How Embedded Linux Solves Baud Rate Limitations in Traditional Serial Communication

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If you have worked on high-speed serial communication projects, you must have encountered the issue where traditional <span>termios</span> cannot configure high baud rates for serial communication parameters. So how can we solve this problem?

In Linux systems, <span>struct termios2</span> is a data structure used for advanced serial communication control. It is an extended version of the traditional <span>termios</span> structure, designed to support non-standard baud rates and more precise terminal attribute control, which can solve this problem. Below is a detailed introduction:

1. Core Functions

1. Support for Non-Standard Baud Rates (Arbitrary Baud Rates)

• The traditional <span>termios</span> can only set standard baud rates (such as 9600, 115200, etc.), while <span>termios2</span> allows setting any value for baud rates (such as 250000, 3Mbps, etc.).
• This is achieved by directly manipulating the <span>c_ispeed</span> (input speed) and <span>c_ospeed</span> (output speed) fields.

Extended Control Capabilities add features on top of <span>termios</span>, such as more refined flow control, timeout control, and other advanced serial communication features.

2. Key Fields (Comparison with <span><span>termios</span></span>)

#include <asm/termbits.h>  // Header file location

struct termios2 {
    tcflag_t c_iflag;       // Input mode flags
    tcflag_t c_oflag;       // Output mode flags
    tcflag_t c_cflag;       // Control mode flags
    tcflag_t c_lflag;       // Local mode flags
    cc_t     c_line;        // Line discipline
    cc_t     c_cc[NCCS];    // Control character array
    speed_t  c_ispeed;      // Input baud rate (non-standard value)
    speed_t  c_ospeed;      // Output baud rate (non-standard value)
};

• Core Extension: <span>c_ispeed</span> and <span>c_ospeed</span> directly store the actual baud rate values (such as <span>250000</span>), rather than traditional predefined constants (such as <span>B115200</span>).

3. Use Cases

1. High-Speed/Non-Standard Serial Devices such as industrial equipment, custom hardware (Arduino/embedded development), drone communication, special sensors, etc., which may require precise non-standard baud rates.

2. Scenarios Requiring Precise Timing Control such as communication protocols that require precise timing (e.g., high-speed variants of DMX512, Modbus).

4. Operation Method (via <span><span>ioctl</span></span>)

It is necessary to use the <span>ioctl()</span> system call in conjunction with special commands to operate on <span>termios2</span>:

Operation	<span>ioctl</span> Command	Description
Read Current Settings	<span>TCGETS2</span>	Get current <span>termios2</span> configuration
Apply Settings Immediately	<span>TCSETS2</span>	Effective unconditionally
Effective After Output Completion	<span>TCSETSW2</span>	Effective after flushing the output buffer
Effective After Input/Output Completion	<span>TCSETSF2</span>	Effective after flushing the input/output buffer

5. Code Example (Setting Non-Standard Baud Rate)

#include <fcntl.h>
#include <asm/termbits.h>
#include <sys/ioctl.h>

int main() {
    int fd = open("/dev/ttyS0", O_RDWR);  // Open serial device

    struct termios2 tio;
    ioctl(fd, TCGETS2, &tio);  // Read current configuration

    // Set non-standard baud rate 250000
    tio.c_cflag &= ~CBAUD;     // Clear standard baud rate flags
    tio.c_cflag |= BOTHER;     // Enable non-standard baud rate mode
    tio.c_ispeed = 250000;     // Input baud rate
    tio.c_ospeed = 250000;     // Output baud rate

    // Other configurations (8N1)
    tio.c_cflag &= ~PARENB;    // No parity
    tio.c_cflag &= ~CSTOPB;    // 1 stop bit
    tio.c_cflag |= CS8;        // 8 data bits

    ioctl(fd, TCSETS2, &tio);  // Apply configuration
    close(fd);
    return 0;
}

6. Notes

1. Linux Specific<span>struct termios2</span> is an extension specific to Linux and is not part of the POSIX standard.
2. Kernel Support requires kernel version ≥ 2.6.31 (some features depend on driver support).
3. Header File Path is usually located in <span><asm/termbits.h></span> or <span><sys/ioctl.h></span>.

END

Author: Mr. Deng

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