Stoppbit

Stoppbit, in asynchronous serial communication, is a bit that marks the end of a character frame. It follows the data bits (and an optional parity bit) and provides a gap before the next start bit, helping the receiver to re-synchronize and to detect the frame boundary.

In a typical UART frame, a character consists of a start bit (low), a number of data bits (commonly 7 or 8), an optional parity bit, and one or more stop bits. The stop bit(s) keep the line in the “mark” or idle state (high for RS-232) for a minimum duration, ensuring the receiver can correctly identify the end of the character and prepare for the next start bit.

Common configurations include one stop bit (the most widespread), two stop bits (used when extra timing margin is needed at the cost of throughput), and, in some hardware, 1.5 stop bits. The 1.5 stop-bit option is supported only by certain UARTs and at specific baud rates. Availability depends on the device’s hardware and baud-rate generator.

Impact on performance and reliability: More stop bits reduce the effective data rate because fewer bits per unit time are devoted to payload. They can improve robustness in noisy environments or with limited timing margins. Conversely, too few stop bits can increase the risk of framing errors if timing drifts.

Implementation and configuration: Stop bit length is chosen as part of the serial port configuration, typically alongside data bits and parity (for example, 8 data bits, no parity, one stop bit, or 8N1). The stop bit is generated by the UART hardware.