MSP430G2553: I2C master transmit code, both SCL and SDA pulled up and stay in this state.

Part Number:MSP430G2553

I am struggling with the I2C interface programming with MSP430G2553 MCU. I did not plug the slave device to test it, but I expected to have at least the slave address to be send over the I2C. I use internal resistors pull ups and ACLK to clock USCI. Here is my code:

#include <msp430.h>

int main(void) {
  WDTCTL = WDTPW + WDTHOLD;                 // Stop watchdog timer
  DCOCTL  = CALDCO_1MHZ;
  BCSCTL1 = CALBC1_1MHZ;

  // ACLK setup: VLO/8
  BCSCTL1 &= ~XTS;                          // Select Low-Frequency clock
  BCSCTL3 = (BCSCTL3 & ~LFXT1S_3) | LFXT1S_2; // Select VLO
  BCSCTL1 = (BCSCTL1 & ~DIVA_3) | DIVA_3;   // VLO/8 divider

  UCB0CTL1 |= UCSWRST;
  UCB0CTL0 = UCMST + UCMODE_3 + UCSYNC;
  UCB0CTL1 |=  UCSSEL1; // Use ACLK
  UCB0BR0 = 180;
  UCB0BR1 = 0;
  UCB0I2CSA = 0x52;
  P1SEL  |= BIT7 + BIT6;
  P1SEL2 |= BIT7 + BIT6;
  P1REN |= BIT6 + BIT7;
  P1OUT |= BIT6 + BIT7;
  UCB0CTL1 &= ~UCSWRST;
  __delay_cycles(200);
  while (UCB0STAT & UCBBUSY);

  UCB0CTL1 |= UCTR + UCTXSTT;
  while (UCB0CTL1 & UCTXSTT);
  UCB0TXBUF = 0x36;
  while (!(UCB0TXIFG & UCB0TXIFG));
  while (UCB0STAT & UCBBUSY);
  UCB0CTL1 |= UCTXSTP;
  while (UCB0CTL1 & UCTXSTP);
  while(1);
}

When debugging I see that the code gets stuck at the following line:

while (UCB0CTL1 & UCTXSTT);

The logic sniffer with the BusPirate device, shows that after initialization, both SDA and SCL are pulled high and stay in this state.

Here the Channel-0 is P1.6 (SCL); Channel-1 is P1.7 (SDA) and Channel-3 is PIN16 (RST). I use MSP-EXP430G2 Rev 1.5 launchpad for the test.