Abstract:
A method for correcting the duty cycle of a clock signal uses two-dual-slope integrators with two comparators; each comparator is connected to both integrators and configured to include a “dead band” when the input pulse duty cycle is at or near 50%. One comparator detects when duty cycle is high and the other comparator detects when the duty cycle is low. When the duty cycle is within the “dead band” range, neither comparator goes valid. This provides an analog filter where the output comparators will not instantaneously switch between opposite duty cycle correction states. When the duty cycle is greater or less than 50%, the integrated voltages on the two integrators move in opposite directions producing twice the signal magnitude on differential inputs of the comparators, as compared with using a single integrator architecture.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit and method for correcting the duty cycle of a clock signal, the circuit and method including a dead band when the duty cycle of the clock signal is at or close to a 50% duty cycle. 
     Duty Cycle Correction (DCC) solutions typically use a dual-slope integrator where one phase of the clock charges the integrator positive and the other phase of the clock discharges the integrator. When one dual-slope integrator is used, a comparator compares the integrated voltage to a reference voltage, and mid-point referencing is usually used. With this approach, the comparator outputs a logical “1” or “0” to indicate if the integrated voltage is above or below the reference voltage; a logical “1” or “0” identifies if the duty cycle is above or below the 50%. Adjusting duty cycle using a two-state detector typically results with “dithering” around the desired solution point. 
     In some cases, two dual-slope integrators have been used (See U.S. Pat. No. 7,570,094 B2). The two integrators are isolated where one integrator directly adjusts the bias on a delay stage to control pull-down slew rate and the other integrator controls the delay stage pull-up slew rate. This is an analog method that directly uses the integrated voltages to bias the delay stage. This method constantly adjusts the bias voltages to acquire a 50% duty cycle from the delay stage(s). In many applications, it is undesirable to constantly chase a clock that jitters or has other small instantaneous variations with the clock duty cycle. This approach does not use a digital solution to correct duty cycle distortion, but generates two analog voltages that are directly applied to the delay stage block to constantly adjust for a 50% output duty cycle. 
     What is desired, therefore, is an improved duty cycle detector circuit that eliminates the undesirable dithering about the 50% duty cycle operating condition. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, a circuit and method for correcting the duty cycle of a clock signal uses two dual-slope integrators with two comparators; each comparator is connected to both integrators and is configured to include a “dead band” when the input pulse duty cycle is at or near 50%. One comparator detects when duty cycle is high and the other comparator detects when the duty cycle is low. When the duty cycle is within the “dead band” range, neither comparator goes valid. This provides an analog filter where the output comparators will not instantaneously switch between opposite duty cycle correction states. When the duty cycle is greater or less than 50%, the integrated voltages on the two integrators move in opposite directions producing twice the signal magnitude on differential inputs of the comparators, as compared with using a single integrator architecture. An additional advantage of the interconnected comparators and integrators is that the ratio of the resistors used for the dual-slope integrators can be equally adjusted based on the ideal common mode trip point of the comparators. The operation and the “dead band” are maintained over a relatively large pull-up/pull-down resistor ratio. An additional feature of this invention is the inclusion of “offset” programming of the individual integrator resistor ratios. This programming provides a method to offset the integration voltage on one integrator differently than the other which can be used to reduce or increase the “dead band” range or compensate for device characteristic variances. Additionally, both integrators can be programmed in the same direction to adjust for the ideal common-mode voltage of the comparators. A reset circuit is included to preset the integrators to near the 50% bias point (near or within the “dead band”) to accelerate the time for initial solution of the Duty Cycle Corrector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a schematic diagram of a duty cycle detector according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a comparator that is used in the duty cycle detector of  FIGS. 1A and 1B ; 
         FIG. 3  is a schematic diagram of digital decoding circuit that can be used in conjunction with the duty cycle detector of  FIGS. 1A and 1B ; 
         FIGS. 4-5  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 50% duty cycle; 
         FIGS. 6-7  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 53% duty cycle; 
         FIGS. 8-9  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 47% duty cycle; and 
         FIG. 10  is a waveform diagram associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with missing clock pulses. 
     
