Abstract: An all digital circuit which operates with an electronic oscillator of the type that receives an input signal and synchronizes its oscillations to transitions in the input signal, comprises: a pulse-generating circuit 20 coupled to the oscillator for digitally forming periodic pulses in synchronization with selected oscillations of the oscillator; a detecting circuit 30 coupled to receive the pulses and the input signal for digitally detecting whether a transition occurs in the input signal in the absence of a pulse; and a counting circuit 40 coupled to the detecting circuit for digitally counting so long as the detecting circuit fails to detect a transition in the absence of a pulse and for indicating the oscillator is synchronized when the count reaches a predetermined number.