Phase syncronizing PLL output across reference and VCO clock domains
Frequency synthesis is based on phase synchronizing PLL output across REFERENCE and VCO clock domains (including outputs for multiple PLLs), based on an input (REF-Domain) SYNC signal (phase timing reference). A frequency synthesizer includes a VCO to generate VCO_clk and a PLL output circuit, such as a channel divider, to generate PLL_OUT based on VCO_clk (in the VCO-Domain). The VCO loop includes a PD to phase compare an input PD_clock based on REF_CLK, and a VCO feedback signal based on divided VCO_clk (NDIV_out). SYNC alignment circuitry generates a SYNC alignment signal based on SYNC, PD_clk, and NDIV_out (REF-Domain), which is used to synchronize the PLL output circuit and PLL_OUT to SYNC. If a reference divider generates PD_clk, the SYNC alignment circuitry generates a reset to SYNC-align the reference divider. If the VCO loop uses fractional divide, the SYNC alignment circuitry resets the fractional modulator to a known sequence.
1. A circuit for frequency synthesis, comprising:
input terminals to receive a reference clock (REF_CLK) with a REF_CLK frequency, and a synchronization signal (SYNC);
PLL circuitry including:
a VCO Loop including a phase detector (PD), voltage controlled oscillator (VCO), and feedback divider (NDIV),
the VCO responsive to a VCO frequency tuning signal to output a VCO clock signal (VCO_clk) with a controlled frequency derived from the REF_CLK frequency,
the PD to generate a phase error signal corresponding to a phase error between:
an input PD clock signal (PD_clk) based on REF_CLK, and
a VCO feedback signal from NDIV (NDIV_out) corresponding to VCO_clk selectively divided by NDIV down to a frequency corresponding to the REF_CLK frequency,
the VCO frequency tuning signal based on the phase error signal; and
a PLL output circuit coupled to the VCO, to generate a PLL output clock (PLL_OUT) based on VCO_clk;
a REF_CLK domain (REF-Domain) and a VCO_clk domain (VCO-Domain), the REF-Domain including REF_CLK, SYNC, PD_clk, and NDIV_out, the VCO-Domain including VCO_clk and PLL_OUT; and
a SYNC alignment block, including:
SYNC capture circuitry to latch SYNC (SYNC_latched), and
SYNC alignment circuitry responsive to SYNC_latched, PD_clk, and NDIV_out to generate a SYNC alignment signal;
the PLL output circuit responsive to the SYNC alignment signal to synchronize PLL_OUT with the REF-Domain SYNC.
2. The circuit of claim 1 , wherein:
the circuit further comprises a reference divider (RDIV) to selectively divide REF_CLK to generate the PD_clk; and
the SYNC alignment circuitry to generate an RDIV reset signal to reset RDIV, and synchronize PD_clk with SYNC.
3. The circuit of claim 1 , wherein:
NDIV is a fractional divider [N.F]DIV with an integer divider NDIV and a fractional modulator; and
the SYNC alignment circuitry further generates a fractional modulator reset signal based on SYNC to reset the fractional modulator to a known fractional-divide sequence, and thereby synchronize VCO_clk with SYNC.
4. The circuit of claim 1 , wherein:
the PLL output circuit is a channel divider circuit (CDIV) to generate PLL_OUT be selectively dividing VCO_clk; and
CDIV is responsive to the SYNC alignment signal to reset, and thereby synchronize PLL_OUT with SYNC.
5. The circuit of claim 1 , wherein:
the SYNC alignment circuitry comprises a D flip-flop that receives as CLR input a REF-Domain signal based on SYNC_latched, and a CLK input based on NDIV_out, and generates the SYNC alignment signal.
6. The circuit of claim 1 , wherein the circuit is a PLL/synthesizer, and wherein:
the PLL/synthesizer is linked to at least a second PLL/synthesizer;
each PLL/synthesizer receiving REF_CLK and SYNC, and providing a respective PLL_OUT;
each PLL/synthesizer generating its respective PLL_OUT synchronized with SYNC.
