Serializer/Deserializer (SerDes) lanes with lane-by-lane datarate independence
A circuit and method enables multiple serializer/deserializer (SerDes) data lanes of a physical layer device (PHY) to operate across a broad range of diversified data rates that are independent from lane to lane. The multiple SerDes data lanes may operate at data rates independent from one another. A single low frequency clock is input to the PHY. A frequency of the single low frequency clock is increased via a common integer-N phase-locked loop (PLL) on the PHY to produce a higher frequency clock. Each of the SerDes data lanes is operated, independently, as a fractional-N PLL that employs the higher frequency clock. Use of the common integer-N PLL enables modulation noise of the fractional-N PLLs to be suppressed by moving the modulation noise to higher frequencies where a level of the modulation noise is filtered, avoiding use of high risk noise cancellation techniques.
1. A circuit on a chip for serial data applications, the circuit comprising:
an integer phase-locked loop (PLL) having a multiplying factor; and
a serializer/deserializer (SerDes) lane, the SerDes lane including a fractional-N (frac-N) PLL, the frac-N PLL including an out-of-band parasitic pole, the multiplying factor in combination with the out-of-band parasitic pole configured to suppress quantization noise introduced by modulating the frac-N PLL.
2. The circuit of claim 1 , wherein the integer PLL is configured to produce an on-chip reference clock signal from an off-chip reference clock signal and wherein the on-chip reference clock signal is distributed to the frac-N PLL and is higher in frequency relative to the off-chip reference clock signal.
3. The circuit of claim 1 , wherein the multiplying factor is configured to effect a frequency of the quantization noise to enable the frac-N PLL to filter at least a portion of the quantization noise.
4. The circuit of claim 1 , wherein the frac-N PLL further includes a divider with a divide value, wherein the multiplying factor is configured to suppress an initial portion of the quantization noise, and wherein modulating the frac-N PLL includes modulating the divide value of the divider.
5. The circuit of claim 4 , wherein the out-of-band parasitic pole is configured to suppress a remaining portion of the quantization noise, the remaining portion at higher offset frequencies relative to offset frequencies of the initial portion, the initial portion and the remaining portion suppressed below a noise level of noise generated from intrinsic phase detector and voltage controlled oscillator (VCO) noise sources on the chip.
6. The circuit of claim 1 , wherein the frac-N PLL includes a divider and sigma-delta modulator, the sigma-delta modulator configured to modulate, randomly, a divide value of the divider, wherein the frac-N PLL has an order that is lower relative to that of the sigma-delta modulator, and wherein the order of the frac-N PLL is two and that of the sigma-delta modulator is three.
7. The circuit of claim 1 , wherein the integer PLL is a first integer PLL, wherein the out-of-band parasitic pole is configured high enough to not affect closed-loop behavior of the frac-N PLL, and wherein the out-of-band parasitic pole in combination with configuration of the multiplying factor enable jitter and phase noise performance of the frac-N PLL to be comparable to that of a second integer PLL that employs a same integer divide value as the frac-N PLL.
8. The circuit of claim 1 , wherein the frac-N PLL further includes a spread spectrum clock (SSC) pattern generator configured to modulate the frac-N PLL, directly, according to an SSC profile.
9. The circuit of claim 1 , wherein:
the chip is a physical layer device (PHY);
the SerDes lane includes a transmitter configured to operate at a transmit data rate and a receiver configured to operate at a receive data rate;
the SerDes lane is configured to employ the transmit and receive data rates to communicate with a media access controller (MAC); and
the transmit and receive data rates are configured independent of data rates employed by other SerDes lanes of the PHY for communicating with other MACs.
10. A circuit on a chip for serial data applications, the circuit comprising:
a first integer phase-locked loop (PLL) having a multiplying factor; and
a serializer/deserializer (SerDes) lane, the SerDes lane including a fractional-N (frac-N) PLL configured to employ an integer divide value, the frac-N PLL including an out-of-band parasitic pole, the multiplying factor in combination with the out-of-band parasitic pole configured to enable jitter or phase noise performance of the frac-N PLL to be comparable to that of a second integer PLL, the second PLL configured to employ the integer divide value.
11. The circuit of claim 10 , wherein the integer PLL is configured to produce an on-chip reference clock signal from an off-chip reference clock signal and wherein the on-chip reference clock signal is distributed to the frac-N PLL and is higher in frequency relative to the off-chip reference clock signal.
12. The circuit of claim 10 , wherein the multiplying factor is configured to effect a frequency of the quantization noise to enable the frac-N PLL to filter at least a portion of the quantization noise.
13. The circuit of claim 10 , wherein the multiplying factor in combination with the out-of-band parasitic pole is further configured to suppress quantization noise introduced by modulating the frac-N PLL, wherein the frac-N PLL further includes a divider with the integer divide value, wherein the multiplying factor is configured to suppress an initial portion of the quantization noise, and wherein modulating the frac-N PLL includes modulating the integer divide value of the divider.
14. The circuit of claim 13 , wherein the out-of-band parasitic pole is configured to suppress a remaining portion of the quantization noise, the remaining portion at higher offset frequencies relative to offset frequencies of the initial portion, the initial portion and the remaining portion suppressed below a noise level of noise generated from intrinsic phase detector and voltage controlled oscillator (VCO) noise sources on the chip.
15. The circuit of claim 10 , wherein the frac-N PLL includes a divider with the integer divide value and a sigma-delta modulator, the sigma-delta modulator configured to modulate, randomly, the integer divide value of the divider, wherein the frac-N PLL has an order that is lower relative to that of the sigma-delta modulator, and wherein the order of the frac-N PLL is two and that of the sigma-delta modulator is three.
16. The circuit of claim 10 , wherein the out-of-band parasitic pole is configured high enough to not affect closed-loop behavior of the frac-N PLL.
17. The circuit of claim 10 , wherein the frac-N PLL further includes a spread spectrum clock (SSC) pattern generator configured to modulate the frac-N PLL, directly, according to an SSC profile.
18. The circuit of claim 10 , wherein:
the chip is a physical layer device (PHY);
the SerDes lane includes a transmitter configured to operate at a transmit data rate and a receiver configured to operate at a receive data rate;
the SerDes lane is configured to employ the transmit and receive data rates to communicate with a media access controller (MAC); and
the transmit and receive data rates are configured independent of data rates employed by other SerDes lanes of the PHY for communicating with other MACs.
19. A circuit on a chip for serial data applications, the circuit comprising:
an integer phase-locked loop (PLL) having a multiplying factor; and
a serializer/deserializer (SerDes) lane, the SerDes lane including a fractional-N (frac-N) PLL, the frac-N PLL including a divider with a divide value, the multiplying factor configured to suppress a portion of quantization noise, the portion introduced by modulating the divide value of the divider.
20. A circuit on a chip for serial data applications, the circuit comprising:
a plurality of serializer/deserializer (SerDes) lanes; and
an integer phase-locked loop (PLL) configured to produce a reference clock signal coupled to the plurality of serializer/deserializer (SerDes) lanes, a SerDes lane of the plurality of SerDes lanes including a respective fractional-N (frac-N) PLL, the reference clock signal produced by the integer PLL distributed to the respective frac-N PLL, the respective frac-N PLL including a respective sigma-delta modulator, the respective frac-N PLL configured to have an order that is lower relative to that of the respective sigma-delta modulator.