Phase-locked loop control circuit

1. An electrical circuit, comprising:

a phase locked loop circuit; and

a PLL start-up circuit configured to selectively provide a reference signal to said phase locked loop circuit based upon relative frequencies of an input signal to said phase locked loop circuit and an output signal of said phase locked loop circuit, said relative frequencies being determined using at least two frequency comparators, wherein two of said frequency comparators comprise frequency adjustable inputs for adjusting said frequency of said input signal and said frequency of said output signal.

2. The electrical circuit of claim 1 , wherein at least one of said frequency comparators comprises at least one frequency adjustable input.

3. The electrical circuit of claim 2 , wherein said at least one frequency adjustable input comprises a frequency divider.

4. The electrical circuit of claim 1 , further comprising a frequency divider configured to modify at least one of said input signal and said output signal and to provide said modified signal to said frequency comparator.

5. The electrical circuit of claim 1 , further comprising a set/reset latch responsive to said at least two frequency comparators, said set/reset latch providing a latch output signal dependent upon the output signals of said at least two frequency comparators.

6. The electrical circuit of claim 5 , wherein said reference signal is selectively provided to said phase locked loop circuit in response to said latch output signal.

7. The electrical circuit of claim 5 , further comprising a gate that selectively provides a reference voltage to said phase locked loop circuit in response to said latch output signal.

8. An electrical circuit for controlling a phase locked loop circuit, comprising:

at least two comparators configured to compare respective frequencies of an input signal to the phase locked loop circuit and an output signal of the phase locked loop circuits wherein at least two of said at least two comparators include frequency dividers for dividing said input signal and said output signal; and

a reference signal configured to be selectively provided to the phase looked loop circuit in response to said comparison of said respective frequencies of said input signal and said output signal.

9. The electrical circuit of claim 8 , wherein said reference signal is configured to be provided to the phase locked loop circuit when said frequency of said output signal falls below said frequency of said input signal by a lower limit factor.

10. A method for controlling a phase locked loop circuit, comprising the step of:

providing a first divided input signal of the phase locked loop circuit and a first divided output signal of the phase locked loop circuit to a first frequency comparator;

providing a second divided input signal of the phase locked loop circuit and a second divided output signal of the phase locked loop circuit to a second frequency comparator; and

selectively providing a reference signal to the phase locked loop circuit in response to the output of said first frequency comparator and the output of said second frequency comparator.

11. The method of claim 10 , further comprising the step of:

providing said reference signal if said frequency of said input signal of the phase locked loop circuit exceeds said frequency of said output signal of the phase locked loop circuit by a first determined factor.

12. An electrical circuit, comprising:

a phase locked loop circuit;

first means for comparing relative frequencies of an input signal to said phase locked loop circuit and an output signal of said phase locked loop circuit;

second means for comparing relative frequencies of said input signal to said phase locked loop circuit and said output signal of said phase locked loop circuit; and

means for selectively providing a reference signal to the phase locked loop circuit in response to said first means for comparing and said second means for comparing, wherein said means for selectively providing a reference signal comprises:

means for adjusting said respective frequencies of said input signal and said output signal; and

means for comparing said modified frequencies of said input signal and said output signal.

13. An electrical circuit for controlling a phase locked loop circuit, comprising:

at least one comparator configured to compare respective frequencies of an input signal to the phase locked loop circuit and an output signal of the phase locked loop circuit;

a reference signal configured to be selectively provided to the phase locked loop circuit in response to said comparison of said respective frequencies of said input signal and said output signal;

wherein said reference signal is configured to be provided to the phase locked loop circuit when said frequency of said output signal falls below said frequency of said input signal by a lower limit factor; and

wherein said reference signal is configured to be removed from the phase locked loop circuit when said frequency of said output signal rises above said frequency of said input signal by an upper limit factor.

14. The electrical circuit of claim 13 , wherein said lower limit factor is less than one; and wherein said upper limit factor is greater than one.

15. The electrical circuit of claim 13 , further comprising a first frequency comparator and a second frequency comparator, said first and second frequency comparators being configured to compare respective frequencies of said input signal and said output signal.

16. The electrical circuit of claim 15 , further comprising at least one frequency divider communicating with each of said first and second frequency comparators, said frequency divider being configured to adjust said respective frequencies of at least one of said input and output signals prior to being provided to said first and second frequency comparators.

17. The electrical circuit of claim 16 , further comprising a set/reset latch circuit connected between said first and second frequency comparators and said reference signal.

18. The electrical circuit of claim 15 , further comprising a gate connected between said set/reset latch and said reference signal, said gate being configured to selectively provide said reference signal to the phase locked loop circuit in response to an output of said set/reset latch.

19. A method for controlling a phase locked loop circuit, comprising the step of:

selectively providing a reference signal to the phase locked loop circuit in response to relative frequencies of an input signal to the phase locked loop circuit and an output signal of the phase locked loop circuit;

providing said reference signal if said frequency of said input signal exceeds said frequency of said output signal by a first determined factor; and

removing said reference signal from the phase locked loop if said frequency of said output signal exceeds said frequency of said input signal by a second determined factor.

20. The method of claim 19 , wherein neither said first determined factor nor said second determined factor is one.

21. The method of claim 19 , wherein said first determined factor is less than said second determined factor.