    
    
     DETAILED DESCRIPTION 
     The invention uses two integrators  102  and  104  as shown in  FIGS. 1A and 1B . The top integrator  102  (N 1 , R 1 , R 2 , R 3 , P 1 , C 1  and C 2 ) uses CLKB (Low True Clock) and the bottom integrator  104  (N 2 , R 4 , R 5 , R 6 , P 2 , C 3 , C 4 ) uses CLK (High True Clock). Each integrator has a resistor stack where the resistor values of resistors in the top integrator  102  (R 1 , R 2 , and R 3 ) are equal to the values of the respective resistors in the bottom integrator (R 4 , R 5 , and R 6 ). Typically R 1 =R 3  and R 4 =R 6 , but this is not a limitation and other values can be used. For example, R 1 &lt;R 3  and R 4 &lt;R 6  which results with the voltage levels into the comparators  106  and  108  being shifted down (lowering the common mode trip point) and when R 1 &gt;R 3  and R 4 &gt;R 6 , the input levels to the comparators  106  and  108  are shifted up. Resistors R 2  and R 5  determine the “dead band” range (R 2 &lt;&lt;R 3  and R 2 &lt;&lt;R 1 ). 
     A reset circuit including reset circuit portion  108  (N 4 , P 4 , R 9  and R 10  are resetting HI_TOP) and reset circuit portion  106  (N 3 , P 3 , R 7  and R 8  for resetting LO_TOP), is included to initialize the voltages of the integrators on HI_TOP and LO_TOP to values near the “dead band” determined by the values of integrator resistor stack. Although not directly connected to the reset circuit, the voltages on HI_BOT and LO_BOT are also preset by the reset circuit through resistors R 2  and R 5 . The exact voltage during reset signal activation is not critical; the purpose of reset is to decrease the time to reach the integrated voltage level on each of the signals connected to the inputs of the comparators. The voltages applied to the comparator inputs are determined by the alternately enabled transistors N 1  and P 1  in the top integrator and N 2  and P 2  in the bottom integrator. The reset circuit can be eliminated when solution speed is not an issue. 
     The top integrator  102  uses the inverted clock phase, CLKB, to alternately enable transistors P 1  and N 1 . When CLKB is low, P 1  sources current to charge the integrating capacitors through R 3  (and R 2 ) raising the voltage on capacitors C 2  and C 1 . When CLKB is high, N 1  sinks current to ground, discharging capacitors C 2  and C 1  through R 1  (and R 2 ) lowering the voltage on capacitors C 2  and C 1 . In this way, the integrated voltages, HI_TOP and H_BOT will increase with increasing CLK duty cycles (increasing CLK high time relative to CLK low time) and HI_TOP and H_BOT will decrease with decreasing CLK duty cycles (decreasing CLK high time relative to CLK low time). Similarly, the bottom integrator  104  works in an identical manner, except the true clock phase is used, CLK, instead of CLKB. When CLK has a 50% duty cycle, the voltage on H-TOP will be the same as the voltage on L-TOP (voltage on HI_BOT will also be the same as the voltage on LO_BOT). So, the 50% CLK condition results in both comparator outputs going low since HI_BOT and LO_BOT are at lower voltage potentials than LO_TOP and HI_TOP respectively. When the duty cycle is greater than 50%, the integrated voltages on HI_TOP and HI_BOT increase, while the voltages on LO_TOP and LO_BOT decrease. When the duty cycle is sufficiently greater than 50% to cause the voltage on HI_BOT to be greater than the voltage on LO_TOP, the HIGH-DC comparator will switch to a “1”, indicating that the duty cycle was greater than 50%. Conversely, when the duty cycle is sufficiently less than 50%, then the output of the LOW-DC comparator will go high indicating that the duty cycle is less than 50%. 
     The Outputs HIGH-DC and LOW-DC provide control signals to duty cycle correcting circuits; HIGH-DC high indicates that the CLOCK duty cycle is too high and the duty cycle correcting circuit needs to reduce the duty cycle (decrease CLOCK high time), LOW-DC high indicates that the CLOCK duty cycle is too low and the duty cycle correcting circuit needs to increase the duty cycle (increase CLOCK high time). Neither HIGH-DC high or LOW-DC high (both low) indicates that no duty cycle correction is needed. 
