Calculate the Serial Communication speed for 8051 Microcontrollers in Mode 1.
0 (Normal Baud Rate)
1 (Double Baud Rate)
Calculated Baud Rate
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How to Calculate Baud Rate for UART in 8051
The 8051 microcontroller typically uses Timer 1 in Mode 2 (8-bit auto-reload) to generate the baud rate for serial communication (UART Mode 1 and 3). The speed of data transfer is determined by the crystal frequency and the value loaded into the TH1 register.
The Mathematical Formula
The standard formula for calculating the baud rate in Mode 1 is:
Crystal Frequency (fosc): The clock speed of your crystal oscillator. Standard values include 11.0592 MHz (ideal for standard baud rates) and 12 MHz.
SMOD (PCON.7): A bit in the Power Control register. If set to 1, it doubles the baud rate.
TH1: The 8-bit value loaded into Timer 1 High byte. It determines the reload frequency.
12: The machine cycle divider for the standard 8051 architecture.
Common Baud Rate Values (at 11.0592 MHz)
Target Baud Rate
SMOD
TH1 (Hex)
TH1 (Decimal)
9600
0
0xFD
253
4800
0
0xFA
250
2400
0
0xF4
244
19200
1
0xFD
253
Why use 11.0592 MHz?
Using a 12 MHz crystal results in non-integer values for the TH1 register when trying to achieve standard speeds like 9600 bps. This causes baud rate error. 11.0592 MHz is divisible by standard UART speeds, resulting in 0% error, ensuring stable serial communication between the 8051 and other devices like PCs or GSM modules.
function calculate8051Baud() {
var foscMHz = parseFloat(document.getElementById('crystalFreq').value);
var smod = parseInt(document.getElementById('smodVal').value);
var th1 = parseInt(document.getElementById('th1Val').value);
var resultDiv = document.getElementById('resultArea');
var baudDisplay = document.getElementById('baudResult');
var errorDisplay = document.getElementById('errorMargin');
if (isNaN(foscMHz) || isNaN(th1) || th1 255) {
alert("Please enter a valid Crystal Frequency and a TH1 value between 0 and 255.");
return;
}
if (th1 === 256) {
alert("TH1 cannot be 256. The maximum is 255.");
return;
}
// Convert MHz to Hz
var fosc = foscMHz * 1000000;
// Formula: Baud = (2^SMOD / 32) * (fosc / (12 * (256 – TH1)))
var denominator = 32 * 12 * (256 – th1);
var numerator = Math.pow(2, smod) * fosc;
var baudRate = numerator / denominator;
baudDisplay.innerText = baudRate.toFixed(2) + " bps";
// Check for common standard rates to show "closeness"
var standards = [1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200];
var closest = standards.reduce(function(prev, curr) {
return (Math.abs(curr – baudRate) < Math.abs(prev – baudRate) ? curr : prev);
});
var errorPercent = ((baudRate – closest) / closest) * 100;
if (Math.abs(errorPercent) < 5) {
errorDisplay.innerHTML = "Approx. " + closest + " Baud (Error: " + errorPercent.toFixed(2) + "%)";
errorDisplay.style.color = Math.abs(errorPercent) < 1 ? "#28a745" : "#e67e22";
} else {
errorDisplay.innerText = "Custom/Non-standard baud rate calculated.";
errorDisplay.style.color = "#333";
}
resultDiv.style.display = "block";
}