Abstract:
A delay locked loop (DLL) circuit that includes a delay line having a plurality of delay elements. Each delay element can be adapted to receive a clock input signal and generate a clock output signal, where the phase of each clock output signal is offset from the clock input signal. The delay line can be configured so that one clock input signal is a reference input clock signal and at least one clock output signal is a delay-line output clock signal. The feedback portion of the circuit can be configured to generate delay adjust signals based upon the phase offsets between pairs of clock signals. The delay adjust signals are fed back to the delay elements causing the reference input clock signal and the clock output signals to be phase-shifted apart equally about 360 degrees.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to integrated circuits for high-speed data applications, and more particularly to a delay-locked loop having accurate clock phase multiplication, and systems and methods employing such a delay-locked loop. 
   BACKGROUND OF THE INVENTION 
   A delay-locked loop is capable of generating multiple clock phase outputs from a clock signal input. It is desirable to have a delay-locked loop which has reduced circuit component matching requirements while generating multiple clock outputs equally spaced about 360 degrees. 
   Prior art inventions disclose delay locked loops that utilize chains of delay elements and relatively complex control schemes to generate a set of clock signals. The delay-locked loop of U.S. Patent Publication No. 2004-0223571 receives an input clock signal, then generates a single delay adjust signal by means of a phase detector, and control circuitry. The single delay adjust signal then controls the delay time of all of the delay elements in the chain. At a minimum, the control circuitry of this application includes a counter control circuit with digital memory, and a digital-to-analog converter. The set of generated clock signals is then further processed by additional circuitry that includes a phase interpolator, selection circuitry, and other feedback controls. 
   It would be desirable to provide a delay lock loop that improved on existing designs and is better suited for high speed data applications. 
   SUMMARY OF THE INVENTION 
   The present invention provides a delay-locked loop which generates multiple clock phases equally spaced about 360 degrees. The phase of each generated clock is individually measured and adjusted, and this reduces circuit matching requirements of certain parts of the clock-generation circuit. In one embodiment, the delay-locked loop comprises a delay line which generates multiple clock phase outputs equally spaced about 360 degrees, and a feedback loop including a feedback circuit which measures the multiple clock phase outputs and provides an adjusted control output signal for each clock phase to the delay line such that the multiple clock phase outputs are equally spaced about 360 degrees. 
   In another embodiment, the present invention provides multiple delay locked loops coupled together to achieve clock phase multiplication where the spacing between generated clock phases is less than the delay through a single delay element in a delay-locked loop. In this embodiment the multiple delay-locked loops generate multiple clock phase outputs equally spaced about 360 degrees, where the spacing between generated clock phases is less than the delay through a single delay element in a delay-locked loop. The system may also include a delay line which generates multiple clock phase outputs equally spaced about 360 degrees, a feedback loop including a feedback circuit which measures the multiple clock phase outputs and provides an adjusted control output signal for each clock phase to the delay line such that the multiple clock phase outputs are equally spaced about 360 degrees, and a second delay-locked loop coupled to the first delay-locked loop such that the rising edges of its clock phases lie exactly between the rising edges of the first delay-locked loop&#39;s clock phases. 
   In a third embodiment, the present invention provides a delay-locked loop and a time division multiplexer, where a portion of the multiplexer itself is embedded within the control loop of the delay-locked loop. In this embodiment, the invention is a multiplexer system comprising a delay-locked loop similar to the delay locked loops of the first and second embodiments, and a time-division multiplexer, where a portion of the multiplexer is embedded within the control loop of the delay-locked loop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating one exemplary embodiment of a delay locked loop according to the present invention. 
       FIG. 2  is a diagram illustrating one exemplary embodiment of a delay line used in a delay-locked loop according to the present invention. 
       FIG. 3  is a timing diagram. 
       FIG. 4  is a diagram illustrating one exemplary embodiment of a delay circuit used in a delay-locked loop according to the present invention. 
       FIG. 5  is a diagram illustrating another exemplary embodiment of a delay circuit used in a delay-locked loop according to the present invention. 
       FIG. 6A  is a diagram illustrating one exemplary embodiment of a feedback circuit used in a delay-locked loop according to the present invention. 
       FIG. 6B  is a diagram illustrating an alternate embodiment of a feedback circuit used in a delay-locked loop according to the present invention. 
       FIG. 6C  is a diagram illustrating another alternate embodiment of a feedback circuit used in a delay-locked loop according to the present invention. 
       FIG. 7  is a timing diagram. 
       FIG. 8  is a diagram illustrating one exemplary embodiment of a duty cycle monitor. 
