+#if HAS_SERIAL_4
+extern struct ao_samd21_usart ao_samd21_usart4;
+
+char
+ao_serial4_getchar(void);
+
+int
+_ao_serial4_pollchar(void);
+
+uint8_t
+_ao_serial4_sleep_for(uint16_t timeout);
+
+void
+ao_serial4_putchar(char c);
+
+void
+ao_serial4_drain(void);
+
+void
+ao_serial4_set_speed(uint8_t speed);
+#endif
+
+#if HAS_SERIAL_5
+extern struct ao_samd21_usart ao_samd21_usart5;
+
+char
+ao_serial5_getchar(void);
+
+int
+_ao_serial5_pollchar(void);
+
+uint8_t
+_ao_serial5_sleep_for(uint16_t timeout);
+
+void
+ao_serial5_putchar(char c);
+
+void
+ao_serial5_drain(void);
+
+void
+ao_serial5_set_speed(uint8_t speed);
+#endif
+