Abstract:
A circuit comprising a first circuit and a state machine. The first circuit may be configured to generate a plurality of state inputs in response to (i) a first clock signal, (ii) a second clock signal delayed from the first clock signal, and (iii) a data signal. The state machine may be configured to generate a pump up signal and a pump down signal in response to (i) said data signal and (ii) a plurality of quadrants defined by a number of possible combinations of the state inputs. The state machine may be further configured to transition between any of the quadrants.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application may relate to co-pending application Ser. No. 09/171,914, Ser. No. 09/470,668 and Ser. No. 09/471,576, filed concurrently and each hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an architecture and/or method for implementing a frequency detector generally and, more particularly, to a method and/or architecture for implementing a data frequency detector. 
     BACKGROUND OF THE INVENTION 
     Frequency detectors are used in analog phase-locked loops for data recovery, clock recovery and frequency synthesis applications. One conventional approach for implementing a frequency detector may be found in an ISSCC99 article entitled “A 1 Gb/s CMOS Clock and Data Recovery Circuit” by Hui Wang, Richard Nottenburg which is hereby incorporated by reference in its entirety. 
     FIG. 1 illustrates an example of a conventional frequency detector  10 . The frequency detector  10  has an input  11  that receives a signal DATA, an input  12  that receives a clock signal CLK and an input  13  that receives a signal QCLK. The frequency detector  10  has an output  14  that presents a signal UP and an output  15  that presents a signal DN. FIG. 2 illustrates a timing diagram of the conventional frequency detector of FIG. 1 illustrating a signal DATA, a signal CLK and a signal QCLK. A number of quadrants I, II, III and IV are defined between a number of vertical lines  16   a-   16   e . For example, quadrant I is defined as the time between the vertical line  16   b  and  16   c.  Quadrants II, III and IV are similarly defined. A lock window is defined as a time between the vertical line  16 c and the vertical line  16   e.    
     The quadrants are defined such that, during a particular quadrant, the signal CLK is generally at either a logic high or a logic low and the signal QCLK is at either a logic high or a logic low. Therefore, the vertical lines  16   a-   16   e  generally occur at the transitions of the signals CLK and QCLK. 
     In general, the signal QCLK is 90 degrees out of phase with the signal CLK. The four quadrants generally represent the various combinations of a digital high (e.g., a “1”) and a digital low (e.g., a “0”) of the signal CLK and the signal QCLK. For example, in the quadrant I, the signal CLK is low and the signal QCLK is high. In the quadrant II, the signal CLK is low and the signal QCLK is low. In the quadrant III, the signal QCLK is low and the signal CLK is high. In the quadrant IV, the signal QCLK is high and the signal CLK is high. The particular polarities of the high and low signals can be inverted. However, with the two signals CLK and QCLK operating at 90 degrees out of phase, only four possible combinations can be implemented. The four illustrated quadrants represent the four various combinations of the signal CLK and the signal QCLK. The signal DATA is shown having a transition  18  and a transition  20 . The transition  18  generally occurs in one of the four quadrants (shown in the quadrant III in FIG. 2, where the signal CLK is high and the signal QCLK is low). When the data transition  18  occurs in the quadrant I or the quadrant IV, a lock condition may be present. 
     Referring to FIG. 3, a state machine  20  is shown implementing the various transitions of the timing diagram of FIG.  1 . The state machine  20  comprises a “reset” state  22 , an “up” state  24  and a “down” state  26 . The state machine  20  transitions between the reset state  22  and the down state  26  during (i) a transition between quadrant II and quadrant III or (ii) a transition between quadrant I and quadrant II. The reset state  22  transitions to the up state  24  during (i) the transition between quadrant IV and quadrant III or (ii) a transition between quadrant III and quadrant II. The states  24  and  26  transition back to the reset state  22  during (i) a transition between quadrant I and III (or vice versa), (ii) a transition between quadrant II and IV (or vice versa), (iii) quadrant I or (iv) quadrant IV. The state machine  20  requires the state  24  and the state  26  to transition back to the reset state  22  after each transition. 
