5.4.2 Fail-Safe Operation

When the external clock fails, the FSCM overwrites the COSC bits to select HFINTOSC (3'b110). The frequency of HFINTOSC would be determined by the previous state of the HFFRQ bits and the NDIV/CDIV bits. The bit flag OSCFIF of the PIR1 register is set. Setting this flag will generate an interrupt if the OSCFIE bit of the PIE1 register is also set. The device firmware can then take steps to mitigate the problems that may arise from a failed clock. The system clock will continue to be sourced from the internal clock source until the device firmware successfully restarts the external oscillator and switches back to external operation, by writing to the NOSC and NDIV bits.