Abstract:
A phase/frequency locked loop (PLL) includes circuitry adapted to detect missing pulses of a reference clock and to control the phase bump of the PLL. The circuitry includes, in part, first and second flip-flops, as well as a one-shot block. The first flip-flop has a data input terminal responsive to a voltage supply, and a clock terminal responsive to an inverse of feedback clock. The second flip-flop has a data input terminal responsive to an output of the first flip-flop, and a clock terminal responsive to the inverse of the feedback clock. The one-shot block generates a pulse in response to a rising edge of the reference clock that is used to generate the feedback clock. The one-shot block generates an output signal applied to a reset terminal of the first flip-flop.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to electronic circuits, and more particularly to controlling the phase bump a phased locked loop. 
         [0002]    A phase locked loop maintains a fixed relationship between the phase and frequency of the signal it receives and those of the signal it generates.  FIG. 1  is a simplified block diagram of a conventional phase locked loop (PLL)  100  adapted to maintain a fixed relationship between the phase and frequency of signal CLK and signal REF. PLL  100  includes, among other components, phase detector  102 , charge pump  104 , loop filter  106  and voltage controlled oscillator (VCO)  108 . The extracted clock signal Clk is supplied at the output terminal of VCO  108 . The operation of PLL  10  is described further below. 
         [0003]    Phase detector  102  receives signals REF and Clk, and in response, generates signals UP and DN that correspond to the difference between the phases of the signals REF and Clk. Charge pump  104  receives signals UP and DN and in response varies the current it supplies to node Vcntrl. Loop filter  106  stores the charge as a voltage, which is then delivered to VCO  108 . 
         [0004]    If signal REF leads signal Clk in phase—indicating that the VCO is running relatively slowly—the duration of pulse signal UP increases, thereby causing charge pump  104  to increase its net output current I until VCO  108  achieves an oscillation frequency at which signal Clk is frequency-locked and phase-locked with signal REF. If, on the other hand, signal REF lags signal Clk in phase—indicating that the VCO is running relatively fast—the duration of pulse signal DN increases—thereby causing VCO  108  achieve an oscillation frequency at which signal Clk is frequency-locked and phase-locked with signal REF. Signal Clk is considered to be locked to signal REF if its frequency is within a predetermined frequency range of signal REF and the phase of signals CLK and REF are aligned. Signal Clk is considered to be out-of-lock with signal REF if its frequency is outside the predetermined frequency range of signal REF. 
         [0005]    When the input reference clock to a PLL changes phase, the PLL must slew to the new phase. Such a condition may happen when, for example, the PLL switches from one reference clock to another clock with the same frequency but a different phase. Such a condition may also happen if the clock that the PLL switches to has a different frequency than the clock the PLL switches from. Furthermore, in some applications it is desirable to have the PLL output clock switch slowly, and not rapidly, to the new phase so as to enable other down-stream circuits to maintain proper operation. 
         [0006]    When the input clock to a PLL misses a pulse or becomes inactive, the output of the Phase-Frequency detector  102  gets stuck in the down state until such time as the input clock becomes active again. Referring to  FIGS. 1 and 2  concurrently, a clock signal, such as REF ideal , applied to a PLL ideally should not have missing pulses. However, in practical applications, a clock signal such as REF actual , actually received by a PLL includes missing pulses. The phase of the feedback signal CLK generated in response to clock signal REF actual  begins to vary as a result of the missing pulses. These phase shifts Δφ 1  and Δφ 2  are shown in  FIG. 2  relative to the ideal clock signal REF ideal . 
         [0007]    When signal DN remains in a high state as a result of the missing pulses, the charge pump disposed in the PLL starts to remove charge from the loop filter. This causes signal Vcntrl generated by charge pump  104  to droop, in turn causing the VCO output phase to move away from its ideal value. 
         [0008]    In accordance with the technique described in U.S. Pat. No. 6,393,596, missing pulses are detected by applying the reference clock to a filter and applying the filter&#39;s output to a comparator. Missing pulses cause the output voltage of the filter to shift. When the output voltage of the filter exceeds a threshold value, the comparator trips to indicate the detection of missing pulses. One drawback of this technique is that the filter reduced the sensitivity of the detection circuit, rendering it slow to respond. Accordingly, a number of missing pulses may be required before the detection. 
