Methods and apparatus to increase the resolution of a clock synthesis circuit that uses feedback interpolation
A frequency synthesis circuit includes a phase locked loop and an interpolator circuit. The phase locked loop circuit receives a reference clock and a feedback clock and generates an output clock with a frequency based on the reference clock and the feedback clock. An interpolator circuit is coupled in the feedback path of the phase locked loop circuit. An interpolator control circuit generates an interpolator control word that specifies a variable time delay for the interpolator circuit. The interpolator circuit receives the output clock, and generates the feedback clock by introducing a variable time delay in the output clock in accordance with the interpolator control word. The time variable delay varies the frequency of the output circuit. Embodiments for frequency synthesis circuits that include a spread spectrum frequency clock generator, frequency modulators, and a fixed frequency clock generator circuit are disclosed.
1. A method to control an interpolator control circuit including mapping an X bit number to a plurality of Y bit numbers, the plurality of Y bit numbers being associated with a Y bit counter, the method comprising:
designating, using a pulse density modulator, most significant bit positions and least significant bit positions of the X bit number;
detecting, using the pulse density modulator, a bit transition from zero to one in a bit position of the Y bit counter when the Y bit counter is incremented;
generating, using the pulse density modulator, a hot mask for the bit position of the Y bit counter;
generating, using the pulse density modulator, a bit sequence based upon reordering bits in the least significant bit positions of the X bit number; and
performing, using the pulse density modulator, first and second Boolean operations between bits of the hot mask and bits of the bit sequence to generate an output of the pulse density modulator, the second Boolean operation being performed on results of the first Boolean operation;
wherein the output of the pulse density modulator is a first output value when the second Boolean operations produces a zero value and a second output value when the second Boolean operation produces a non-zero value.
2. The method of claim 1 , wherein the hot mask generating includes setting the bit position of the Y bit counter to a value of one.
3. The method of claim 1 , wherein the reordering includes reversing the least significant bit positions of the X bit number.
4. The method of claim 1 , wherein the first Boolean operation includes a bitwise Boolean AND operation.
5. The method of claim 1 , wherein the second Boolean operation includes a bitwise OR operation.
6. The method of claim 1 , wherein the first output value is equal to a value of the most significant bit positions of the X bit number.
7. The method of claim 1 , wherein the second output value is equal to a value of the most significant bit positions of the X bit number+1.
8. The method of claim 1 , wherein the X bit number is updated at a first interval rate and the plurality of Y bit numbers are updated at a second interval rate.
9. The method of claim 8 , wherein the second interval rate is faster than the first interval rate.
10. The method of claim 9 , wherein the second interval rate comprises a rate equal to 2 N times greater than the first interval rate, and wherein a value of N is a difference between a number of bits in the X bit number and a number of bits in one of the plurality of Y bit numbers.
11. A clock synthesis system configured to map an X bit number to a plurality of Y bit numbers, the plurality of Y bit numbers being associated with a Y bit counter, the system comprising:
an interpolator control circuit configured to generate an interpolator control word based on a control signal; and
a pulse density modulator configured to:
designate most significant bit positions and least significant bit positions of the X bit number;
detect a bit transition from zero to one in a bit position of the Y bit counter when the Y bit counter is incremented;
generate a hot mask for the bit position of the Y bit counter;
reorder bits in the least significant bit positions of the X bit number to generate a bit sequence; and
perform first and second Boolean operations between bits of the hot mask and bits of the bit sequence to generate the control signal, the second Boolean operation being performed on results of the first Boolean operation, wherein the control signal is a first output value when the second Boolean operations produces a zero value and a second output value when the second Boolean operation produces a nonzero value.
12. The system of claim 11 , wherein the generation of the hot mask includes setting the bit position of the Y bit counter to a value of one.
13. The system of claim 11 , wherein the reordering of the bits includes reversing the least significant bit positions of the X bit number.
14. The system of claim 11 , wherein the first Boolean operation includes a bitwise AND operation.
15. The system of claim 11 , wherein the second Boolean operation includes a bitwise OR operation.
16. The system of claim 11 , wherein the first output value is equal to a value of the most significant bit positions of the X bit number.
17. The system of claim 11 , wherein the second output value is equal to a value of the most significant bit positions of the X bit number+1.
18. The system of claim 11 , wherein the X bit number is updated at a first interval rate and the plurality of Y bit numbers are updated at a second interval rate.
19. The system of claim 18 , wherein the second interval rate is faster than the first interval rate.
20. The system of claim 19 , wherein the second interval rate comprises a rate equal to 2 N times greater than the first interval rate, and wherein a value of N is a difference between a number of bits in the X bit number and a number of bits in one of the plurality of Y bit numbers.