     When a high duty cycle exists and HI_BOT goes sufficiently higher than LO_TOP, the HIGH-AMP will switch from a “0” to a “1” indicating that a greater than 50% duty cycle occurred. Likewise, when a low duty cycle exists and LO_BOT goes sufficiently higher than HI_TOP, the LOW-AMP will switch from a “0” to a “1” indicating that a less than 50% duty cycle occurred. The differential voltage between HI_BOT and LO_TOP and between LO_BOT and HI_TOP determines the output state of the comparators. The relative offset between the “BOT” nodes and the “TOP” nodes can be programmed by selecting the tap point on R 1  or R 4 , (R 1 &lt;&lt;R 2 , R 4 &lt;&lt;R 5 , R 1  may be different than R 4 ). If R 1  or R 4  is programmed to a higher resistance value, then the associated TOP and BOT node voltages will increase and programming R 1  or R 4  to a lower resistance value will decrease the associated TOP and BOT node voltages will decrease. The “HI” integrator and the “LO” integrator are independently programmable that can be used to increase or decrease the “dead band” using R 1  or R 4  programming. R 1  programming is done by selecting the appropriate NOR gate path, I 4  through I 6 , connected to transistors N 1 A or N 1 B or N 1 C. R 4  programming is done be selecting the appropriate NAND/Inverter gate path, I 7  through I 12 , connected to transistors N 2 A or N 2 B. 
     Resistor R 1  is thus divided into resistor components R 1 A, R 1 B, and R 1 C, with corresponding tap points HTAP&lt; 0 &gt;, HTAP&lt; 1 &gt;, and HTAP&lt; 2 &gt;. Transistors N 1 A, N 1 B, and N 1 C receive the gated HCLK&lt; 0 &gt;, HCLK&lt; 1 &gt;, and HCLK&lt; 2 &gt; signals and are in turn coupled to the resistor R 1  tap points. Similarly, resistor R 4  is thus divided into resistor components R 4 A, R 4 B, and R 4 C, with corresponding tap points LTAP&lt; 0 &gt;, LTAP&lt; 1 &gt;, and LTAP&lt; 2 &gt;. Transistors N 4 A, N 4 B, and N 4 C receive the gated LCLK&lt; 0 &gt;, LCLK&lt; 1 &gt;, and LCLK&lt; 2 &gt; signals and are in turn coupled to the resistor R 4  tap points. 
     Programming selection signals, HENB&lt; 0 : 2 &gt; and LEN&lt; 0 : 2 &gt; are generated in a DCC-PGM logic block shown in  FIG. 3 . NOR gates I 4 , I 6 , and I 5  receive the CLK signal as well as the HENB&lt; 0 : 2 &gt; signals and generate the HCLK&lt; 0 : 2 &gt; signals. NAND gates I 218 , I 216 , and I 11  receive the CLK signal as well as the LEN&lt; 0 : 2 &gt; signals and are coupled to inventers I 14 , I 13 , and I 10 , which in turn generate the LCLK&lt; 0 : 2 &gt; signals. NOR gate I 1  receives the CLK and VSS signals and generates the INB signal at the gate of transistor P 1 . NAND gate I 227  receives the CLK and VINT signals and is coupled to inverter I 16 , which generates the IN signal at the gate of transistor P 2 . For NOR gate I 1 , in  FIGS. 1A and 1B , VSS is used as a logic zero so that the INB path is identical to the HCLK&lt; 0 : 2 &gt; paths. In a similar fashion, VINT or a logic one is used as an input to NAND gate I 227 , so that identical NAND gates drive IN and LCLK&lt; 0 : 1 &gt;. The gate matching should not be an absolute requirement, but is the preferred method so that the delays match. It should also be noted in  FIGS. 1A and 1B  that inverter I 15  is used to receive the reset RST signal and to generate an inverted RSTB signal required for the proper operation of reset signal portions  106  and  108 . 
     Referring now to  FIG. 2 , a comparator  200  suitable for use in the duty cycle detector circuit of  FIGS. 1A and 1B  is shown. Comparator  200  includes an active load including P-channel transistors P 21  and P 22  coupled to VCC (VCCE), a differential pair including N-channel transistors N 22  and N 23 , and tail current transistor N 21  coupled to VSS or ground. The gate of transistor N 21  is coupled to VCC (VCCE). The gate of transistor defines the PLUS input nodes, the gate of transistor N 23  defines the MINUS input node, and the drain of transistor N 23  defines the OUT output node. The sources of transistors N 22  and N 23 , and the drain of transistor N 21  are coupled together to define the NTAIL node. The simple comparator of  FIG. 2  is used to save integrated circuit die area, but other comparators can be used as is known in the art. 