         [0009]    In accordance with the technique described in U.S. Pat. No. 6,590,949, the reference clock signal is digitally compared against the feedback clock. However, detection is made only after a number of transitions of the reference clock signal have been missing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    A phase/frequency locked loop (PLL) includes a circuit adapted to detect missing pulses of a reference clock applied to the PLL. The circuit includes, in part, first and second flip-flops, as well as a one-shot block. The first flip-flop has a data input terminal responsive to a voltage supply, and a clock terminal responsive to an inverse of a feedback clock. The second flip-flop has a data input terminal responsive to an output of the first flip-flop, and a clock terminal responsive to the inverse of the feedback clock. The one-shot block generates a pulse in response to a rising edge of the reference clock that is used to generate the feedback clock. The one-shot block generates an output signal applied to a reset terminal of the first flip-flop. 
         [0011]    The circuit further includes, in part, third and fourth flip-flops. The third flip-flop has a data input terminal responsive to the voltage supply, and a clock terminal responsive to the feedback clock. The fourth flip-flop has a data input terminal responsive to an output of the third flip-flop and a clock terminal responsive to the feedback clock. The reset terminal of the third flip-flop receives the output signal of the one-shot block. 
         [0012]    The circuit further includes, in part, fifth and sixth flip-flops as well as a second one-shot block. The fifth flip-flop has a data input terminal responsive to the voltage supply, and a clock terminal responsive to the inverse of feedback clock. The sixth flip-flop has a data input terminal responsive to an output of the fifth flip-flop, and a clock terminal responsive to the inverse of the feedback clock. The second one-shot block generates a pulse in response to a falling edge of the reference clock. The second one-shot block generates an output signal applied to a reset terminal of the fifth flip-flop. 
         [0013]    The circuit further includes, in part, seventh and eight flip-flops. The seventh flip-flop has a data input terminal responsive to the voltage supply, and a clock terminal responsive to the feedback clock. The eight flip-flop has a data input terminal responsive to an output of the seventh flip-flop and a clock terminal responsive to the feedback clock. The reset terminal of the seventh flip-flop receives the output signal of the second one-shot block. 
         [0014]    The circuit further includes, in part, first, second and third logic gates. The first logic gate performs a NOR operation on output signals of the second and fourth flip-flops. The second logic gate performs a NOR operation on output signals of the sixth and eight flip-flops. The third logic gate performs a NAND operation on output signals of the first and second logic gates. A cross-coupled NOR logic also disposed in the circuit is responsive to an output of the third logic gate. The reset terminals of the second, fourth, sixth and eight flip-flops are responsive to a reset signal to which said cross-coupled NOR logic is also responsive. 
         [0015]    The PLL embodying the circuit further includes, in part, a phase/frequency detector responsive to a phase/frequency of each said feedback and the reference clock, a first pulse-width limiter adapted to generate a second pulse in response to a first output of the phase/frequency detector, a second pulse-width limiter adapted to generate a third pulse in response to a second output of the phase/frequency detector, a third pulse-width limiter adapted to generate a fourth pulse in response to an alarm signal; and a fourth logic gate performing an OR operation on said second and fourth pulses. The PLL further includes a charge pump responsive to the second pulse and the fourth logic gate; and an oscillator adapted to generate the feedback clock in response to the charge pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram of a PLL, as known in the prior art. 
           [0017]      FIG. 2  is a timing diagram of a number of signals associated with the PLL of  FIG. 1 . 
           [0018]      FIG. 3  is a schematic diagram of a missing pulse detection circuit, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 4  is a timing diagram of a number of signals associated with circuit of  FIG. 3  when the reference clock signal is initially in sync with the feedback clock signal but is subsequently stuck in a high level. 
           [0020]      FIG. 5  is a timing diagram of a number of signals associated with circuit of  FIG. 3  when the reference clock signal is initially in sync with the feedback clock signal but is subsequently stuck in a low level. 
