In industrial equipment front-end AC signal monitoring systems, zero-crossing detection (ZCD) methodology proves particularly valuable for real-time power failure detection. The implementation involves passing the AC signal through a full-bridge rectifier configuration, where the rectified output flows through a high-value current-limiting resistor. Clamping diodes are strategically employed to restrict the voltage amplitude within safe operational boundaries while verifying signal integrity. The voltage after being clamped by the internal diode of the MCU is shown in Figure 3-1.

For standard 220V and 50Hz mains monitoring, the voltage typically rises from 0V to VDD within approximately 50μs after zero-crossing. By implementing a software algorithm that starts a 100μs watchdog timer upon zero-cross detection, any failure to recover high voltage within this window reliably indicates power failure. Additionally, precise AC frequency measurement can be derived from the time intervals between consecutive zero-crossings. This approach effectively detects both complete outages and frequency variations while requiring minimal computational resources.

Figure 3-1 Voltage of MCU Input Pin