     Referring again to  FIG. 2 , VCC (VCCE) is shown to be the supply voltage for the differential P-Channel transistors of the comparator/differential amplifier  200  and the signal connected to the gate of transistor N 21  for biasing the amplifier  200 . The VCCE voltage (External Supply) is different from the VINT voltage (internal regulated supply). In an embodiment of the invention, the higher voltage (VCCE) was used for the comparator  200  to increase signal margin for the amplifiers. Programmable logic, logic driving the integrators, the integrators, and the reset circuits are powered by VINT (regulated voltage) and the amplifiers/comparators powered by VCCE. While an embodiment of the present invention is shown in  FIGS. 1A-3  using two power supplies, two separate power supplies are not a requirement and a single power supply can be used. 
     As mentioned above, programming selection signals, HENB&lt; 0 : 2 &gt; and LEN&lt; 0 : 2 &gt; are generated in a DCC-PGM logic block  300  as is shown in  FIG. 3 . TMDCC mnemonic indicates Test Mode Duty Cycle Correction signals. TMDCC&lt; 0 : 3 &gt; and generated TMDCCB&lt; 0 : 3 &gt; signals provide a method to adjust the integration offsets. These signals can be used for test purposes or to selectively adjust the integration offsets for normal operation. The default values for TMDCC&lt; 0 : 3 &gt; are all zero. It should be mentioned that programmability, via HENB and LEN signals are not needed if programmability of the integrator is not required. In this case, only one pull-down N-channel transistor per integrator would be required and the taps at the bottom of the integrators would be replaced with a single resistor per integrator. Inverter  180  receives the TMDCC&lt; 0 : 3 &gt; signal and generates the TMDCCB&lt; 0 : 3 &gt; signal. Inverter  180  has four separate inverters, one for each of the four outputs TMDCCB&lt; 0 : 3 &gt;. In the LOW DECODE section of logic block  300 , NAND gate I 270  receives the LENB&lt; 2 &gt; and LENB&lt; 1 &gt; signals to generate the LENB&lt; 0 &gt; signal for a LOW DEFAULT setting. NAND gate I 241  receives the TMDCCB&lt; 1 &gt; and TMDCC&lt; 0 &gt; signals to generate the LENB&lt; 1 &gt; signal for a LOW REDUCE VOLTAGE setting. NAND gate I 267  receives the TMDCC&lt; 1 &gt; and TMDCCB&lt; 0 &gt; signals to generate the LENB&lt; 2 &gt; signal in a LOW INCREASE VOLTAGE setting. In the HIGH DECODE section of logic block  300 , NAND gate  1271  receives the HENB&lt; 2 &gt; and HENB&lt; 1 &gt; signals to generate the HENB&lt; 0 &gt; signal and the HEN&lt; 0 &gt; signal through inverter I 109  for a HIGH DEFAULT setting. NAND gate I 242  receives the TMDCCB&lt; 3 &gt; and TMDCC&lt; 2 &gt; signals to generate the HENB&lt; 1 &gt; signal and the HEN&lt; 1 &gt; signal through inverter I 110  for a HIGH REDUCE VOLTAGE setting. NAND gate I 243  receives the TMDCC&lt; 3 &gt; and TMDCCB&lt; 2 &gt; signals to generate the HENB&lt; 2 &gt; signal and the HEN&lt; 2 &gt; signal through inverter  1111  in a HIGH INCREASE VOLTAGE setting. HENB&lt; 0 &gt;=0 and LEN&lt; 0 &gt;=1 are default cases, with TMDCC&lt; 0 : 3 &gt;=0. HENB&lt; 0 : 2 &gt; programs the CLK-high integrator,  102 , and LEN&lt; 0 : 2 &gt; programs the CLK-low integrator,  104 . When the default cases are selected, HENB&lt; 0 &gt;=0 and LEN&lt; 0 &gt;=1, HCLK&lt; 1 &gt; and LCLK&lt; 1 &gt; both go high enabling HTAP&lt; 1 &gt; and LTAP&lt; 1 &gt;, which are the mid-tap points on resistors R 1 &lt;a,b,c&gt; and R 4 &lt;a,b,c&gt; respectively. Programming HENB&lt; 1 &gt;=0 reduces HI_TOP and HI_BOT voltages relative to the default voltages by selecting HTAP&lt; 2 &gt;. The CLK-low integrator,  104 , is programmed in a similar way, except LEN&lt; 0 : 2 &gt;, high true signals, selects integrated voltage levels on LOW_TOP and LOW_BOT nodes. It should be mentioned that the purposed of the TMDCC programming is to adjust for mismatches that can exist when the circuit is implemented in silicon; TMDCC is primarily used to compensate for circuit imbalances. 
     Various signal and node names are used in this specification. The following table shows the signal name used in the duty cycle detector versus the same signal name used on the waveforms in a DLL circuit used in  FIGS. 4-10 . 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Duty Cycle Detector 
                 DLL Circuit 
               
