock
+            self.ref_period = 1/self.external_clock
             self.comm.switch_clock(True)
         self.initial_time = int64(self.runtime_env.warmup_time/self.ref_period)