How do I calculate the STM32 BRR value?

The BRR value is calculated based on the peripheral clock frequency (PCLK) and the desired baud rate. The formula depends on the oversampling mode (8x or 16x). For 16x oversampling, USARTDIV = PCLK / (16 * Baud Rate).

What is the maximum acceptable baud rate error?

Generally, a baud rate error of less than 2% to 3% is acceptable for standard UART communication. However, for high-speed or long-distance communication, aim for less than 1% error.

Does this tool support all STM32 families?

This tool uses the standard fractional baud rate generation formula found in most STM32 families like F0, F1, F3, F4, L0, L4, G0, G4, etc. Always verify with your specific MCU datasheet.

STM32 Baud Rate Calculator | BRR Register & Error % Tool

Configuration

Peripheral Clock (PCLK)

Target Baud Rate

Oversampling Mode

Calculation Results

USARTDIV Value -

Mantissa (12-bit) -

Fraction (4-bit) -

BRR Register (Hex) -

Actual Baud Rate -

Error Percentage -

Generated C Code

Ready-to-use initialization code for your STM32 project.

// Click Calculate to generate code

Common Baud Rates Analysis

Error percentages for standard baud rates with current PCLK.

Baud Rate	Actual Rate	Error %	BRR (Hex)	Status
Enter PCLK and Calculate to see table

About STM32 Baud Rate Calculation

Configuring the UART/USART peripheral on STM32 microcontrollers requires setting the Baud Rate Register (BRR) correctly. The BRR value is derived from the peripheral clock frequency (PCLK) and the desired baud rate. Since the BRR uses a fractional divider, it's not always possible to achieve the exact baud rate, leading to a small error percentage.

This tool helps you calculate the precise Mantissa and Fraction values for the BRR register, ensuring your communication is reliable. It supports both standard 16x oversampling and 8x oversampling modes found in STM32F1, F4, L4, and other series.

How to Use

Peripheral Clock (PCLK): Enter the frequency of the APB bus driving your USART. Be careful! This is often different from the system core clock (HCLK). Check your clock tree.
Target Baud Rate: Select a standard rate like 9600 or 115200, or choose "Custom" to enter a specific value.
Oversampling: Default is 16 samples. Use 8 samples if you need higher speeds and can tolerate slightly less noise immunity.
Analyze Results: Check the "Error %". Ideally, this should be below 2.5% for reliable communication. If it's red, try changing your PCLK or Baud Rate.

Frequently Asked Questions

What is the formula for STM32 Baud Rate?

For 16x oversampling: Tx/Rx Baud = f_CK / (16 * USARTDIV).
For 8x oversampling: Tx/Rx Baud = f_CK / (8 * USARTDIV).

Why is my Error % so high?

This usually happens when the PCLK is not divisible by the baud rate nicely. For example, generating 115200 baud from an 8MHz clock results in some error. Try using a clock frequency that is a multiple of the baud rate, or simply a higher clock frequency to increase resolution.

What is the difference between OVER8=0 and OVER8=1?

OVER8=0 (Oversampling by 16) is the default and offers better noise immunity. OVER8=1 (Oversampling by 8) allows for higher maximum baud rates but is slightly less robust against noise.

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