Independently clocking digital loop filter by time-to-digital converter in digital phase-locked loop
A time-to-digital converter (TDC) circuit includes phase error calculation circuitry to: determine phase error values based on a time difference between a input reference clock and a feedback clock of a digital phase-locked loop (DPLL) circuit, the input reference clock and the feedback clock being unsynchronized; and provide the phase error values to a digital loop filter (DLF) of the DPLL circuit. The TDC circuit further includes clock generation circuitry to: generate a filter clock that asserts a clock pulse in response to detecting each last-received pulse of the input reference clock and the feedback clock; and provide the filter clock to the DLF concurrently with providing the phase error values to the DLF that are synchronized to the filter clock.
1. A time-to-digital converter (TDC) circuit comprising:
phase error calculation circuitry to:
determine phase error values based on a time difference between a input reference clock and a feedback clock of a digital phase-locked loop (DPLL) circuit, the input reference clock and the feedback clock being unsynchronized; and
provide the phase error values to a digital loop filter (DLF) of the DPLL circuit; and
clock generation circuitry to:
generate a filter clock that asserts a clock pulse in response to detecting each last-received pulse of the input reference clock and the feedback clock; and
provide the filter clock to the DLF concurrently with providing the phase error values to the DLF that are synchronized to the filter clock.
2. The TDC circuit of claim 1 , wherein the phase error values comprise a series of digital bits.
3. The TDC circuit of claim 1 , wherein the clock generation circuitry comprises:
a first latch that is triggered by the input reference clock;
a second latch that is triggered by the feedback clock; and
an AND gate that receives outputs of each of the first latch and the second latch and outputs the filter clock.
4. The TDC circuit of claim 3 , wherein each of the first latch and the second latch are gated D-latches.
5. The TDC circuit of claim 3 , wherein the clock generation circuitry further comprises a pulse width modulation (PWM) circuit coupled to an output of the AND gate, the PWM circuit to:
set a pulse width of each clock pulse of the filter clock; and
reset the first latch and the second latch at an end of each pulse width.
6. The TDC circuit of claim 3 , wherein a one value is provided to inputs of each of the first latch and the second latch.
7. A digital phase-locked loop (DPLL) circuit comprising:
digital loop filter (DLF) configured to operate according to a filter clock generated by a time-to-digital converter (TDC);
the TDC operatively coupled to the DLF and comprising:
phase error calculation circuitry to:
determine phase error values based on a time difference between a input reference clock and a feedback clock of the DPLL circuit, the input reference clock and the feedback clock being unsynchronized; and
provide the phase error values to the DLF; and
clock generation circuitry to:
generate the filter clock that asserts a clock pulse in response to detecting each last-received pulse of the input reference clock and the feedback clock; and
provide the filter clock to the DLF concurrently with providing the phase error values to the DLF that are synchronized to the filter clock; and
a digitally-controlled oscillator (DCO) coupled to the DLF and configured to generate an output signal that is convertible to the feedback clock.
8. The DPLL circuit of claim 7 , wherein the phase error values comprise a series of digital bits.
9. The DPLL circuit of claim 7 , wherein the clock generation circuitry comprises:
a first latch that is triggered by the input reference clock;
a second latch that is triggered by the feedback clock; and
an AND gate that receives outputs of each of the first latch and the second latch and outputs the filter clock.
10. The DPLL circuit of claim 9 , wherein each of the first latch and the second latch are gated D-latches.
11. The DPLL circuit of claim 9 , wherein the clock generation circuitry further comprises a pulse width modulation (PWM) circuit coupled to an output of the AND gate, the PWM circuit to:
set a pulse width of each clock pulse of the filter clock; and
reset the first latch and the second latch at an end of each pulse width.
12. The DPLL circuit of claim 9 , wherein a one value is provided to inputs of each of the first latch and the second latch.
13. The DPLL circuit of claim 7 , further comprising a feedback divider to generate the feedback clock by reducing a frequency of the output signal by an integer value.
14. A method of operating a digital phase-locked loop (DPLL) circuit, the method comprising:
generating, by a time-to-digital converter (TDC), phase error values based on a time difference between an input reference clock and a feedback clock of the DPLL circuit, the input reference clock and the feedback clock being unsynchronized;
generating, by the TDC, a filter clock that asserts a clock pulse in response to detecting each last-received pulse of the input reference clock and the feedback clock;
providing, by the TDC to a digital loop filter (DLF), the phase error values concurrently with the clock pulses of the filter clock; and
generating, by the DLF, filtered phase error values that causes a digitally-controlled oscillator (DCO) to generate an output signal that is convertible to the feedback clock.
15. The method of claim 14 , further comprising synchronizing, by the TDC, the generating the phase error values with the generating the clock pulses, wherein the phase error values comprise a series of digital bits.
16. The method of claim 14 , wherein the TDC comprises a first latch, a second latch, and an AND gate coupled to outputs of the first latch and the second latch, and wherein the method of operating the DPLL circuit further comprises:
causing the first latch to be triggered by the input reference clock;
causing the second latch to be triggered by the feedback clock; and
causing outputs of each of the first latch and the second latch to be provided to the AND gate.
17. The method of claim 16 , wherein each of the first latch and the second latch are gated D-latches.
18. The method of claim 16 , wherein the TDC further comprises a pulse width modulation (PWM) circuit coupled to an output of the AND gate, and the method of operating the DPLL circuit further comprises:
causing the PWM circuit to set a pulse width of each clock pulse of the filter clock; and
causing the first latch and the second latch to each be reset at an end of each pulse width.
19. The method of claim 16 , wherein the method of operating the DPLL circuit further comprises providing a one value to inputs of each of the first latch and the second latch.
20. The method of claim 14 , wherein the DPLL circuit further comprises a feedback divider, and the method of operating the DPLL circuit further comprises generating, by the feedback divider, the feedback clock by reducing a frequency of the output signal by an integer value.