Digital integrated frequency discriminator of external clock and

Abstract

A digital integrated circuit normally operated from an external clock signal and including an internal reference oscillator for providing a reference signal as an internally generated signal backup in the event that the external clock signal is not present or is too low in frequency for safe operation of the integrated circuit. The integrated circuit has a frequency comparator for receiving the external clock signal and the internal reference signal as inputs and producing an output indicative of the state of the external clock signal. A two-way switch is connected to the output of the frequency comparator and selects either the external clock signal or the internal reference signal for transmission to the integrated circuit based upon the output from the frequency comparator. The frequency comparator includes a frequency discriminator which has two parts for providing respective complementary output logic signals when the external clock signal is consistently at either the high logic level or the low logic level, and for respectively providing the same output logic signal when the external clock signal alternates between the high and low logic levels. An output circuit is connected to the outputs of the first and second parts of the frequency discriminator for combining the output signals of the first and second parts in producing the output of the frequency comparator as provided to the two-way switch.