串口是嵌入式开发中最常前的外设设备，既可以用作不同单片机之间的通信，也可以用作在STM32 MCU和PC机之间的通信，STM32F407的串口功能非常强大，可以接红外，可以接流控，也可以接SIM卡接口，但我这里只介绍我们最常用的UART通信的一点调试经验，以STM32F407为例，对其它STM32芯片也适用，希望对大家有所帮助，如有错误不当之处欢迎大家联系指正。
一、串口的三种工作方式
操作串口一般有两种方式：查询和中断；STM32还支持第三种DMA方式。
（1）查询：串口程序不断地循环查询标志，看看当前有没有数据要它传送或接收。如果有的话进行相应的写操作和读操作进行传送或接收数据。
（2）中断：平时串口只要打开中断即可。如果发现有一个中断来，则意味着有数据需要接收（接收中断）或数据已经发送完成（发送中断）。
（3）DMA方式，设置好DMA工作方式，由DMA来自动接收或发送数据。
一般来说，查询方式的效率是比较低的，并且由于STM32的UART硬件上没有FIFO，如果程序功能比较多，查询不及时的话很容易出现数据丢失的现象， 故实际项目中这种方式用的并不多。
中断方式的话我们可以分别设置接收中断和发送中断，当串口有数据需要接收时才进入中断程序进行读读操，这种方式占用CPU资源比较少，实际项目中比较常用，但需要注意中断程序不要太复杂使执行时间太长，如果执行时间超过一个字符的时间的话也会出现数据丢失的现象，这个波特率比较高的串口编程中比较容易出现，可以考虑用循环BUF方法，在中断程序中只负责实时地接收实数数和发送时的填数（写发送寄存器），其它操作放在中断外处理。
STM32还提供了第三种DMA方式用来支持高速地串口传输。这种方式只要设置好接收和发送缓冲位置，可以由DMA来自动接收和发送数据，这可以最小化占用CPU时间。
二、串口的使用步骤
（1）中断方式
基本步骤是初试化时钟，脚位、波特率设置、安装中断服务程序、开中断等，参考代码如下：
void uart_init(void)
{
USART_InitTypeDef USART_InitStructure;
NVIC_InitTypeDef NVIC_InitStructure;
GPIO_InitTypeDef GPIO_InitStructure;
/* Enable GPIO clock */
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOC, ENABLE);
/* Enable USART clock */
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);
/* Connect USART pins to AF7 */
GPIO_PinAFConfig(GPIOC, GPIO_PinSource10, GPIO_AF_USART3);
GPIO_PinAFConfig(GPIOC, GPIO_PinSource11, GPIO_AF_USART3);
/* Configure USART Tx and Rx as alternate function push-pull */
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
GPIO_Init(GPIOC, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
GPIO_Init(GPIOC, &GPIO_InitStructure);
/* USARTx configuration ----------------------------------------------------*/
/* USARTx configured as follow:
- BaudRate = 3750000 baud
- Maximum BaudRate that can be achieved when using the Oversampling by 8
is: (USART APB Clock / 8)
Example:
- (USART3 APB1 Clock / 8) = (30 MHz / 8) = 3750000 baud
- (USART1 APB2 Clock / 8) = (60 MHz / 8) = 7500000 baud
- Maximum BaudRate that can be achieved when using the Oversampling by 16
is: (USART APB Clock / 16)
Example: (USART3 APB1 Clock / 16) = (30 MHz / 16) = 1875000 baud
Example: (USART1 APB2 Clock / 16) = (60 MHz / 16) = 3750000 baud
- Word Length = 8 Bits
- one Stop Bit
- No parity
- Hardware flow control disabled (RTS and CTS signals)
- Receive and transmit enabled
*/
USART_InitStructure.USART_BaudRate = 115200;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART3, &USART_InitStructure);
/* NVIC configuration */
/* Configure the Priority Group to 2 bits */
NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);
/* Enable the USARTx Interrupt */
NVIC_InitStructure.NVIC_IRQChannel = USART3_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
/* Enable USART */
USART_Cmd(USART3, ENABLE);
USART_ITConfig(USART3, USART_IT_RXNE, ENABLE);
}
中断服务程序如下：
void USART3_IRQHandler(void)
{
unsigned char ch;
if(USART_GetITStatus(USART3, USART_IT_RXNE) != RESET)
{
/* Read one byte from the receive data register */
ch = (USART_ReceiveData(USART3));
printf("in[%c].\r\n",ch);
}
}
直接把接收到的字符打印出来。
（2）DMA方式
基本步骤同中断方式，额外需要DMA的初始化配置，参考代码如下：
void uart_init(void)
{
USART_InitTypeDef USART_InitStructure;
NVIC_InitTypeDef NVIC_InitStructure;
GPIO_InitTypeDef GPIO_InitStructure;
DMA_InitTypeDef DMA_InitStruct;
