Texas Instruments MSP430x1xx manual Sourcing Mclk from a Crystal

Basic Clock Module Operation

Sourcing MCLK from a Crystal

After a PUC, the basic clock module uses DCOCLK for MCLK. If required, MCLK may be sourced from LFXT1 or XT2.

The sequence to switch the MCLK source from the DCO clock to the crystal clock (LFXT1CLK or XT2CLK) is:

1)Switch on the crystal oscillator

2)Clear the OFIFG flag

3)Wait at least 50 ∝s

4)Test OFIFG, and repeat steps 1-4 until OFIFG remains cleared.

; Select LFXT1 (HF mode) for MCLK

	BIC	#OSCOFF,SR	; Turn on osc.
	BIS.B	#XTS,BCSCTL1	; HF mode
L1	BIC.B	#OFIFG,&IFG1	; Clear OFIFG
	MOV	#0FFh,R15	; Delay
L2	DEC	R15	;
	JNZ	L2	;
	BIT.B	#OFIFG,&IFG1	; Re−test OFIFG
	JNZ	L1	; Repeat test if needed
	BIS.B	#SELM1+SELM0,&BCSCTL2 ; Select LFXT1CLK

4-12Basic Clock Module