diff --git a/src/mcu_program/spi.cpp b/src/mcu_program/spi.cpp
index 24590a6b1..e2f8f2e1e 100644
--- a/src/mcu_program/spi.cpp
+++ b/src/mcu_program/spi.cpp
@@ -5,8 +5,8 @@
 
 static lime::LMS7002M* serPort;
 
-extern float bandwidthRF;
-extern float RefClk;
+extern "C" float bandwidthRF;
+extern "C" float RefClk;
 
 void SetupCalibrations(lime::LMS7002M* chip, double BW)
 {