LOCK-IN DETECTING CIRCUIT HAVING VARIABLE WINDOW FOR CHECKING PHASE LOCKED LOOP AND METHOD USED THEREIN

摘要

A phase locked loop makes a system clock signal synchronous to a horizontal synchronizing signal for a display unit, and a lock-in detecting circuit monitors said phase locked loop to see whether or not a phase difference takes place between the system clock signal and the horizontal synchronizing signal, wherein the lock-in detecting circuit measures the unlocked state between the system clock signal and the horizontal synchronizing signal in a window defined in a vertical synchronizing period and, thereafter, compares the time period of the unlocked state with a critical value to see whether or not the unlocked state is due to a temporary phenomenon or a phase difference to be corrected so that an detecting signal of the lock-in detecting circuit is reliable.