Multiple reference clock synthesizer
A clock synthesizer for dividing a source clock by N.R including a logic circuit, a delay line, a select circuit, an accumulator, and a clock divider circuit. The logic circuit divides N.R by 2 M to get NNEW.RNEW in which NNEW is zero and RNEW is at least 0.5. The delay line receives a first clock and has multiple delay taps, where the first clock is based on the source clock. The select circuit selects the delay taps based on a tap select value and provides a delayed clock. The accumulator adds RNEW for each cycle of the delayed clock and performs a modulo function on a sum value to generate the tap select value. The clock divider circuit transitions an output clock based on selected transitions of the delayed clock, which is achieved by dividing the first clock or the delayed clock by 2 M−1 .
1. A clock synthesizer that divides a source clock by a fractional divide ratio N.R which is at least one, comprising:
a logic circuit which determines an integer M such that the fractional divide ratio N.R divided by 2 M to provides a new digital value NNEW.RNEW in which said NNEW value is zero and said RNEW value is at least 0.5;
a delay line having an input receiving a first clock and having a plurality of delay taps, wherein said first clock has a frequency which is the frequency of the source clock divided by a first value;
a select circuit having a plurality of selectable inputs coupled to said plurality of delay taps, a select input receiving a tap select value, and an output providing a delay clock;
an accumulator which adds said RNEW value to a sum value for each cycle of said delay clock and that performs a modulo function on said sum value to generate said tap select value; and
a first clock divider circuit that transitions an output clock based on selected transitions of said delay clock wherein said transitions are selected to divide the frequency of the delay clock by a second value in which said first value multiplied by said second value is 2 M .
2. The clock synthesizer of claim 1 , wherein said first clock has a frequency which is the same as the source clock, and wherein said first clock divider circuit comprises:
a counter having a clock input receiving said delay clock and an output providing an enable signal every 2 M−1 cycles of said delay clock; and
a divide-by-two circuit having a clock input receiving said delay clock, a data input coupled to an inverting output, an enable input receiving said enable signal, and a non-inverting output providing said output clock.
3. The clock synthesizer of claim 2 , wherein the source clock is provided as said first clock to said input of said delay line.
4. The clock synthesizer of claim 2 , further comprising a duty cycle reduction circuit having an input for receiving the source clock and an output providing said first clock having the same frequency as the source clock.
5. The clock synthesizer of claim 2 , wherein said divide-by-two circuit comprises a flip flop.
6. The clock synthesizer of claim 1 , further comprising:
a second clock divider circuit having a clock input for receiving the source clock and an output providing said first clock with a frequency that is equal to the source clock frequency divided by 2 M−1 ; and
wherein said first clock divider circuit comprises a flip flop having a clock input receiving said delay clock, a data input coupled to an inverting output, and a non-inverting output providing said output clock.
7. The clock synthesizer of claim 6 , wherein said second clock divider circuit further comprises a duty cycle reduction circuit which reduces a duty cycle said first clock provided to said input of said delay line.
8. The clock synthesizer of claim 1 , wherein said accumulator uses a number of most significant bits of said RNEW value for said modulo function commensurate with a number of said plurality of delay taps of said delay line.
9. The clock synthesizer of claim 8 , wherein said accumulator uses an additional most significant bit of said RNEW value for said modulo function for rounding said tap select value.
10. The clock synthesizer of claim 1 , wherein said delay line comprises:
decode logic that receives and converts said tap select value into a plurality of tap enable signals; and
a plurality of latches, each having a data input receiving a corresponding one of said plurality of tap enable signals, an enable input coupled to a corresponding one of said plurality of delay taps, and a tap enable output.
11. A clock synthesizer that divides a source clock by a binary fractional divide ratio N.R which is at least one, comprising:
a logic circuit which right shifts N.R an integer number M times to provide a new digital value NNEW.RNEW in which said NNEW value is zero and in which a most significant bit of said RNEW value is 1;
a delay line having an input receiving a first clock and having a plurality of delay taps, wherein said first clock has a frequency which is the frequency of the source clock divided by a first value;
a select circuit having a plurality of selectable inputs coupled to said plurality of delay taps, a select input receiving a tap select value, and an output providing a second clock;
an accumulator which adds said RNEW value to a sum value for each cycle of said second clock and that performs a modulo function on said sum value to generate said tap select value; and
a clock counter circuit that transitions an output clock based on selected transitions of said second clock wherein said transitions are selected to divide the frequency of the second clock by a second value in which the first value multiplied by the second value is 2 M .
12. The clock synthesizer of claim 11 , wherein said clock counter circuit comprises:
a counter having a clock input receiving said second clock and an output providing an enable signal every 2 M−1 cycles of said second clock; and
a divide-by-two circuit having a clock input receiving said second clock, a data input coupled to an inverting output, an enable input receiving said enable signal, and a non-inverting output providing said output clock.
13. The clock synthesizer of claim 11 , further comprising:
a clock divider circuit having a clock input for receiving the source clock and an output providing said first clock with a frequency that is equal to the source clock frequency divided by 2 M−1 ; and
wherein said clock counter circuit comprises a divide-by-two circuit having a clock input receiving said second clock, a data input coupled to an inverting output, and a non-inverting output providing said output clock.
14. The clock synthesizer of claim 11 , wherein said delay line comprises:
decode logic that receives and converts said tap select signal into a plurality of tap enable signals; and
a plurality of latches, each having a data input receiving a corresponding one of said plurality of tap enable signals, an enable input coupled to a corresponding one of said plurality of delay taps, and a tap enable output.
15. A method of dividing a source clock by a fractional divide ratio N.R which is at least one, comprising:
determining an integer M such that the fractional divide ratio N.R divided by 2 M provides a new digital value NNEW.RNEW in which the NNEW value is zero and the RNEW value is at least 0.5;
providing a free-running first clock through a delay line having a plurality of delay taps, wherein the first clock has a frequency that is the frequency of the source clock divided by a first value;
selecting one of the plurality of delay taps based on a tap select signal and providing a delayed clock based on selected delay taps;
accumulating the RNEW value for each cycle of said delayed clock to a sum value;
performing a modulo function of the sum value to generate the tap select signal; and
transitioning an output clock based on selected transitions of the delayed clock wherein the transitions are selected to divide the delayed clock by a second value in which the first and second values multiplied together is 2 M .
16. The method of claim 15 , wherein:
said providing a free-running first clock through a delay line comprises generating the first clock with the same frequency as the source clock; and
wherein said transitioning an output clock comprises:
providing an enable signal every 2 M−1 cycles of the delayed clock; and
toggling the output clock during a transition of the delayed clock while the enable signal is provided.
17. The method of claim 15 , wherein:
said providing a free-running first clock through a delay line comprises dividing a frequency of the source clock by 2 M−1 ; and
wherein said transitioning an output clock comprises toggling the output clock during each cycle of the delayed clock.
18. The method of claim 15 , further comprising reducing the duty cycle of the first clock provided to the delay line.
19. The method of claim 15 , further comprising delaying each tap enable of the delay line by approximately the same delay as the first clock propagating through the delay line to a corresponding delay tap.
20. The method of claim 15 , further comprising enabling each delay tap of the delay line when it is at a predetermined logic level.