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Clock Failure Detect (CFD)

What is CFD?

CFD enables the microcontroller (MCU) to detect if the primary (main) clock source stops and then automatically switch over to an alternative internal clock source. This adds an additional layer of robustness in situations where the external crystal is damaged or the external clock signal stops. Once configured, the switch happens automatically, and an interrupt can be generated for the MCU to take appropriate actions. 

Example APM Timing Diagram

Design Benefits of CFD

CFD allows the MCU to utilize an external clock/crystal, but in the event that the source fails, the CFD will automatically switch the MCU to an internal clock to continue operation. This allows the MCU to go into recovery mode, taking the actions needed to set the application into a safe state or to generate necessary error signaling. Since the internal clock speed may not be equal to the external clock speed, the CFD interrupt could also be used to make the necessary speed adjustments to continue normal operation.

Configuring CFD

The CFD system is “set it and forget it”. First, select which clock source to monitor. This clock source could be external or internal. Next, set the interrupt enable flag to generate an interrupt if the source fails. Finally, enable the CFD. The application is now set up to switch over to an internal clock.

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