Patent Description:
The subject disclosure relates to a method and apparatus for clock and data alignment that reduces power consumption.

The demand for greater bandwidth from data centers continues to increase, necessitating the need for faster optical and electrical communication hardware. Despite the demand for faster communication, there is a limitation on the power that the hardware can consume fueled by capacity and environmental concerns. Existing data centers are equipped to handle a limited amount of power from the grid and current estimates suggest that data centers will consume <NUM>% of the world's total power by <NUM>.

The subject disclosure describes, among other things, illustrative embodiments for a method of pipelining phase rotators that results in power and area savings. Other embodiments are described in the subject disclosure.

One or more aspects of the subject disclosure, not covered by the appended claims, include combination of pipelining and twin Phase Rotators (PRs) to create a low power, jitter and area Clock-and-Data Recovery (CDR) loop.

One or more aspects of the subject disclosure, not covered by the appended claims, include the use of a Finite Impulse Response (FIR) filter in a CDR loop that helps suppress phase noise and reduce jitter.

Techniques herein include a method of pipelining PRs that results in power and area savings. A combination of pipelining and twin PRs creates a low power, jitter and area CDR loop. The use of an FIR filter in a CDR loop helps to suppress phase noise and reduce jitter. The combination of these features enables the use of PRs in <NUM>-Gb/s CDRs where strict jitter requirements have resulted in LC Voltage Controlled Oscillators (LCVCOs) being the only proven option. These features also result in the lowest possible area and power for a CDR, both of which are at a premium in high-speed wireline transceivers.

The method of pipelining PRs introduced herein results in lower power and area consumption than in the traditional PR or tournament style PR. For example, a two-stage pipelined PR implemented with the method disclosed herein and with equal bits in stage one and stage two has a power and area reduction factor of (<NUM>)(<NUM>-N/<NUM>). This power and area reduction factor is further improved if more bits are used in stage two or if additional stages are added to the PR.

The method of pipelining PRs disclosed herein is combined with the concept of twin PRs to further improve the PR non-linearity - namely the Integral Non-Linearity (INL) which directly translates to jitter during plesiochronous operation. Since power and area is at a premium in high-speed wireline transceivers, traditional twin PRs are not common in Serializer-Deserializers (SerDes). Here the use of twin PRs is enabled by the low power and area consumption of the pipelined PR.

An FIR filter is added to a CDR loop to suppress phase noise and filter jitter. This strategy has been used in fractional-N frequency synthesizers to suppress ΣΔ noise but never in a CDR to suppress noise resulting from plesiochronous operation.

<NUM>-Gb/s+ SerDes have stringent jitter requirements which have only been met with LCVCO-based CDRs. However, such implementations suffer from high power and area consumption. The method of pipelining PRs described herein combined with the utilization of twin PRs and FIR filters in the CDR show a way for PRs to be used in <NUM>-Gb/s+ SerDes.

A CDR loop is implemented in <NUM> FinFET CMOS to test the concepts described herein. The twin pipelined PR achieves a peak-peak INL of <NUM>-fs while consuming <NUM>-mA from a <NUM>. During plesiochronous operation the PR phase noise was measured and integrated from DC to Nyquist resulting in an RMS jitter of <NUM>-fs. The addition of a <NUM>-tap Kaiser-Bessel FIR filter at the PR output further reduces the PR RMS jitter to <NUM>-fs. Once shaped by a complete CDR loop, the jitter specifications of this PR will enable a low power and area CDR, and in turn a <NUM>-Gb/s+ wireline transceiver that meets the <75fs RMS random jitter standard.

The PR concepts described herein are not limited to usage within a CDR loop. In other places where PRs are present the same concepts can be used to achieve low jitter, power, and area. For example, these PR concepts can be used in inner clock generation for an Analog-to-Digital Converter (DAC) or Digital-to-Analog Converter (ADC).

To limit the total power consumed in data centers, key hardware, namely ADCs, DACs and SerDes, must only increase their power at the same rate as their speed. For example, <NUM> Gigabit Per Second (Gb/s) Very Short Reach (VSR) SerDes are expected to consume <NUM>-mW total which corresponds to a power efficiency of <NUM> Picojoules Per Bit (pJ/b).

A SerDes (<FIG>) consists of two main blocks: a Transmitter (Tx) and a Receiver (Rx). The Tx's main responsibility is to serialize many low-speed data paths into one high-speed data path. Conversely, the Rx deserializes the high-speed data path into many low-speed data paths. As transmission speeds increase, SerDes become increasingly reliant on high-speed medium-resolution DACs and ADCs to perform their serialization and deserialization.

High-speed DACs and ADCs (<FIG>) can be further split into two parts: the data path and the clock path. The subject disclosure addresses the clock path which consumes a large portion of the DAC and ADC power budget.