               
                   
                   
               
             
             
               
                   
                 CLK 
                 RCLK 
               
               
                   
                 HIGH_DC 
                 FSTF, FSTFL 
               
               
                   
                 LOW_DC 
                 SLWF, SLWFL 
               
               
                   
                 RST 
                 RSTDLL, DLLRST 
               
               
                   
                 N/A 
                 SYNC (DLL Locked Signal) 
               
               
                   
                 N/A 
                 DCCOK (CLOCK OK, Clock Running) 
               
               
                   
                 N/A 
                 SYNC (DLL Locked Clock with DC 
               
               
                   
                   
                 Correction) 
               
               
                   
                   
               
             
          
         
       
     
     The function of the reset circuits  106  and  108  discussed above with respect to  FIGS. 1A and 1B  is described in further detail below. The reset circuits are used to establish a near mid voltage on the HI_TOP and HI_BOT nodes. If no other influences existed, HI_BOT would be equal or lower to HI_TOP and LO_BOT would be equal or lower to HI_BOT. With a very small differential voltage difference on the comparators, the comparators could switch to either state, during or immediately after reset. However, in the present invention, the reset circuit is not the only mechanism that presets the integration nodes. In addition to the reset circuit, the transistors driving the integrators are connected during reset and impact the integrator node voltages. If the clock started out low for an extended period while in reset, for example, the voltage on HI_TOP would tend to go lower than the voltage on LO_BOT which would result in LOW_DC going active during reset. If the clock had been high during reset, the opposite comparator would have gone active. With the clock running, the voltages on the integrator can still be impacted away from the ideal reset voltage. Any of these conditions can cause the reset voltages on the integrator to deviate from the ideal reset voltage resulting in either a HIGH_DC or LOW_DC state; during or immediately after reset, one or the other comparator could go to a logic “1” until adequate time has occurred for a stable integrated solution. The intent of the reset circuit is to speed up the integrating process. Without reset, these nodes would have to integrate from the supply voltage or from ground toward the dead-band voltage zone which would increase the time to a stable solution. So, even though there can be an invalid solution during reset or immediately after reset, the time to a stable solution is significantly decreased by the addition of the reset circuit. There are solutions to eliminate the non-dead-band condition that may occur during reset, but more circuit complexity to eliminate this condition is probably not warranted. 
     As noted above,  FIGS. 4-5  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 50% duty cycle.  FIG. 4  shows the RSTDLL, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), HI_TOP, and LO_BOT signals. Note that during an initial time period, the SLWF signal goes high, but thereafter the SLWF and FSTF signals are both at a logic zero state. Similarly,  FIG. 5  shows the RSTDLL, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), LO_TOP, and HI_BOT signals. Note that during an initial time period, the SLWF signal goes high, but thereafter the SLWF and FSTF signals are both at a logic zero state. When SLWF and FSTF are both low (in the dead band region) this occurs when the duty cycle is at or near 50% duty cycle. When both are low, no correction is made to the duty cycle with the DLL. Having the dead band results in corrections being made only when the sampled clock goes slightly above or slightly below 50% duty cycle. The deviation required before a high or low duty cycle correction is detected is determined by the dead band of the integrators. The outputs of the duty cycle detector, LOW_DC and HIGH_DC are only valid after the clock has been constantly running for a period of time long relative to the integration time constant of the integrator. 
