diff --git a/bsp/rm48x50/rtconfig.h b/bsp/rm48x50/rtconfig.h
index 86fc630c8c..d681ef736e 100644
--- a/bsp/rm48x50/rtconfig.h
+++ b/bsp/rm48x50/rtconfig.h
@@ -212,6 +212,7 @@
 // </section>
 
 #define RT_VFP_LAZY_STACKING
+#define RT_USING_CPU_FFS
 // </RDTConfigurator>
 
 #endif
diff --git a/libcpu/arm/cortex-r4/cpu.c b/libcpu/arm/cortex-r4/cpu.c
index 569f7d0c29..3aa7cd35de 100644
--- a/libcpu/arm/cortex-r4/cpu.c
+++ b/libcpu/arm/cortex-r4/cpu.c
@@ -39,4 +39,17 @@ void rt_hw_cpu_shutdown()
 	while (1);
 }
 
+#ifdef RT_USING_CPU_FFS
+int __rt_ffs(int value)
+{
+	if (value == 0)
+		return value;
+
+	__asm("    rsb r1, r0, #0");
+	__asm("    and r1, r1, r0");
+	__asm("    clz r1, r1");
+	__asm("    rsb r0, r1, #32");
+}
+#endif
+
 /*@}*/