Abstract:
A duty cycle detector comprising a first circuit configured to receive clock cycles including a first level and a second level. The first circuit is configured to obtain a first value based on the length of the first level and to obtain second and third values based on the length of the second level. The first value is compared to the second and the third values to determine a duty cycle range of the clock cycles.

Description:
BACKGROUND  
       [0001]     Many digital circuits receive a clock signal to operate. One type of circuit that receives a clock signal to operate is a memory circuit, such as a dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), or double data rate synchronous dynamic random access memory (DDR-SDRAM). In a memory circuit operating at high frequencies, it is important to have a clock signal that has about a 50% duty cycle. This provides the memory circuit with approximately an equal amount of time on the high level phase and the low level phase of a clock cycle for transferring data, such as latching rising edge data and latching falling edge data into and out of the memory circuit.  
         [0002]     Typically, a clock signal is provided by an oscillator, such as a crystal oscillator, and clock circuitry. The oscillator and clock circuitry often provide a clock signal that does not have a 50% duty cycle. For example, the clock signal may have a 45% duty cycle, where the high level phase is 45% of one clock cycle and the low level phase is the remaining 55% of the clock cycle. To correct or change the duty cycle of the clock signal, a duty cycle detector can indicate the duty cycle of the clock signal and the output of the duty cycle detector can be provided to the clock circuitry that corrects the clock signal to have about a 50% duty cycle.  
         [0003]     For these and other reasons there is a need for the present invention.  
       SUMMARY  
       [0004]     One aspect of the present invention provides a duty cycle detector comprising a first circuit configured to receive clock cycles including a first level and a second level. The first circuit is configured to obtain a first value based on the length of the first level and to obtain second and third values based on the length of the second level. The first value is compared to the second and the third values to determine a duty cycle range of the clock cycles. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is block diagram illustrating one embodiment of an electronic system according to the present invention.  
         [0006]      FIG. 2  is a block diagram illustrating one embodiment of a duty cycle detector according to the present invention.  
         [0007]      FIG. 3  is a diagram illustrating one embodiment of a phase length detector circuit.  
         [0008]      FIG. 4  is a diagram illustrating one embodiment of a comparator circuit.  
         [0009]      FIG. 5  is a timing diagram illustrating the operation of one embodiment of a duty cycle detector according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]     In the following Detailed Description, 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. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized 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.  
         [0011]      FIG. 1  is a block diagram illustrating one embodiment of an electronic system  20  according to the present invention. Electronic system  20  includes a host  22  and a memory circuit  24 . Host  22  is electrically coupled to memory circuit  24  via memory communications path  26 . Host  22  can be any suitable electronic host, such as a computer system including a microprocessor or a microcontroller. Memory circuit  24  can be any suitable memory, such as a memory that utilizes a clock signal to operate. In one embodiment, memory circuit  24  comprises a random access memory, such as a dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), or double data rate synchronous dynamic random access memory (DDR-SDRAM).  
         [0012]     Memory circuit  24  includes a duty cycle detector  28  that receives a clock signal CLK at  30  and an inverted clock signal bCLK at  32 . Clock signal CLK at  30  is the inverse of inverted clock signal bCLK at  32 . In one embodiment, duty cycle detector  28  receives clock signal CLK at  30  and/or inverted clock signal bCLK at  32  from host  22  via memory communications path  26 . In other embodiments, duty cycle detector  28  receives clock signal CLK at  30  and/or inverted clock signal bCLK at  32  from any suitable device, such as a dedicated clock circuit that is part of memory circuit  24  or situated outside memory circuit  24 .  