       FIG. 9  is a diagram illustrating one exemplary embodiment of a reset circuit. 
       FIG. 10  is a timing diagram. 
       FIG. 11  is a diagram illustrating a second exemplary embodiment of a delay-locked loop according to the present invention where multiple delay-locked loops are coupled together to achieve clock phase multiplication. 
       FIG. 12  is a timing diagram. 
       FIG. 13  is a diagram illustrating a second delay line B suitable for use in the delay-locked loop illustrated in  FIG. 12 . 
       FIG. 14  is a diagram illustrating one exemplary embodiment of feedback circuit suitable for use with a second delay line according to the present invention. 
       FIG. 15  is a diagram illustrating one exemplary embodiment of a duty cycle monitor. 
       FIG. 16  is a diagram illustrating one exemplary embodiment of a reset circuit. 
       FIG. 17  is a diagram illustrating a third exemplary embodiment of a delay locked loop where a time division multiplexer is embedded within the delay-locked loop according to the present invention. 
       FIG. 18  is a diagram illustrating one exemplary embodiment of a time division multiplexer used with a delay-locked loop according to the present invention. 
       FIG. 19  is a diagram illustrating one exemplary embodiment of a delay line used in a delay-locked loop according to the present invention. 
       FIG. 20  is a diagram illustrating one exemplary embodiment of a feedback circuit used in a delay-locked loop according to the present invention. 
       FIG. 21  is a timing diagram. 
       FIG. 22  is a diagram illustrating one exemplary embodiment of a reset circuit used with a delay-locked loop according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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. 
     FIG. 1  is a diagram illustrating one exemplary embodiment of a delay-locked loop according to the present invention. The delay-locked loop generates multiple clock phases equally spaced about 360°. The phase of each generated clock is individually measured and adjusted, reducing clock-generation circuit matching requirements. The delay-locked loop  40  includes a delay line  42  and a feedback circuit  44 . In operation, delay line  42  receives a clock signal input  50 . Delay-line  42  provides multiple clock phase outputs P[ 0 : 2 ] and P[ 0 : 2 ] to a high speed data application, such as a mixer or other system  52  (e.g., a high-speed serializer or deserializer). The multiple clock phases P[ 0 : 2 ] and P[ 0 : 2 ] are measured by feedback control circuit  44  as part of a feedback control loop, indicated at  54 ,  56 . Feedback control circuit  44  provides feedback control signals BIASNI and BIASN 2  such that the rising edge of clock phases P[ 0 : 2 ] are evenly distributed about 360°. 
     FIG. 2  illustrates one exemplary embodiment of delay line  42  suitable for use in a delay-locked loop according to the present invention. The delay-locked loop  40  provides accurate clock phase multiplication, where the multiple clock phases output from the delay line  42  are equally spaced about 360 degrees. BIASNI and BIASN 2  are controlled such that the rising edge of clock phases P[ 0 : 2 ] are evenly distributed about 360°, illustrated in  FIG. 3 . 
   The “duty-cycle monitor”  43  controls BIASP such that the duty cycle at P 2  (and, by extension, at P 0  and PI) is substantially equal to 50%. Other circuits, some not based on a duty-cycle measurement, can also be used to generate BIASP. Dynamic duty-cycle control is generally not as important when a larger number of clock phases (N=8 or larger, for example) are generated by the delay-locked loop  40 . 
     FIGS. 4 and 5  illustrate two exemplary embodiments of a delay cell suitable for use with a delay-locked loop according to the present invention. These delay cells  46  and  48  may be used separately or in combination. Other delay cell designs suitable for use with the present invention will become apparent to one skilled in the art after reading the present application. 
     FIG. 6A  is a circuit diagram illustrating one exemplary embodiment of feedback circuit  44  used within a delay-locked loop according to the present invention. Feedback circuit  44  controls BIASN 1  and BIASN 2  such that the rising edge of the multiple clock phases generated by the delay line are equally spaced about 360 degrees. 
   At the delay-locked loop&#39;s stable operating point, the time-averaged currents I 0 =I 1 =I 2 . When these currents are equal, then it follows that the time from the rising edge of P 2  to the falling edge of P 0  must be equal to both (a) the time from the rising edge of P 0  to the falling edge of P 1  and to (b) the time from the rising edge of P 1  to the falling edge of P 2 . In addition, by including cascade transistors  66  and  68  connecting to BIASNC and BIASPC, systematic current mirror gain (which can result in phase errors in P[ 0 : 2 ]) is reduced. The voltage on BIASNC and BIASPC is nominally Vdd/2, but other voltages can be used with good results. 
