diff --git a/Core/Src/syscalls.c b/Core/Src/syscalls.c
index 970afaa..9fdcefd 100644
--- a/Core/Src/syscalls.c
+++ b/Core/Src/syscalls.c
@@ -64,7 +64,7 @@ GETCHAR_PROTOTYPE
 
 void delay_us(uint16_t us)
 {
-    uint32_t delay = (HAL_RCC_GetHCLKFreq() / 4000000 * us);
+    volatile uint32_t delay = (HAL_RCC_GetHCLKFreq() / 4000000 * us);
     while (delay--);
 }