8.4.4 PLL Lock Detect

The PLL lock detection circuit is a digital detection circuit which detects any frequency error, even a single cycle slip. The PLL unlock is signalized when a certain number of cycle slips have been exceeded, at which point the counter is reset. A frequency error of 2% will cause PLL unlock to stay low. A 0.5% frequency error shows up as toggling the PLL lock output with roughly 50% duty cycle at roughly 1/1000 ^th of the PFD update frequency to the device. A frequency error of 1ppm would show up as rare toggling low for a duration of approximately 1000 PFD update clock cycles. If the system plans using PLL lock to toggle a system reset, then consider adding an RC filter on the PLL LOCK output (Status 1 or Status 0) to avoid rare cycle slips from triggering an entire system reset.