           [0021]      FIG. 6  is a timing diagram of a number of signals associated with circuit of  FIG. 3  when the reference clock signal is initially 180 degrees out of phase with respect to the feedback clock signal but is subsequently stuck in a low level. 
           [0022]      FIG. 7  is a timing diagram of a number of signals associated with circuit of  FIG. 3  when the reference clock signal is initially  180  degrees out of phase with respect to the feedback clock signal but is subsequently stuck in a high level. 
           [0023]      FIG. 8  is a block diagram of a PLL in which the circuit of  FIG. 3  is disposed, in accordance with one embodiment of the present invention. 
           [0024]      FIG. 9  is a timing diagram of a number of signals associated with the PLL of  FIG. 8 , in accordance with one embodiment of the present invention. 
           [0025]      FIG. 10  is a block diagram of a pulse-width limiter disposed in the PLL of  FIG. 8 , in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 3  is a schematic diagram of a missing pulse detection circuit  150 , in accordance with one embodiment of the present invention. When a missing pulse is detected by circuit  150 , the output signal Alarm of NOR gate  44  is set. Output signal ALARM is reset when signal ALARM_RESET applied to OR gate  12  is asserted. 
         [0027]    One-shot block  10  generates a pulse on each rising edge of the reference clock signal REF and applies this pulse to OR gate  12 . Likewise, one-shot block  14  generates a pulse on each falling edge of the reference clock signal REF and applies this pulse to OR gate  16 . The output signal RISING of OR gate  12  is applied to the clear input terminals CLR of flip-flops  22  and  26 . The output signal FALLING of OR gate  16  is applied to the clear input terminals CLR of flip-flops  30  and  34 . Signal ALARM_RESET is applied to the CLR input terminals of flip-flops  24 ,  28 ,  32  and  36 . 
         [0028]    The input clock terminals of flip-flops  22  and  24  receive clock signal FB 1  that is the inverse of feedback clock FEEDBACK_CLOCK supplied by the VCO disposed in a PLL embodying circuit  150 . The input clock terminals of flip-flops  30  and  32  receive clock signal FB 2  that is the inverse of clock FEEDBACK_CLOCK. Signal FEEDBACK_CLOCK is applied to the clock input terminals of flip-flops  26 ,  28 ,  34  and  36 . The output data of flip-flop  22  is applied to the input data of flip-flop  24 ; the output data of flip-flop  26  is applied to the input data of flip-flop  28 ; the output data of flip-flop  30  is applied to the input data of flip-flop  32 ; the output data of flip-flop  34  is applied to the input data of flip-flop  36 . The output data of flip-flops  24  and  28  are applied to NOR gate  38 . The output data of flip-flops  32  and  36  are applied to NOR gate  40 . The outputs of NOR gates  38  and  40  are applied to NAND gate  42  whose output is applied to cross-coupled NOR gates  44 ,  46 . NOR gate  44  generates output signal ALARM, as described above. Flip-flops  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34  and  36  are reset when their respective reset signal CLR is asserted. 
         [0029]      FIG. 4  is a timing diagram of a number of signals associated with circuit  150  when the reference clock REF is initially in sync with the feedback clock FEEDBACK_CLOCK but is subsequently stuck in a low level. Referring concurrently to  FIGS. 3 and 4 , with each rising edge of the reference clock signal REF, a RISING pulse is generated. Similarly, with each falling edge of the reference clock signal REF, a FALLING pulse is generated. Because the feedback clock signal generated by the VCO is always present, on each falling edge of signal FEEDBACK_CLOCK, a logic high is transferred from the input data terminal of flip-flop  22  to its output terminal. In other words, with each falling edge of signal FEEDBACK_CLOCK, signal V 1 A goes high. Because signal RISING is applied to the CLR terminal of flip-flop  22 , with each rising edge of signal RISING, V 1 A is reset to zero. For example, in response to falling edge  402  of signal FEEDBACK_CLOCK, signal V 1 A goes high  404 , and in response to rising edge  406  of signal RISING, signal V 1 A goes low  408 . In response to edge  410  of signal FEEDBACK_CLOCK, signal V 1 A goes high  412 . On the next falling edge  414  of signal FEEDBACK_CLOCK, the high level of signal V 1 A causes signal V 1 B to go high  416 , which in turn causes NOR gate  38 , NAND gate  42  and NOR gates  44  and  46  to set signal ALARM to a high level  450  to indicate detection of the missing pulse on the reference clock signal CLK. In response to the next rising edge  418  of the reference clock signal REF, signal RISING goes high  420 , thereby causing signal V 1 A to go low  422 . On the next falling edge  424  of signal FEEDBACK_CLOCK, the low level of signal V 1 A causes signal V 1 B to go low  426 . 