     As noted above,  FIGS. 6-7  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 53% duty cycle.  FIG. 6  shows the DLLRST, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), HI_TOP, and LO_BOT signals. Note that during an initial time period, the SLWF signal goes high, then goes low, and thereafter the SLWF signal stays low and the FSTF signal switches to a logic one state. Similarly,  FIG. 7  shows the RSTDLL, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), LO_TOP, and HI_BOT signals. Note that during an initial time period, the SLWF signal goes high, then goes low, and thereafter the SLWF signal stays low and the FSTF signal switches to a logic one state. 
     As noted above,  FIGS. 8-9  are waveform diagrams associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with a 47% duty cycle.  FIG. 8  shows the RSTDLL, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), HI_TOP, and LO_BOT signals. Note the SLWF signal quickly switches to a logic one state, and the FSTF signal quickly switches to a logic zero state. The simulation in  FIG. 8  starts with the clock running with a duty cycle of 47%. At the start of the simulation, a RST signal occurs to initialize the integration nodes to near mid-point. When the RST is released, the integrator immediately monitors the clock duty cycle. In the case of  FIG. 8 , the initialization set the integration levels very close to the integration level with the clock constantly running. Even though an almost immediate solution was found, the output of the duty cycle detector should not be used until sufficient sampling time has expired. In the present case, the DLL circuit holds off monitoring the duty cycle detector until the DLL has locked on the rising edge of the clock.  FIGS. 6 and 7  illustrate this point where RST initializes the integrator to a predetermined voltage, determined by the reset circuits  106  and  108 . In the present case, where there is a high duty cycle, the duty cycle detector does not settle on the correct solution immediately, but requires an integration period to determine the solution, as is shown in  FIG. 6 . Switching when the clock is first applied is not significant. Similarly,  FIG. 9  shows the RSTDLL, SLWF (associated with the LOW_DC output node), FSTF (associated with the HIGH_DC output node), LO_TOP, and HI_BOT signals. Note the SLWF signal quickly switches to a logic one state, and the FSTF signal quickly switches to a logic zero state. No initial switching occurred because the reset state was the same as the integrated clock solution. 
     As noted above,  FIG. 10  is a waveform diagram associated with the duty cycle detector circuit of  FIGS. 1A and 1B  having an input clock signal with missing clock pulses.  FIG. 10  includes the CLK, HI_BOT, LO_TOP, HI_TOP, LO_BOT, LOW_DC, and HI_DC signals. The plot of  FIG. 10  is for the full DLL circuit to demonstrate how missing clocks are treated. When a missing clock is detected, by a circuit (not shown), a RST occurs to preset the integration nodes in preparation for when the clock restarts. The signals are DLL level signals. RCLK=CLK as previously mentioned, DCCOK is high when the clock is continuously running, and DCCOK is generated by a circuit in the DLL. When DCCOK goes low, RST (DCCRST) goes active. SYNC is “locked” and the duty cycle is corrected in the DLL circuit. 
     The method used in the present invention allows the ratio of the resistors in the pull-up and pull-down sections of each integrator to be different than one. This means that the voltages on the nodes connected to the comparators will be different for different resistor ratios. This is an important aspect of the present invention, since dual slope integrators that use a reference voltage require the ratio to match the reference voltage. In the present invention, the ratio can be changed, by the TMDCC programming or by changing the resistor ratio during design without specifically changing a separate reference voltage or voltages. This is true because the present invention does not have a conventional reference voltage. The reference voltage for the present invention is generated from the opposite integrator; the only requirement is that the two integrators have the same resistor ratio (or close to the same ratio). The resistor ratios of both integrators can be both changed to increase or decrease the voltages on the integrating nodes (at 50% DC) to the most favorable operating point of the comparators. 
     It will be apparent to those skilled in the art, therefore, that various modifications and variations can be made to the invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims.