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MSP430FR4133 eUSCI_A UART setting of UCAxMCTLW register

Nightingale
Intellectual 440 points

Hi,recently I use fr4133 code_example: msp430fr413x_euscia0_uart_01 and found some quention of UCA0MCTLW register. Here is the code_example below.

/**********************************************************************************/

UCA0CTLW0 |= UCSWRST;
UCA0CTLW0 |= UCSSEL__SMCLK;

// Baud Rate calculation
// 8000000/(16*9600) = 52.083
// Fractional portion = 0.083
// User's Guide Table 14-4: UCBRSx = 0x49
// UCBRFx = int ( (52.083-52)*16) = 1


UCA0BR0 = 52; // 8000000/16/9600
UCA0BR1 = 0x00;
UCA0MCTLW = 0x4900 | UCOS16 | UCBRF_1;

UCA0CTLW0 &= ~UCSWRST; // Initialize eUSCI
UCA0IE |= UCRXIE; // Enable USCI_A0 RX interrupt

/**********************************************************************************/

The  Fractional portion is 0.083,and i look up for User's Guide Table 14-4,UCBRSx=0x04, but the code_example says UCBRSx=0x49,why?

  • 0
    TI__Prodigy 485 points

    Hi,

    As UG described, UCBRSx can be found by looking up the fractional part of N ( = N - INT(N) ) in table 15-4.
    N=8000000/9600=833.3333, fractional part of N ( = N - INT(N) ) should be 0.3333. Looking up in table 15-4, UCBRSx=0x49.
    The comment "Fractional portion = 0.083" is a little confused here. 0.083 is not fractional part of N.

    Regards,
    Darren

  • 0 in reply to Darren Lu
    Intellectual 440 points

    Thanks for your reply. Now, I get confused in it. As you say ,if I setting like this:
    /***********************************************/
    // Baud Rate calculation
    // 8000000/(16*38400) = 13.020
    // Fractional portion = 0.020
    // User's Guide Table 14-4: UCBRSx = 0x49
    // UCBRFx = int ( (13.020-13)*16) = 0
    UCA0BR0 = 13; // 8000000/16/38400
    UCA0BR1 = 0x00;
    UCA0MCTLW = 0x4900 | UCOS16 | UCBRF_0;
    /**********************************************/
    here ,800000/38400=208.333, so UCBRSx should be 0x49,right? But I look up for table 14-5, it says UCBRSx =0x84 ,why?

  • 0 in reply to Nightingale
    TI__Prodigy 485 points

    UCBRSx is the 8-bit modulation pattern that will impact TX/RX neg/pos error. Table 14-4 provides some patterns according to different fractions.  Table 14-5 also provide some patterns according to each typical baud rate settings and also TX/RX neg/pos error from error calculation. Both two UCBRSx values(different pattern) can work well.

    Best Regards,
    Darren

  • 0 in reply to Darren Lu
    Intellectual 440 points
    Thank you so much.
  • 0 in reply to Darren Lu
    Guru 325350 points

    For any combination of clock and baud rate (not only those in the table), you can easily compute the register values and the worst-case errors with the USCI UART Calculator in the Wiki.

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