MSP430FR6989: backchannel UART problem

The backchannel UART on the FR6989 Launchpad is UCA1, rather than UCA0. [Ref Launchpad guide (SLAU627A) Sec 2.2.4] That the Example uses UCA0 does seem like a missed opportunity.

Perhaps the simplest solution is to remove the RXD/TXD jumpers from the "bridge" header (J101) and patch the P2.0 (J1.8) pin to TXD and P2.1 (J2.19) pin to RXD.

Alternatively, you could change the program to use UCA1 instead of UCA0. Besides the register references, you need to change the PSEL references to set P3.4/5 ("P3SEL0 |= (BIT4|BIT5)") [Ref Data Sheet (SLAS789D) Table 6-25] and change to "vector=USCI_A1_VECTOR".

Correct

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//******************************************************************************
//  MSP430FR69xx Demo - eUSCI_A0 UART echo at 9600 baud using BRCLK = 8MHz
//
//  Description: This demo echoes back characters received via a PC serial port.
//  SMCLK/ DCO is used as a clock source and the device is put in LPM3
//  The auto-clock enable feature is used by the eUSCI and SMCLK is turned off
//  when the UART is idle and turned on when a receive edge is detected.
//  Note that level shifter hardware is needed to shift between RS232 and MSP
//  voltage levels.
//
//  The example code shows proper initialization of registers
//  and interrupts to receive and transmit data.
//  To test code in LPM3, disconnect the debugger.
//
//  ACLK = VLO, MCLK =  DCO = SMCLK = 8MHz
//
//                MSP430FR6989
//             -----------------
//       RST -|     P2.0/UCA0TXD|----> PC (echo)
//            |                 |
//            |                 |
//            |     P2.1/UCA0RXD|<---- PC
//            |                 |
//
//   William Goh
//   Texas Instruments Inc.
//   April 2014
//   Built with IAR Embedded Workbench V5.60 & Code Composer Studio V6.0
//******************************************************************************
// modified by: Peter Spevak
//
// last change: April 7th 2020
//
// 04/07/2020 changing the UART to eUSCIA1, as this is the UART for
//            backchannel on virtual COM port to PC
//
//*****************************************************************************
#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW | WDTHOLD;                 // Stop Watchdog

  // Configure GPIO
  //P2SEL0 |= BIT0 | BIT1;                    // USCI_A0 UART operation         // 04/07/2020
  //P2SEL1 &= ~(BIT0 | BIT1);                                                   // 04/07/2020
  P3SEL0 |= BIT4 + BIT5;                    // eUSCI_A1 UART                    // 04/07/2020
  P3SEL1 &= ~(BIT4 + BIT5);                 // eUSCI_A1 UART                    // 04/07/2020

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

  // Startup clock system with max DCO setting ~8MHz
  CSCTL0_H = CSKEY >> 8;                    // Unlock clock registers
  CSCTL1 = DCOFSEL_3 | DCORSEL;             // Set DCO to 8MHz
  CSCTL2 = SELA__VLOCLK | SELS__DCOCLK | SELM__DCOCLK;
  CSCTL3 = DIVA__1 | DIVS__1 | DIVM__1;     // Set all dividers
  CSCTL0_H = 0;                             // Lock CS registers

  // Configure USCI_A0 for UART mode
  /*UCA0CTLW0 = UCSWRST;                      // Put eUSCI in reset             // 04/07/2020
  UCA0CTLW0 |= UCSSEL__SMCLK;               // CLK = SMCLK
  // Baud Rate calculation
  // 8000000/(16*9600) = 52.083
  // Fractional portion = 0.083
  // User's Guide Table 21-4: UCBRSx = 0x04
  // UCBRFx = int ( (52.083-52)*16) = 1
  UCA0BR0 = 52;                             // 8000000/16/9600
  UCA0BR1 = 0x00;
  UCA0MCTLW |= UCOS16 | UCBRF_1 | 0x4900;
  UCA0CTLW0 &= ~UCSWRST;                    // Initialize eUSCI
  UCA0IE |= UCRXIE;                         // Enable USCI_A0 RX interrupt
*/

  // Configure USCI_A1 for UART mode
  UCA1CTLW0 = UCSWRST;                      // Put eUSCI in reset               // 04/07/2020
  UCA1CTLW0 |= UCSSEL__SMCLK;               // CLK = SMCLK                      // 04/07/2020
  // Baud Rate calculation
  // 8000000/(16*9600) = 52.083
  // Fractional portion = 0.083
  // User's Guide Table 21-4: UCBRSx = 0x04
  // UCBRFx = int ( (52.083-52)*16) = 1
  UCA1BR0 = 52;                             // 8000000/16/9600                  // 04/07/2020
  UCA1BR1 = 0x00;                                                               // 04/07/2020
  UCA1MCTLW |= UCOS16 | UCBRF_1 | 0x4900;                                       // 04/07/2020
  UCA1CTLW0 &= ~UCSWRST;                    // Initialize eUSCI                 // 04/07/2020
  UCA1IE |= UCRXIE;                         // Enable USCI_A0 RX interrupt      // 04/07/2020

  __bis_SR_register(LPM3_bits | GIE);       // Enter LPM3, interrupts enabled
  __no_operation();                         // For debugger
}

#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=USCI_A0_VECTOR
__interrupt void USCI_A0_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_A0_VECTOR))) USCI_A0_ISR (void)
#else
#error Compiler not supported!
#endif
{
  switch(__even_in_range(UCA0IV, USCI_UART_UCTXCPTIFG))
  {
    case USCI_NONE: break;
    case USCI_UART_UCRXIFG:
      while(!(UCA0IFG&UCTXIFG));
      UCA0TXBUF = UCA0RXBUF;
      __no_operation();
      break;
    case USCI_UART_UCTXIFG: break;
    case USCI_UART_UCSTTIFG: break;
    case USCI_UART_UCTXCPTIFG: break;
  }
}

#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)            // 04/07/2020
#pragma vector=USCI_A1_VECTOR
__interrupt void USCI_A1_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(USCI_A1_VECTOR))) USCI_A1_ISR (void)
#else
#error Compiler not supported!
#endif
{
  switch(__even_in_range(UCA1IV, USCI_UART_UCTXCPTIFG))
  {
    case USCI_NONE: break;
    case USCI_UART_UCRXIFG:
      while(!(UCA1IFG&UCTXIFG));
      UCA1TXBUF = UCA1RXBUF;
      __no_operation();
      break;
    case USCI_UART_UCTXIFG: break;
    case USCI_UART_UCSTTIFG: break;
    case USCI_UART_UCTXCPTIFG: break;
  }
}

Hello Yannick,

the code sequence you have written there cannot work. You need to check, whether the UART TX channel is still busy or not, as the TX buffer will always try transmitting at the point of writing to it. This might corrupt the state machine of the eUSCI or the content of the data you're transmitting you'll be writing at the speed of the code execution to TX buffer, while the data rate is 9600.

You need this kind of instruction sequence.e.g. when inserting this into the ISR I have posted you can see as a response not the echo but the "U" from MSP430.

while(!(UCA1IFG&UCTXIFG));
      //UCA1TXBUF = UCA1RXBUF;                                                  // 04/07/2020
      UCA1TXBUF = 0x55;                                                         // 04/07/2020

Best regards

Peter