/* Enable GPIO clock */
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOC, ENABLE);
/* Enable USART clock */
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);
/* Connect USART pins to AF7 */
GPIO_PinAFConfig(GPIOC, GPIO_PinSource10, GPIO_AF_USART3);
GPIO_PinAFConfig(GPIOC, GPIO_PinSource11, GPIO_AF_USART3);
/* Configure USART Tx and Rx as alternate function push-pull */
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
GPIO_Init(GPIOC, &GPIO_InitStructure);
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
GPIO_Init(GPIOC, &GPIO_InitStructure);
/* USARTx configuration ----------------------------------------------------*/
/* USARTx configured as follow:
- BaudRate = 3750000 baud
- Maximum BaudRate that can be achieved when using the Oversampling by 8
is: (USART APB Clock / 8)
Example:
- (USART3 APB1 Clock / 8) = (30 MHz / 8) = 3750000 baud
- (USART1 APB2 Clock / 8) = (60 MHz / 8) = 7500000 baud
- Maximum BaudRate that can be achieved when using the Oversampling by 16
is: (USART APB Clock / 16)
Example: (USART3 APB1 Clock / 16) = (30 MHz / 16) = 1875000 baud
Example: (USART1 APB2 Clock / 16) = (60 MHz / 16) = 3750000 baud
- Word Length = 8 Bits
- one Stop Bit
- No parity
- Hardware flow control disabled (RTS and CTS signals)
- Receive and transmit enabled
*/
USART_InitStructure.USART_BaudRate = 115200;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART3, &USART_InitStructure);
/* DMA_Configuration */
DMA_DeInit(DMA1_Stream1);
DMA_InitStruct.DMA_Channel = DMA_Channel_4;
DMA_InitStruct.DMA_PeripheralBaseAddr = (u32)&USART3->DR; //source buf
DMA_InitStruct.DMA_Memory0BaseAddr = (u8)pdata; //target buf
DMA_InitStruct.DMA_DIR = DMA_DIR_PeripheralToMemory;
DMA_InitStruct.DMA_BufferSize = lenght; //BuffSize;
DMA_InitStruct.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA_InitStruct.DMA_MemoryInc = DMA_MemoryInc_Enable;
DMA_InitStruct.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Word;
DMA_InitStruct.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
DMA_InitStruct.DMA_Mode = DMA_Mode_Circular; //DMA_Mode_Normal;
DMA_InitStruct.DMA_Priority = DMA_Priority_High;
DMA_InitStruct.DMA_FIFOMode = DMA_FIFOMode_Disable;
DMA_InitStruct.DMA_FIFOThreshold = DMA_FIFOThreshold_HalfFull;
DMA_InitStruct.DMA_MemoryBurst = DMA_MemoryBurst_Single;
DMA_InitStruct.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
DMA_Init(DMA1_Stream1, &DMA_InitStruct);
/* NVIC configuration */
/* Configure the Priority Group to 2 bits */
NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);
/* Enable the USARTx Interrupt */
NVIC_InitStructure.NVIC_IRQChannel = DMA1_Stream1_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
NVIC_Init(&NVIC_InitStructure);
/* Open DMA interrupt*/
DMA_ITConfig(DMA1_Stream1, DMA_IT_TC, ENABLE);
DMA_Cmd(DMA1_Stream1, ENABLE);
USART_Cmd(USART3, ENABLE);
USART_DMACmd(USART3,USART_DMAReq_Rx,ENABLE);
}
DMA中断服务程序如下：
void DMA1_Stream1_IRQHandler(void) //UART3_RX
{
static short i;
//When a Transfer Complete
if(SET == DMA_GetITStatus(DMA1_Stream1, DMA_IT_TCIF1))
{
DMA_ClearITPendingBit(DMA1_Stream1, DMA_IT_TCIF1);
i++;
}
}
上面程序只配了DMA接收，发送类似。
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