The basic purpose of a DAC is to receive an N-bit binary bus and convert it to a single analog signal. Modern DACs also perform serialization via cascaded multiplexers (MUXs) which use progressively higher-speed clocks as their select bit to combine several low-speed data paths.

The basic purpose of an ADC is to receive a single analog signal and convert it to an N-bit binary bus. Modern ADCs use time-interleaved structures where a Sampling Front-End (SFE) first deserializes the data into lower-speed paths before parallel sub-ADCs, each operating at <MAT>, perform the actual data conversion. Fs is the overall sampling rate of the ADC. Rank <NUM> and Rank <NUM> are integers that represent the number of low-speed data paths after the first and second stages of interleaving respectively.

Modern DACs and ADCs, see <FIG> and <FIG>, have sampling rates in the range of <NUM>-to-<NUM> Gigasamples Per Second (GS/s) and could require multi-phase clocks operating anywhere from <MAT> to <MAT>. As an example, a <NUM>-GS/s DAC and ADC are required to perform PAM4 encoded data transmission at <NUM>-Gb/s. A common approach for the DAC is to utilize <NUM>:<NUM>, <NUM>:<NUM> and <NUM>:<NUM> MUX stages. This requires four-phase clocks at <MAT>, eight-phase clocks at <MAT> and <NUM>-phase clocks at <MAT>. For the ADC it is common for Rank <NUM>=<NUM> and Rank <NUM>=<NUM>, requiring eight-phase clocks at <MAT> and <NUM>-phase clocks at <MAT>.

Another important aspect of SerDes is the clock-to-data alignment to ensure sampling is occurring at the optimal point. A CDR loop (<FIG>) is used to perform this alignment. The basic operation of a CDR is as follows. Data is recovered and compared to its sampling clock using a Phase Detector (PD). The PD outputs pulses equivalent to the phase mismatch between the data and sampling clock. These pulses are then filtered and used to drive either a PR or an LCVCO.

Of these two strategies, LCVCO-based CDRs are less common because of their high power and area consumption. However, CDRs have begun using LCVCOs to meet the stringent jitter requirements. For example, <NUM>-Gb/s SerDes implementations are expected to target <75fs, rms random jitter. This shift is largely due to the difficulty in designing a PR that can meet this jitter requirement. However, the subject disclosure presents new concepts that make the implementation of PR-based CDRs possible at <NUM>-Gb/s and beyond.

At its simplest, a PR (<FIG>) takes a weighted sum of two input clocks, CKin1 and CKin2 with phases θ<NUM> and θ<NUM> respectively, to generate the output CKout with phase θout. Considering the clocks as phasors, the output is related to the input by CKout = (<NUM> - α)(cos θ<NUM> + j sin θ<NUM>) + α(cos θ<NUM> + j sin θ<NUM>). The weights of the two clocks are arranged such that α is between <NUM> and <NUM>, so when the weight of CKin1 is increased, the weight of CKin2 is decreased by the same amount. This results in θout being closer to θ<NUM> when α is low, and closer to θ<NUM> when α is high.

When CKin1 and CKin2 are separated by <NUM>°, the output clock simplifies to CKout = (<NUM> - α) + jα with output phase <MAT>. One defining characteristic of PRs is their INL, which is a measure of the output phase deviation from the ideal output phase. The ideal output phase of a quadrature PR is given as θout,ideal = (α)(<NUM>°) thus INL can be defined as: <MAT>.

INL is perhaps more useful when defined in seconds. Given an input clock period of TCK and N bits dedicated to phase selection, INL(s) = <MAT> where α can be increased from <NUM> to <NUM> in steps of <NUM>°/<NUM>N. From this formula, it is clear that to improve INL the number of bits dedicated to phase selection must be increased or the input clock period be decreased. For each additional bit added the phase rotator power consumption doubles. The phase rotator power consumption also increases linearly with the input clock frequency. Furthermore, there are diminishing returns when increasing the PR bits. Beyond <NUM> or <NUM> bits for phase selection the PR peak-to-peak INL stops improving in realistic implementations.

Another method of improving PR linearity is to have CKin1 and CKin2 separated by <NUM>°. In this scenario, the output clock simplifies to CKout = (<NUM> - α) + <MAT> with output phase <MAT>. Now INL becomes <MAT> and <MAT>. While the INL is improved, an octagonal PR implementation requires <NUM> input phases which adds significant complexity and power earlier in the CDR.

