Abstract:
A phase control circuit includes a signal generator sub-circuit that generates a set of phase reference signals having phase angles generally distributed over a phase angle adjustment range. A controller sub-circuit produces weighting signals that assign relative priority for each of the phase reference signals, and includes at least one incremental adjustment input. The controller sub-circuit is adapted to maintain the weighting signals in a generally steady state when receiving signaling on the adjustment input that represents no adjustment, and to adjust relative intensities of the weighting signals based on stimulation of the adjustment input. The phase control circuit further includes a mixer sub-circuit that is coupled to the set of phase reference signals and to weighting signals that collectively control a mix of the phase reference signals. The mixer sub-circuit is adapted to produce an output signal having a phase angle that is based on the mix of the phase reference signals.

Description:
THE FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to integrated circuits, and more particularly to a mixer having up/down phase control, and systems and methods utilizing such a mixer. In one embodiment, the present invention provides a system and method for clock extraction from serial data.  
       BACKGROUND OF THE INVENTION  
       [0002]     A mixer is generally used to combine two or more input signals to form a single output signal. These input signals can differ in frequency and phase, or in phase only. By selectively weighting the combination of these input signals, and in some cases varying those weights in time, a useful and complex output signal can be generated. A mixer with multiple inputs of equal frequency and distributed phase can generate an output signal of a similar frequency and of any phase through the selective weighting of each mixer input. To form a clock recovery system, it is desirable to have a mixer where the selective weighting of the mixer inputs can be directly controlled by a traditional data phase detector and its UP/DOWN logic outputs.  
       SUMMARY OF THE INVENTION  
       [0003]     According to one aspect of the invention, a phase control circuit includes a signal generator sub-circuit that generates a set of phase reference signals having phase angles generally distributed over a phase angle adjustment range. A mixer sub-circuit is coupled to the set of phase reference signals and to weighting signals that collectively control a mix of the phase reference signals. The mixer sub-circuit is adapted to produce an output signal having a phase angle that is based on the mix of the phase reference signals. The phase control circuit further includes a controller sub-circuit that produces the weighting signals, and includes at least one adjustment input. The controller sub-circuit is adapted to maintain the weighing signals in a generally steady state when receiving signaling on the adjustment input that represents no adjustment, and to adjust relative intensities of the weighting signals based on stimulation of the adjustment input.  
         [0004]     According to another aspect of the invention, a mixer control circuit includes an arrayed set of control signal driver elements, each element having a set of at least one UP/DOWN input that receives UPI/DOWN signaling common to all array elements, a feedback loop that receives a feedback signal common to all array elements representing at least an aggregation of control signal outputs of the array elements, a set of at least one control signal input that receives control signal outputs from other elements of the array, and a set of at least one control signal output that produces at least one control signal. The at least one control signal is produced such that an aggregation of the control signals of the array is proportional to a reference amplitude signal according to a first ratio. Also, a control signal having the highest amplitude among the control signals of the array is proportional to the reference amplitude signal according to a second ratio. Furthermore, a signal pulse on the at least one UP/DOWN input affects relative amplitudes of the control signals driven by the array elements.  
         [0005]     A method of recovering a clock from a high-speed serial data transmission according to another aspect of the invention includes generating a plurality of phase reference signals, and controlling an amplitude of each of the phase reference signals to assign a relative weight to each of the phase reference signals. The controlling includes generating a plurality of amplitude control signals corresponding to the plurality of phase reference signals; stabilizing relative and absolute amplitudes of the amplitude control signals to maintain steady state in an absence of a change in input signal; inter-relating the amplitude control signals such that a change in any one amplitude control signal propagates to effect changes in the other amplitude control signals; mixing the weighted phase reference signals to produce a local clock signal having a desired phase angle; and providing an input signal representing a comparison between the local clock signal and the serial data transmission. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  illustrates one exemplary embodiment of a mixer with up/down phase control, built from a multi-phase mixer and an array of mixer control subcircuit A, according to the present invention.  
         [0007]      FIG. 2  illustrates exemplary embodiments of two buffer cells from which the mixer can be built.  
         [0008]      FIG. 3A  illustrates one exemplary embodiment of mixer control subcircuit A.  
         [0009]      FIG. 3B  is a block diagram illustrating one exemplary arrayed arrangement of mixer control subcircuits A.  
         [0010]      FIG. 4  illustrates exemplary embodiments of the voltage levels of BIASN[0:4] in an initial state, after a  DOWN  pulse, and after an  UP  pulse.  
         [0011]      FIG. 5  illustrates one exemplary embodiment of a mixer reset circuit.  
         [0012]      FIG. 6  illustrates one exemplary embodiment of a clock recovery system with a data phase detector using a single clock input and a mixer with a single  UP  and a single  DOWN  input, according to the present invention.  
         [0013]      FIG. 7  illustrates one exemplary embodiment of a mixer with multiple up/down phase control inputs, built from a multi-phase mixer and mixer control subcircuit B.  
         [0014]      FIG. 8  illustrates one exemplary embodiment of mixer control subcircuit B.  