         [0013]     Duty cycle detector  28  provides two output signals, OUTPUT 1  at  34  and OUTPUT 2  at  36 , to indicate a duty cycle range of clock signal CLK at  30 . Duty cycle detector  28  provides output signals, OUTPUT 1  at  34  and OUTPUT 2  at  36 , to indicate whether the duty cycle of clock signal CLK at  30  is within a duty cycle range, greater than the duty cycle range, or less than the duty cycle range. Duty cycle detector  28  provides the output signals, OUTPUT 1  at  34  and OUTPUT 2  at  36 , to the source of clock signal CLK at  30  and inverted clock signal bCLK at  32 . The source, such as host  22  or a dedicated clock circuit that is part of memory circuit  24  or outside memory circuit  24 , corrects the clock signal CLK at  30  and inverted clock signal bCLK at  32  to have a duty cycle within the duty cycle range. In one embodiment, the duty cycle range is centered around a 50% duty cycle.  
         [0014]      FIG. 2  is a block diagram illustrating one embodiment of duty cycle detector  28  according to the present invention. Duty cycle detector  28  includes a phase length detector circuit  52  and a comparator circuit  54 . Phase length detector circuit  52  is electrically coupled to comparator circuit  54  via comparator communications path  56 .  
         [0015]     Phase length detector circuit  52  receives clock signal CLK at  58  and inverted clock signal bCLK at  60  and provides three values to comparator circuit  54  via comparator communications path  56 . Clock signal CLK at  58  is the inverse of inverted clock signal bCLK at  60 . One of the three values represents the length of one phase of clock signal CLK at  58  and the other two of the three values represents the other phase of clock signal CLK at  58 .  
         [0016]     Comparator circuit  54  receives the three values and compares the one value that represents the length of one phase of clock signal CLK at  58  to each of the other two values. Comparator circuit  54  provides output signals, OUTPUT 1  at  62  and OUTPUT 2  at  64 , to indicate a duty cycle range of clock signal CLK at  58 .  
         [0017]      FIG. 3  is a diagram illustrating one embodiment of phase length detector circuit  52 . Phase length detector circuit  52  receives clock signal CLK at  102  and inverted clock signal bCLK at  104  and  106 . Clock signal CLK at  102  is the inverse of inverted clock signal bCLK at  104  and  106 . Phase length detector circuit  52  provides voltage values VA at  108 , VB at  110 , and VC at  112  to a comparator circuit, such as comparator circuit  54  (shown in  FIG. 2 ).  
         [0018]     Phase length detector circuit  52  includes a first phase length detector  114 , a second phase length detector  116 , and a third phase length detector  118 . First phase length detector  114  receives clock signal CLK at  102  and provides voltage value VA at  108  that represents the length of the high level phase of clock signal CLK at  102 . Second phase length detector  116  receives inverted clock signal bCLK at  104  and provides voltage value VB at  110  that is one representation of the length of the high level phase of inverted clock signal bCLK at  104 , which is the length of the low level phase of clock signal CLK at  102 . Third phase length detector  118  receives inverted clock signal bCLK at  106  and provides voltage value VC at  112  that is another representation of the length of the high level phase of inverted clock signal bCLK at  106 , which is the length of the low level phase of clock signal CLK at  102 . In other embodiments, first phase length detector  114  can receive inverted clock signal bCLK and second and third phase length detectors  116  and  118  can receive clock signal CLK.  
         [0019]     First phase length detector  114  includes a first capacitor C 1  at  120 , a first switching transistor  122 , a first bias transistor  124 , a first logic gate  126 , and a first reset transistor  128 . First switching transistor  122  and first bias transistor  124  are n-channel metal oxide semiconductor (NMOS) transistors and first reset transistor  128  is a p-channel metal oxide semiconductor (PMOS) transistor. Also, first logic gate  126  is an AND gate. In other embodiments, first switching transistor  122 , first bias transistor  124 , and first reset transistor  128  can be any suitable type of transistor and first logic gate  126  can be any suitable logic gate.  
         [0020]     One side of the drain-source path of first reset transistor  128  is electrically coupled to power VCC at  130  and the other side of the drain-source path of first reset transistor  128  is electrically coupled at  108  to one side of the drain-source path of first switching transistor  122  and one side of first capacitor C 1  at  120 . The other side of the drain-source path of first switching transistor  122  is electrically coupled at  132  to one side of the drain-source path of first bias transistor  124 . The other side of the drain-source path of first bias transistor  124  is electrically coupled to a reference, such as ground, at  134  and the other side of first capacitor C 1  at  120  is electrically coupled to the reference at  134 .  
