接上篇有了uart設(shè)備對(duì)象及rt_uart_ops結(jié)構(gòu)體以后，我們就需要提供對(duì)應(yīng)的真實(shí)的能操控硬件的函數(shù)，其實(shí)真正操作的硬件的函數(shù)并不多，創(chuàng)建的結(jié)構(gòu)體對(duì)象如下：

static const struct rt_uart_ops stm32_uart_ops =
{
    .configure = stm32_configure,
    .control = stm32_control,
    .putc = stm32_putc,
    .getc = stm32_getc,
    .dma_transmit = stm32_dma_transmit
};

      先來看下基于hal庫代碼實(shí)現(xiàn)的stm32_configure的完整代碼，當(dāng)你需要更換其它單片機(jī)時(shí)，再系統(tǒng)沒有提供對(duì)應(yīng)的bsp的情況下，你可以仿照代碼進(jìn)行重構(gòu)該函數(shù)用于適用于特殊的硬件，系統(tǒng)也提供了很多種不同的芯片的bsp：

static rt_err_t stm32_configure(struct rt_serial_device *serial, struct serial_configure *cfg)
{
    struct stm32_uart *uart;
    RT_ASSERT(serial != RT_NULL);
    RT_ASSERT(cfg != RT_NULL);

    uart = rt_container_of(serial, struct stm32_uart, serial);

    uart->handle.Instance          = uart->config->Instance;
    uart->handle.Init.BaudRate     = cfg->baud_rate;
    uart->handle.Init.HwFlowCtl    = UART_HWCONTROL_NONE;
    uart->handle.Init.Mode         = UART_MODE_TX_RX;
    uart->handle.Init.OverSampling = UART_OVERSAMPLING_16;
    switch (cfg->data_bits)
    {
    case DATA_BITS_8:
        uart->handle.Init.WordLength = UART_WORDLENGTH_8B;
        break;
    case DATA_BITS_9:
        uart->handle.Init.WordLength = UART_WORDLENGTH_9B;
        break;
    default:
        uart->handle.Init.WordLength = UART_WORDLENGTH_8B;
        break;
    }
    switch (cfg->stop_bits)
    {
    case STOP_BITS_1:
        uart->handle.Init.StopBits   = UART_STOPBITS_1;
        break;
    case STOP_BITS_2:
        uart->handle.Init.StopBits   = UART_STOPBITS_2;
        break;
    default:
        uart->handle.Init.StopBits   = UART_STOPBITS_1;
        break;
    }
    switch (cfg->parity)
    {
    case PARITY_NONE:
        uart->handle.Init.Parity     = UART_PARITY_NONE;
        break;
    case PARITY_ODD:
        uart->handle.Init.Parity     = UART_PARITY_ODD;
        break;
    case PARITY_EVEN:
        uart->handle.Init.Parity     = UART_PARITY_EVEN;
        break;
    default:
        uart->handle.Init.Parity     = UART_PARITY_NONE;
        break;
    }

#ifdef RT_SERIAL_USING_DMA
    uart->dma_rx.last_index = 0;
#endif

    if (HAL_UART_Init(&uart->handle) != HAL_OK)
    {
        return -RT_ERROR;
    }

    return RT_EOK;
}

      代碼很長(zhǎng)，但是功能很簡(jiǎn)單就是對(duì)串口進(jìn)行初始化，設(shè)置串口波特率，硬件流控，串口模式、串口采樣、數(shù)據(jù)位、停止位、DMA相關(guān)參數(shù)、最后調(diào)用HAL_UART_Init()完成串口的初始化功能。

      接下來看下stm32_control函數(shù)的代碼，代碼雖長(zhǎng)單功能并不復(fù)雜：

static rt_err_t stm32_control(struct rt_serial_device *serial, int cmd, void *arg)
{
    struct stm32_uart *uart;
#ifdef RT_SERIAL_USING_DMA
    rt_ubase_t ctrl_arg = (rt_ubase_t)arg;
#endif

    RT_ASSERT(serial != RT_NULL);
    uart = rt_container_of(serial, struct stm32_uart, serial);

    switch (cmd)
    {
    /* disable interrupt */
    case RT_DEVICE_CTRL_CLR_INT:
        /* disable rx irq */
        NVIC_DisableIRQ(uart->config->irq_type);
        /* disable interrupt */
        __HAL_UART_DISABLE_IT(&(uart->handle), UART_IT_RXNE);

#ifdef RT_SERIAL_USING_DMA
        /* disable DMA */
        if (ctrl_arg == RT_DEVICE_FLAG_DMA_RX)
        {
            HAL_NVIC_DisableIRQ(uart->config->dma_rx->dma_irq);
            if (HAL_DMA_Abort(&(uart->dma_rx.handle)) != HAL_OK)
            {
                RT_ASSERT(0);
            }

            if (HAL_DMA_DeInit(&(uart->dma_rx.handle)) != HAL_OK)
            {
                RT_ASSERT(0);
            }
        }
        else if(ctrl_arg == RT_DEVICE_FLAG_DMA_TX)
        {
            HAL_NVIC_DisableIRQ(uart->config->dma_tx->dma_irq);
            if (HAL_DMA_DeInit(&(uart->dma_tx.handle)) != HAL_OK)
            {
                RT_ASSERT(0);
            }
        }
#endif
        break;
    /* enable interrupt */
    case RT_DEVICE_CTRL_SET_INT:
        /* enable rx irq */
        HAL_NVIC_SetPriority(uart->config->irq_type, 1, 0);
        HAL_NVIC_EnableIRQ(uart->config->irq_type);
        /* enable interrupt */
        __HAL_UART_ENABLE_IT(&(uart->handle), UART_IT_RXNE);
        break;