         [0015]      FIG. 9  illustrates one exemplary embodiment of a clock recovery system with a data phase detector using a multi-phase clock input and a mixer with multiple  UP  inputs and multiple  DOWN  inputs. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be used and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
         [0017]     One aspect of the present invention is directed to a signal mixer in which the selective weighting of each mixer input is controlled by incremental up/down adjustment. To form a clock recovery loop in one application, the mixer output can be coupled to the clock input of a data phase detector, while the data phase detector output is coupled to the up and down inputs of the mixer. The clock recovery loop controls the phase and therefore the frequency of the mixer output such that these are matched, or locked, to the incoming serial data.  
         [0018]     In a related embodiment, the selective weights applied to each mixer input are controlled by multiple up inputs and multiple down inputs. Certain specialized data phase detectors use multiple phase sampling methods and will generate multiple up and down output signals. This embodiment is intended to couple to this specialized type of data phase detector.  
         [0019]     One embodiment of the invention is a mixer comprising multiple mixer buffer cells whose outputs are coupled together; a common-mode feedback loop, ensuring that the net strength of the mixer buffer cells results in a mixer bandwidth that substantially matches the frequency of the signal being mixed; and a control loop which in steady-state results in a specific subset of the mixer buffer cells having a substantially high drive strength, with the remainder having a substantially low drive strength. The mixer buffer cell may further comprise a signal input, a signal output, and one or more additional inputs for controlling its output drive strength. Likewise, in the common-mode feedback loop the frequency of the signal being mixed may be converted to a voltage by means of a delay-locked loop, and this voltage may be used to control the mixer bandwidth. The control loop may further comprise control inputs  UP  and  DOWN  which when asserted results in a shift in the strength of the mixer buffer cells, resulting in a shift in output phase.  
         [0020]     In another embodiment, the invention includes a mixer comprising a mixer similar to the mixer described above and additional  UP  and  DOWN  control inputs, operating in parallel with and with similar function as the  UP  and  DOWN  inputs described above.  
         [0021]     Another embodiment of the invention comprises a single-phase clock recovery system including: a data phase detector; a multi-phase clock generator; and a mixer similar to that described above coupled to the data phase detector and the multi-phase clock generator. One embodiment is a multi-phase clock recovery system comprising a multi-phase data phase detector; a multi-phase clock generator; a second multi-phase clock generator; and a mixer similar to that set forth above, coupled to the data phase detector and the multi-phase clock generators.  
         [0022]      FIG. 1  illustrates one example embodiment of a mixer according to one aspect of the present invention. The multi-phase mixer  30  of  FIG. 1  is built from 5 variable strength buffer cells  32  whose outputs ate coupled together. The input signals IN[0:4] are nominally 50% duty-cycle input signals of varying phase, generally evenly distributed about 360 degrees. The accuracy of this distribution is not especially important. The drive strength of each variable-strength buffer cell is set by the voltage on its BIASN and BIASP inputs.  FIG. 2  illustrates two exemplary embodiments of a variable-strength buffer cell  32 . These are both inverting circuits, and can be used separately or in combination. By selectively strengthening one or more of the variable-strength buffer cells  32  and weakening others, the phase of the signal at  OUT  can be set to any value. An optional inverter coupled between  OUT  and OUT can provide for higher mixer output drive capability and a reduction in the mixer&#39;s output rise/fall time.  
         [0023]     Also shown in  FIG. 1  is one embodiment of a mixer control circuit  34 . In one sense, this circuit is an analog state machine controlled by logic control inputs  UP  and  DOWN . The voltage on MAXBIASN assists in the control of the voltages of BIASN[0:4] such that ΣV BIASN[0:4] =K*V MAXBIASN , where this factor K is determined in part by the ratio of certain transistor W/L values. By controlling ΣV BIASN[0:4] , the total drive current at  OUT  is controlled, and then so is the bandwidth at  OUT . For good operation of the mixer, its bandwidth is set to substantially the frequency of the input signals IN[0:4]. To achieve this, the voltage V MAXBIASN  can be derived, for example, from the control voltage of a delay-locked loop operating at the same frequency as the mixer.  
         [0024]      FIG. 3A  illustrates one exemplary embodiment of mixer control sub-circuit A that is arrayed 5 times as indicated in  FIG. 1 .  FIG. 3B  is a block diagram illustrating the organization and interconnectivity of the elements of the array according to one embodiment. The array of  FIG. 3B  includes blocks  42 ,  44 ,  46 ,  48 , and  50 , each of which represents the sub-circuit of  FIG. 3A . The array can be through of as a one-dimensional array of adjacent interconnected blocks, in which block  42  is interconnected with block  50  as if these blocks were physically adjacent. Persons skilled in the relevant arts will recognize that the blocks can be functionally, or operably adjacent without being actually physically adjacent with one another in an integrated circuit (IC). For each block, or array element, the  UP  and  DOWN  inputs are commonly interconnected, as illustrated. Likewise, the BIASNFB input is common to all blocks.  
