The Texas Instruments MSP430 microcontroller family uses the Universal Serial Communication Interface (USCI) or the Universal Synchronous/Asynchronous Receiver/Transmitter (USART) to handle serial communications. To establish a reliable UART connection, the internal clock source (BRCLK) must be divided down to match the target baud rate (e.g., 9600 or 115200 bps).
This calculator determines the values for the Prescaler (UCBRx) and the Modulator stages (UCBRSx and UCBRFx) required to minimize bit timing errors.
Register Definitions
Register/Bit
Description
UCOS16
Oversampling mode bit. If 1, enables 16x oversampling (High Frequency). If 0, uses Low Frequency mode.
UCBRx
Clock Prescaler (16-bit). Split into two 8-bit registers: UCBR0 (low byte) and UCBR1 (high byte).
UCBRFx
First modulation stage. Used only in Oversampling mode (UCOS16=1) to fine-tune the timing within a bit period.
UCBRSx
Second modulation stage. Adjusts the length of specific bit periods to average out the timing error over time.
Calculation Modes
The USCI module supports two modes of baud rate generation:
1. Oversampling Mode (UCOS16 = 1)
This is the preferred mode when the clock frequency is high enough (Frequency > 16 × Baud Rate). It uses a majority vote logic on samples to reduce noise susceptibility.
N = Clock Frequency / Baud Rate
UCBRx = floor(N / 16)
UCBRFx = round((N/16 – UCBRx) × 16)
UCBRSx = Typically set to 0 or determined by lookup table for specific fractional remainders.
2. Low-Frequency Mode (UCOS16 = 0)
Used when the clock frequency is lower (Frequency < 16 × Baud Rate) or to save power.
N = Clock Frequency / Baud Rate
UCBRx = floor(N)
UCBRFx = 0 (Always)
UCBRSx = round((N – UCBRx) × 8)
How to Use These Values
Once calculated, apply these values to your MSP430 initialization code. For example, in C (using TI driver library or direct register access):
UCA0CTL1 |= UCSWRST; // Put state machine in reset
UCA0CTL1 |= UCSSEL_2; // SMCLK
UCA0BR0 = [UCBR0 Value];
UCA0BR1 = [UCBR1 Value];
UCA0MCTL = UCBRS_[Value] + UCBRF_[Value] + UCOS16;
UCA0CTL1 &= ~UCSWRST; // Initialize USCI state machine
function calculateBaudRegisters() {
// 1. Get Inputs
var freqInput = document.getElementById('clockFreq').value;
var unitMultiplier = document.getElementById('clockUnit').value;
var baudInput = document.getElementById('customBaud').value;
// 2. Validate Inputs
if (!freqInput || !baudInput || isNaN(freqInput) || isNaN(baudInput)) {
alert("Please enter valid numeric values for Frequency and Baud Rate.");
return;
}
var clockFreq = parseFloat(freqInput) * parseFloat(unitMultiplier);
var targetBaud = parseFloat(baudInput);
if (clockFreq <= 0 || targetBaud > 8) & 0xFF;
// Calculate actual baud and error for LF
// Formula: Actual = f / (UCBR + UCBRS/8) roughly,
// but physically it modulates. Avg division factor is UCBR + UCBRS/8.
var lf_divider = lf_ucbr + (lf_ucbrs / 8.0);
var lf_actual = (lf_divider > 0) ? (clockFreq / lf_divider) : 0;
var lf_error = ((lf_actual – targetBaud) / targetBaud) * 100;
// — Oversampling Mode Calculation (UCOS16 = 1) —
var os_possible = (N >= 16);
var os_ucbr = 0, os_ucbrf = 0, os_ucbrs = 0, os_ucbr0 = 0, os_ucbr1 = 0;
var os_actual = 0, os_error = 0;
if (os_possible) {
var os_div_val = N / 16.0;
os_ucbr = Math.floor(os_div_val);
var os_fraction = os_div_val – os_ucbr;
os_ucbrf = Math.round(os_fraction * 16);
if (os_ucbrf === 16) {
os_ucbrf = 0;
os_ucbr++;
}
os_ucbrs = 0; // Usually 0 for oversampling unless specific lookup used
os_ucbr0 = os_ucbr & 0xFF;
os_ucbr1 = (os_ucbr >> 8) & 0xFF;
// Calculate actual baud and error for OS
// Divider effective = 16 * (UCBR + UCBRF/16)
var os_divider = 16 * (os_ucbr + (os_ucbrf / 16.0));
os_actual = (os_divider > 0) ? (clockFreq / os_divider) : 0;
os_error = ((os_actual – targetBaud) / targetBaud) * 100;
}
// 4. Update UI
// Helper to format Hex
function toHex(num) {
return "0x" + (num < 16 ? "0" : "") + num.toString(16).toUpperCase();
}
// Helper to format error
function formatError(err) {
var absErr = Math.abs(err);
var colorClass = absErr < 2 ? "good-val" : "bad-val";
return '' + (err > 0 ? "+" : "") + err.toFixed(2) + '%';
}
document.getElementById('results-area').style.display = 'block';
// Update Low Freq DOM
document.getElementById('lf-ucbr').innerText = lf_ucbr;
document.getElementById('lf-ucbrf').innerText = lf_ucbrf;
document.getElementById('lf-ucbrs').innerText = "0x" + lf_ucbrs.toString(16).toUpperCase(); // UCBRS is often written to register as is
document.getElementById('lf-ucbr0').innerText = toHex(lf_ucbr0);
document.getElementById('lf-ucbr1').innerText = toHex(lf_ucbr1);
document.getElementById('lf-actual').innerText = Math.round(lf_actual).toLocaleString();
document.getElementById('lf-error').innerHTML = formatError(lf_error);
// Update Oversampling DOM
var osContainer = document.getElementById('oversampling-result');
if (os_possible) {
osContainer.style.opacity = "1";
osContainer.style.pointerEvents = "auto";
// Highlight recommendation
if (Math.abs(os_error) <= Math.abs(lf_error)) {
osContainer.style.borderLeftColor = "green";
document.getElementById('lowfreq-result').style.borderLeftColor = "#ccc";
} else {
osContainer.style.borderLeftColor = "#ccc";
document.getElementById('lowfreq-result').style.borderLeftColor = "green";
}
document.getElementById('os-ucbr').innerText = os_ucbr;
document.getElementById('os-ucbrf').innerText = os_ucbrf;
document.getElementById('os-ucbrs').innerText = "0x" + os_ucbrs.toString(16).toUpperCase();
document.getElementById('os-ucbr0').innerText = toHex(os_ucbr0);
document.getElementById('os-ucbr1').innerText = toHex(os_ucbr1);
document.getElementById('os-actual').innerText = Math.round(os_actual).toLocaleString();
document.getElementById('os-error').innerHTML = formatError(os_error);
} else {
osContainer.style.opacity = "0.5";
osContainer.style.pointerEvents = "none";
document.getElementById('os-actual').innerText = "N/A (Clock too slow)";
document.getElementById('os-error').innerText = "N/A";
// Force highlight to LF
document.getElementById('lowfreq-result').style.borderLeftColor = "green";
osContainer.style.borderLeftColor = "#ccc";
}
}