USART: Universal Asynchronous Receiver/Transmitter Driver

Overview

The MCUXpresso SDK provides a peripheral UART driver for the Universal Synchronous Receiver/Transmitter (USART) module of MCUXpresso SDK devices. Driver does not support synchronous mode.

The USART driver includes two parts: functional APIs and transactional APIs.

Functional APIs are used for USART initialization/configuration/operation for optimization/customization purpose. Using the functional API requires the knowledge of the USART peripheral and know how to organize functional APIs to meet the application requirements. All functional API use the peripheral base address as the first parameter. USART functional operation groups provide the functional APIs set.

Transactional APIs can be used to enable the peripheral quickly and in the application if the code size and performance of transactional APIs can satisfy the requirements. If the code size and performance are critical requirements, see the transactional API implementation and write custom code. All transactional APIs use the usart_handle_t as the second parameter. Initialize the handle by calling the USART_TransferCreateHandle() API.

Transactional APIs support asynchronous transfer, which means that the functions USART_TransferSendNonBlocking() and USART_TransferReceiveNonBlocking() set up an interrupt for data transfer. When the transfer completes, the upper layer is notified through a callback function with the kStatus_USART_TxIdle and kStatus_USART_RxIdle.

Transactional receive APIs support the ring buffer. Prepare the memory for the ring buffer and pass in the start address and size while calling the USART_TransferCreateHandle(). If passing NULL, the ring buffer feature is disabled. When the ring buffer is enabled, the received data is saved to the ring buffer in the background. The USART_TransferReceiveNonBlocking() function first gets data from the ring buffer. If the ring buffer does not have enough data, the function first returns the data in the ring buffer and then saves the received data to user memory. When all data is received, the upper layer is informed through a callback with the kStatus_USART_RxIdle.

If the receive ring buffer is full, the upper layer is informed through a callback with the kStatus_USART_RxRingBufferOverrun. In the callback function, the upper layer reads data out from the ring buffer. If not, the oldest data is overwritten by the new data.

The ring buffer size is specified when creating the handle. Note that one byte is reserved for the ring buffer maintenance. When creating handle using the following code:

USART_TransferCreateHandle(USART0, &handle, USART_UserCallback, NULL);

In this example, the buffer size is 32, but only 31 bytes are used for saving data.

Typical use case

USART Send/receive using a polling method

uint8_t ch;
USART_GetDefaultConfig(&user_config);
user_config.baudRate_Bps = 115200U;
user_config.enableTx = true;
user_config.enableRx = true;

USART_Init(USART1,&user_config,120000000U);

while(1)
{
    USART_ReadBlocking(USART1, &ch, 1);
    USART_WriteBlocking(USART1, &ch, 1);
}

USART Send/receive using an interrupt method

usart_handle_t g_usartHandle;
usart_config_t user_config;
usart_transfer_t sendXfer;
usart_transfer_t receiveXfer;
volatile bool txFinished;
volatile bool rxFinished;
uint8_t sendData[] = ['H', 'e', 'l', 'l', 'o'];
uint8_t receiveData[32];

void USART_UserCallback(usart_handle_t *handle, status_t status, void *userData)
{
    userData = userData;

    if (kStatus_USART_TxIdle == status)
    {
        txFinished = true;
    }

    if (kStatus_USART_RxIdle == status)
    {
        rxFinished = true;
    }
}

void main(void)
{
    //...

    USART_GetDefaultConfig(&user_config);
    user_config.baudRate_Bps = 115200U;
    user_config.enableTx = true;
    user_config.enableRx = true;

    USART_Init(USART1, &user_config, 120000000U);
    USART_TransferCreateHandle(USART1, &g_usartHandle, USART_UserCallback, NULL);

    // Prepare to send.
    sendXfer.data = sendData
    sendXfer.dataSize = sizeof(sendData);
    txFinished = false;

    // Send out.
    USART_TransferSendNonBlocking(USART1, &g_usartHandle, &sendXfer);

    // Wait send finished.
    while (!txFinished)
    {
    }

    // Prepare to receive.
    receiveXfer.data = receiveData;
    receiveXfer.dataSize = sizeof(receiveData);
    rxFinished = false;

    // Receive.
    USART_TransferReceiveNonBlocking(USART1, &g_usartHandle, &receiveXfer, NULL);