A comparison of phase rotator constellations for an ideal, octagonal and quadrature implementation can be seen in <FIG>. <FIG> compares the theoretical INL for a 6b octagonal and quadrature PR. The worst-case jitter for a CDR occurs during plesiochronous operation, where there exists a frequency mismatch between the incoming data and the sampling clock. This causes the PR to spin and not lock to a single phase. The peak-peak INL measured in seconds translates to random jitter during plesiochronous operation. Thus, for PRs to be used in CDRs for <NUM>-Gb/s links it is critical that new low power methods of improving INL are found. The INL formula gives a local maximum when α=<NUM> and a local minimum when α = <NUM>. One strategy that has been shown to improve PR linearity is the use of twin PRs (<FIG>). The concept is to sum the output of two PRs with αPR<NUM> = αPR<NUM> + <NUM>. The INL of PR<NUM> is at a minimum when the INL of PR<NUM> is at a maximum and vice versa, thus resulting in the INL approximately canceling out. The twin PR constellation diagram can be seen in <FIG>.

<FIG> compares the theoretical INL for a traditional 6b quadrature PR to that of a twin 6b quadrature PR.

Process, Voltage and Temperature (PVT) variations cause the INL of each PR to lose some correlation, so the result is not zero non-linearity, but the INL is significantly lowered. The drawback of this concept is the PR power and area consumption is doubled.

The subject disclosure presents a PR concept that can be used to lower power and area consumption using pipelining. The concept behind PR pipelining is to split the bits dedicated to phase selection across multiple stages. Previously utilized PR pipelining was done in a tournament style where extra PRs are used in each stage and MUXing is used after each stage to decide what phase to forward (<FIG>). While each driver stage in a tournament style PR uses less power, there is an increase in the total number of driver stages resulting in no power or area savings. This style of PR is more akin to multi-phase generation, thus achieving improved linearity (like going from a quadrature PR to an octagonal PR) but the power consumption is equal to or greater than that of the traditional PR.

The method of PR pipelining disclosed herein uses the same number of PRs in stage one as the total number of stages, but in each subsequent stage the total number of PRs decreases. Consider, for example, a two-stage PR separated by a dashed line utilizing the method of pipelining disclosed herein (<FIG>). Stage one, depicted by reference number <NUM>, utilizes two parallel PRs to generate two clock phases offset by a single Least Significant Bit (LSB). Stage two, depicted by reference number <NUM>, then uses a single PR to interpolate between these two phases entering the second stage.

For a comparison, consider a traditional PR where N bits are dedicated to phase selection. Each controllable driver stage requires <NUM>N unit devices so in total (<NUM>)(<NUM>N) total unit devices are used in the PR. In the method of pipelining disclosed herein, <MAT> bits are used in stage one and <MAT> bits are used in stage two. The same resolution is achieved but (<NUM>)(<NUM>N/<NUM>) unit devices are needed in stage one and (<NUM>)(<NUM>N/<NUM>) devices are needed in stage two. Thus, the total number of unit devices is now (<NUM>)(<NUM>N/<NUM>) resulting in a significant power and area reduction factor of (<NUM>)(<NUM>-N/<NUM>). This power and area reduction factor can be further improved if more bits are used in stage two than in stage one. For example, in one non-limiting embodiment, four bits may be used in the first stage and five bits in the second stage. As more stages are added to the method of PR pipelining disclosed herein, the total number of unit devices continues to decrease, and the power and area reduction factor is further improved.

An example of a two stage pipelined twin phase rotator is shown in <FIG>. Stage one, depicted by reference number <NUM>, utilizes twin two parallel PRs <NUM>. Stage two, depicted by reference number <NUM>, uses two parallel PRs. Summation nodes are depicted by reference number <NUM>. This enables the use of twin PRs without a significant power or area penalty. Combining these concepts results in a highly linear, low power PR that can be used in future generations of SerDes.

In another non-limiting embodiment, shown in <FIG>, a three-stage pipelined phase rotator may be implemented. Stage one, depicted by reference number <NUM>, utilizes twin two parallel PRs <NUM>. Stage two, depicted by reference number <NUM>, uses two parallel PRs. Stage three, depicted by reference number <NUM>, uses a single PR. There may be any value of α<NUM> compared to α<NUM> (and α<NUM> to α<NUM>). Optimally these codes are <NUM> LSB offset from each other, but this is not required. They could be any offset while still achieving many benefits. For example, say CKin1 is <NUM>° and Ckin2 is <NUM>° and <NUM> bits are dedicated to stage <NUM>. If α<NUM> and α<NUM> are <NUM> LSB offset then stage <NUM> is interpolating between phases which are offset by <NUM>°/<NUM><NUM>=<NUM>°. If α<NUM> and α<NUM> are offset by <NUM> LSBs then stage <NUM> interpolates between phases which are offset by <NUM>°.

In another non-limiting embodiment, the location of the summation nodes <NUM> in the two stage pipelined phase rotator may change, see for example, <FIG>, and still have some pipelining occur afterwards. Note the rearranging of the alpha values amongst the stages. The benefit of this implementation is the drop of <NUM> of the unit cell groups from stage <NUM> saving an additional 2x2N unit cells in the overall design, where N is the number of bits dedicated to stage <NUM>.