     The state machine  20  transitions to a next state  24  or  26  in response to present transitions between quadrants. The state machine  20  does not use information available in the form of a current state during a transition between two quadrants. Furthermore, the state machine  20  does not check transitions between quadrants II and III due to jitter. The state machine  20  can only transition between the states  22  and  24  or the states  22  and  26 . 
     Conventional frequency detectors implemented with the state machine  20  have a number of drawbacks including (i) leaving unused states, (ii) having next state logic that only depends on a present transition, (iii) a failure to use information available in the form of a current state and/or (iv) a failure to check transitions between particular quadrants (e.g., II and III) due to jitter. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a circuit comprising a first circuit and a state machine. The first circuit may be configured to generate a plurality of state inputs in response to (i) a first clock signal, (ii) a second clock signal delayed from the first clock signal, and (iii) a data signal. The state machine may be configured to generate a pump up signal and a pump down signal in response to (i) said data signal and (ii) a plurality of quadrants defined by a number of possible combinations of the state inputs. The state machine may be further configured to transition between any of the quadrants. 
     The objects, features and advantages of the present invention include providing a method and/or architecture to implement a frequency detector that may (i) improve the gain of the frequency detection, (ii) decrease PLL lock time and/or (iii) increase jitter tolerance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a block diagram of a conventional frequency detector; 
     FIG. 2 is a timing diagram of a data and clock signal; 
     FIG. 3 is a conventional state machine illustrating the operation of the circuit of FIG. 1; 
     FIG. 4 is a state diagram illustrating the operation of a preferred embodiment of the present invention; 
     FIG. 5 is an exemplary implementation of the state machine of FIG. 4; and 
     FIG. 6 is an exemplary logic circuit implementing the state machine of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 4, a state diagram (or state machine)  100  is shown in accordance with a preferred embodiment of the present invention. The state diagram  100  generally comprises a “reset” state  102 , an “up” state  104  and a “down” state  106 . The state diagram  100  may be similar to the state diagram  20  of FIG.  3 . However, the state diagram  100 , may have alternate transitions compared to the state machine  20 . For example, the state machine  100  may remain in the state  104  during a transition between the quadrants II and I or the quadrants III and II. Additionally, the state machine may remain in the state  106  during a transition between the states III and IV or II and III. Additionally, the state machine  100  may respond to a previous state as well as a current transition. For example, a transition between the quadrant III to the quadrant II may (i) cause a transition to the up state  104  if the state machine is in the reset state  102 , (ii) remain in the up state  104  if the state machine  100  is in the up state  104  or (iii) cause a transition to the reset state  102  if the state machine  100  is in the down state  106 . Other transitions have similar multiple responses. 
     The state machine  100  may exit the reset state  102  and enter the down state  106  during (i) a transition between quadrant II and quadrant III or (ii) a transition between quadrant I and quadrant II. The state machine  100  may remain in the down state  106  during (i) a transition between quadrant III and quadrant IV or (ii) a transition between quadrant II and quadrant III. The state machine  100  may exit the down state  106  and enter the reset state  102  during (i) a transition between quadrant II and quadrant I or (ii) a transition between quadrant III and quadrant II. 
     The state machine  100  may exit the reset state  102  and enter the up state  104  during (i) a transition between quadrant IV and quadrant III or (ii) a transition between quadrant III and quadrant II. The state machine  100  generally remains in the up state  104  during (i) a transition between quadrant II and quadrant I or (ii) a transition between quadrant III and quadrant II. The state machine  100  may exit the up state  104  and enter the reset state  102  during (i) a transition between quadrant III and quadrant IV or (ii) a transition between quadrant II and quadrant III. 
