UART

Overview

The Universal Asynchronous Receiver/Transmitter (UART) is a universal serial data bus used for asynchronous communication. It enables bi-directional communication between devices in full-duplex mode.

UART is widely used to print information for debugging or to connect to various external modules such as GPS and Bluetooth.

A UART is connected to other modules through two wires (as shown in Figure 1) or four wires (as shown in Figure 2).

• TX: TX pin of the transmitting UART. It is connected to the RX pin of the peer UART.

• RX: RX pin of the receiving UART. It is connected to the TX pin of the peer UART.

• RTS: Request to Send signal pin. It is connected to the CTS pin of the peer UART and is used to indicate whether the local UART is ready to receive data.

• CTS: Clear to Send signal pin. It is connected to the RTS pin of the peer UART and is used to indicate whether the local UART is allowed to send data to the peer end.

  Figure 1 Two-wire UART communication

  

  Figure 2 Four-wire UART communication

  

• The transmitting and receiving UARTs must ensure that they have the same settings on particular attributes such as the baud rate and data format (start bit, data bit, parity bit, and stop bit) before they start to communicate. During data transmission, a UART sends data to the peer end over the TX pin and receives data from the peer end over the RX pin. When the size of the buffer used by a UART for storing received data reaches the preset threshold, the RTS signal of the UART changes to 1 (data cannot be received), and the peer UART stops sending data to it because its CTS signal does not allow it to send data.

• The UART interface defines a set of common functions for operating a UART port, including obtaining and releasing device handles, reading and writing data of a specified length, and obtaining and setting the baud rate, as well as the device attributes.

Available APIs

Table 1 UART driver APIs

NOTE
All APIs described in this document can be called only in kernel mode.

Usage Guidelines

How to Use

The figure below illustrates how to use the APIs.

Figure 3 Using UART driver APIs

Opening a UART Device Handle

Before performing UART communication, call UartOpen to obtain a UART device handle. This function returns the pointer to the UART device handle with the specified port number.

DevHandle UartOpen(uint32_t port);

Table 2 Description of UartOpen

Example: Obtain the device handle of UART port 3.

DevHandle handle = NULL;    /* UART device handle */
uint32_t port = 3;          /* UART port number */
handle = UartOpen(port);
if (handle == NULL) {
    HDF_LOGE("UartOpen: failed!\n");
    return;
}

Setting the UART Baud Rate

Call UartSetBaud() to set the UART baud rate.

int32_t UartSetBaud(DevHandle handle, uint32_t baudRate);

Table 3 Description of UartSetBaud

Example: Set the UART baud rate to 9600.

int32_t ret;
/* Set the UART baud rate to 9600. */
ret = UartSetBaud(handle, 9600);
if (ret != 0) {
    HDF_LOGE("UartSetBaud: failed, ret %d\n", ret);
}

Obtaining the UART Baud Rate

Call UartGetBaud() to obtain the UART baud rate.

int32_t UartGetBaud(DevHandle handle, uint32_t *baudRate);

Table 4 Description of UartGetBaud

Example: Obtain the UART baud rate.

int32_t ret;
uint32_t baudRate;
/* Obtain the UART baud rate. */
ret = UartGetBaud(handle, &baudRate);
if (ret != 0) {
    HDF_LOGE("UartGetBaud: failed, ret %d\n", ret);
}

Setting UART Device Attributes

Call UartSetAttribute() to set UART device attributes.

int32_t UartSetAttribute(DevHandle handle, struct UartAttribute *attribute);

Table 5 Description of UartSetAttribute

Example: Set UART device attributes.

int32_t ret;
struct UartAttribute attribute;
attribute.dataBits = UART_ATTR_DATABIT_7;   /* Enable 7 bits to be transferred each time. */
attribute.parity = UART_ATTR_PARITY_NONE;   /* Disable parity check. */
attribute.stopBits = UART_ATTR_STOPBIT_1;   /* Set the stop bit to 1. */
attribute.rts = UART_ATTR_RTS_DIS;          /* Disable the RTS signal. */
attribute.cts = UART_ATTR_CTS_DIS;          /* Disable the CTS signal. */
attribute.fifoRxEn = UART_ATTR_RX_FIFO_EN;  /* Enable RX FIFO. */
attribute.fifoTxEn = UART_ATTR_TX_FIFO_EN;  /* Enable TX FIFO. */
/* Set UART device attributes. */
ret = UartSetAttribute(handle, &attribute);
if (ret != 0) {
    HDF_LOGE("UartSetAttribute: failed, ret %d\n", ret);
}

Obtaining UART Device Attributes

Call UartGetAttribute() to obtain the current UART device attributes.

int32_t UartGetAttribute(DevHandle handle, struct UartAttribute *attribute);

Table 6 Description of UartGetAttribute

Example: Obtain UART device attributes.

int32_t ret;
struct UartAttribute attribute;
/* Obtain UART device attributes. */
ret = UartGetAttribute(handle, &attribute);
if (ret != 0) {
    HDF_LOGE("UartGetAttribute: failed, ret %d\n", ret);
}