Another embodiment of the subject disclosure, not covered by the appended claims, includes improving PR linearity by adding an FIR filter to the output of the PR. FIR filters have been used in the feedback path of fractional-N frequency synthesizers to suppress ΣΔ noise. The subject disclosure presents utilizing FIR filters in a CDR. An FIR filter can be seen in <FIG>. In the basic FIR filter all α coefficients are equal resulting in a stop band attenuation of <NUM> log<NUM> N at fsample/<NUM> and a <NUM>-dB BW that is proportional to fsample/N where N is the number of FIR taps and fsample = <NUM>/TD where TD is the z-<NUM> delay.

In an analog implementation, increasing FIR taps beyond tens of taps can be difficult and prone to error but this still provides sufficient stop band attenuation and bandwidth to significantly reduce the phase noise introduced by a PR. Other types of FIR filters, for example a Kaiser-Bessel FIR, can be used to improve the stop band attenuation and <NUM>-dB bandwidth without increasing the number of taps. In such an implementation the coefficients are calculated using a zeroth order modified Bessel function of the first kind. An example of a <NUM> tap Kaiser-Bessel FIR frequency response with fsample = <NUM> can be seen in <FIG>. The stop band attenuation is around <NUM>-dB and the <NUM>-dB bandwidth is around <NUM>. An example of an analog FIR filter implementation can be seen in <FIG>. Summation is done in the voltage domain and the resistor values are modified to set the tap coefficients.

To test the concepts disclosed herein, an <NUM>-bit, <NUM>-GHz version of the phase rotator from <FIG> was implemented in <NUM>-nm FinFET CMOS and simulated using Cadence Spectre. The layout of the phase rotator can be seen in <FIG>. The first stage is depicted by reference number <NUM> and the second stage is depicted by reference number <NUM>. The two stage pipelined twin phase rotator has an active area of <NUM>µm × <NUM>µm.

A simulation was run with the PR in plesiochronous operation. The resulting phase noise plot can be seen in <FIG>. The RMS jitter was measured to be <NUM>-fs integrating from DC-to-fCK/<NUM>. This number is also state of the art. In another simulation with the PR in static operation the jitter was measured to be <NUM>-fs. Another simulation was then run with an FIR filter following the PR. The resulting phase noise can also be seen in <FIG>, and the improvement in the phase noise with the FIR filter is clear. This improvement is again shown by measuring the RMS jitter which dropped from <NUM>-fs to <NUM>-fs.

The use of terms "first," "second," "third," and so forth, as used in the claims, unless otherwise clear by context, is for clarity only and does not otherwise indicate or imply any order.

In addition, the words "example" and "exemplary" are used herein to mean serving as an instance or illustration. Any embodiment or design described herein as "example" or "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs. Rather, use of the word example or exemplary is intended to present concepts in a concrete fashion. As used in this application, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or". That is, unless specified otherwise or clear from context, "X employs A or B" is intended to mean any of the natural inclusive permutations. In addition, the articles "a" and "an" as used in this application and the appended claims should generally be construed to mean "one or more" unless specified otherwise or clear from context to be directed to a singular form.

As employed herein, the term "processor" can refer to substantially any computing processing unit or device comprising, but not limited to comprising, single-core processors; single-processors with software multithread execution capability; multi-core processors; multi-core processors with software multithread execution capability; multi-core processors with hardware multithread technology; parallel platforms; and parallel platforms with distributed shared memory. Additionally, a processor can refer to an integrated circuit, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), a programmable logic controller (PLC), a complex programmable logic device (CPLD), a discrete gate or transistor logic, discrete hardware components or any combination thereof designed to perform the functions described herein. Processors can exploit nano-scale architectures such as, but not limited to, molecular and quantum-dot based transistors, switches and gates, in order to optimize space usage or enhance performance of user equipment. A processor can also be implemented as a combination of computing processing units.

What has been described above includes mere examples of various embodiments. Accordingly, the embodiments disclosed and/or claimed herein are intended to embrace many alterations, modifications and variations that fall within the scope of the appended claims. Furthermore, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term "comprising" as "comprising" is interpreted when employed as a transitional word in a claim.

Claim 1:
A multiple stage pipelined phase rotator, comprising:
a first stage (<NUM>, <NUM>, <NUM>) comprising a first number of phase rotators in parallel generating respective clock phases offset by a fixed amount;
a second stage (<NUM>, <NUM>, <NUM>) comprising a second number of phase rotators receiving outputs from the first number of phase rotators of the first stage, the second stage outputting a first weighted sum of respective clock phases generated by the second number of phase rotators;
wherein the second number of phase rotators is less than the first number of phase rotators, and
wherein a total number of bits dedicated to phase selection is split across the first stage and the second stage.