I am having issues with the UART on the MSP430FR5969 microcontroller. I am sending about 85 characters 100 times per second for a data acquisition system. I am using 115200 baud rate which should be sufficient. When I view the serial output in a terminal program, I get chunks of ~4000 characters every few hundred milliseconds. Since the data is time sensitive, I would like it to come in steadily.

Here is the guts of the code with the data acquisition specifics removed.

#include <msp430.h>

unsigned char counter = 0;

int main(void) {
    init();             // Initialize MSP430

    while(1) {

void init() {
    WDTCTL = WDTPW | WDTHOLD;    // Stop watchdog timer

    P2SEL0 &= ~(BIT5);           // USCI_A1 UART TX line
    P2SEL1 |=  (BIT5);

    // Disable the GPIO power-on default high-impedance mode to activate
    // previously configured port settings
    PM5CTL0 &= ~LOCKLPM5;

    // Clock System Setup
    // ---------------------------------------------------------------------
    CSCTL0_H = CSKEY >> 8;                    // Unlock CS registers
    CSCTL1 = DCOFSEL_4 | DCORSEL;             // Set DCO to 16MHz
                                              // ACLK = VLOCLK
    CSCTL3 = DIVA__2 | DIVS__1 | DIVM__2;     // Set all dividers to 1
    CSCTL0_H = 0;                             // Lock CS registers
    // ---------------------------------------------------------------------

    // Configure USCI_A1 for UART mode
    // ---------------------------------------------------------------------
    UCA1CTLW0 = UCSWRST;                       // Put eUSCI in reset
    UCA1CTLW0 |= UCSSEL__SMCLK;                // CLK = SMCLK
    UCA1BR0 = 8;                               // 115200 baud rate
    UCA1BR1 = 0x00;
    UCA1MCTLW |= UCOS16 | UCBRF_10 | 0xF700;
    UCA1CTLW0 &= ~UCSWRST;                     // Initialize eUSCI

void write_data_to_uart() {
    char i;
    for(i = 0; i < 85; i++) {
    uart_write_byte(0x0D);                  // Carriage return
    uart_write_byte(0x0A);                  // Line feed

void uart_write_byte(char x) {
    while(!(UCA1IFG & UCTXIFG));
    UCA1TXBUF = x;

I feel like the issue has to do with a misconception on my part about UART. Is there some sort of timer setup I should be using to steadily feed the data into the TX buffer?

  • 3
    \$\begingroup\$ Are you using a USB to serial converter? It's probably some sort of buffering going on with that, maybe try a different one if you can get one. Could be the terminal program too I guess, might be worth trying a different one. \$\endgroup\$
    – PeterJ
    Jan 6, 2015 at 3:29
  • 1
    \$\begingroup\$ Came here to say buffering on host side but @PeterJ already did :) Can you look at the uart lines with a scope or logic analyzer? You could prove it to yourself that way. \$\endgroup\$ Jan 6, 2015 at 3:32
  • \$\begingroup\$ I don't know why I didn't think to try that... been a long day! But it appears that there is steady data coming from the microcontroller - I don't see any long pauses. Thank you! \$\endgroup\$
    – dgreenheck
    Jan 6, 2015 at 3:44
  • \$\begingroup\$ You might need to consider flow control. \$\endgroup\$
    – downbeat
    Jan 6, 2015 at 4:27

1 Answer 1


You may have removed it but I do not see any delay between your calls to write_data_to_uart(). Are you sleeping for sufficient time between your calls?

What is on the receiving side of the UART? An acutal serial port or a USB Serial converter?

The best way to pace the data going out of the uC is to write your data to a memory location, hook up to a timer interrupt and kick off a DMA transfer using the DMA from memory to UCA1TXBUF. This method has caused the least jitter in the past and I was able to put uC in a low-power state.


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