MSP430G2553 Problem to feed external clock from XIN to MCLK

Hello MSP430-Experts !

I'm very happy with my LaunchPad for some time now.

At the moment I try to switch MCLK from DCO-Clock of 8 MHz   to  an external XTAL-Oscillator clock of 2 MHz.

As a cross check I'm using a toggling pin and a scope - - -   five clocks per toggle instruction can be observed.

However, I don't see any change in behavior of the toggling - it stays at  5 x 125 ns  !

My code is:

BCSCTL1 = CALBC1_8MHZ;   // Set DCO to 8 MHz

DCOCTL = CALDCO_8MHZ;

// P2.6 is normally INPUT by default ! now used as XIN only ...

_BIC_SR( OSCOFF );  // reset bit in status register

BCSCTL3 = LFXT1S_3 + XCAP_0; 

IFG1 &= ~OFIFG;   // Clear OSCFault flag

__bis_SR_register(SCG1 + SCG0);   // Stop DCO

BCSCTL2 |= SELM_3 + SELS;

..............................

Is there any statement missing or is the sequence wrong ??

Thank you very much in advance !!

With best regards, Uli