         [0025]     Referring again to  FIG. 3A , the voltages BIASN[0:4] are aggregated to achieve a signal representing the total output. In the embodiment shown, the voltages are summed by means of first converting each to a current (using M 3 ), then mirroring each current (using M 2  and M 1 ) and summing those currents at BIASNFB. By means of a common-mode feedback loop including transistors M 1 -M 5 , and by setting W/L of M 1  equal to W/L of M 2  and W/L of M 0  (in  FIG. 1 ) equal to 2 times W/L of M 3 , the voltage on BIASNFB will settle to a voltage such that ΣI M3 =I M0 . Other ratios of M 0  to M 3  other than 2 can also be used with good results.  
         [0026]     In addition to the conmon-mode feedback loop, there is a second loop which controls the voltage relationship between each element in BIASN[0:4]. The BIASPUP and BIASPDOWN inputs of each mixer control subcircuit are connected to the BIASP outputs of an adjacent mixer control subcircuit as follows: BIASPUP[N] is coupled to BIASP[N−1], and BIASPDOWN[N] is coupled to BLASP[N+1], for N=0, 1, 2, 3, 4 (where N+1=5, use 0). With both loops operational,  FIG. 4   a  shows a representative steady-state condition for BIASN[0:4]. Referring again to the arrayed mixer control subcircuit of  FIGS. 3A and 3B , consider what happens to the state of BLASN[0:4] in  FIG. 4   a  when a  DOWN  pulse is applied: BIASN 3  will receive a pulse of current proportional to V BIASN2  by means of an inverting amplifier formed by M 3  and M 2  and coupling BIASN 2  to BIASP 2 . This pulse of current is then integrated by capacitor Cf, raising V BIASN3 . In a similar fashion, BIASN 2  will receive a brief pulse of current proportional to V BIASN1 , by means of the inverting amplifier formed by M 3  and M 2  and coupling BLASNI to BLASPI. This pulse of current is then integrated by another capacitor Cf, raising V BIASN2 , though by a lesser amount than V BIASN3  was raised. Also, in a similar fashion, BIASN 4  will receive a brief pulse of current proportional to V BIASN3  by means of another inverting amplifier formed by M 3  and M 2  and coupling BIASN 3  to BIASP 3 . This pulse of current is then integrated by another capacitor Cf, raising V BIASN4 , though again by a lesser amount than V BIASN3  was raised. During the assertion of  DOWN  and at the same time that these described events which are tending to raise the voltage on some of the nets BIASN[0:4], all of these nets are discharged by an equal amount as determined by the common-mode feedback loop which includes discharge transistors M 4  and M 5 .  
         [0027]      FIG. 4   b  illustrates the state of V BIASN[0:4]  after pulsing  DOWN  low for a period of time and giving rise to the events described above. And after pulsing  UP  low for a period of time and giving rise to events similar yet complementary to those described above, the state of V BIASN[0:4]  is as illustrated in  FIG. 4   c.    
         [0028]     Note that each of the 3 states in  FIG. 4  are stable and do not drift (say, to the state of  FIG. 4   a ) when subject to no adjustment such as when receiving an equal percentage of  UP  and  DOWN  pulses. Put another way, there is no inherent systematic offset in this mixer control circuit, and this is of particular value when the mixer is integrated into a clock recovery loop.  
         [0029]     Also note that this disclosure shows the mixing of P phases where P=5, though any number P≧3 can be used.  
         [0030]     Also note that a reset or startup circuit is helpful to pull the circuit out of the undesirable state when V BIASN[0:4]  all equal 0. The RESET circuit in  FIG. 5  is suitable for use with the present invention, though other circuits are possible, and some do not require an explicit RESET input.  
         [0031]      FIG. 6  illustrates a clock recovery system using a mixer according to the present invention. The mixer is coupled between a data phase detector having DATA and CLK inputs and  UP  and  DOWN  outputs. The system will adjust the phase of the recovered clock until its phase and frequency match that of the incoming data. A multi-phase clock generator is used to generate the mixer&#39;s clock inputs from a single reference clock.  
         [0032]      FIG. 7  illustrates another embodiment of a mixer, similar to that in  FIG. 1 , but with multiple  UP  and  DOWN  phase control inputs. This mixer uses a new mixer control subcircuit B, illustrated in  FIG. 8 . Here, M 5 , M 7 , and M 8  consist of arrays of 5 transistors connected in parallel. For example, M 7  indicates an array of 5 transistors with the drains shorted together, the sources shorted together, and the gates coupled to the bus  UP[0:4] .  
         [0033]     This new mixer is used in an alternate clock recovery system, illustrated in  FIG. 9 . Here, the clock recovery system uses a specialized data phase detector having multiple CLK inputs and multiple  UP  and  DOWN  outputs. To generate the multiple clock phases used by this specialized data phase detector, a second multiphase clock generator is used and is coupled between the mixer output and the data phase detector. Though both are shown here to be equal to 5, the number of phases in the two multi-phase clock generators do not necessarily need to be equal.  
         [0034]     Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the chemical, mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein.