在外部中断中将printf写入，外部中断LED可以翻转，但串口不可显示

```/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "dma.h"
#include "usart.h"
#include "gpio.h"
#include "stdio.h"
/* Private includes ----------------------------------------------------------*/

void SystemClock_Config(void);

int main(void)
{
HAL_Init();
SystemClock_Config();
MX_GPIO_Init();
MX_DMA_Init();
MX_USART1_UART_Init();

while (1)
{

printf("key down\r\n");

}
/* USER CODE END 3 */
}/**@brief System Clock Configuration@retval None
*/
void SystemClock_Config(void)
{
RCC_OscInitTypeDef RCC_OscInitStruct = {0};
RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};/** Configure the main internal regulator output voltage
*/
HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1_BOOST);/** Initializes the RCC Oscillators according to the specified parametersin the RCC_OscInitTypeDef structure.
*/
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
RCC_OscInitStruct.HSEState = RCC_HSE_ON;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV6;
RCC_OscInitStruct.PLL.PLLN = 85;
RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
{
Error_Handler();
}/** Initializes the CPU, AHB and APB buses clocks
*/
RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
|RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_4) != HAL_OK)
{
Error_Handler();
}
}/* USER CODE BEGIN 4 *//* USER CODE END 4 *//**@brief  This function is executed in case of error occurrence.@retval None
*/
void Error_Handler(void)
{
/* USER CODE BEGIN Error_Handler_Debug */
/* User can add his own implementation to report the HAL error return state */
__disable_irq();
while (1)
{
}
/* USER CODE END Error_Handler_Debug */
}#ifdef  USE_FULL_ASSERT
/**@brief  Reports the name of the source file and the source line number    where the assert_param error has occurred.
    where the assert_param error has occurred.
@param  file: pointer to the source file name@param  line: assert_param error line source number@retval None
*/
void assert_failed(uint8_t *file, uint32_t line)
{
/* USER CODE BEGIN 6 */
/* User can add his own implementation to report the file name and line number,
ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
/* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */
int fputc(int ch,FILE *f)
{
while((USART1->ISR&0X40)==0);//中断状态寄存器
USART3->TDR=(uint8_t)ch;//传输数据寄存器return ch;
}
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin)
{
UNUSED(GPIO_Pin);
if(bule_key_Pin==GPIO_Pin)
{
HAL_GPIO_TogglePin(led2_GPIO_Port,led2_Pin);

printf("key down\r\n");
}
}```

```

fputc函数里用了uart1和uart3，怎么这样子？

检查一下对应初始化的是哪个串口


判断和发送的串口号不一致

可以参考库中的例程

先保证UART的字符能在串口终端，这就要求你所用片内UART跟串口助手TX/RX连接OK，
再就是波特率、帧格式两边一致；


如果要通过重定向做打印输出，写法及用法可以参考相关例程，比方：


\STM32Cube_FW_F4_V1.28.1\Projects\STM32F401RE-Nucleo\Examples\UART\UART_Printf
\STM32Cube_FW_G4_V1.6.0\Projects\NUCLEO-G474RE\Examples\UART\UART_Printf
\STM32Cube_FW_F1_V1.8.6\Projects\STM3210E_EVAL\Examples\UART\UART_Printf
......


其实也可以使用sprintf做字符转换后直接调用UART发送。