Method of frequency search for DCO and decoder using the same

1. A method of frequency search for a digitally controlled oscillator (DCO) with multiple sub-bands, the method comprising:

dividing an output signal received from a loop filter to generate multiple workable pre-control codes corresponding to a frequency of the DCO for selection, wherein each pre-control code comprises a most significant bit (MSB);

selecting one of the workable pre-control codes according to the MSBs thereof; and

providing the selected pre-control code to the DCO.

2. The method as claimed in claim 1 , wherein selection of the workable pre-control codes further comprises:

comparing MSBs of each workable pre-control code with an MSB of a previously selected control code; and

selecting the pre-control code with a minimum difference between the MSB of the pre-control code and the MSB of the previously selected control code.

3. The method as claimed in claim 2 , wherein the previously selected control code is selected in a most recent cycle.

4. A method of frequency search for a digitally controlled oscillator (DCO) with multiple sub-bands each corresponding to a first predetermined number of delay units, comprising:

dividing an output signal of a filter by a second predetermined number, wherein the second predetermined number is half of the first predetermined number;

providing a quotient of the division and an integral part of a remainder of the division as first and second parts of a first workable pre-control code respectively;

generating a first part of a second workable pre-control code by subtracting the first part of the first second workable pre-control code by one;

generating a second part of the second workable pre-control code by adding the second part of the first second workable pre-control code by the second predetermined number;

selecting one of the workable pre-control codes according to the first parts thereof; and

providing the selected pre-control code to the DCO.

5. The method as claimed in claim 4 , wherein selection of the workable pre-control codes further comprises:

comparing the first parts of each workable pre-control code with a first part of a previously selected control code; and

selecting the pre-control code with a minimum difference between the first part of the pre-control code and the first part of the previously selected control code.

6. The method as claimed in claim 5 , wherein the previously selected control code is selected in a most recent cycle.

7. A method of frequency search for a digitally controlled oscillator (DCO) with overlapped sub-bands each corresponding to a first predetermined number of delay units, comprising:

dividing an output signal of a filter by a second predetermined number, wherein the second predetermined number is half of the first predetermined number;

providing a quotient of the division and an integral part of a remainder of the division as first and second parts of a first workable pre-control code respectively;

generating a first part of a second workable pre-control code by adding the first part of the first second workable pre-control code by one;

generating a second part of the second workable pre-control code by subtracting the second part of the first second workable pre-control code by the second predetermined number;

selecting one of the workable pre-control codes according to the first parts thereof; and

providing the selected pre-control code to the DCO.

8. The method as claimed in claim 7 , wherein selection of the workable pre-control codes further comprises:

comparing the first parts of each workable pre-control code with a first part of a previously selected control code; and

selecting the pre-control code with a minimum difference between the first part of the pre-control code and the first part of the previously selected control code.

9. The method as claimed in claim 8 , wherein the previously selected control code is selected in a most recent cycle.

10. A decoder providing a control code for a digitally controlled oscillator (DCO) with multiple sub-bands, the decoder comprising:

a first pre-control code generator dividing an output signal received from a loop filter to generate a first pre-control code comprising first and second parts;

a second pre-control code generator generating a second pre-control code comprising first and second parts according to an output of the first pre-control code generator;

a comparator receiving the first parts of the first and second pre-control codes and generating a control signal; and

a multiplexer receiving the first and second parts of the first and second pre-control codes and selecting one of the same as the control code according to the control signal.

11. The decoder as claimed in claim 10 , wherein the second pre-control code is generated according to the first pre-control code.

12. The decoder as claimed in claim 10 , wherein the first pre-control code generator is a divider.

13. The decoder as claimed in claim 10 , further comprising a delay receiving a first part of the control code from the multiplexer and providing the same to the comparator.

14. A decoder providing a control code for a digitally controlled oscillator (DCO) with multiple sub-bands, the decoder comprising:

a first pre-control code generator arranged to receive an output signal of a filter and divide the output signal to generate a first pre-control code;

a second pre-control code generator arranged to receive the first pre-control code of the first pre-control code generator;

a comparator coupled to the first and second pre-control code generators; and

a multiplexer coupled to the comparator.

15. The decoder as claimed in claim 14 , wherein the second pre-control code generator is coupled to the filter via the first pre-control code generator.

16. The decoder as claimed in claim 14 , wherein the first pre-control code generator is a divider.

17. The decoder as claimed in claim 16 , further comprising a delay coupled between the multiplexer and the comparator.

18. A method of frequency search for a digitally controlled oscillator (DCO) with multiple sub-bands, comprising:

dividing a current output signal of a filter to generate first and second workable pre-control codes at different frequency search paths for selection, respectively, wherein periods of the DCO set by the first and second workable pre-control codes are the almost same;

selecting one of the first and second workable pre-control codes according to a previous output signal of the filter; and

providing the selected pre-control code to the DCO.

19. The method as claimed in claim 18 , wherein the step of selecting one of the first and second workable pre-control codes further comprises:

comparing most significant bits (MSBs) of each workable pre-control code with an MSB of a previously selected control code generated by the previous output signal; and

selecting the pre-control code with a minimum difference between the MSB of the pre-control code and the MSB of the previously selected control code.

20. The method as claimed in claim 18 , wherein the step of dividing the current output signal of the filter to generate the first and second workable pre-control codes comprises:

providing a quotient of division and an integral part of a remainder of division to serve as first and second parts of the first workable pre-control code respectively;

generating a first part of the second workable pre-control code by adding or subtracting the first part of the first workable pre-control code by one; and

generating a second part of the second workable pre-control code by subtracting or adding the second part of the first workable pre-control code by a predetermined number.

21. The method as claimed in claim 20 , wherein the step of selecting one of the first and second workable pre-control codes further comprises:

comparing the first parts of each workable pre-control code with a first part of a previously selected control code generated by the previous output signal; and

selecting the pre-control code with a minimum difference between the first part of the pre-control code and the first part of the previously selected control code.

22. A method of frequency search for a digitally controlled oscillator (DCO) with multiple sub-bands, comprising:

dividing a current output signal of a loop filter to determine first and second frequency search paths for a frequency of the DCO;

selecting one of the first and second frequency search paths according to a previous frequency search path based on a previous output signal of the filter; and

controlling the frequency of the DCO according to the selected frequency search path.

23. The method as claimed in claim 22 , wherein the selected frequency search path is selected according to a path correlation between the first frequency search path and the previous frequency search path, and a path correlation between the second frequency search path and the previous frequency search path.

24. The method as claimed in claim 23 , wherein the step of selecting one of the first and second frequency search paths further comprises:

comparing first parts of workable pre-control codes at the first and second frequency search paths with a first part of a previously selected control code at the previous frequency search path to obtain the path correlations; and

selecting the frequency search path with a minimum difference between the pre-control code and the previously selected control code.