         [0021]     First logic gate  126  receives clock signal CLK at  102  and a gating signal GATE 1  at  136 . The output of first logic gate  126  is electrically coupled at  138  to the gate of first switching transistor  122 . Also, the gate of first reset transistor  128  receives an active low reset signal bRESET at  140  and the gate of first bias transistor  124  receives a bias voltage VBIAS at  142 .  
         [0022]     Clock signal CLK at  102  and gating signal GATE 1  at  136  are provided to first logic gate  126 . If gating signal GATE 1  at  136  is at a low logic level, the output of first logic gate  126  is at a low logic level that turns off first switching transistor  122 . With first switching transistor  122  turned off, reset signal bRESET at  140  is provided at a low voltage level to turn on first reset transistor  128  and charge first capacitor C 1  at  120  to a high voltage level. Reset signal bRESET at  140  is switched to a high voltage level to turn off first reset transistor  128  and terminate charging of first capacitor C 1  at  120 . Also, bias voltage VBIAS at  142  is provided to the gate of first bias transistor  124  to bias first bias transistor  124  to conduct current.  
         [0023]     Gating signal GATE 1  at  136  is provided at a high logic level for one or more high level phases of clock signal CLK at  102 . With gating signal GATE 1  at  136  at a high logic level, the output of first logic gate  126  follows clock signal CLK at  102 . If clock signal CLK at  102  is at a high logic level, the output of first logic gate  126  is at a high logic level to turn on first switching transistor  122  and current flows through first switching transistor  122  and first bias transistor  124  to the reference at  134 . First capacitor C 1  at  120  discharges with first switching transistor  122  turned on and the voltage value VA at  108  represents the length of the high level phase of clock signal CLK at  102 .  
         [0024]     Second phase length detector  116  includes a second capacitor C 2  at  144 , a second switching transistor  146 , a second bias transistor  148 , a second logic gate  150 , and a second reset transistor  152 . Second switching transistor  146  and second bias transistor  148  are NMOS transistors and second reset transistor  152  is a PMOS transistor. Also, second logic gate  150  is an AND gate. In other embodiments, second switching transistor  146 , second bias transistor  148 , and second reset transistor  152  can be any suitable type of transistor and second logic gate  150  can be any suitable logic gate.  
         [0025]     One side of the drain-source path of second reset transistor  152  is electrically coupled to power VCC at  130  and the other side of the drain-source path of second reset transistor  152  is electrically coupled at  110  to one side of the drain-source path of second switching transistor  146  and one side of second capacitor C 2  at  144 . The other side of the drain-source path of second switching transistor  146  is electrically coupled at  154  to one side of the drain-source path of second bias transistor  148 . The other side of the drain-source path of second bias transistor  148  is electrically coupled to the reference at  134  and the other side of second capacitor C 2  at  144  is electrically coupled to the reference at  134 .  
         [0026]     Second logic gate  150  receives inverted clock signal bCLK at  104  and gating signal GATE 2  at  156 . The output of second logic gate  150  is electrically coupled at  158  to the gate of second switching transistor  146 . Also, the gate of second reset transistor  152  receives active low reset signal bRESET at  140  and the gate of second bias transistor  148  receives bias voltage VBIAS at  142 .  
         [0027]     Inverted clock signal bCLK at  104  and gating signal GATE 2  at  156  are provided to second logic gate  150 . If gating signal GATE 2  at  156  is at a low logic level, the output of second logic gate  150  is at a low logic level that turns off second switching transistor  146 . With second switching transistor  146  turned off, reset signal bRESET at  140  is provided at a low voltage level to turn on second reset transistor  152  and charge second capacitor C 2  at  144  to a high voltage level. Reset signal bRESET at  140  is switched to a high voltage level to turn off second reset transistor  152  and terminate charging of second capacitor C 2  at  144 . Also, bias voltage VBIAS at  142  is provided to the gate of second bias transistor  148  to bias second bias transistor  148  to conduct current.  
