MSP430FR4133: Drive XT1 -> ACLK -> UART?

Hello,

I am using CCS 10.1.0.00010  in Windows with MSP430FR4133 LaunchPad™ Development Kit (MSP-EXP430FR4133)

I am trying to drive the UART from clock derived from XT1.

    CS_turnOnXT1(CS_XT1_DRIVE_1);

    CS_initClockSignal(
        CS_ACLK,
        CS_XT1CLK_SELECT,
        CS_CLOCK_DIVIDER_1
        );

I have XT1 -> ACLK -> TimerA with period of 32767 and duty cycle 16383.
As expected, I see the LED flashing at once per second.

In the same program, the UART works as expected when driven from UCSSEL__SMCLK.
I haven't found a way to drive the UART with ACLK.  The choices are:

#define UCSSEL_0               (0x0000)       /* USCI 0 Clock Source: 0 */
#define UCSSEL_1               (0x0040)       /* USCI 0 Clock Source: 1 */
#define UCSSEL_2               (0x0080)       /* USCI 0 Clock Source: 2 */
#define UCSSEL_3               (0x00C0)       /* USCI 0 Clock Source: 3 */
#define UCSSEL__UCLK           (0x0000)       /* USCI 0 Clock Source: UCLK */
#define UCSSEL__MODCLK         (0x0040)       /* USCI 0 Clock Source: MODCLK */
#define UCSSEL__SMCLK          (0x0080)       /* USCI 0 Clock Source: SMCLK */

>>>>> How do I drive the UART directly from XT1 -> ACLK?

The underlying problem was the SMCLK/UART data was intermittent when running the code in CCS debug mode.
I was thinking that was really a problem with SMCLK in debug mode and thought XT1 would be a more stable source.

>>>>> Is there another way to fix intermittent SMCLK/UART data in debug mode?

I have an oscilloscope on the bench with board if that is helpful.

Thanks,
Dave