Phase locked loop circuit including digital voltage-controlled oscillator, ring oscillator and selector

1. A phase locked loop (PLL) circuit, comprising:

a polyphase reference clock output circuit, which outputs multiple reference clocks, each clock being of different phase;

a digital voltage-controlled oscillator (VCO), which, using any one of said multiple reference clocks chosen as an operating clock, outputs an output clock whose frequency varies according to a value of a frequency control signal, and which outputs a delay amount data representing a phase difference between a phase of said output clock and an ideal phase computed based on the value of said frequency control signal; and

a selection circuit, which is responsive to said delay amount data to select and output said output clock synchronized with one of said multiple reference clocks.

2. The PLL circuit according to claim 1 , wherein a frequency of said reference clock is higher than the highest frequency of said output clock.

3. The PLL circuit according to claim 1 , wherein said selection circuit outputs said output clock, based on phase adjustment steps set corresponding to phase differences between one and another of said multiple reference clocks.

4. The PLL circuit according to claim 1 , wherein:

said polyphase reference clock output circuit includes an odd number of inverters connected in series;

an output from an inverter in a last stage is connected to an input of an inverter in a first stage; and

an output from each of said odd number of inverters is outputted respectively as a reference clock.

5. The PLL circuit according to claim 1 , wherein:

said polyphase reference clock output circuit comprises a plurality of delay cells connected in series, each of said delay cells including a differential input and a differential output, an output from a last stage is connected to an input of said delay cell in a first stage, and an output from each of said plurality of delay cells is outputted respectively as a reference clock.

6. The PLL circuit according to claim 1 , wherein said selection circuit comprises:

a plurality of latch circuits connected to said multiple reference clocks, respectively, said plurality of latch circuits latching said output clock, taking a corresponding one of said reference clocks as a trigger signal; and

a selector which selects and outputs any one of a plurality of delayed clocks outputted from said plurality of latch circuits.

7. The PLL circuit according to claim 6 , wherein said selection circuit changes over said delayed clock to one to be selected, at a timing before a logical level of said output clock is changed over.

8. The PLL circuit according to claim 1 , wherein said digital VCO outputs said output clock whose frequency becomes higher as said value of said frequency control signal input increases.

9. A phase locked loop circuit, comprising:

a ring oscillator which comprises a plurality of inverters which are connected to form a ring connection, and which outputs a plurality of clock signals with different phase from one another;

a digital voltage-controlled oscillator which is responsive to at least one of said clock signals and a frequency control signal, to output an output clock and a delay amount data; and

a selection circuit comprising:

a plurality of latch circuits, each of which latches said output clock in response to a corresponding one of said plurality of clock signals to output a respective latched output clock; and

a selector which receives the latched output clocks and said delay amount data, and which, based on said delay amount data, selects and outputs one of the latched output clock signals.

10. The PLL circuit according to claim 9 , wherein said selection circuit changes the selected and outputted one of the latched output clock signals, at a timing before a logical level of said output clock is changed over.