         [0030]      FIG. 5  is a timing diagram of a number of signals associated with circuit  150  when the reference clock REF is initially in sync with the feedback clock FEEDBACK_CLOCK but is subsequently stuck in a high level. Referring concurrently to  FIGS. 3 and 5 , with each rising edge of the reference clock signal REF, a RISING pulse is generated. Similarly, with each falling edge of the reference clock signal REF, a FALLING pulse is generated. Because the feedback clock signal generated by the VCO is always present, on each falling edge of signal FEEDBACK_CLOCK, a logic high is transferred from the input data terminal of flip-flop  34  to its output terminal. In other words, with each falling edge of signal FEEDBACK_CLOCK, signal V 4 A goes high. Because signal FALLING is applied to the CLR terminal of flip-flop  34 , with each rising edge of signal FALLING, signal V 4 A is reset to zero. For example, in response to rising edge  502  of signal FEEDBACK_CLOCK, signal V 4 A goes high  504 , and in response to rising edge  506  of signal FALLING, signal V 4 A goes low  508 . In response to edge  510  of signal FEEDBACK_CLOCK, signal V 4 A goes high  512 . On the next rising edge  514  of signal FEEDBACK_CLOCK, the high level of signal V 4 A causes signal V 4 B to go high  516 , which in turn causes NOR gate  40 , NAND gate  42  and NOR gates  44  and  46  to set signal ALARM to a high level  550  to indicate detection of the missing pulse on the reference clock signal CLK. In response to the next falling edge  518  of the reference clock signal REF, signal FALLING goes high  520 , thereby causing signal V 4 A to go low  522 . On the next rising edge  524  of signal FEEDBACK_CLOCK, the low level of signal V 4 A causes signal V 4 B to go low  526 . 
         [0031]      FIG. 6  is a timing diagram of a number of signals associated with circuit  150  when the reference clock REF is initially 180 degrees out of phase with respect to the feedback clock FEEDBACK_CLOCK but is subsequently stuck in a low level. Referring concurrently to  FIGS. 3 and 6 , with each rising edge of the reference clock signal REF, a RISING pulse is generated. Similarly, with each falling edge of the reference clock signal REF, a FALLING pulse is generated. Because the feedback clock signal generated by the VCO is always present, on each rising edge of signal FEEDBACK_CLOCK, a logic high is transferred from the input data terminal of flip-flop  26  to its output terminal. In other words, with each rising edge of signal FEEDBACK_CLOCK, signal V 2 A goes high. Because signal RISING is applied to the CLR terminal of flip-flop  26 , with each rising edge of signal RISING, signal V 2 A is reset to zero. For example, in response to rising edge  602  of signal FEEDBACK_CLOCK, signal V 2 A goes high  604 , and in response to rising edge  606  of signal RISING, signal V 2 A goes low  608 . In response to edge  610  of signal FEEDBACK_CLOCK, signal V 2 A goes high  612 . On the next rising edge  614  of signal FEEDBACK_CLOCK, the high level of signal V 2 A causes signal V 2 B to go high  616 , which in turn causes NOR gate  38 , NAND gate  42  and NOR gates  44  and  46  to set signal ALARM to a high level  650  to indicate detection of the missing pulse on the reference clock signal CLK. In response to the next rising edge  618  of the reference clock signal REF, signal RISING goes high  620 , thereby causing signal V 2 A to go low  622 . On the next rising edge  624  of signal FEEDBACK_CLOCK, the low level of signal V 2 A causes signal V 2 B to go low  626 . 
