串口实验，编译之后显示错误如下[mw_shl_code=applescript,true]linking...
..OBJTemplate.axf: Error: L6218E: Undefined symbol USART_Cmd (referred from usart.o).
..OBJTemplate.axf: Error: L6218E: Undefined symbol USART_GetITStatus (referred from usart.o).
..OBJTemplate.axf: Error: L6218E: Undefined symbol USART_ITConfig (referred from usart.o).
..OBJTemplate.axf: Error: L6218E: Undefined symbol USART_Init (referred from usart.o).
..OBJTemplate.axf: Error: L6218E: Undefined symbol USART_ReceiveData (referred from usart.o).[/mw_shl_code]


求指导！

[mw_shl_code=applescript,true]#ifndef __USART_H
#define __USART_H
#include "stdio.h"       
#include "stm32f4xx_conf.h"
#include "stm32f4xx.h"
#include "sys.h"
#include "stm32f4xx_usart.h"[/mw_shl_code]