int stm32_hw_usart_init(void)
{struct stm32_uart *uart;struct serial_configure config = RT_SERIAL_CONFIG_DEFAULT;RCC_Configuration();GPIO_Configuration();#ifdef RT_USING_UART2uart = &uart2;serial2.ops    = &stm32_uart_ops;serial2.config = config;NVIC_Configuration(&uart2);/* register UART2 device */rt_hw_serial_register(&serial2,"uart2",RT_DEVICE_FLAG_RDWR | RT_DEVICE_FLAG_INT_RX, //dev->flag (not dev->open_flag)
                          uart);
#endif /* RT_USING_UART2 */#ifdef RT_USING_UART3
...
#endif /* RT_USING_UART3 */return 0;
}
//INIT_BOARD_EXPORT(stm32_hw_usart_init); //it must be invoked in board.c(rt_hw_board_init for setting CONSOLE_DEVICE)

/* STM32 uart driver */
struct stm32_uart
{USART_TypeDef *uart_device;IRQn_Type irq;
};

static const struct rt_uart_ops stm32_uart_ops =
{stm32_configure,stm32_control,stm32_putc,stm32_getc,
};

struct serial_configure config = RT_SERIAL_CONFIG_DEFAULT;

/* register UART2 device */
rt_hw_serial_register(&serial2,"uart2",RT_DEVICE_FLAG_RDWR | RT_DEVICE_FLAG_INT_RX, //dev->flag (not dev->open_flag)uart);

/** Serial FIFO mode */
struct rt_serial_rx_fifo
{/* software fifo */rt_uint8_t *buffer;rt_uint16_t put_index, get_index;
};struct rt_serial_tx_fifo
{struct rt_completion completion;
};/* * Serial DMA mode*/
struct rt_serial_rx_dma
{rt_bool_t activated;
};struct rt_serial_tx_dma
{rt_bool_t activated;struct rt_data_queue data_queue;
};struct rt_serial_device
{struct rt_device          parent;const struct rt_uart_ops *ops;struct serial_configure   config;void *serial_rx;void *serial_tx;
};
typedef struct rt_serial_device rt_serial_t;

/*** uart operators*/
struct rt_uart_ops
{rt_err_t (*configure)(struct rt_serial_device *serial, struct serial_configure *cfg);rt_err_t (*control)(struct rt_serial_device *serial, int cmd, void *arg);int (*putc)(struct rt_serial_device *serial, char c);int (*getc)(struct rt_serial_device *serial);rt_size_t (*dma_transmit)(struct rt_serial_device *serial, const rt_uint8_t *buf, rt_size_t size, int direction);
};

/** serial register*/
rt_err_t rt_hw_serial_register(struct rt_serial_device *serial,const char              *name,rt_uint32_t              flag,void                    *data)
{struct rt_device *device;RT_ASSERT(serial != RT_NULL);device = &(serial->parent);device->type        = RT_Device_Class_Char;device->rx_indicate = RT_NULL;device->tx_complete = RT_NULL;device->init        = rt_serial_init;device->open        = rt_serial_open;device->close       = rt_serial_close;device->read        = rt_serial_read;device->write       = rt_serial_write;device->control     = rt_serial_control;device->user_data   = data;/* register a character device */return rt_device_register(device, name, flag);
}

#if defined(RT_USING_UART2)
/* UART2 device driver structure */
struct stm32_uart uart2 =
{USART2,USART2_IRQn,
};
struct rt_serial_device serial2;void USART2_IRQHandler(void)
{struct stm32_uart *uart;uart = &uart2;/* enter interrupt */rt_interrupt_enter();if (USART_GetITStatus(uart->uart_device, USART_IT_RXNE) != RESET){rt_hw_serial_isr(&serial2, RT_SERIAL_EVENT_RX_IND);//USART_IT_RXNE is cleared automatically by reading USART_DR in stm32_getc()
    }if (USART_GetITStatus(uart->uart_device, USART_IT_TC) != RESET){/* clear interrupt */USART_ClearITPendingBit(uart->uart_device, USART_IT_TC);}/* leave interrupt */rt_interrupt_leave();
}
#endif /* RT_USING_UART2 */

