UART Serial Frame

UART Frame Calculator

Build a UART serial frame from a data value and framing configuration. Calculate the start bit, LSB-first data sequence, optional parity bit, stop bits and complete transmitted binary frame.

Start Bit 5–9 Data Bits Even / Odd Parity Stop Bits LSB First

UART Frame Builder Asynchronous Serial

Data Value Enter hexadecimal data, for example 55, A5 or 1FF.

Data Bits Select the number of UART payload bits.

Parity Optional parity bit follows the data bits.

Stop Bits Stop bits are transmitted as logic 1.

UART frames normally begin with one logic-0 start bit. Data bits are transmitted least-significant bit first, followed by optional parity and one or more logic-1 stop bits.

UART Frame Result

–

Data Hex –

Data Decimal –

Total Frame Bits –

Payload Efficiency –

Data Bits –

Parity –

Stop Bits –

Overhead Bits –

Start 0

Data LSB First –

Parity Bit –

Stop –

UART Transmission Frame — Left to Right –

What Is a UART Frame?

UART, or Universal Asynchronous Receiver/Transmitter, transfers serial data without a shared clock signal. Instead, the transmitter and receiver agree on framing settings such as the number of data bits, parity and stop bits.

Each transmitted data unit is surrounded by framing bits so the receiver can determine where the serial data begins and ends.

UART Frame Structure

A basic UART frame normally contains a start bit, several data bits and one or more stop bits. A parity bit may optionally appear between the data and stop fields.

Start → Data bits → Optional parity → Stop bit(s)

The UART line is normally idle at logic 1. A logic-0 start bit signals the beginning of a new frame.

UART Data Is Sent LSB First

UART commonly transmits the least-significant data bit first.

For example, hexadecimal 55 is:

Hex: 55

Normal binary display: 01010101

Bit 0 is already the rightmost bit in normal binary notation, so the transmitted data sequence is produced by sending bit 0 first, followed by bit 1 and continuing toward the most-significant data bit.

UART Start Bit

The start bit is normally logic 0 and occupies one bit period. The transition from the idle-high state to the start bit allows the receiving UART to synchronize its sampling with the incoming character.

Idle: 1

Start: 0

UART Parity Bit

Parity provides a simple error-detection mechanism. The parity bit is calculated from the number of 1 bits in the transmitted data field.

Parity	Rule
None	No parity bit is transmitted
Even	Total number of 1s including parity is even
Odd	Total number of 1s including parity is odd

Even Parity Example

Suppose an eight-bit data value contains three logic-1 bits.

Data 1-count: 3

Even parity requires total 1-count: Even

Parity bit: 1

Final 1-count: 4

If the data already contains an even number of ones, the even parity bit is 0.

Odd Parity Example

Odd parity works in the opposite direction. The parity bit is selected so the total count of 1 bits becomes odd.

Data 1-count: 4

Odd parity requires: Odd number of 1s

Parity bit: 1

Final 1-count: 5

UART Stop Bits

Stop bits are transmitted as logic 1 and indicate the end of a frame. This calculator supports one or two complete stop bits.

1 stop bit: 1

2 stop bits: 11

UART 8N1 Frame

One of the most common UART configurations is written as 8N1.

8 = 8 data bits
N = No parity
1 = 1 stop bit

The complete frame therefore contains:

1 start bit + 8 data bits + 0 parity bits + 1 stop bit

Total: 10 bits

UART 8E1 Frame

An 8E1 configuration uses eight data bits, even parity and one stop bit.

Start: 1 bit

Data: 8 bits

Parity: 1 bit

Stop: 1 bit

Total: 11 bits

UART Frame Overhead

Not every bit in a UART frame contains payload data. Start, parity and stop bits are framing overhead.

8N1:

Payload: 8 bits

Frame: 10 bits

Overhead: 2 bits

The payload efficiency is the percentage of total transmitted frame bits that belong to the data field.

Common UART Frame Formats

Format	Data Bits	Parity	Stop Bits	Total Bits
8N1	8	None	1	10
8N2	8	None	2	11
8E1	8	Even	1	11
8O1	8	Odd	1	11
7E1	7	Even	1	10

Where UART Frames Are Used

UART serial communication is widely used in microcontrollers, embedded devices, development boards, sensors, GPS receivers, Bluetooth modules, industrial controllers, debugging ports and many other electronic systems.

Understanding the exact frame bits can be useful when debugging logic-analyzer captures or checking whether transmitter and receiver framing settings agree.

Important UART Frame Notes

Important: this calculator builds one UART data frame.

The start bit is assumed to be logic 0.

Data is transmitted least-significant bit first.

The calculator supports 5 through 9 data bits.

Parity can be None, Even or Odd.

Stop bits are represented as logic 1.

The tool supports one or two stop bits.

The data value must fit within the selected number of data bits.

This calculator does not calculate baud rate or transmission time.

Actual UART hardware may support additional framing modes such as mark parity, space parity or fractional stop-bit configurations.

UART Frame Calculator FAQs

What does this UART Frame Calculator do?

It constructs a UART binary frame from a hexadecimal data value, data-bit width, parity setting and number of stop bits.

What is the UART start bit?

A UART frame normally begins with one logic-0 start bit.

Does UART transmit the LSB or MSB first?

UART commonly transmits the least-significant data bit first.

What does 8N1 mean?

8N1 means eight data bits, no parity and one stop bit.

How many bits are in an 8N1 UART frame?

An 8N1 frame contains one start bit, eight data bits and one stop bit, for a total of ten bits.

How does even parity work?

The parity bit is chosen so the total number of 1 bits across the data and parity fields is even.

How does odd parity work?

The parity bit is chosen so the total number of 1 bits across the data and parity fields is odd.

Are UART stop bits 0 or 1?

UART stop bits are normally logic 1.

Does this calculator calculate baud rate?

No. It calculates the binary structure and length of the UART frame only.