     FIG. 6B  is a circuit diagram illustrating an alternate embodiment of feedback circuit  44  used within a delay-locked loop according to the present invention. The use of multiple operational amplifiers with negative feedback provides additional current stabilization by eliminating the error created by the current mirrors. 
     FIG. 6C  is a circuit diagram illustrating another alternate embodiment of feedback circuit  44  used within a delay-locked loop according to the present invention. This feedback circuit includes one embodiment of a fixed tail bias circuit, and alternative capacitor locations. 
     FIG. 7  illustrates one exemplary embodiment of a timing diagram, including P 0 , P 1 , P 2 , I 0 , I 1  and I 2 . 
     FIG. 8  is a circuit diagram illustrating one exemplary embodiment of a duty cycle monitor  43 . The duty-cycle monitor  43  is important in that it helps ensure that a 50% clock duty cycle is maintained through the delay line. 
   There are other circuits that can also be used to generate BIASN 1  and BIASN 2  from clocks P 0 , PI, and P 2 . Though this disclosure describes a delay locked loop generating 3 clock phases, similar circuits can be used to generate N clock phases as long as N≧2:3. 
   The present invention provides a single-ended delay-locked loop  40  where the delays from one rising edge to the next rising edge are controlled. All falling edges are controlled as a group (by means of controlling BIASP) such that the duty cycle of each clock is substantially 50%. The present invention provides a delay line where the time from each clock&#39;s rising edge to the next clock phase rising edge is converted to a current. A reference current is generated that is proportional to the time from the rising clock edge of the last clock phase in the delay line to the first clock phase in the delay line  42 . A feedback control circuit then adjusts individual delay line element delays  46 , or alternatively  48 , until all of these currents (representing the spacing between rising edges of adjacent phases) are equal. 
   To help prevent a false-lock, a reset function can be used at start-up.  FIG. 9  is a circuit diagram  90  illustrating one exemplary embodiment of a reset function suitable for use with the present invention. A circuit can be designed to detect a false-lock state and then momentarily apply reset (i.e., force RESET low). 
   A false-lock condition occurs when the total delay in the delay line is some multiple of 360° where that multiple is an integer &gt;1. In the exemplary timing diagram of  FIG. 10 , the clock phases P 0 , P 1 , P 2  are evenly distributed about 720°, which is not desirable. 
     FIGS. 11 and 12  are diagrams illustrating a second exemplary embodiment of a delay-locked loop for generating multiple clock phases evenly spaced about 360° according to the present invention. In one aspect illustrated, clock phase multiplication is achieved with multiple delay-locked loops  40   a  and  40   b  coupled together where the spacing between generated clock phases is less than the delay through a single delay element within each delay-locked loop. The present invention overcomes the problem where the minimum clock phase spacing that you could achieve was limited to the minimum delay-element delay. A first delay-locked loop  40   a  is constructed as previously described herein under the heading “DELAY-LOCKEDLOOP”. A second delay-locked loop  40   b  is coupled to the first delay-locked loop such that the rising edges of its three clock phases lie exactly distributed between the rising edges of the first delay-locked loop&#39;s clock phases, illustrated generally in the timing diagram of  FIG. 12 . 
     FIG. 13  is a diagram illustrating one exemplary embodiment of the second delay line  42   b  used in a coupled delay-locked loop according to the present invention.  FIG. 14  is a diagram illustrating one exemplary embodiment of feedback circuit  44   b  for the second delay line  42   b  used in coupled-delay locked loop  40   a.  A control circuit converts the time delay from the rising edge of P 0  to the falling edge of P 0 ′ to a first current and the rising edge of P 0 ′ to the falling edge of P 1  to a second current. A comparison circuit continuously compares the value of the first current to the value of the second current. When the first current is greater than the second current, the voltage on BIAS N 0 ′ increases, advancing the P 0 ′ rising edge in time, thereby reducing the first current and increasing the second current. This feedback circuit will settle into a state with the first current equal to the second current, and thus with the rising edge of P 0 ′ mid-point between (in time) the rising edges of P 0  and P 1 , as desired. In a similar fashion, the rising edge of P 1 ′ is adjusted until mid-point between the rising edges of P 1  and P 2 , and the rising edge of P 2 ′ is adjusted until mid-point between the rising edges of P 2  and P 0 . 