         [0028]     Gating signal GATE 2  at  156  is provided at a high logic level for one or more high level phases of inverted clock signal bCLK at  104 . With gating signal GATE 2  at  156  at a high logic level, the output of second logic gate  150  follows inverted clock signal bCLK at  104 . If inverted clock signal bCLK at  104  is at a high logic level, the output of second logic gate  150  is at a high logic level to turn on second switching transistor  146 . Current flows through second switching transistor  146  and second bias transistor  148  to the reference at  134 . Second capacitor C 2  at  144  discharges with second switching transistor  146  turned on and voltage value VB at  110  represents the length of the high level phase of inverted clock signal bCLK at  104 , which is the low level phase of clock signal CLK at  102 .  
         [0029]     Third phase length detector  118  includes a third capacitor C 3  at  160 , a third switching transistor  162 , a third bias transistor  164 , a third logic gate  166 , and a third reset transistor  168 . Third switching transistor  162  and third bias transistor  164  are NMOS transistors and third reset transistor  168  is a PMOS transistor. Also, third logic gate  166  is an AND gate. In other embodiments, third switching transistor  162 , third bias transistor  164 , and third reset transistor  168  can be any suitable type of transistor and third logic gate  166  can be any suitable logic gate.  
         [0030]     One side of the drain-source path of third reset transistor  168  is electrically coupled to power VCC at  130  and the other side of the drain-source path of third reset transistor  168  is electrically coupled at  112  to one side of the drain-source path of third switching transistor  162  and one side of third capacitor C 3  at  160 . The other side of the drain-source path of third switching transistor  162  is electrically coupled at  170  to one side of the drain-source path of third bias transistor  164 . The other side of the drain-source path of third bias transistor  164  is electrically coupled to the reference at  134  and the other side of third capacitor C 3  at  160  is electrically coupled to the reference at  134 .  
         [0031]     Third logic gate  166  receives inverted clock signal bCLK at  106  and gating signal GATE 2  at  172 . The output of third logic gate  166  is electrically coupled at  174  to the gate of third switching transistor  162 . Also, the gate of third reset transistor  168  receives active low reset signal bRESET at  140  and the gate of third bias transistor  164  receives bias voltage VBIAS at  142 .  
         [0032]     Inverted clock signal bCLK at  106  and gating signal GATE 2  at  172  are provided to third logic gate  166 . If gating signal GATE 2  at  172  is at a low logic level, the output of third logic gate  166  is at a low logic level that turns off third switching transistor  162 . With third switching transistor  162  turned off, reset signal bRESET at  140  is provided at a low voltage level to turn on third reset transistor  168  and charge third capacitor C 3  at  160  to a high voltage level. Reset signal bRESET at  140  is switched to a high voltage level to turn off third reset transistor  168  and terminate charging of third capacitor C 3  at  160 . Also, bias voltage VBIAS at  142  is provided to the gate of third bias transistor  164  to bias third bias transistor  164  to conduct current.  
         [0033]     Gating signal GATE 2  at  172  is provided at a high logic level for one or more high level phases of inverted clock signal bCLK at  106 . With gating signal GATE 2  at  172  at a high logic level, the output of third logic gate  166  follows inverted clock signal bCLK at  106 . If inverted clock signal bCLK at  106  is at a high logic level, the output of third logic gate  166  is at a high logic level to turn on third switching transistor  162  and current flows through third switching transistor  162  and third bias transistor  164  to the reference at  134 . Third capacitor C 3  at  160  discharges with third switching transistor  162  turned on and the voltage value VC at  112  represents the length of the high level phase of inverted clock signal bCLK at  106 , which is the low level phase of clock signal CLK at  102 .  
