Texas Instruments MSP430x1xx Basic Clock Module Fail-Safe Operation, 9. Oscillator-Fault Signal

Basic Clock Module Operation

4.2.6Basic Clock Module Fail-Safe Operation

The basic clock module incorporates an oscillator-fault detection fail-safe feature. The oscillator fault detector is an analog circuit that monitors the LFXT1CLK (in HF mode) and the XT2CLK. An oscillator fault is detected when either clock signal is not present for approximately 50 ∝s. When an oscillator fault is detected, and when MCLK is sourced from either LFXT1 in HF mode or XT2, MCLK is automatically switched to the DCO for its clock source. This allows code execution to continue, even though the crystal oscillator has stopped.

When OFIFG is set and OFIE is set, an NMI interrupt is requested. The NMI interrupt service routine can test the OFIFG flag to determine if an oscillator fault occurred. The OFIFG flag must be cleared by software.

Note: No Oscillator Fault Detection for LFXT1 in LF Mode

Oscillator fault detection is only applicable for LFXT1 in HF mode and XT2.

There is no oscillator fault detection for LFXT1 in LF mode.

OFIFG is set by the oscillator fault signal, XT_OscFault. XT_OscFault is set at POR, when LFXT1 has an oscillator fault in HF mode, or when XT2 has an oscillator fault. When XT2 or LFXT1 in HF mode is stopped with software the XT_OscFault signal becomes active immediately, remains active until the oscillator is re-started, and becomes inactive approximately 50 ∝s after the oscillator re-starts as shown in Figure 4−8.

Figure 4−9. Oscillator-Fault Signal

VCC

XT1OFF/

XT2OFF

LFXT1CLK/

XT2CLK

XT_OscFault

software enables OSC software disables OSC

OSC faults

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4-10Basic Clock Module