MSP430FR5969: Unable to get consistent I2C behavior with the MSP430FR5969 - UCSCLLOW doesn't change after setting UCTXSTT.

Tool/software:

Hi all,

I've been scratching my head on this one. Long story short, I am teaching myself I2C on the MSP430FR5969. I made the following circuit, consisting of an Arduino UNO and a MSP430FR5969 Launchpad. I connected P1.6 (SDA) and P1.7 (SCL) on the launchpad to SDA and SCL pins on the Arduino UNO. I also set the Launchpad to be a master transmitter and the Arduino Uno to be a slave receiver. I also have two 4.7 kOhm resistors connecting the SDA and SCL to a 5 V rail. 

What I have noticed is that the Launchpad would communicate through I2C initially because I can see the characters being received by the Arduino Uno via the serial monitor. But in the second time, the Launchpad will not run the ISR. I added some breakpoints and noticed that UCSCLLOW would not change. It remains 0. But when I reset the Arduino UNO and run the Launchpad with breakpoints again, UCSCLLOW would change to 1 right after I set UCTXSTT to the UCB0CTLW0 register.

I'm really confused. Some help would be greatly appreciated. I attached the two codes below. The address of the Arduino UNO is set to be 4.

#include <msp430.h>
#include <stdbool.h>
#include <stdint.h>

static inline
void launchpad_clk_config(void) {
    // The SMCLK will be used for both the SPI communication and the timers.
    // Due to the FRAM, we need to add in a wait condition so that higher frequencies can be used.

    CSCTL0_H =  CSKEY >> 8;                                  // Unlock CS registers for modification.
    CSCTL1   = ~DCORSEL & ~DCOFSEL_2;                        // Set DCO to 1 MHz
    CSCTL2   =  SELA__VLOCLK | SELS__DCOCLK | SELM__DCOCLK;  //
    CSCTL3   =  DIVA__1 | DIVS__1 | DIVM__1;                 // Set all dividers
    CSCTL0_H =  0;                                           // Lock CS registers back.
}

#define LP_SDA  BIT6
#define LP_SCL  BIT7

static inline
void launchpad_env_config(void) {
    P1SEL1    |=  LP_SDA;
    P1SEL0    &= ~LP_SDA;

    P1SEL1    |=  LP_SCL;
    P1SEL0    &= ~LP_SCL;

    UCB0CTL1  |=  UCSWRST;
    UCB0CTLW0 |=  UCMODE_3 + UCSYNC + UCMST + UCSSEL_2; // Using the SMCLK, and transmit.
    UCB0BRW    =  10;                                   // SMCLK will generate a clock of 12 MHz, which will be divided by 30 to yield 400 KHz
    UCB0CTLW1  =  UCASTP_2;                             // automatic STOP assertion
    UCB0CTL1  &= ~UCSWRST;                              // eUSCI_B in operational state
    UCB0IFG    =  0;
}

static inline
void launchpad_env_write(const char addr, const char bytes) {
    UCB0CTL1  |=  UCSWRST;
    UCB0TBCNT  =  bytes;
    UCB0I2CSA  =  addr;
    UCB0CTL1  &= ~UCSWRST;

    UCB0CTLW0 |=  UCTR;
    UCB0IE    |=  UCTXIE0 + UCSTTIE + UCNACKIE;
    UCB0CTLW0 |=  UCTXSTT;
}

char num = '9';

void main(void) {
    WDTCTL = WDTPW | WDTHOLD;
    PM5CTL0 &= ~LOCKLPM5;

    launchpad_clk_config();
    launchpad_env_config();

    __enable_interrupt();

    launchpad_env_write(4, 1);
    launchpad_env_write(4, 1);
}

#pragma vector=USCI_B0_VECTOR
__interrupt void USCI_B0_ISR(void) {
    switch(__even_in_range(UCB0IV, USCI_I2C_UCBIT9IFG)) {
        case USCI_NONE:
            __no_operation();
            break;
        case USCI_I2C_UCNACKIFG:
            __no_operation();
            break;
        case USCI_I2C_UCTXIFG0:
            UCB0TXBUF = num++;
            break;
        case USCI_I2C_UCRXIFG0:
            __no_operation();
            break;
        default:
            break;
    }
    return;
}

// Wire Slave Receiver
// by Nicholas Zambetti <http://www.zambetti.com>

// Demonstrates use of the Wire library
// Receives data as an I2C/TWI slave device
// Refer to the "Wire Master Writer" example for use with this

// Created 29 March 2006

// This example code is in the public domain.

#include <Wire.h>

void setup() {
  Wire.begin(4);                // join i2c bus with address #4
  Wire.onReceive(receiveEvent); // register event
  Serial.begin(9600);           // start serial for output
}

void loop() {
  delay(1);
}

void receiveEvent(int howMany) {
  while (Wire.available()) {
    char c = Wire.read(); // receive byte as a character
    Serial.print(c);      // print the character
  }
}