diff --git a/port/stm32-f4discovery-cc256x/Makefile b/port/stm32-f4discovery-cc256x/Makefile
index 426cb315f..926cd54c3 100644
--- a/port/stm32-f4discovery-cc256x/Makefile
+++ b/port/stm32-f4discovery-cc256x/Makefile
@@ -22,7 +22,7 @@ TARGET = stm32f4-discovery-cc256x
 # debug build?
 DEBUG = 1
 # optimization
-OPT = -O2
+OPT = -Os
 
 
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