Xmega Baud Rate Calculator

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XMEGA USART Baud Rate Calculator

Example: 2000000 for 2MHz, 32000000 for 32MHz

Optimal Settings

BSEL (Baud Rate Select) –
BSCALE (Baud Scale) –

Actual Baud Rate: – bps

Error: –%

C-Code Implementation


    

    

    

        
Understanding XMEGA Baud Rate Generation

        
The AVR XMEGA series microcontrollers utilize a unique fractional baud rate generator. Unlike standard AVRs where you only have an integer divider, the XMEGA system uses two parameters: BSEL (a 12-bit value) and BSCALE (a 4-bit 2's complement value from -7 to 7).

        
The Formula

        
The relationship between the frequency, baud rate, and parameters depends on the sign of BSCALE:

        
    
            
  • When BSCALE ≥ 0:
    Baud Rate = f_per / (16 * ((2^BSCALE * BSEL) + 1))
    • 
            
  • When BSCALE < 0:
    Baud Rate = f_per / (16 * 2^BSCALE * (BSEL + 1))
    • 
        

        
Why use negative BSCALE?

        
Negative BSCALE values act as a "fractional" multiplier, allowing for much higher precision and lower error rates at high baud rates or low clock frequencies. Our calculator iterates through all 15 possible BSCALE values (-7 to 7) to find the combination that yields the lowest possible percentage error for your specific hardware setup.

        
Example: 115200 bps @ 32MHz

        
If you are running an ATxmega32A4U at 32MHz and need 115200 baud:

        
    
            
  • Desired: 115,200 bps
    • 
            
  • Calculated BSEL: 1047
    • 
            
  • Calculated BSCALE: -6
    • 
            
  • Actual Rate: 115,207 bps (0.01% error)
    • 
        

        
Implementation Tips

        
After calculating your values, they must be written to the BAUDCTRLA and BAUDCTRLB registers. Note that the lower 8 bits of BSEL go into BAUDCTRLA, while the upper 4 bits of BSEL and the 4 bits of BSCALE are combined into BAUDCTRLB. Always ensure your peripheral clock (f_per) matches your actual system clock configuration (e.g., DFLL settings) for accurate timing.

    




function calculateXmegaBaud() {
    var f_per = parseFloat(document.getElementById('f_per').value);
    var target = parseFloat(document.getElementById('target_baud').value);
    if (!f_per || !target || f_per <= 0 || target <= 0) {
        alert("Please enter valid numbers for Clock and Baud Rate.");
        return;
    }
    var bestBSEL = 0;
    var bestBSCALE = 0;
    var minError = 100;
    var bestActual = 0;
    // Iterate through all possible BSCALE values -7 to 7
    for (var bscale = -7; bscale = 0) {
            // bsel = ((f_per / (16 * target)) – 1) / (2^bscale)
            bsel = Math.round(((f_per / (16 * target)) – 1) / Math.pow(2, bscale));
            if (bsel >= 0 && bsel = 0 && bsel <= 4095) {
                actual = f_per / (16 * Math.pow(2, bscale) * (bsel + 1));
            } else { continue; }
        }
        var error = Math.abs((actual – target) / target) * 100;
        if (error > 8) & 0x0F;
    var ctrlB = (bscale_bits << 4) | bsel_high;
    var ctrlA = bestBSEL & 0xFF;
    var code = "// XMEGA USART Configuration\n";
    code += "USART_BAUDCTRLA = 0x" + ctrlA.toString(16).toUpperCase().padStart(2, '0') + "; // BSEL Lower 8 bits\n";
    code += "USART_BAUDCTRLB = 0x" + ctrlB.toString(16).toUpperCase().padStart(2, '0') + "; // BSCALE << 4 | BSEL High 4 bits";
    document.getElementById('code_snippet').innerText = code;
    document.getElementById('results-area').style.display = 'block';
}
		


				
			




			

				


		

		
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