UART Frame Format

UART sends data in frames. A frame is usually one character or byte plus framing bits.

Generic UART frame

For the common 8N1 format:

8: 8 data bits
N: no parity
1: 1 stop bit

Example 8N1 frame

The UART line is normally idle in the mark state, the start bit is the active low/space state, stop bits are mark state, and bits are commonly transmitted least-significant bit first. Microchip’s generic asynchronous UART documentation describes NRZ mark/space behavior, idle mark state, start/stop bits, bit period, and LSb-first transmission for asynchronous UART operation.

Frame Fields

Field	Typical value	Purpose
Idle	Logic 1 / mark	Line state when no frame is active
Start bit	Logic 0 / space	Tells receiver a new frame has started
Data bits	Commonly 7 or 8; implementation-dependent options may vary	Payload bits
Parity bit	Optional	Simple error detection
Stop bit(s)	Logic 1 / mark	Confirms frame end and gives receiver recovery time

Note: UART endpoints must agree on baud rate, number of data bits, parity mode, and stop-bit setting. A mismatch can produce corrupted data or framing/parity errors.

UART Protocol