Texas Instruments MSP430x1xx - Manual (Page 108)

LFXT1 and XT2 Oscillators
7-8
Figure 7–8. Oscillator Fault in Oscillator Error Condition at Start-Up
V
CC
XT2CLK
LFXT1CLK
XT_OscFault
XT1Off/
XT2Off
7.2.4 Select DCO Oscillator for MCLK on XT Oscillator Fault
The DCO oscillator is selected automatically for MCLK if either one of the
oscillators LFXT1 (in HF mode only), or XT2 is selected for MCLK source and
this oscillator fails. Since the DCO oscillator is now selected, the NMI
requested by the oscillator fault can be processed. The DCO oscillator is
switched on and the DCOCLK is switched to be the source for MCLK. An NMI
is processed even if the CPU is switched off (CPUOFF=1).
The MSP430 ultralow-power system allows any enabled interrupt to be
serviced from any low-power mode, including LPM4. MCLK automatically
becomes active inside of an interrupt service routine.
An NMI interrupt routine written by the user, which handles the oscillator fault,
has some important steps that should be used for proper handling of the fault
situation. First, if the NMI interrupt routine detects the oscillator fault and
selects the DCO as clock source for the system clock MCLK. Second, the user
programs a proper algorithm to detect if the XT oscillator is working again and
reselects an XT oscillator for the system clock MCLK if desired.