7. A system including multiple frequency synthesizers, each comprising:
input terminals to receive a reference clock (REF_CLK) with a REF_CLK frequency, and a synchronization signal (SYNC);
PLL circuitry including:
a VCO Loop including a phase detector (PFD), voltage controlled oscillator (VCO), and feedback divider (NDIV),
the VCO responsive to a VCO frequency tuning signal to output a VCO clock signal (VCO_clk) with a controlled frequency derived from the REF_CLK frequency,
the PD to generate a phase error signal corresponding to a phase error between
an input PD clock signal (PD_clk) based on REF_CLK, and
a VCO feedback signal from NDIV (NDIV_out) corresponding to VCO_clk selectively divided by NDIV down to a frequency corresponding to the REF_CLK frequency,
the VCO frequency tuning signal based on the phase error signal; and
a PLL output circuit coupled to the VCO, to generate a PLL output clock (PLL_OUT) based on VCO_clk;
a REF_CLK domain (REF-Domain) and a VCO_clk domain (VCO-Domain), the REF-Domain including REF_CLK, SYNC, PD_clk, and NDIV_out, the VCO-Domain including VCO_clk and PLL_OUT; and
a SYNC alignment block, including:
SYNC capture circuitry to latch SYNC (SYNC_latched), and
SYNC alignment circuitry responsive to SYNC_latched, PD_clk, and NDIV_out to generate a SYNC alignment signal;
the PLL output circuit responsive to the SYNC alignment signal to synchronize PLL_OUT with the REF-Domain SYNC;
the multiple frequency synthesizers each generating a respective PLL_OUT that is synchronized to SYNC.
8. The system of claim 7 , wherein:
the frequency synthesizer further comprises a reference divider (RDIV) to selectively divide REF_CLK to generate the PD_clk; and
the SYNC alignment circuitry to generate an RDIV reset signal to reset RDIV, and synchronize PD_clk with SYNC.
9. The system of claim 7 , wherein:
NDIV is a fractional divider [N.F]DIV with an integer divider NDIV and a fractional modulator; and
the SYNC alignment circuitry further generates a fractional modulator reset signal based on SYNC to reset the fractional modulator to a known fractional-divide sequence, and thereby synchronize VCO_clk with SYNC.
10. The system of claim 7 , wherein:
the PLL output circuit is a channel divider circuit (CDIV) to generate PLL_OUT be selectively dividing VCO_clk; and
CDIV is responsive to the SYNC alignment signal to reset, and thereby synchronize PLL_OUT with SYNC.
11. The system of claim 7 , wherein:
the SYNC alignment circuitry comprises a D flip-flop that receives as CLR input a REF-Domain signal based on SYNC_latched, and a CLK input based on NDIV_out, and generates the SYNC alignment signal.
12. A method of frequency synthesis, comprising:
receiving a reference clock (REF_CLK) with a REF_CLK frequency, and a synchronization signal (SYNC);
generating a VCO clock signal (VCO_clk), in response to a VCO frequency tuning signal corresponding to a phase error between:
an input PD clock signal (PD_clk) based on REF_CLK, and
a VCO feedback signal (NDIV_out) corresponding to VCO_clk selectively divided down to a frequency corresponding to the REF_CLK frequency;
generating a PLL output clock (PLL_OUT) based on VCO_clk;
generating a SYNC alignment signal based on SYNC, PD_clk, and NDIV_out; and
synchronizing the generation of PLL_OUT with SYNC based on the SYNC alignment signal.
13. The method of claim 12 , further comprising:
selectively dividing REF_CLK to generate the PD_clk; and
generating an RDIV reset signal to synchronize the generation of PD_clk with SYNC.
14. The method of claim 12 , wherein:
the VCO feedback signal NDIV_out is generated by a fractional divider including an integer divider and a fractional modulator; and
the method further comprises generating a fractional modulator reset signal based on SYNC to reset the fractional modulator to a known fractional-divide sequence.
15. The method of claim 12 , wherein:
PLL_OUT is generated by selectively dividing VCO_clk with a channel divider (CDIV); and
the method further comprising generating a CDIV reset signal to reset CDIV based on SYNC.
16. The method of claim 12 , wherein:
the method is used in a system with multiple frequency synthesizers, each receiving REF_CLK and SYNC, and providing a respective PLL_OUT;
each frequency synthesizer generating its respective PLL_OUT synchronized with SYNC.