VxWorks UART Programming: Serial Port Configuration and I/O

VxWorks UART Programming: Serial Port Configuration and I/O

UART (Universal Asynchronous Receiver/Transmitter) remains a fundamental interface for debugging, control, and device communication in embedded systems. While the programming model is broadly consistent across operating systems, VxWorks exposes UART functionality through its I/O system and ioctl interface.

This guide provides a practical walkthrough of configuring serial ports, performing read/write operations, and implementing a complete UART task in VxWorks.

🛠️ Serial Port Configuration

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Opening the Serial Device

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Serial ports in VxWorks are exposed as device files:

fd = open("/tyCo/0", O_RDWR, 0);

• /tyCo/0: First serial device
• O_RDWR: Enables both read and write access

Device naming may vary depending on BSP and hardware configuration.

Configuring Mode and Buffers

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Basic configuration is performed using ioctl():

ioctl(fd, FIOSETOPTIONS, OPT_RAW);
ioctl(fd, FIOFLUSH, 0);

• OPT_RAW: Disables line processing (raw data mode)
• FIOFLUSH: Clears input/output buffers

Common ioctl Commands

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Setting Line Parameters

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Hardware-level configuration is applied using:

ioctl(fd, SIO_HW_OPTS_SET, CS8 | PARENB | CLOCAL | CREAD);

Key flags:

• CS8: 8 data bits
• PARENB: Enable parity
• PARODD: Odd parity (optional)
• CLOCAL: Ignore modem control
• CREAD: Enable receiver

🔄 UART Read and Write Operations

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Once configured, UART communication uses standard POSIX-style APIs:

int read(int fd, char *buffer, size_t maxbytes);
int write(int fd, char *buffer, size_t nbytes);

Parameters

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• fd: File descriptor from open()
• buffer: Data buffer
• maxbytes / nbytes: Transfer size

These APIs integrate seamlessly with the VxWorks I/O system.

💻 Complete UART Example

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The following example demonstrates a UART task that periodically sends data while coordinating access via shared memory and semaphores.

#include "vxWorks.h"
#include "stdio.h"
#include "ioLib.h"
#include "taskLib.h"
#include "sioLib.h"
#include "sdLib.h"
#include "semLib.h"
#include "msgQLib.h"

#define DEV_NAME "/tyCo/2"
#define MAX_BUF_SIZE 20
#define SHARE_DATA_LENGTH 20

typedef struct unix_clock_struct 
{
    UINT32 sec;    
    UINT32 msec;   
    UINT8 quality; 
} UNIX_CLOCK_STRUCT;

char *comdata;
SEM_ID mutexComdata;

int set_serial(int fd);

void taskUart(void)
{
    int ret;
    int fd = open(DEV_NAME, O_RDWR, 0);
    UNIX_CLOCK_STRUCT w_buff;

    if (fd < 0)
        printf("open failed.\n");

    if (set_serial(fd) < 0)
        printf("serial config failed.\n");

    while (1)
    {
        semTake(mutexComdata, WAIT_FOREVER);

        ioctl(fd, FIOFLUSH, 0);
        bzero(&w_buff, sizeof(w_buff));
        memcpy(&w_buff, comdata, sizeof(w_buff));

        if (write(fd, &w_buff.sec, sizeof(w_buff.sec)) < 0)
            printf("write failed.\n");
        else
            printf("write success: %d\n", w_buff.sec);

        semGive(mutexComdata);
        taskDelay(sysClkRateGet() * 2);
    }
}

int set_serial(int fd)
{
    if (fd < 0)
        return -1;

    if (ioctl(fd, FIOBAUDRATE, 9600) < 0)
        return -1;

    if (ioctl(fd, SIO_HW_OPTS_SET, CREAD | CS8 | CLOCAL) < 0)
        return -1;

    return 0;
}

🔍 Key Implementation Insights

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Synchronization

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• Use semaphores (semTake, semGive) to protect shared data
• Prevent concurrent access to UART resources

Buffer Management

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• Clear buffers before transmission (FIOFLUSH)
• Avoid stale or partial data reads

Task-Based Design

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• Run UART logic in a dedicated task
• Control execution rate using taskDelay()

⚠️ Common Pitfalls

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Incorrect Device Name

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• /tyCo/x mapping depends on BSP configuration
• Verify using system device listing

Misconfigured Line Settings

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• Incorrect parity or data bits can corrupt communication
• Ensure both ends match configuration

Blocking Behavior

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• read() may block if no data is available
• Consider non-blocking modes or timeouts

✅ Best Practices

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• Always validate file descriptors and return values
• Use raw mode for binary communication
• Encapsulate configuration into reusable functions
• Separate UART logic from application logic
• Log errors for easier debugging in production systems

📌 Conclusion

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UART programming in VxWorks builds on a familiar POSIX-style model while leveraging powerful configuration via ioctl. By combining proper device setup, robust synchronization, and task-based design, developers can implement reliable serial communication for debugging, control, and data exchange.

This foundation can be extended to support interrupt-driven drivers, DMA-based transfers, or higher-level communication protocols in more advanced embedded systems.

Reference: VxWorks UART Programming: Serial Port Configuration and I/O