Enter the frequency of the APB bus connected to the USART (e.g., 8000000, 16000000, 84000000).
16 Samples (OVER8 = 0) – Standard
8 Samples (OVER8 = 1) – High Speed
USART_BRR Register Value0x0000
USARTDIV (Float):–
Mantissa (Integer):–
Fraction (Integer):–
Actual Baud Rate:–
Error Deviation:–
Generating Accurate Baud Rates on STM32
Configuring serial communication on STM32 microcontrollers requires precise calculation of the USART_BRR (Baud Rate Register) value. Unlike high-level libraries where you simply pass a number like 9600, bare-metal programming requires understanding how the peripheral clock is divided to generate the transmission bits.
The Calculation Logic
The STM32 USART peripheral derives its timing from the bus clock (APB1 or APB2, depending on the specific USART instance). This frequency is denoted as fPCLK. The Baud Rate Register defines a divider, known as USARTDIV, which determines how many clock cycles constitute one bit period.
Formula (Oversampling by 16):
USARTDIV = fPCLK / (16 × Desired Baud Rate)
Formula (Oversampling by 8):
USARTDIV = fPCLK / (8 × Desired Baud Rate)
Understanding the Register Structure
The USART_BRR is a 16-bit register split into two parts:
Mantissa (12 bits): The integer part of the division.
Fraction (4 bits): The fractional part of the division.
Because integer division loses precision, the fractional part is crucial for minimizing communication errors. The calculator above computes the exact float value, extracts the integer mantissa, and rounds the remainder to the nearest 4-bit fraction (or 3-bit if using 8x oversampling).
Why Error Percentage Matters
Serial communication is asynchronous, meaning there is no shared clock signal. The receiver relies on internal timing to sample bits. If the difference between the actual baud rate and the target rate exceeds the tolerance (typically ±2% to ±5%), bit errors will occur, leading to corrupted data frames.
Always verify that the Error Deviation is within acceptable limits (usually less than 2%) for stable communication. If the error is too high, consider changing your PCLK frequency or using a different oversampling mode.
function calculateBaudRate() {
// 1. Get input values
var pclkInput = document.getElementById('pclkFreq');
var baudInput = document.getElementById('targetBaud');
var oversamplingInput = document.getElementById('oversampling');
var resultsPanel = document.getElementById('resultsPanel');
var pclk = parseFloat(pclkInput.value);
var targetBaud = parseFloat(baudInput.value);
var oversampling = parseInt(oversamplingInput.value);
// 2. Validate inputs
if (isNaN(pclk) || pclk <= 0 || isNaN(targetBaud) || targetBaud = oversampling) {
mantissa += 1;
fraction = 0;
}
// 7. Calculate Actual Baud Rate achieved
// Actual = PCLK / (Oversampling * (Mantissa + Fraction/Oversampling))
var effectiveDivisor = mantissa + (fraction / oversampling);
var actualBaud = pclk / (oversampling * effectiveDivisor);
// 8. Calculate Error Percentage
var errorPercent = ((actualBaud – targetBaud) / targetBaud) * 100;
// 9. Construct BRR Hex Value
var brrValue = 0;
if (oversampling === 16) {
// Standard assignment: Mantissa << 4 | Fraction
brrValue = (mantissa << 4) | fraction;
} else {
// Oversampling by 8 assignment
// In OVER8 mode, bits [2:0] are the fraction. Bit [3] must be 0.
// Mantissa is still shifted by 4 in the register map typically.
// Fraction is strictly 3 bits (0-7).
var fraction3Bit = fraction & 0x07;
brrValue = (mantissa << 4) | fraction3Bit;
}
// 10. Update DOM
resultsPanel.style.display = "block";
// Format Hex to 4 digits (e.g., 0x0271)
var hexString = "0x" + brrValue.toString(16).toUpperCase().padStart(4, '0');
document.getElementById('brrHexResult').innerText = hexString;
document.getElementById('usartDivResult').innerText = usartDiv.toFixed(4);
document.getElementById('mantissaResult').innerText = mantissa;
document.getElementById('fractionResult').innerText = fraction + " / " + oversampling;
document.getElementById('actualBaudResult').innerText = Math.round(actualBaud) + " bps";
var errorElem = document.getElementById('errorResult');
var absError = Math.abs(errorPercent);
errorElem.innerText = errorPercent.toFixed(2) + "%";
// Color code error
if (absError < 1.0) {
errorElem.className = "result-value error-good";
} else if (absError < 2.5) {
errorElem.className = "result-value error-warn";
} else {
errorElem.className = "result-value error-bad";
}
}