    // Wait receive finished.
    while (!rxFinished)
    {
    }

    // ...
}

USART Receive using the ringbuffer feature

#define RING_BUFFER_SIZE 64
#define RX_DATA_SIZE     32

usart_handle_t g_usartHandle;
usart_config_t user_config;
usart_transfer_t sendXfer;
usart_transfer_t receiveXfer;
volatile bool txFinished;
volatile bool rxFinished;
uint8_t receiveData[RX_DATA_SIZE];
uint8_t ringBuffer[RING_BUFFER_SIZE];

void USART_UserCallback(usart_handle_t *handle, status_t status, void *userData)
{
    userData = userData;

    if (kStatus_USART_RxIdle == status)
    {
        rxFinished = true;
    }
}

void main(void)
{
    size_t bytesRead;
    //...

    USART_GetDefaultConfig(&user_config);
    user_config.baudRate_Bps = 115200U;
    user_config.enableTx = true;
    user_config.enableRx = true;

    USART_Init(USART1, &user_config, 120000000U);
    USART_TransferCreateHandle(USART1, &g_usartHandle, USART_UserCallback, NULL);
    USART_TransferStartRingBuffer(USART1, &g_usartHandle, ringBuffer, RING_BUFFER_SIZE);
    // Now the RX is working in background, receive in to ring buffer.

    // Prepare to receive.
    receiveXfer.data = receiveData;
    receiveXfer.dataSize = sizeof(receiveData);
    rxFinished = false;

    // Receive.
    USART_TransferReceiveNonBlocking(USART1, &g_usartHandle, &receiveXfer);

    if (bytesRead = RX_DATA_SIZE) /* Have read enough data. */
    {
        ;
    }
    else
    {
        if (bytesRead) /* Received some data, process first. */
        {
            ;
        }

        // Wait receive finished.
        while (!rxFinished)
        {
        }
    }

    // ...
}

USART Send/Receive using the DMA method

usart_handle_t g_usartHandle;
dma_handle_t g_usartTxDmaHandle;
dma_handle_t g_usartRxDmaHandle;
usart_config_t user_config;
usart_transfer_t sendXfer;
usart_transfer_t receiveXfer;
volatile bool txFinished;
volatile bool rxFinished;
uint8_t sendData[] = ['H', 'e', 'l', 'l', 'o'];
uint8_t receiveData[32];

void USART_UserCallback(usart_handle_t *handle, status_t status, void *userData)
{
    userData = userData;

    if (kStatus_USART_TxIdle == status)
    {
        txFinished = true;
    }

    if (kStatus_USART_RxIdle == status)
    {
        rxFinished = true;
    }
}

void main(void)
{
    //...

    USART_GetDefaultConfig(&user_config);
    user_config.baudRate_Bps = 115200U;
    user_config.enableTx = true;
    user_config.enableRx = true;

    USART_Init(USART1, &user_config, 120000000U);

    // Set up the DMA
    DMA_Init(DMA0);
    DMA_EnableChannel(DMA0, USART_TX_DMA_CHANNEL);
    DMA_EnableChannel(DMA0, USART_RX_DMA_CHANNEL);

    DMA_CreateHandle(&g_usartTxDmaHandle, DMA0, USART_TX_DMA_CHANNEL);
    DMA_CreateHandle(&g_usartRxDmaHandle, DMA0, USART_RX_DMA_CHANNEL);

    USART_TransferCreateHandleDMA(USART1, &g_usartHandle, USART_UserCallback, NULL, &g_usartTxDmaHandle, &g_usartRxDmaHandle);

    // Prepare to send.
    sendXfer.data = sendData
    sendXfer.dataSize = sizeof(sendData);
    txFinished = false;

    // Send out.
    USART_TransferSendDMA(USART1, &g_usartHandle, &sendXfer);

    // Wait send finished.
    while (!txFinished)
    {
    }

    // Prepare to receive.
    receiveXfer.data = receiveData;
    receiveXfer.dataSize = sizeof(receiveData);
    rxFinished = false;

    // Receive.
    USART_TransferReceiveDMA(USART1, &g_usartHandle, &receiveXfer);

    // Wait receive finished.
    while (!rxFinished)
    {
    }

    // ...
}

Modules
	USART DMA Driver
	USART Driver
	USART FreeRTOS Driver