PATENT ABSTRACT
The present invention provides a method and an apparatus for generating a phase error signal from a reference signal and a feedback signal using a modified reset generation mechanism. An input circuit receives a reference signal and a feedback signal. A phase error detector circuit generates a phase error signal based on the reference signal and feedback signal. The input circuit is reset and, after a delay, the phase error detector circuit is reset. The delay is selected so that there is no jitter associated with the dead zone.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to a phase-lock loop (PLL) circuit, and more particularly to reducing jitter in a PLL operating at high frequencies. 
     2. Description of the Related Art 
     A phase-lock loop (PLL) is typically used to synchronize (‘lock’) an internal voltage-controlled oscillator (VCO) to an external reference signal. A PLL thus keeps a circuit operating at a specific frequency, and is used in a wide variety of electronic circuits for this purpose. 
     One of the key components of a PLL is a phase-frequency detector (PFD) circuit, which compares the VCO signal to the reference signal and generates a phase error signal that is a measure of their phase difference. The VCO generates a periodic signal with a frequency that is controlled by the phase error signal. The VCO output is coupled to the feedback input of the PFD, thereby forming a feedback loop. If the frequency of the feedback signal is not equal to the frequency of the reference signal, the phase error signal causes the VCO frequency to shift toward the frequency of the reference signal, until the VCO finally locks onto the frequency of the reference. 
     For very small phase differences, for example when the PLL is in a steady-state condition, the dead zone is the region in which the phase error signal is insensitive to phase-difference changes. Thus one problem with a PFD is that jitter is introduced into the loop due to the dead zone. Most approaches to minimizing the dead zone are particularly complicated, and do not allow the PFD to operate at high frequencies with zero dead zone. 
     Therefore, there is a need for a phase/frequency detector that operates in high frequency circuits with zero dead zone. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and an apparatus for generating a phase error signal from a reference signal and a feedback signal using a modified reset generation mechanism. An input circuit receives a reference signal and a feedback signal. A phase error detector circuit generates a phase error signal based on the reference signal and feedback signal. The input circuit is reset and, after a delay, the phase error detector circuit is reset. The delay is selected so that there is no jitter associated with the dead zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a high frequency phase/frequency detector; 
     FIG. 2 is a block diagram of a dynamic AND circuit; 
     FIG. 3 is a block diagram of a latch circuit; 
     FIG. 4 is a block diagram of a pulse shaping circuit; 
     FIGS. 5A and 5B are timing diagrams showing the status of various inputs and outputs when signal A arrives before signal B; and 
     FIGS. 6A and 6B are timing diagrams showing the status of various inputs and outputs when signal B arrives before signal A. 
    
    
     DETAILED DESCRIPTION 
     In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning network communications, electromagnetic signaling techniques, and the like, have been omitted inasmuch as such details are not considered to be within the understanding of persons of ordinary skill in the relevant art. 
     In the remainder of this description, a processing unit (PU) may be a sole processor of computations in a device. In such a situation, the PU is typically referred to as am MPU (main processing unit). The processing unit may also be one of many processing units that share the computational load according to some methodology or algorithm developed for a given computational device. For the remainder of this description, all references to processors shall use the term MPU whether the MPU is the sole computational element in the device or whether the MPU is sharing the computational element with other MPUs. 
     It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combination thereof. In a preferred embodiment, however, the functions are performed by a processor such as a computer or an electronic data processor in accordance with code such as computer program code, software, and/or integrated circuits that are coded to perform such functions, unless indicated otherwise. 
     Referring to FIG. 1 of the drawings, the reference numeral  100  generally designates a phase/frequency detector (PFD). The circuit  100  comprises two inputs  102  and  104  for receiving a reference signal and a feedback signal, respectively, two latches  106  and  108 , two NOR gates  110  and  112 , two dynamic AND circuits  114  and  116 , a reset circuit  118  and a pulse shaping circuit  120  with two outputs. 
     Latch circuit  106  is shown to receive a reference signal from input  102  and a first reset signal (bothb) and to generate a latched reference signal (disa). Latch circuit  108  is shown to receive a feedback signal from input  104  and the first reset signal (bothb) and to generate a latched feedback signal (disb). When there is no input present, the output (disa) for latch circuit  106  is low. When the reference signal input to latch circuit  106  goes high, the latched reference signal output goes high and remains high until latch circuit  106  receives a first reset signal. Latch circuit  108  behaves similarly. 
