UART Timing Calculator

Calculator

Example data table

Formula used

How to use this calculator

1. Enter your target baud rate and frame format settings.
2. Provide the peripheral clock and select an oversampling factor.
3. Add an inter-frame gap if your protocol inserts idle time.
4. Set a payload size to estimate total transmission duration.
5. Press Calculate to view timing, throughput, and baud error.
6. Use CSV or PDF exports to share results with your team.

Clock-divisor planning across targets

UART baud generation is constrained by the clock and the selected oversampling factor. The calculator estimates an ideal divisor then rounds it, producing an actual baud and percent error. When the clock is coarse or the baud is high, a small divisor change can shift the baud noticeably. Keep combined transmitter and receiver mismatch low so sampling stays near the bit center.

Framing overhead and effective payload rate

Each transmitted character carries more than payload bits. A typical 8N1 frame uses one start bit and one stop bit, so only 8 of 10 bits are payload, giving 80% efficiency before any gaps. Adding parity drops efficiency further. The calculator reports bits per frame, frames per second, and payload bytes per second so you can compare formats when bandwidth is tight.

Stop-bit margin versus timing budget

Stop bits extend the idle period after data, allowing receivers extra time to resynchronize and tolerate line noise or interrupt latency. Moving from 1 to 2 stop bits increases frame length and reduces throughput, but may improve robustness on long cables or mixed-clock systems. Use the frame time and half-bit timing outputs to judge whether firmware latency can meet required sampling windows.

Oversampling choice and sampling point stability

Oversampling affects both noise immunity and achievable baud. A 16x sampler provides more timing resolution for start-bit detection and can average jitter, but requires a higher clock-to-baud ratio. An 8x sampler can reach higher bauds for a given clock, yet leaves less margin for drift and edge uncertainty. The calculator’s divisor estimate helps you see which mode yields a smaller baud error.

Batch transfer time and latency expectations

Transmission time matters for control loops, logging, and bootloaders. By entering a payload size, the calculator multiplies per-frame time by the number of bytes and includes any configured inter-frame gap. This shows how protocol features, like deliberate spacing between bytes, affect latency. Use the estimate to select buffer sizes, choose DMA thresholds, and predict worst-case turnaround when acknowledgments follow a burst. For reliability, validate results with oscilloscope captures and real cable loads during testing.

FAQs

What does “bits per frame” include?

It includes the start bit, selected data bits, optional parity bit, and chosen stop bits. This total drives frame time and throughput calculations.

How much baud error is usually acceptable?

Many links work well within about ±2% combined error. Noisy channels, long cables, or high speeds often need tighter margins.

Why does adding parity reduce throughput?

Parity adds an extra bit to every frame without adding payload. That increases frame time and lowers effective bytes per second.

When should I use 2 stop bits?

Use them when you need extra recovery time, improved tolerance to timing drift, or more spacing on shared or noisy lines.

How does inter-frame gap affect results?

A gap adds idle time between characters. It reduces frames per second and payload throughput, while increasing total transmit time.

Will every device use the same divisor formula?

No. Some peripherals use fractional divisors or different scaling. Treat the estimate as guidance and confirm with your device documentation.