         [0034]     In phase length detector circuit  52 , each of the capacitors including first capacitor C 1  at  120 , second capacitor C 2  at  144 , and third capacitor C 3  at  160  has a different capacitive value as compared to the other capacitors. First capacitor C 1  at  120  has a capacitive value that is situated midway between the capacitive value of second capacitor C 2  at  144  and the capacitive value of third capacitor C 3  at  160 . First capacitor C 1  at  120  has a capacitive value of CV, second capacitor C 2  at  144  has a capacitive value of CV times (1−X), and third capacitor C 3  at  160  has a capacitive value of CV times (1+X), where X is a percentage of capacitive value CV, such as 4%. Capacitive value CV can be in any suitable capacitive value range, such as the picofarad range or the nanofarad range. In other embodiments, first capacitor C 1  at  120  can have any suitable capacitive value in relation to the capacitive values of second capacitor C 2  at  144  and third capacitor C 3  at  160 .  
         [0035]     In operation, phase length detector circuit  52  receives clock signal CLK at  102  and inverted clock signal bCLK at  104  and  106 . Also, phase length detector circuit  52  receives bias voltage VBIAS at  142  to bias each of the bias transistors, including first bias transistor  124 , second bias transistor  148 , and third bias transistor  164 , to the same bias voltage level. Thus, each of the bias transistors is biased to conduct the same amount of current.  
         [0036]     The gating signals GATE 1  at  136  and GATE 2  at  156  and  172  are provided at a low logic level to turn off each of the switching transistors, including first switching transistor  122 , second switching transistor  146 , and third switching transistor  162 . With each of the switching transistors turned off, the active low reset signal bRESET at  140  is provided at a low voltage level to turn on the reset transistors, including first reset transistor  128 , second reset transistor  152 , and third reset transistor  168 . With each of the reset transistors turned on, the capacitors, including first capacitor C 1  at  120 , second capacitor C 2  at  144 , and third capacitor C 3  at  160 , are charged to a high voltage level, such as close to VCC. After charging the capacitors, the active low reset signal bRESET at  140  is set to a high voltage level to turn off the reset transistors and terminate charging the capacitors.  
         [0037]     Next, gating signal GATE 1  at  136  is provided at a high logic level to gate clock signal CLK at  102  to first switching transistor  122 . Also, gating signal GATE 2  at  156  and  172  is provided at a high logic level to gate inverted clock signal bCLK at  104  and  106  to second switching transistor  146  and third switching transistor  162 . Gating signal GATE 1  at  136  is provided at a high logic level from before a high phase level in clock signal CLK at  102  until after a high phase level in clock signal CLK at  102 . Gating signal GATE 2  at  156  and  172  is provided at a high logic level from before a high phase level in inverted clock signal bCLK at  104  and  106  until after a high phase level in inverted clock signal bCLK at  104  and  106 . Gating signal GATE 1  at  136  and gating signal GATE 2  at  156  and  172  are provided at a high logic level for the same number of high phase levels.  
         [0038]     For example, gating signal GATE 1  at  136  is provided at a high logic level for one high phase level of clock signal CLK at  102 . With gating signal GATE 1  at  136  at a high logic level, clock signal CLK at  102  transitions to a high logic level that turns on first switching transistor  122 . With first switching transistor  122  turned on to conduct current, first capacitor  120  discharges through first switching transistor  122  and first bias transistor  124 . As clock signal CLK at  102  transitions to a low logic level, first switching transistor  122  is turned off and first capacitor  120  discontinues discharging. Gating signal GATE 1  at  136  is switched to a low logic level and the resulting voltage value VA at  108  represents the length of the high level phase of clock signal CLK at  102 .  
