Lock/unlock indicator for pll circuits

Abstract

A lock circuit for indicating a locked/unlocked condition of phased lock loops circuits, which uses a reference clock signal input to a good-cycle counter and to a bad-cycle counter to signal a set/reset latch to output a signal indicating whether or not the phase of the output signal and input clock are in phase. Phase detector inputs are XOR gated to produce a pulse when the phase locked output clock is in a bad cycle indicated by the phase locked output clock not being in-phase with its input clock. Pulses on the XOR gate output on a bad cycle feed a single cycle counter reset circuit. The single cycle counter reset circuit on every cycle resets one of the bad and good counters based on its detection of a bad cycle pulse from the XOR gate. The good-cycle counter's output is to the SET input of the set/reset latch, and the bad-cycle counter's output is to the RESET input of the set/reset latch . We enable specific cycling of both the good and bad counters.