Because the serial communication between the UART host controller and the selected device is asynchronous, Start and Stop bits are used on the serial data to synchronize the two devices. The structure of serial data accompanied by Start and Stop bits is referred to as a character.

An additional parity bit may be added to the serial character. This bit appears after the last data bit and before the stop bit(s) in the character structure to provide the UART Host Controller with the ability to perform simple error checking on the received data.

The UART Host Controller Line Control Register (LCR) is used to control the serial character characteristics. The individual bits of the data word are sent after the Start bit, starting with the least significant bit (LSB). These are followed by the optional parity bit, followed by the Stop bit(s), which can be 1, 1.5, or 2.

The Stop bit duration implemented by UART host controller may appear longer due to idle time inserted between characters for some configurations and baud clock divisor values in the transmit direction.

All bit in the transmission (with exception to the half stop bit when 1.5 stop bits are used) are transmitted for exactly the same time duration (which is referred to as Bit Period or Bit Time). One Bit Time equals to 16 baud clocks.

To ensure stability on the line, the receiver samples the serial input data at approximately the midpoint of the Bit Time once the start bit has been detected.