Pll frequency synthesizer having a power saving circuit

Claim 1

A PLL frequency synthesizer comprising: an oscillator generating a first oscillation signal; reference counter means, coupled to said oscillator, for generating a reference signal by frequency-dividing the first oscillation signal; a voltage-controlled oscillator generating a second oscillation signal; prescaler means, coupled to said voltage-controlled oscillator, for generating a first frequency-divided signal by frequency-dividing the second oscillation signal; programmable counter means, coupled to said prescaler means, for generating a second frequency-divided signal by frequency-dividing the first frequency-divided signal; phase comparator means, coupled to said reference counter means and said programmable counter means, for detecting a phase difference between the reference signal the second frequency-divided signal and for outputting a voltage signal based on the phase difference to the voltage controlled oscillator; initial phase detection means, coupled to said prescaler means and said oscillator, for detecting a state where an edge of the first frequency-divided signal and an edge of the first oscillation signal are within a predetermined time range and for outputting a detection signal to said programmable counter means, said reference counter means and said phase comparator means when said state is detected, said programmable counter means, said reference counter means and said phase comparator means starting to operate in response to said detection signal; reset means, coupled to said oscillator and said prescaler means, for determining whether or not said oscillator starts to normally generate the first oscillation signal in response to a predetermined external signal which intermittently operates said oscillator in a standby mode and for outputting a reset signal to said prescaler when it is determined that said oscillator normally generates the first oscillation signal; and hold means, coupled to said prescaler means and said initial phase detection means, for preventing the first frequency-divided signal from being output to said programmable counter means and said initial phase detection means until said prescaler means is reset to an initial state in response to said reset signal and starts to normally generate said first frequency-divided signal.