diff --git a/bsps/arm/stm32h7/boards/stm/stm32h743i-eval/ext-mem-ctl.c b/bsps/arm/stm32h7/boards/stm/stm32h743i-eval/ext-mem-ctl.c
index a2ab9d8f1f..a16bc84083 100644
--- a/bsps/arm/stm32h7/boards/stm/stm32h743i-eval/ext-mem-ctl.c
+++ b/bsps/arm/stm32h7/boards/stm/stm32h743i-eval/ext-mem-ctl.c
@@ -38,6 +38,10 @@
 #define DATA_IN_ExtSRAM
 #define DATA_IN_ExtSDRAM
 
+#ifdef __rtems__
+/* needs prototype and do not want to include <bsp.h> */
+void  SystemInit_ExtMemCtl(void);
+#endif
 void  SystemInit_ExtMemCtl(void)
 {