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UART Baud Rate Calculator
Calculate actual baud rate and error percentage from MCU system clock.
| Baud Rate | Actual | Error | Status |
|---|
UART (Universal Asynchronous Receiver-Transmitter) requires both devices to agree on a baud rate within tight timing tolerances.
Formulas:
- Exact Baud Rate:
Baud = Clock / (Oversampling * USART_DIV) - Error %:
((Actual - Target) / Target) * 100 - Frame Time:
(Data + Parity + Stop + Start bits) / Baud
Usage: Input the system clock and target baud rate. An error rate of ≤ 0.5% is excellent, ≤ 2% is acceptable, and anything above 2% may cause data corruption.
When you need it: Choosing a baud rate the MCU's clock can actually generate within tolerance, or diagnosing garbled bytes that come from too much baud error between two devices.
Worked example: 16 MHz clock, 115200 baud, 16× oversampling → USARTDIV = 16e6 / (16 × 115200) = 8.68. Rounding to 9 gives an actual 16e6 / (16 × 9) = 111111 baud, an error of (111111 − 115200) / 115200 = −3.5% — over budget. A fractional baud generator or 8× oversampling closes the gap.
Tips & gotchas:
- Keep total baud error under about ±2%; a UART samples mid-bit and accumulates error across ~10 bits per frame, so ±2.5% is roughly the breaking point.
- Both ends must agree within their combined tolerance — a −1.5% transmitter and +1.5% receiver already eat the whole margin.
- Fractional (fixed-point) baud dividers on modern MCUs cut the error dramatically versus integer-only dividers.
- High baud rates (≥ 921600) need a clean crystal-derived clock; internal RC oscillators drift with temperature and voltage.