         [0039]     Also, gating signal GATE 2  at  156  and  172  is provided at a high logic level for one high phase level of inverted clock signal bCLK at  104  and  106 . As clock signal CLK at  102  transitions to a low logic level, inverted clock signal bCLK at  104  and  106  transitions to a high logic level that turns on second switching transistor  146  and third switching transistor  162 . With second switching transistor  146  turned on to conduct current, second capacitor  144  discharges through second switching transistor  146  and second bias transistor  148 . With third switching transistor  162  turned on to conduct current, third capacitor  160  discharges through third switching transistor  162  and third bias transistor  164 . As inverted clock signal bCLK at  104  and  106  transitions to a low logic level, second switching transistor  146  and third switching transistor  162  are turned off and second capacitor  144  and third capacitor  160  discontinue discharging. Gating signal GATE 2  is provided at a low logic level and the resulting voltage values VB at  110  and VC at  112  are representations of the length of the high level phase of inverted clock signal bCLK at  104  and  106 , which is the length of the low level phase of clock signal CLK at  102 .  
         [0040]     The capacitive value of second capacitor C 2  at  144  is smaller than the capacitive value of third capacitor C 3  at  160  and second capacitor C 2  at  144  discharges faster than third capacitor C 3  at  160 . Thus, the resulting voltage value VB at  110  is less than the resulting voltage value VC at  112 . If the resulting voltage value VA at  108  is between the resulting voltage value VB at  110  and the resulting voltage value VC at  112 , clock signal CLK at  102  has a duty cycle within a predetermined duty cycle range defined by the capacitive values of the capacitors, including first capacitor C 1  at  120 , second capacitor C 2  at  144 , and third capacitor C 3  at  160 . In one embodiment, if the capacitive value of first capacitor C 1  at  120  is capacitive value CV and the capacitive value of second capacitor C 2  at  144  is capacitive value CV minus 4% and the capacitive value of third capacitor C 3  at  160  is capacitive value CV plus 4%, a resulting voltage value VA at  108  between the resulting voltage value VB at  110  and the resulting voltage value VC at  112  indicates a duty cycle in the range of 49% to 51% (or 50% plus or minus 1%). In other embodiments, the relationship between the capacitive values of the capacitors and the duty cycle range can be any suitable relationship.  
         [0041]     If the resulting voltage value VA at  108  is less than the resulting voltage value VB at  110 , the high level phase is high for a longer length of time than the low level phase and the duty cycle of clock signal CLK at  102  is greater than the predetermined duty cycle range.  
         [0042]     If the resulting voltage value VA at  108  is greater than the resulting voltage value VC at  112 , the high level phase is high for a shorter length of time than the low level phase and the duty cycle of clock signal CLK at  102  is less than the predetermined duty cycle range.  
         [0043]      FIG. 4  is a diagram illustrating one embodiment of a comparator circuit  54 . Comparator circuit  54  receives voltage value VA at  202  and  204 , voltage value VB at  206 , and voltage value VC at  208 . Comparator circuit  54  compares voltage value VA at  202  and  204  to voltage value VB at  206  and to voltage value VC at  208  and provides outputs OUTPUT 1  at  210  and OUTPUT 2  at  212 . The outputs indicate the duty cycle range of clock signal CLK, such as clock signal CLK  102  (shown in  FIG. 3 ).  
         [0044]     Comparator circuit  54  includes a first comparator  214 , a second comparator  216 , an OR gate  218 , and an AND gate  220 . The negative input of first comparator  214  receives voltage value VA at  202  and the positive input of first comparator  214  receives voltage value VB at  206 . The output of first comparator  214  is electrically coupled to one input of OR gate  218  and to one input of AND gate  220  via first output path  222 . Also, first comparator  214  receives an enable signal EVALUATE at  224  that enables first comparator  214  to provide an output on first output path  222 .  
         [0045]     The negative input of second comparator  216  receives voltage value VA at  204  and the positive input of second comparator  216  receives voltage value VC at  208 . The output of second comparator  216  is electrically coupled to one input of OR gate  218  and to one input of AND gate  220  via second output path  226 . Also, second comparator  216  receives enable signal EVALUATE at  224  that enables the second comparator  216  to provide an output on second output path  226 .  
