UART Baud Rate & Frame Timing Calculator

Calculate UART frame timing, throughput, and USART BRR register divisor from baud rate, data format, and MCU clock frequency. Identify baud rate error for reliable serial communication.

Loading calculator...

Formula

BRR = \frac{f_{clk}}{16 \times B}, \quad T_{bit} = \frac{1}{B}, \quad N_{frame} = 1 + D + P + S

Reference: STM32 Reference Manual RM0008 §27.3.4; ST AN2908

B — Target baud rate (bps)

f_{clk} — MCU peripheral clock frequency (Hz)

BRR — Baud rate register divisor (integer)

T_{bit} — Duration of one bit (s)

N_{frame} — Total bits per UART frame (bits)

D — Data bits (bits)

P — Parity bits (0 or 1) (bits)

S — Stop bits (1 or 2) (bits)

How It Works

Universal Asynchronous Receiver/Transmitter (UART) communication relies on precisely timed serial data transmission between devices. The baud rate determines the communication speed, representing the number of signal changes per second. Each UART frame consists of multiple bits: a start bit, data bits (typically 5-9), optional parity for error checking, and stop bit(s). The clock frequency of the microcontroller determines the achievable baud rates through a divisor calculation. Lower baud rates allow more reliable communication over longer distances, while higher rates enable faster data transfer. The bit timing must be synchronized between transmitter and receiver within a small error tolerance, typically less than 2-3% to ensure reliable communication.

Worked Example

Consider a microcontroller with a 16 MHz system clock, targeting a 9600 baud rate. First, calculate the frame length: 1 start bit + 8 data bits + no parity + 1 stop bit = 10 total bits. To calculate the Baud Rate Register (BRR), divide the clock frequency by 16 times the desired baud rate: 16,000,000 / (16 × 9600) = 104.1667. Rounding to the nearest integer gives 104. To verify error percentage: (|9600 - (16,000,000 / (16 × 104))| / 9600) × 100 = 0.16%, which is well within acceptable communication parameters.

Practical Tips

✓Always use integer division and rounding for BRR calculation
✓Choose standard baud rates to minimize configuration complexity
✓Verify actual baud rate with oscilloscope or logic analyzer

Common Mistakes

✗Failing to account for system clock frequency variations
✗Overlooking fractional baud rate generation requirements
✗Not verifying actual versus target baud rate error percentage

Frequently Asked Questions

What is the most common UART configuration?

8-N-1 configuration is standard: 8 data bits, no parity, 1 stop bit. This provides a good balance between data transmission efficiency and error detection.

How does parity bit affect UART communication?

Parity provides basic error detection by adding an extra bit that ensures the total number of 1 bits is even or odd. It catches single-bit transmission errors.

What causes high baud rate errors?

Crystal oscillator tolerances, temperature variations, and system clock inaccuracies can cause baud rate deviations. High-precision oscillators minimize these issues.

Can I use non-standard baud rates?

While possible, non-standard baud rates may require more complex clock generation and can increase communication error probability.

What's the maximum practical UART baud rate?

Typical microcontrollers support up to 115,200 baud. Beyond this, signal integrity becomes challenging without specialized hardware.