In this lesson, you will learn how to configure low-level UART parameters using the termios structure to ensure your Linux-based embedded system can reliably exchange data with external hardware.
Configuring the Serial Interface
When interacting with serial devices (like sensors, modems, or microcontrollers) in user space, Linux exposes the UART as a character device node (e.g., /dev/ttyS0). However, simply opening the file descriptor is insufficient; you must synchronize the communication parameters—specifically baud rate, parity, data bits, and stop bits—to match your external device.
In Linux, these settings are managed through the termios structure. By using tcgetattr to fetch the current configuration and tcsetattr to apply changes, you act as the orchestrator of the physical communication layer.
Modifying Baud Rate and Framing
To establish a connection, you must define the framing. For instance, to set a baud rate of 115200 and configure 8 data bits with no parity, you manipulate the c_cflag (control mode flag) member of the termios struct:
struct termios tty;
tcgetattr(fd, &tty);
// Set baud rate to 115200
cfsetospeed(&tty, B115200);
cfsetispeed(&tty, B115200);
// 8 data bits, no parity, 1 stop bit (8N1)
tty.c_cflag &= ~PARENB; // Clear parity bit
tty.c_cflag &= ~CSTOPB; // Clear stop field (1 stop bit)
tty.c_cflag &= ~CSIZE; // Clear current size
tty.c_cflag |= CS8; // Set 8 data bits
// Apply settings immediately
tcsetattr(fd, TCSANOW, &tty);
Essential Control Modes
Beyond the basic framing, you must often switch the terminal into "raw" mode. By default, Linux serial ports are configured as terminal lines, which perform character processing (like interpreting carriage returns or flow control signals). For raw data transfer between embedded systems, you should clear the c_lflag (local mode) and c_iflag (input mode) flags to disable echo and input signal processing, ensuring your raw binary data reaches the application intact.
Always verify the handshake requirements; if the external device requires hardware flow control, you must enable CRTSCTS in the c_cflag field, or the communication will likely hang due to buffer overruns or missing status signals.
Key Takeaway: Precise configuration of the termios structure is the prerequisite for reliable UART communication, ensuring that both the embedded Linux host and the external peripheral interpret the signal voltage transitions as identical bitstreams.