         [0046]     In operation, voltage value VA at  202  and  204 , voltage value VB at  206 , and voltage value VC at  208  are provided to comparator circuit  54  from a phase length detector circuit, such as phase length detector circuit  52  (shown in  FIG. 2 ) and phase length detector circuit  52  of  FIG. 3 . Also, first comparator  214  and second comparator  216  receive enable signal EVALUATE at  224  to enable the outputs of first comparator  214  and second comparator  216 .  
         [0047]     If voltage value VA at  202  and  204  is greater than voltage value VB at  206  and less than voltage value VC at  208 , the output of first comparator  214  is at a low logic level and the output of second comparator  216  is at a high logic level. In response, the output of OR gate  218  provides a high logic level output signal OUTPUT 1  at  210  and the output of AND gate  220  provides a low logic level output signal OUTPUT 2  at  212 . A high output signal OUTPUT 1  at  210  and a low output signal OUTPUT 2  at  212  indicate voltage value VA at  202  and  204  is between voltage value VB at  206  and voltage value VC at  208  and in the predetermined duty cycle range, such as between 49% and 51%.  
         [0048]     If voltage value VA at  202  and  204  is less than voltage value VB at  206 , then voltage value VA at  202  and  204  is also less than voltage value VC at  208 . The output of first comparator  214  is at a high logic level and the output of second comparator  216  is at a high logic level. In response, the output of OR gate  218  provides a high logic level output signal OUTPUT 1  at  210  and the output of AND gate  220  provides a high logic level output signal OUTPUT 2  at  212 . A high output signal OUTPUT 1  at  210  and a high output signal OUTPUT 2  at  212  indicate voltage value VA at  202  and  204  is less than voltage value VB at  206  and voltage value VC at  208  and clock cycle CLK has a duty cycle that is greater than the predetermined duty cycle range, such as greater than 51%.  
         [0049]     If voltage value VA at  202  and  204  is greater than voltage value VC at  208 , then voltage value VA at  202  and  204  is also greater than voltage value VB at  206 . The output of first comparator  214  is at a low logic level and the output of second comparator  216  is at a low logic level. In response, the output of OR gate  218  provides a low logic level output signal OUTPUT 1  at  210  and the output of AND gate  220  provides a low logic level output signal OUTPUT 2  at  212 . A low output signal OUTPUT 1  at  210  and a low output signal OUTPUT 2  at  212  indicate voltage value VA at  202  and  204  is greater than voltage value VB at  206  and voltage value VC at  208  and clock cycle CLK has a duty cycle that is less than the predetermined duty cycle range, such as less than 49%.  
         [0050]      FIG. 5  is a timing diagram illustrating the operation of one embodiment of a duty cycle detector according to the present invention. The duty cycle detector is similar to duty cycle detector  28  of  FIG. 2 . The duty cycle detector includes a phase length detector circuit, such as phase length detector circuit  52  of  FIG. 3 , and a comparator circuit, such as comparator circuit  54  of  FIG. 4 .  
         [0051]     The phase length detector circuit receives clock signal CLK at  300  and inverted clock signal bCLK at  302 , which is the inverse of clock signal CLK at  300 . Also, the phase length detector circuit receives gating signals GATE 1  at  304  and GATE 2  at  306  and the active low reset signal bRESET at  308 . In addition, the phase length detector circuit receives a bias voltage (not shown), such as bias voltage VBIAS (shown in  FIG. 3 ), to bias the bias transistors  124 ,  148 , and  164  to conduct current.  
         [0052]     The phase length detector provides voltage values at  310 , including voltage value VA at  312 , voltage value VB at  314 , and voltage value VC at  316  to the comparator circuit. An enable signal EVALUATE at  318  is received by the comparator circuit to enable outputs from the comparators. The comparator circuit provides output signals OUTPUT 1  at  320  and OUTPUT 2  at  322 .  
