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 an input reference clock and a feedback clock of a digital phase-locked loop (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 a digital loop filter (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:
are based on a time difference between the input reference clock and the feedback clock, which are unsynchronized; and
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;
a time-to-digital converter operatively coupled to the DLF and comprising:
phase error calculation circuitry to determine phase error values based on an input reference clock and a feedback clock; 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:
are based on a time difference between the input reference clock and the feedback clock, which are unsynchronized; and
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 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
synchronizing, by the TDC, the generating the phase error values with generating clock pulses of the filter clock.
15. The method of claim 14 , wherein the generating the phase error values comprises determining a difference between the input reference clock and the feedback clock, which are unsynchronized, and 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 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.
18. 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.
19. The method of claim 14 , further comprising 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.
20. The method of claim 19 , 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.