     NOR circuit  110  is coupled to the first latch circuit  106  for receiving the latched reference signal (disa) and a derived reference signal (qouta) and for generating a first NOR signal. NOR circuit  112  is coupled to the second latch circuit  108  for receiving the latched feedback signal (disb) and a derived feedback signal (qoutb) and for generating a second NOR signal. When both inputs to NOR circuit  110  are low, the output of NOR circuit  110  is high. As is well known in the art, if either or both inputs of a NOR circuit are high, the output for the NOR circuit is low, and only when both inputs are low is the NOR circuit output high. Both NOR circuits  110  and  112  behave this way. 
     Dynamic AND circuit  114  receives the reference signal from input  102  and is coupled to the first NOR circuit  110  for receiving the first NOR signal. Dynamic AND circuit  114  is also configured to receive a second reset signal and to generate the derived reference signal. The second reset signal is a delayed signal of the first reset signal. Dynamic AND circuit  116  is shown to receive the feedback signal from input  104  and is coupled to NOR circuit  112  for receiving the second NOR signal. Dynamic AND circuit  116  is also configured to receive the second reset signal and to generate the derived feedback signal. As is well known in the art, the output of an AND circuit is high only when all inputs to the AND circuit are high, and the output is low when one or more of the inputs is low. Both dynamic AND circuits  114  and  116  behave this way. 
     Reset circuit  118  is coupled to first and second dynamic AND circuits  114  and  116  for receiving the derived reference signal and derived feedback signal, respectively, and coupled to the first and second latch circuits  106  and  108  for generating the first reset signal. The reset circuit  118  is also coupled to first and second dynamic AND circuits  114  and  116  for providing the second reset signal. When the derived reference signal and the derived feedback signal both go high, the first reset signal is output, and after a delay tau, the second reset signal is output. The delay tau is proportional to the period of the reference signal and may be varied between 5% to 25% of the period of the reference signal. For simplicity, the delay tau may be fixed at 10% of the period of the reference signal, for example 25 picoseconds for a 4 GHz clock. 
     Pulse shaping circuit  120  is coupled to dynamic AND circuits  114  and  116  for receiving the derived reference signal and derived feedback signal, respectively, and configured for generating first and second output pulses, wherein, if the reference signal arrives before the feedback signal, the first output pulse UP has a duration which is proportional to a time delay between the reference signal and the feedback signal, and if the feedback signal arrives before the reference signal, the second output pulse DN has a duration which is proportional to a time delay between the reference signal and the feedback signal. Pulse shaping circuit  120  has a dead zone associated with very small phase differences between the reference signal and the feedback signal and a means for reducing the dead zone by changing the durations of the first and second output pulses. The pulse shaping circuit used in the present invention is well known in the art. 
     Initially, the PFD is waiting for input. If the reference signal arrives first, the PFD circuit sets itself, using the derived reference signal (qouta), NOR circuit  110 , and the first NOR signal (ena), to ignore input  102 , and waits for the feedback signal to arrive. Similarly, if the feedback signal arrives first, the PFD circuit sets itself, using the derived feedback signal (qoutb), NOR circuit  112  and the second NOR signal (enb), to ignore input  104 , and waits for the reference signal to arrive. Once both the reference signal and the feedback signal have arrived, the phase error between the two signals is detected. 
     Once the phase error is detected, two events occur at substantially the same time. First, the phase error is sent to pulse shaping circuit  120 . Second, reset circuit  118  issues a first reset signal to latches  106  and  108  and, after a delay, a second reset signal to dynamic ANDs  114  and  116 . The two reset signals cause the PFD to reset to its initial stage so that the cycle can begin again. 
     Now referring to FIG. 2, the reference numeral  200  generally designates a dynamic AND circuit comprising inputs  202 ,  204 ,  206  and  208 , combinatorial logic  210 , memory  212 , error output  214  and output  216 . The combinatorial logic circuit  210  is shown to receive the second reset signal at input  202 , the reference signal at input  204 , the first NOR signal at input  206 , and the derived reference signal  208 , and is configured for generating a clogic signal if successful in logically combining inputs  202 ,  204 ,  206 , and  208 , and a clogic error signal if unsuccessful. The memory circuit  212  is coupled to the first combinatorial logic circuit for receiving the clogic signal and generating the derived reference signal. Error output  214  is coupled to combinatorial logic circuit  210  for receiving a clogic error signal. 
     Now referring to FIG. 3, the reference numeral  300  generally designates a latch circuit comprising NAND circuits  302  and  304 , inputs  306  and  308  and outputs  310  and  312 . NAND circuit  302  is configured to receive the first reset signal at input  308  and a first latch enable signal, and to generate a first latch disable signal, which is coupled to output  312 . NAND circuit  304  is coupled to the first NAND circuit for receiving the first latch disable signal and coupled to input  306  for receiving an input signal and generating a first latch enable signal, which is coupled to output  310  and coupled to the input of NAND  302 . Input  306  is typically coupled to either a reference signal or a feedback signal. 