         [0053]     To begin, gating signals GATE 1  at  304  and GATE 2  at  306  are provided at a low logic level at  324  to turn off switching transistors  122 ,  146 , and  162 . The reset signal bRESET at  308  is at a low logic level at  326  to turn on reset transistors  128 ,  152 , and  168  and charge capacitors  120 ,  144 , and  160  to high voltage levels, indicated at  328 . To obtain a duty cycle range, reset signal bRESET at  308  is switched to a high voltage level at  330  to turn off reset transistors  128 ,  152 , and  168  and discontinue charging capacitors  120 ,  144 , and  160 .  
         [0054]     Gating signal GATE 1  at  304  transitions to a high logic level at  332 , while clock signal CLK at  300  is at a low level. At  334 , clock signal CLK at  300  switches to a high level that turns on first switching transistor  122  and begins to discharge first capacitor  120 . Voltage value VA at  312  drops at  336  as first capacitor  120  discharges. At  338 , clock signal CLK at  300  switches to a low level that turns off first switching transistor  122  and discontinues discharging first capacitor  120 . At  340 , gating signal GATE 1  at  304  switches to a low logic level and at  342  the resulting voltage value VA at  312  on first capacitor  120  represents the length of the high level phase of clock signal CLK at  300 .  
         [0055]     Gating signal GATE 2  at  306  transitions to a high logic level at  344 , while inverted clock signal bCLK at  302  is at a low level. At  346 , inverted clock signal bCLK at  302  transitions to a high level, which turns on second switching transistor  146  and third switching transistor  162 . With second switching transistor  146  and third switching transistor  162  turned on, second capacitor  144  and third capacitor  160  begin to discharge. At  348 , since the capacitive value of second capacitor  144  is smaller than the capacitive value of third capacitor  160 , voltage value VB at  314  discharges faster than voltage value VC at  316 . At  350 , inverted clock signal bCLK at  302  transitions to a low level that turns off second switching transistor  146  and third switching transistor  162 . Turning off second switching transistor  146  and third switching transistor  162  discontinues discharging second capacitor  144  and third capacitor  160 . At  352 , gating signal GATE 2  at  306  switches to a low logic level. At  354 , the resulting voltage value VB at  314  on second capacitor  144  is one representation of the length of the high level phase of inverted clock signal bCLK at  302 , which is the length of the low level phase of clock signal CLK at  300 . At  356 , the resulting voltage value VC at  316  on third capacitor  160  is another representation of the length of the high level phase of inverted clock signal bCLK at  302 , which is the length of the low level phase of clock signal CLK at  300 .  
         [0056]     At  358 , enable signal EVALUATE at  318  transitions to a high voltage level to enable first comparator  214  and second comparator  216 . Output signal OUTPUT 1  at  320  and output signal OUTPUT 2  at  322  become valid at  360 . With voltage value VA at  312  between voltage value VB at  314  and voltage value VC at  316 , output signal OUTPUT 1  at  320  is at a high logic level at  362  and output signal OUTPUT 2  at  322  is at a low logic level at  364 . Enable signal EVALUATE at  318  transitions to a low voltage level at  366  to tri-state first comparator  214  and second comparator  216 . Output signal OUTPUT 1  at  320  and output signal OUTPUT 2  at  322  become invalid at  368 . Reset signal bRESET at  308  transitions to a low level at  370  that charges capacitors  120 ,  144 , and  160  and voltage values, voltage value VA at  312 , voltage value VB at  314 , and voltage value VC at  316 , to high voltage levels at  372 .  
         [0057]     The duty cycle detector provides the output signals, OUTPUT 1  at  320  and OUTPUT 2  at  322 , to the source of clock signal CLK at  300  and inverted clock signal bCLK at  302 . The source receives the valid output signals, OUTPUT 1  at  320  and OUTPUT 2  at  322 , between  360  and  368 . If the valid output signals, OUTPUT 1  at  320  and OUTPUT 2  at  322 , indicate the duty cycle of clock signal CLK at  300  is not within the duty cycle range, the source corrects the clock signal CLK at  300  and inverted clock signal bCLK at  302  to have a duty cycle closer to and eventually within the duty cycle range.  
         [0058]     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a 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. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.