         [0032]      FIG. 7  is a timing diagram of a number of signals associated with circuit  150  when the reference clock REF is initially 180 degrees out of phase with respect to the feedback clock FEEDBACK_CLOCK but is subsequently stuck in a high level. Referring concurrently to  FIGS. 3 and 7 , with each rising edge of the reference clock signal REF, a RISING pulse is generated. Similarly, with each falling edge of the reference clock signal REF, a FALLING pulse is generated. Because the feedback clock signal generated by the VCO is always present, on each falling edge of signal FEEDBACK_CLOCK, a logic high is transferred from the input data terminal of flip-flop  30  to its output terminal. In other words, with each falling edge of signal FEEDBACK_CLOCK, signal V 3 A goes high. Because signal FALLING is applied to the CLR terminal of flip-flop  30 , with each rising edge of signal FALLING, signal V 3 A is reset to zero. For example, in response to falling edge  702  of signal FEEDBACK_CLOCK, signal V 3 A goes high  704 , and in response to rising edge  706  of signal FALLING, signal V 3 A goes low  708 . In response to edge  710  of signal FEEDBACK_CLOCK, signal V 3 A goes high  712 . On the next falling edge  714  of signal FEEDBACK_CLOCK, the high level of signal V 3 A causes signal V 3 B to go high  716 , which in turn causes NOR gate  40 , NAND gate  42  and NOR gates  44  and  46  to set signal ALARM to a high level  750  to indicate detection of the missing pulse on the reference clock signal CLK. In response to the next falling edge  718  of the reference clock signal REF, signal FALLING goes high  720 , thereby causing signal V 3 A to go low  722 . On the next falling edge  724  of signal FEEDBACK_CLOCK, the low level of signal V 3 A causes signal V 3 B to go low  726 . Referring to  FIGS. 4-7 , it is seen that signal ALARM is set half a clock cycle after a missing pulse occurs on signal REF. 
         [0033]      FIG. 8  is a schematic diagram of a PLL  200 , in accordance with one embodiment of the present invention. PLL  200  is shown as including a phase/frequency detector  202 , missing pulse detection circuit  150  (see  FIG. 3 ), pulse-width limiters  204 ,  206 ,  208  and OR gate  210 . Pulse-width limiter  206  limits the width of signal DN received from phase/frequency detector  202  to generate signal DN_X. Pulse-width limiter  204  limits the width of signal UP_L received from phase/frequency detector  202  to generate signal UP_L. Pulse-width limiter  208  limits the width of signal ALARM received from circuit  150  shown in  FIG. 3  to generate signal ALARM_L. 
         [0034]      FIG. 9  is a timing diagram of a number of signal associated with PLL  200 . Reference clock signal REF is shown as having missing pulses. Because transitions  900  and  902  of signals FEEDBACK_CLOCK and REF are aligned, phase/frequency detector  202  generates both UP and DN pulses  904 ,  906 . Accordingly, pulses  908  and  910  also appear on signals UP_X and DN_X. Similarly, because transitions  930  and  932  of signals CLK and REF are aligned, phase/frequency detector  102  generates both UP and DN pulses  934 ,  936 , in response to which, pulses  938  and  940  appear on signals UP_X and DN_X. 
         [0035]    Since there is no transition on signal REF during the next two cycles of signal CLK, signal DN goes high  952  in response to transition  950  of signal CLK. Accordingly, pulse width limiter  206  generates pulse  954  on signal DN_X. Because reference clock signal REF was in sync with feedback clock signal FEEDBACK_CLOCK before being stuck at a low level, on the next falling edge  952  of signal FEEDBACK_CLOCK, pulse-width detection circuit  150  causes signal ALARM to go high  956 , as was described in detail above with references to  FIGS. 3 and 4 . In response, pulse width limiter  208  generates a pulse  956  on signal ALARM_L, which in turn causes a pulse  958  to appear on signal UP_X. Pulse  958  of signal UP_X reduces the phase bump generated as a result of pulse  954  on signal DN_X. 
         [0036]    The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the type of pulse-width limiting, slew detection, etc. The invention is not limited by the number of current sources or current sinks. The invention is not limited by the type of integrated circuit in which the present disclosure may be disposed. Nor is the disclosure limited to any specific type of process technology, e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture the present disclosure. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.