     Now referring to FIG. 4, the reference numeral  400  generally designates a pulse shaping circuit comprising two inputs for receiving a derived reference signal and a derived feedback signal, NOT circuits  402 ,  404 ,  406 , and  408 , null delay circuits  410  and  412 , NAND circuits  414  and  416 , and two outputs UP and DN. If the reference signal arrives before the feedback signal, the pulse shaping circuit generates a pulse at output UP with pulse width proportional to the delay between the reference signal and feedback signal. If the feedback signal arrives before the reference signal, the pulse shaping circuit generates a pulse at output DN with pulse width proportional to the delay between the reference signal and feedback signal. 
     In the pulse shaping circuit, NOT circuit  402  is coupled to dynamic AND circuit  116  for receiving the derived feedback signal and generating a first NOT feedback signal. Null delay circuit  410  is coupled to dynamic AND  114  for receiving the derived reference signal and generating a delayed reference signal. NAND circuit  414  is coupled to NOT circuit  402  and null delay circuit  410  for receiving the first NOT feedback signal and the delayed reference signal, respectively, and generating a UPB signal. NOT circuit  404  is coupled to NAND circuit  414  for receiving the UPB signal and generating an UP signal. NOT circuit  406  is coupled to dynamic AND circuit  114  for receiving the derived reference signal and generating a first NOT reference signal. Null delay circuit  412  is coupled to dynamic AND circuit  116  for receiving the derived feedback signal and generating a delayed feedback signal. NAND circuit  416  is coupled to NOT circuit  406  and null delay circuit  412  for receiving the first NOT reference signal and the delayed feedback signal, respectively, and generating a DNB signal. NOT circuit  408  is coupled to NAND circuit  416  for receiving the DNB signal and generating a DN signal. 
     Now referring to FIGS. 5A and 5B, a timing diagram is shown, illustrating arrival of the reference signal before the feedback signal. When the reference signal arrives at input  102 , the output of dynamic AND circuit  114 , the derived reference signal, goes high and disables dynamic AND circuit  114 . Dynamic AND circuit  114  is disabled as a result of the output of NOR circuit  110 , the first NOR signal, going low in response to the derived reference signal going high. When a feedback signal arrives at input  104 , the output of dynamic AND circuit  116 , the derived feedback signal, goes high and disables dynamic AND circuit  116 . Dynamic AND circuit  116  is disabled as a result of the output of NOR circuit  112 , the second NOR signal, going low in response to the derived feedback signal going low. Once the derived reference signal and the derived feedback signal are both high, pulse shaping circuit  120  generates a pulse at output UP, with width proportional to the delay between the reference signal and feedback signal. At the same time, when the derived reference signal and derived feedback signal are both high, the output of NAND circuit  122  goes low generating the first reset signal, causing the latched reference signal and latched feedback signal to go high, holding the first and second NOR signals low. 
     After the first reset signal is generated, transport delay  124  generates the second reset signal, causing dynamic AND circuits  114  and  116  to reset and the derived reference signal and derived feedback signal to go low. Once input  102  goes low, the latched reference signal goes low, causing the first NOR signal to go high, enabling dynamic AND  114  for the next time input  102  goes high. Similarly, once input  104  goes low, the latched feedback signal goes low, causing the second NOR signal to go high, enabling dynamic AND  116  for the next time input  104  goes high. The cycle is complete and the process repeats for subsequent cycles. 
     Now referring to FIGS. 6A and 6B, a timing diagram is shown, illustrating arrival of the feedback signal before the reference signal. When the feedback signal arrives at input  104 , it causes the derived feedback signal to go high, disabling dynamic AND circuit  116  because of the second NOR signal going low. Once the reference signal arrives at input  102 , it similarly disables dynamic AND  114 . When both the derived feedback signal and the derived reference signal are high, pulse shaping circuit  120  generates a pulse at output DN, with width proportional to the delay between the feedback signal and reference signal. At the same time, when both the derived feedback signal and the derived reference signal are high, the first reset signal is generated, resetting latches  106  and  108 , and then the second reset signal is generated, resetting dynamic AND  114  and  116 . 
     The present invention, allows a PFD to achieve zero dead zone in high speed circuits by using a modified, dual-stage reset mechanism. The dual-stage nature of the reset allows for a highly responsive reset. One application of this feature is constructing a PLL for use in a high-speed clock circuit with little or no dead zone. Achievable is a dead zone of less than one picosecond at cycle times of less than 5 FO4 delays. 
     It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. This description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.