#ifdef RT_SERIAL_USING_DMA
    case RT_DEVICE_CTRL_CONFIG:
        stm32_dma_config(serial, ctrl_arg);
        break;
#endif

    case RT_DEVICE_CTRL_CLOSE:
        if (HAL_UART_DeInit(&(uart->handle)) != HAL_OK )
        {
            RT_ASSERT(0)
        }
        break;

    }
    return RT_EOK;
}

      主要看一下幾個(gè)控制參數(shù)：RT_DEVICE_CTRL_CLR_INT關(guān)閉中斷，如果有使用dma則關(guān)閉DMA。RT_DEVICE_CTRL_SET_INT：開啟終端，如果有使用dma則采用dma_config功能打開dma。RT_DEVICE_CTRL_CLOSE:關(guān)閉串口，就這幾個(gè)功能。

      發(fā)送函數(shù)功能最簡(jiǎn)單，輕松TC中斷標(biāo)記，將數(shù)據(jù)放入DR數(shù)據(jù)寄存器中，while循環(huán)等待TC中斷觸發(fā)（DR數(shù)據(jù)寄存器為空）：

static int stm32_putc(struct rt_serial_device *serial, char c)
{
    struct stm32_uart *uart;
    RT_ASSERT(serial != RT_NULL);

    uart = rt_container_of(serial, struct stm32_uart, serial);
    UART_INSTANCE_CLEAR_FUNCTION(&(uart->handle), UART_FLAG_TC);
#if defined(SOC_SERIES_STM32L4) || defined(SOC_SERIES_STM32F7) || defined(SOC_SERIES_STM32F0) \
    || defined(SOC_SERIES_STM32L0) || defined(SOC_SERIES_STM32G0) || defined(SOC_SERIES_STM32H7) \
    || defined(SOC_SERIES_STM32G4)
    uart->handle.Instance->TDR = c;
#else
    uart->handle.Instance->DR = c;
#endif
    while (__HAL_UART_GET_FLAG(&(uart->handle), UART_FLAG_TC) == RESET);
    return 1;
}

      接下來是讀取數(shù)據(jù)，這里有個(gè)有意思的現(xiàn)象，看過源碼你會(huì)發(fā)現(xiàn)讀取數(shù)據(jù)和寫入數(shù)據(jù)訪問的實(shí)際的硬件寄存器都是一樣的，DR or RDR，那么你會(huì)不會(huì)有個(gè)疑問，那么某一刻讀取的數(shù)據(jù)會(huì)不會(huì)是剛寫入的數(shù)據(jù)，代碼之后揭曉：

static int stm32_getc(struct rt_serial_device *serial)
{
    int ch;
    struct stm32_uart *uart;
    RT_ASSERT(serial != RT_NULL);
    uart = rt_container_of(serial, struct stm32_uart, serial);

    ch = -1;
    if (__HAL_UART_GET_FLAG(&(uart->handle), UART_FLAG_RXNE) != RESET)
    {
#if defined(SOC_SERIES_STM32L4) || defined(SOC_SERIES_STM32F7) || defined(SOC_SERIES_STM32F0) \
    || defined(SOC_SERIES_STM32L0) || defined(SOC_SERIES_STM32G0) || defined(SOC_SERIES_STM32H7) \
    || defined(SOC_SERIES_STM32G4)
        ch = uart->handle.Instance->RDR & 0xff;
#else
        ch = uart->handle.Instance->DR & 0xff;
#endif
    }
    return ch;
}

      揭曉答案啦：這里以DR為例，其實(shí)它是一對(duì)神奇的雙胞胎，一個(gè)名字對(duì)應(yīng)著兩個(gè)真實(shí)的寄存器，當(dāng)你寫入數(shù)據(jù)時(shí)實(shí)際寫入的是發(fā)送DR,當(dāng)你讀取數(shù)據(jù)時(shí)實(shí)際訪問的是接收DR，是通過你對(duì)硬件的讀or寫操作，硬件自行對(duì)應(yīng)的，當(dāng)年在這個(gè)問題上耗死了好些的腦細(xì)胞，也不知道是哪個(gè)天才設(shè)計(jì)的機(jī)制。

      最后到了還有連續(xù)傳送字節(jié)（發(fā)送）的實(shí)現(xiàn)，借助的硬件的DMA技術(shù)實(shí)現(xiàn)的：

static rt_size_t stm32_dma_transmit(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
{
    struct stm32_uart *uart;
    RT_ASSERT(serial != RT_NULL);
    RT_ASSERT(buf != RT_NULL);
    uart = rt_container_of(serial, struct stm32_uart, serial);

    if (size == 0)
    {
        return 0;
    }

    if (RT_SERIAL_DMA_TX == direction)
    {
        if (HAL_UART_Transmit_DMA(&uart->handle, buf, size) == HAL_OK)
        {
            return size;
        }
        else
        {
            return 0;
        }
    }
    return 0;
}

      帶著問題結(jié)束本篇吧，發(fā)送很簡(jiǎn)單 一個(gè)接一個(gè)的發(fā)送出去，那么接收呢，DR中有沒有數(shù)據(jù)，是不是需要頻繁的觸發(fā)讀取函數(shù)，或者讀不及時(shí)會(huì)不會(huì)被下一個(gè)來的數(shù)據(jù)將DR中的數(shù)據(jù)覆蓋，或者數(shù)據(jù)直接被丟掉呢？