Setting the UART Transmission Mode

Call UartSetTransMode() to set the UART transmission mode.

int32_t UartSetTransMode(DevHandle handle, enum UartTransMode mode);

Table 7 Description of UartSetTransMode

Example: Set the UART transmission mode to UART_MODE_RD_BLOCK.

int32_t ret;
/* Set the UART transmission mode. */
ret = UartSetTransMode(handle, UART_MODE_RD_BLOCK);
if (ret != 0) {
    HDF_LOGE("UartSetTransMode: failed, ret %d\n", ret);
}

Writing Data to a UART Device

Call UartWrite() to write data of the specified length to a UART device.

int32_t UartWrite(DevHandle handle, uint8_t *data, uint32_t size);

Table 8 Description of UartWrite

Example: Write data to a UART device.

int32_t ret;
uint8_t wbuff[5] = {1, 2, 3, 4, 5};
/* Write 5-byte data to the UART device. */
ret = UartWrite(handle, wbuff, 5);
if (ret != 0) {
    HDF_LOGE("UartWrite: failed, ret %d\n", ret);
}

Reading Data from a UART Device

Call UartRead() to read data of the specified length from a UART device.

int32_t UartRead(DevHandle handle, uint8_t *data, uint32_t size);

Table 9 Description of UartRead

Example: Read data of the specified length from a UART device.

int32_t ret;
uint8_t rbuff[5] = {0};
/* Read 5-byte data from the UART device. */
ret = UartRead(handle, rbuff, 5);
if (ret < 0) {
    HDF_LOGE("UartRead: failed, ret %d\n", ret);
}

CAUTION
Data is successfully read from the UART device if a non-negative value is returned. If 0 is returned, no valid data can be read from the UART device. A value greater than 0 indicates the length of the data read from the UART device. The data length must be less than or equal to the value of size and cannot exceed the maximum length of the data to read at a time specified by the UART controller in use.

Closing a UART Device Handle

Call UartClose() to close a UART device handle.

void UartClose(DevHandle handle);

This function releases the resources requested by UartOpen.

Table 10 Description of UartClose

Example: Close a UART device handle.

UartClose(handle); /* Close the UART device handle. */

Example

The following example shows how to open a UART device handle, set the baud rate, device attributes, and transmission mode, read data from or write data into the UART device, and then close the UART device handle.

#include "hdf_log.h"
#include "uart_if.h"

void UartTestSample(void)
{
    int32_t ret;
    uint32_t port;  
    DevHandle handle = NULL;
    uint8_t wbuff[5] = { 1, 2, 3, 4, 5 };
    uint8_t rbuff[5] = { 0 };
    struct UartAttribute attribute;
    attribute.dataBits = UART_ATTR_DATABIT_7;   /* Enable 7 bits to be transferred each time. */
    attribute.parity = UART_ATTR_PARITY_NONE;   /* Disable parity check. */
    attribute.stopBits = UART_ATTR_STOPBIT_1;   /* Set the stop bit to 1. */
    attribute.rts = UART_ATTR_RTS_DIS;          /* Disable the RTS signal. */
    attribute.cts = UART_ATTR_CTS_DIS;          /* Disable the CTS signal. */
    attribute.fifoRxEn = UART_ATTR_RX_FIFO_EN;  /* Enable RX FIFO. */
    attribute.fifoTxEn = UART_ATTR_TX_FIFO_EN;  /* Enable TX FIFO. */
    /* Enter the UART port number. */
    port = 1; 
    /* Open the UART device handle based on the port number. */
    handle = UartOpen(port);
    if (handle == NULL) {
        HDF_LOGE("UartOpen: failed!\n");
        return;
    }
    /* Set the UART baud rate to 9600. */
    ret = UartSetBaud(handle, 9600);
    if (ret != 0) {
        HDF_LOGE("UartSetBaud: failed, ret %d\n", ret);
        goto _ERR;
    }
    /* Set UART device attributes. */
    ret = UartSetAttribute(handle, &attribute);
    if (ret != 0) {
        HDF_LOGE("UartSetAttribute: failed, ret %d\n", ret);
        goto _ERR;
    }
    /* Set the UART transmission mode to non-blocking mode. */
    ret = UartSetTransMode(handle, UART_MODE_RD_NONBLOCK);
    if (ret != 0) {
        HDF_LOGE("UartSetTransMode: failed, ret %d\n", ret);
        goto _ERR;
    }
    /* Write 5-byte data to the UART device. */
    ret = UartWrite(handle, wbuff, 5);
    if (ret != 0) {
        HDF_LOGE("UartWrite: failed, ret %d\n", ret);
        goto _ERR;
    }
    /* Read 5-byte data from the UART device. */
    ret = UartRead(handle, rbuff, 5);
    if (ret < 0) {
        HDF_LOGE("UartRead: failed, ret %d\n", ret);
        goto _ERR;
    }
_ERR:
    /* Close the UART device handle. */
    UartClose(handle); 
}