UART Timing Calculator
Analyze UART bit time, frame time, character time, packet transmission time, receiver sampling position, baud-rate error, and timing differences between common serial baud rates.
COM-002 focuses on ideal UART timing. It does not calculate MCU clock dividers, UART registers, FIFO behavior, DMA timing, buffer depth, or interrupt latency.
Engineering tool
UART Timing Calculator
Analyze UART bit time, frame time, packet timing, sampling position, baud-rate error, and timing differences between baud rates.
UART timing summary
10 bits/frame
9,600 baud, 8N1
Result console
- Bit time
- 104.167µs
- Frame bits
- 10bits
- Frame time
- 1.042ms
- Character time
- 1.042ms
- Characters per second
- 960chars/s
Result is valid for the selected ideal UART timing model.
Formula reference
UART Timing Formulas
UART is asynchronous communication. Timing is derived from baud rate and complete frame format.
Bit Time: tbit = 1 / BaudRateFrameBits = 1 + DataBits + ParityBits + StopBitsFrame Time = FrameBits / BaudRatePacket Time = PacketBits / BaudRatePacketBits = PacketBytes × FrameBitsSampling Instant = BitTime × SamplingPositionBaud Error = (Actual - Expected) / Expected × 100%Variable definitions
- Start bit
- fixed 1 bit at the beginning of a UART frame
- Data bits
- 5, 6, 7, 8, or 9 payload bits
- Parity bits
- 0 for None, 1 for Even or Odd
- Stop bits
- 1, 1.5, or 2 end-of-frame bit periods
- Character time
- one complete UART frame time
- Baud-rate error
- percent difference between generated and target baud rate
Worked Examples
9600 baud, 8N1
Bit time = 1 / 9600 = 104.167 µs. Frame bits = 1 + 8 + 0 + 1 = 10. Frame time = 10 / 9600 = 1.04167 ms.
115200 baud, 8N1
Bit time = 1 / 115200 = 8.68056 µs. Frame time = 10 / 115200 = 86.806 µs.
921600 baud, 8N1
Bit time = 1.08507 µs. A 10-bit 8N1 frame takes about 10.851 µs.
57600 baud, 7E1
Frame bits = 1 start + 7 data + 1 parity + 1 stop = 10 bits. Frame time = 173.611 µs.
38400 baud, 8O2
Frame bits = 1 + 8 + 1 + 2 = 12 bits. Frame time = 312.5 µs.
1000 bytes at 115200 baud, 8N1
Packet bits = 1000 × 10 = 10000 bits. Packet time = 10000 / 115200 = 86.806 ms.
Baud error: 115200 expected vs 114000 actual
Error = (114000 - 115200) / 115200 × 100% = -1.0417%. Verify whether both UART endpoints tolerate this mismatch.
Compare 9600 vs 115200 baud
With the same frame format, 115200 baud has one twelfth the bit time of 9600 baud and is 12× faster.
Engineering Notes
- UART is asynchronous, so the receiver does not receive a separate clock signal.
- Bit time is determined directly by baud rate.
- Frame time depends on the start bit, data bits, optional parity bit, and stop bits.
- Character time is the complete frame duration, not only the payload byte duration.
- Packet timing must include UART framing overhead for every byte.
- Larger baud-rate mismatch reduces communication reliability.
- Real MCU tolerance depends on the UART implementation, oscillator accuracy, sampling method, and frame length.
- Use a logic analyzer or oscilloscope to verify actual bit timing in hardware.
Common Mistakes
- Ignoring stop bits when calculating frame time.
- Ignoring parity overhead in 8E1 or 8O1 formats.
- Assuming character time equals bit time.
- Treating baud rate as bytes per second.
- Ignoring baud-rate error between transmitter and receiver.
- Assuming every UART accepts the same timing error.
- Forgetting that packet time grows with every framed byte.
- Using ideal timing estimates without hardware validation.
Support reference
FAQ
What is UART timing?
UART timing describes the bit period, frame duration, sampling point, and total transfer time of an asynchronous serial link.
How is frame time calculated?
Frame time is calculated by adding the start bit, data bits, optional parity bit, and stop bits, then dividing the total frame bits by the baud rate.
What is character time?
Character time is the duration of one complete UART frame, not just one data byte. It includes start, data, parity, and stop bits.
How do I calculate packet transmission time?
Multiply the packet byte count by the number of bits in each UART frame, then divide by the baud rate.
What is UART baud error?
Baud error is the percent difference between the actual generated baud rate and the expected baud rate.
How much baud error is acceptable?
There is no universal guaranteed value. Acceptable baud error depends on the UART hardware, oscillator accuracy, frame length, receiver sampling, wiring, and noise environment.
Why is UART asynchronous?
UART does not transmit a shared clock. The receiver detects the start bit and samples incoming bits using its own local timing.
How does frame format affect transmission time?
More data bits, parity bits, or stop bits increase the number of transmitted bits per character, which increases frame time and reduces payload throughput.
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Engineering Disclaimer
This calculator provides ideal UART timing estimates. Real designs must be validated against oscillator tolerance, receiver sampling behavior, signal integrity, wiring, firmware behavior, and hardware measurements.