     FIG. 15  illustrates one exemplary embodiment of a duty cycle control circuit  150 . The duty cycle control circuit  150  monitors the duty cycle of P 2 ′ and controls BIASP (and the timing of the falling edge of P 0 ′,P 1 ′,and P 2 ′) such that the duty cycle of P 2 ′ (and, by extension, the duty cycle of P 0 ′ and P 1 ′) is substantially 50%. 
   Though this disclosure describes two coupled delay-locked-loops each generating 3 phases (for a total of 6 clock phases), similar circuits can be used to generate a total of 2N clock phases for any N≧3. 
     FIG. 16  is a circuit diagram illustrating one exemplary embodiment of a reset circuit  160  suitable for use with the present invention. A reset function is needed at startup. 
   One of the advantages of this circuit  40   a  is that it offers the ability to halve the spacing “in time” of the generated clocks while each of the two coupled delay lines operating frequency and stage delay remain unchanged. By contrast, an attempt to get a single delay line to generate twice as many phases while operating at the same frequency would require that the number of delay stages in the delay line be doubled and the delay through each delay stage would be halved. For a delay line already operating close to its speed limit, this is not possible. 
   Also, similar techniques and circuits can be used to couple more than two delay-locked loops. Furthermore, the various alternate embodiments of feedback circuit  44 , including  44   a ,  44   b ,  44   c  and others, could be incorporated into circuit  40   a.    
     FIG. 17  illustrates a third exemplary embodiment of the present invention consisting of a delay-locked loop  66  and time-division multiplexer  60 , where a portion of the multiplexer is embedded within the control loop of the delay-locked loop. The clocks generated by the delay-locked loop can be used to control a time-division multiplexer  60 . To reduce timing errors that can be introduced in such a circuit because of mismatched parallel signal paths through the multiplexer, the delaylocked loop&#39;s feedback loop is expanded to include key components of the time division multiplexer  60 . 
     FIG. 18  illustrates one exemplary embodiment of an embedded time division multiplexer according to the present invention. The circuit begins with the delay line illustrated in  FIG. 19 . The clock phases P[ 0 : 2 ] and P[ 0 : 2 ] are then used to control the time-division multiplexer. 
   In reference to  FIG. 18 , the darkened lines indicate buses, and the darkened circuit elements indicate arrays of elements. In any given “bit-time” (defined here as that period of time between the rising edge of P N  and the falling edge of P N+1 ) either A N  or B N  will pulse high (depending on the state of D N ) while all the other Ax and Bx (x≠n) remain low. The AND gates  62  and  64  can be straightforward CMOS-logic implementations or can be more elaborate constructions. 
   In reference also to  FIG. 19 , BIASNI and BIASN 2  are controlled such that the pulses on nets A[ 0 : 2 ] and B[ 0 : 2 ] in  FIG. 18  are all the same width. The “duty-cycle control” circuit  43  controls BIASP such that the duty cycle at P 2  (and, by extension, at P 0  and P 1 ) is substantially equal to 50%. 
     FIG. 20  is a circuit diagram  44   c  illustrating one exemplary embodiment of feedback circuit C according to the present invention. The loop dominant pole is set by C F    70 . Other methods of compensation are possible. This feedback circuit&#39;s stable operating point is when the time-averaged currents I 0 , I 1  and I 2  are all equal. When equal, this means that the signal pulse-widths on AN and BN (N=0, 1, 2) are all equal. This is the desired condition. By including cascade transistors  72  and  74  connecting to BIASNC and BIASPC, systematic current mirror gain (which can result in phase errors in P[ 0 : 2 ]) is reduced. The voltage on BIASNC and BIASPC is nominally Vdd/2, but other voltages can be used with good results. It will be apparent to one skilled in the art after reading the present application that circuits other than that shown in  FIG. 20  can be used to accomplish substantially the same function. 
   The present invention provides a single-ended delay-locked loop  66 , controlling a time-division multiplexer  60 , where the pulses internal to that multiplexer  60  are used to control the delay-locked loop. “Lock” is achieved (i.e. the feedback loop reaches its stable operating point) when all of the multiplexer&#39;s control pulses (A[ 0 : 2 ] and B[ 0 : 2 ]) are the same width. When the loop is “in lock”, the timing diagram illustrated in  FIG. 21  applies. 
   To prevent a false-lock, a RESET function is applied at startup. One exemplary embodiment of a reset circuit  90  suitable for use with the present invention is illustrated in  FIG. 22 . 
   Though this disclosure describes a delay-locked loop generating 3 phases and controlling a 3:1 time-division multiplexer, similar circuits can be used to create an N-phase DLL controlling an N:1 time-division multiplexer as long as N≧3. 
   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, electromechanical, 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.