Patent ID: 6043749
Filing Date: 2000-03-28
Classification: G01R

Abstract:
A frequency detection circuit comprising:a transistor switch, connected between a first electric potential and a second electric potential, having a gate applied with a clock signal to be measured;a resistor and a capacitor, which both are connected in parallel between the output of said transistor switch and said second electric potential, said capacitor being charged toward said first electric potential through an on-resistance of said transistor switch when the measured clock signal assumes one level and discharged toward said second electric potential through said resistor in accordance with a time constant determined by a resistance of said resistor and a capacitance of said capacitor when the measured clock signal changes to the other level; anda comparator, connected to the output of said transistor switch, for comparing an output voltage of said transistor switch with a predetermined threshold voltage supplied to said comparator and delivering a signal of alternating waveform when the frequency of the measured clock signal is smaller as compared to said time constant and the output voltage of said transistor switch crosses said threshold voltage during discharge of said capacitor, and said comparator delivering a signal of constant level when the frequency of the measured clock signal is larger as compared to said time constant and the output of said transistor switch does not cross said threshold voltage.