Using I2C UCB1 on MSP430F5529

How does your code proceed? In master transmit mode, the USCI will stall the transfer in the ACK cycle (SCL remains low) until you either write data to TXBUF or set UCTXSTP (to not write anything, a 'slave present' check). Only then will UCTXSTT clear. Then check UCNACKIFG for the slave answer and proceed with writing data to TXBUF (as soon as TXIFG is set again) or ending the transfer with setting UCTXSTP (again, when TXIFG indicates that the previous byte is being sent)

Thanks for the response. I am in the state where SCL is low, and remains low. My issue is that I have tried several changes to code and none seem to work. I expect I am not waiting for the right flag to be set before taking an action and the data to be sent out is just overwriting something. So To have a starting point, here is what I tried this morning. I have also disabled TX interrupt use and am just trying to write the TxBUF when I have something to transmit and the buffer is open to receive. This is likely where I am not waiting for the right flag to be set.

void I2C_UCB1_CUR_MON_read(char cur_mon_addr, char cur_mon_reg){
UCB1I2CSA = cur_mon_addr; // Set Slave Address
UCB1CTL1 &= ~UCSWRST; // Clear SW reset, resume operation
I2C_UCB1_Char = cur_mon_reg;
UCB1CTL1 |= UCTR + UCTXSTT; // I2C TX, start condition
while (UCB1CTL1 & UCTXSTT); // Wait start condition, address transmitted
while (!(UCB1IFG & UCTXIFG)); // Wait for TXBUF to be empty
UCB1TXBUF = I2C_UCB1_Char; // Transmit data I2C_UCB1_Char
while (!(UCB1IFG & UCTXIFG)); // Wait for TXBUF to be empty
while (UCB1CTL1 & UCTXSTP); // Ensure stop condition got sent

//------------------------------------------------------------------------------

}

I never see the UCB1TXBUF = I2C_UCB1_Char; I see the ACK from the Slave. I am not sure how to see it in software nor how to respond to it.

The problem is that the USCI waits for something in TXBUF (or UCTXSTP set) before it completes the start byte ACK cycle. But your code waits for the start byte being sent before it writes to TXBUF. So both wait for each other on the first while. The second while is superfluous. As soon as you set UCTXSTT and UCTR, UCTXIFG gets set. You only need to wait for TXIFG for writing the second (and subsequent) byte(s) or setting UCTXSTP to end the transfer after the first byte.