该中断函数在usart.c中，在RTT下的每一个中断服务子程序的入口都调用了rt_interrupt_enter()，在中断函数的子程序的出口则调用了rt_interrupt_leave()。

/* ISR for serial interrupt */
void rt_hw_serial_isr(struct rt_serial_device *serial, int event)
{switch (event & 0xff){case RT_SERIAL_EVENT_RX_IND:{int ch = -1;rt_base_t level;struct rt_serial_rx_fifo* rx_fifo;rx_fifo = (struct rt_serial_rx_fifo*)serial->serial_rx;RT_ASSERT(rx_fifo != RT_NULL);/* interrupt mode receive */RT_ASSERT(serial->parent.open_flag & RT_DEVICE_FLAG_INT_RX);while (1){ch = serial->ops->getc(serial);if (ch == -1) break;/* disable interrupt */level = rt_hw_interrupt_disable();rx_fifo->buffer[rx_fifo->put_index] = ch;rx_fifo->put_index += 1;if (rx_fifo->put_index >= serial->config.bufsz) rx_fifo->put_index = 0;/* if the next position is read index, discard this 'read char' */if (rx_fifo->put_index == rx_fifo->get_index){rx_fifo->get_index += 1;if (rx_fifo->get_index >= serial->config.bufsz) rx_fifo->get_index = 0;}/* enable interrupt */rt_hw_interrupt_enable(level);}/* invoke callback */if (serial->parent.rx_indicate != RT_NULL){rt_size_t rx_length;/* get rx length */level = rt_hw_interrupt_disable();rx_length = (rx_fifo->put_index >= rx_fifo->get_index)? (rx_fifo->put_index - rx_fifo->get_index):(serial->config.bufsz - (rx_fifo->get_index - rx_fifo->put_index));rt_hw_interrupt_enable(level);serial->parent.rx_indicate(&serial->parent, rx_length);}break;}case RT_SERIAL_EVENT_TX_DONE:{struct rt_serial_tx_fifo* tx_fifo;tx_fifo = (struct rt_serial_tx_fifo*)serial->serial_tx;rt_completion_done(&(tx_fifo->completion));break;}case RT_SERIAL_EVENT_TX_DMADONE:{const void *data_ptr;rt_size_t data_size;const void *last_data_ptr;struct rt_serial_tx_dma* tx_dma;tx_dma = (struct rt_serial_tx_dma*) serial->serial_tx;rt_data_queue_pop(&(tx_dma->data_queue), &last_data_ptr, &data_size, 0);if (rt_data_queue_peak(&(tx_dma->data_queue), &data_ptr, &data_size) == RT_EOK){/* transmit next data node */tx_dma->activated = RT_TRUE;serial->ops->dma_transmit(serial, data_ptr, data_size, RT_SERIAL_DMA_TX);}else{tx_dma->activated = RT_FALSE;}/* invoke callback */if (serial->parent.tx_complete != RT_NULL){serial->parent.tx_complete(&serial->parent, (void*)last_data_ptr);}break;}case RT_SERIAL_EVENT_RX_DMADONE:{int length;struct rt_serial_rx_dma* rx_dma;rx_dma = (struct rt_serial_rx_dma*)serial->serial_rx;/* get DMA rx length */length = (event & (~0xff)) >> 8;serial->parent.rx_indicate(&(serial->parent), length);rx_dma->activated = RT_FALSE;break;}}
}

该函数位于serial.c中，默认情况下usart的rt_device结构体中rx_indicate域被置空，因此不会运行这一段代码。如果使用rt_device_set_rx_indicate(rt_device_t dev, rt_err_t(* rx_ind)(rt_device_t dev, rt_size_t size))函数为一个串口设备注册了接收事件回调函数，在该串口接收到数据后，就会调用之前注册的rx_ind函数，将当前设备指针以及待读取的数据长度作为调用参数传递给用户。