     The state machine generally exits either the state  104  or the state  106  and enters the reset state  102  during (i) a transition between quadrants I and III, (ii) a transition between the quadrants II and IV, and/or (iii) a transition between the quadrants I and IV. As illustrated, the next state of the state machine  100  generally depends upon the previous state for each transition between two consecutive quadrants. The state machine  100  may generate every valid up and down transition signal. The state machine  100  may be used to improve lock time when implemented in a frequency detector. A transition of the signal DATA may (i) sample the signal CLK and QCLK (ii) respond with the appropriate action (e.g., either a transition to a new state or remain in the current state). The state machine  100  may provide an improvement in jitter tolerance by allowing transitions between quadrants II and III, which is not present in the state machine  20 . 
     The present invention may be used in clock and data recovery PLL designs that may operate at Gigabit/sec (and higher) data rates. The present invention may enable (i) the implementation of a clock recovery PLL with no reference clock and (ii) an improved lock time. 
     Referring to FIG. 5, an exemplary implementation of the state machine  100  is shown in the context of a circuit  140 . The circuit  140  may generate a number of state inputs (e.g., I, Ipre, Q and Qpre) presented to the state machine  100 . The generation of the state inputs will be described in more detail in connection with FIG.  6 . The circuit  140  may comprise a number of flip-flops  142   a-   142   n.  The flip-flops may be implemented in one example, as double-edge triggered flip-flops. The input signal DATA may be presented to the clock input of each of the flip-flops  142   a-   142   n.  The clock signal CLK may be presented to the D input of the flip-flop  142   a.  The flip-flop  142   a  may present the signal I to the D input of the flip-flop  142   b.  The flip-flop  142   b  may present the signal Ipre. The flip-flop  142   c  may receive the clock signal QCLK at a D input. The flip-flop  142   c  may present the signal Q to a D input of the flip-flop  142   n.  The flip-flop  142   n  may present the signal Qpre. 
     In general, the circuit  140  presents the signal I, the signal Ipre, the signal Q and the signal Qpre to the state machine  100  in response to the signal CLK, the signal QCLK and the signal DATA. The signal QCLK may be, in one example, delayed 90 degrees with respect to the signal CLK. Additionally, the state machine  100  may receive the signal DATA. The state machine  100  generally presents a signal (e.g., UP) and a signal (e.g., DN) in response to the state inputs I, Ipre, Q, Qpre and DATA. 
     Referring to FIG. 6, an exemplary logic circuit implementing the state machine  100  is shown. The state machine  100  generally comprises a number of multiplexers  250   a-   250   n,  a number of gates  252   a-   252   n,  a number of gates  254   a-   254   n  and a number of flip-flops  256   a-   256   n.  In one example, the gates  252   a-   242   n  may be exclusive OR gates. However, other gates may be implemented accordingly to meet the design criteria of a particular implementation. The gates  252   a ,  252   b ,  252   c  and  252   f  generally receive the signal I and the signal Q. The multiplexers  250   a  and  250   c  generally receive inputs from one or more of the gates  252   a-   252   n  and may have a select input that may receive the signal Ipre. The multiplexers  250   b  and  250   d  generally receive inputs from one or more of the gates  252   a-   252   n  and may receive the signal Qpre as the select input. 
     The gate  252   i  may present a signal (e.g., SEL). The multiplexer  250   c  generally presents the signal UP. The multiplexer  250   c  generally receives an output (e.g., U 7 ) from the multiplexer  250   b  and a latched feedback of the signal UP (e.g., UPPRE). The flip-flop  256   a  generally latches the signal UP in response to the signal DATA as the clock. Similarly, the multiplexer  250   n  may present the signal DN in response to a signal from the multiplexer  250   d  (e.g., U 12 ) and a latched feedback of the signal DN (e.g., DNPRE). The flip-flop  256   n  may latch the signal DN in response to the signal DATA. The multiplexers  250   c  and  250   n  generally respond to a signal SEL at the select input. The flip-flops  256   a-   256   n  may be implemented, in one example, as double edge triggered flip-flops. 
     The signals UP and DN may be used as pump up and/or pump down signals that may be presented to a voltage controlled oscillator of a phase-locked loop circuit. By providing more flexibility between transitions, the present invention may improve the performance of such a phase-locked loop by implementing a frequency detector with a low lock time, increased jitter tolerance and a decreased lock time, when compared with conventional approaches. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.