Patent Document

BACKGROUND 
     This disclosure relates to clock signal generators for electronic systems and circuits, and particularly, a duty cycle generator and method of operation for adjusting and setting duty cycle of a signal for timing operations with less jitter and increased accuracy. 
     Clocking signals used in most electronic systems provide the heart beat and pulse lines for correct operation. High speed applications such as SerDes (Serial/Deserializer) and DDR (double data rate) transmitter links sending data on both edges of a reference clock rely highly on its duty cycle. Duty cycle distortions in such applications impact timing margin and performance affecting eye closure. 
     Usually duty cycle distortions occur due to incoming clock duty cycle variations, systematic PFET vs. NFET process mismatch that affect threshold voltages, drive strength, etc, and local PFET vs. NFET device mismatch. They can also be altered by the processing circuit&#39;s architecture. For example, embedded PLLs (phase locked loops) used for clock generation could use LFSR (Linear Feedback Shift Register) divider architectures primarily chosen due to programmability and high speed operation. However the duty cycle (pulse duration) creation mechanics could not necessarily give a 50% output (which can vary based on the divide value chosen). Clock propagation circuits such as DLLs (Delay Locked loops), Delay lines, Phase Rotators, I/O drivers in-turn could cause additional static distortion (across process, voltage and temperature (PVT) ranges that the circuits are specified to run at) to the incoming variation, making the net outgoing static duty cycle variation worse. 
     BRIEF SUMMARY 
     It is an aspect of the present disclosure to provide an improved programmable duty cycle generator and method of operation. 
     The improved programmable duty cycle signal generator can be placed in key pinch points of the critical path where precise duty cycle definition is needed. 
     In one aspect, the duty cycle is not measured, but rather is generated based on a predetermined value. Saw tooth generator/Integrator schemes are used to create the saw type waveforms of the incoming frequency which in conjunction with DAC (Digital to Analog Converter) is used to create the desired duty cycle. The range of the output duty cycle is determined by the number of DAC control bits, for example if a 5 bit DAC is used, the output waveform can have any desired duty cycle from 3% to 97% in steps of about 3%. 
     According to an embodiment, there is provided a programmable duty cycle signal generator comprising: a first integrator circuit for receiving an input clock signal (CLK); the first integrator circuit creating from the input CLK signal a first linear voltage signal representative of a full time period of the input CLK; a digital to analog converter (DAC) receiving bits representing a programmed output signal duty cycle; a sampling circuit generating a voltage supply signal from the first linear voltage signal for input to the DAC, the DAC using the voltage supply signal and the programmed bits to generate a reference signal voltage representative of the programmed duty cycle; an edge pulse detector detecting an edge of the input CLK to create trigger signal and generating a rising edge of an output signal of the duty cycle signal generator; a second integrator for integrating, in real time, the output signal to create a second linear voltage output signal; a comparator device receiving the reference signal voltage and the second linear voltage output signal and generating a compared output signal at a time the second linear voltage output signal exceeds the reference signal, the compared output signal being used to generate a falling edge of the output signal, the output signal rising and falling edge occurring in each time period at the programmed duty cycle. 
     According to one embodiment, there is provided a method for programmable duty cycle signal generation comprising: integrating, using a first integrator circuit, the input CLK signal to create a first linear voltage signal representative of a full time period of the input CLK, and in each period: receiving at a digital to analog converter (DAC) bits representing a programmed output signal duty cycle; generating a voltage supply signal from the first linear voltage signal for input to the DAC; generating, by the DAC using the voltage supply signal and the duty cycle bits, a reference signal voltage representative of the programmed duty cycle; detecting an edge of the input CLK signal to create a rising edge of an output signal of the duty cycle signal generator output; integrating by a second integrator, in real time, the output signal to create a second linear voltage output signal; comparing, in real-time, the reference signal to the second linear voltage output signal; and, at a time the second linear voltage output signal exceeds the reference signal generating a compared output signal, generating, based on the compared output signal, a falling edge of the output signal in the current period of the output signal, the rising and falling edge of the output signal of the duty cycle signal generator timed according to the programmed duty cycle. 
     In an alternate embodiment, there is provided a programmable duty cycle signal generator comprising: a saw tooth conversion circuit for receiving an input clock signal (CLK) and creating from the input CLK signal a first linear increasing voltage signal representative of a full time period of the input CLK, and in each period: a digital to analog converter (DAC) receiving bits representing a programmed output signal duty cycle; a sampling circuit generating a voltage supply signal from the first linear increasing voltage signal for input to the DAC, the DAC using the voltage supply signal and the duty cycle bits to generate a reference signal voltage representative of a programmed duty cycle; an edge pulse detector detecting an edge of the input CLK to create trigger signal and generating a rising edge of an output signal of the duty cycle signal generator; a comparator device receiving the first linear increasing voltage signal and the reference signal voltage and for real-time comparing the first linear increasing voltage output signal to the reference signal, and, at a time the first linear increasing voltage output signal exceeds the reference signal, the comparator device generating a falling edge of the output signal in the current period of the output signal, wherein the rising and falling edges of the output signal of the duty cycle signal generator are timed according to the programmed duty cycle 
     Further to this alternate embodiment, there is provided a method for programmable duty cycle signal generation comprising: receiving an input clock signal (CLK); converting, using a saw tooth converter circuit, the input CLK signal to create a linear rising voltage signal representative of a full time period of the input CLK; and in each period: receiving at a digital to analog converter (DAC) bits representing a programmed output signal duty cycle; generating a voltage supply signal from the linear increasing voltage signal for input to the DAC; generating, by the DAC using the voltage supply signal and duty cycle bits, a reference signal voltage representative of a programmed duty cycle; detecting an edge of the input CLK to create trigger signal and generating a rising edge of an output signal of the duty cycle signal generator; comparing, in real-time, the reference signal voltage to the linear increasing voltage signal, and, at a time the linear rising voltage signal exceeds the reference signal voltage, generating a falling edge of the output signal in the current period of the output signal, the rising and falling edge of the output signal of the duty cycle signal generator timed according to the programmed duty cycle. 
     In embodiments, the programmable duty cycle generator comprise interconnections of semiconductor structures and circuits, including CMOS or MOSFET structures. 
     Certain embodiments of the presented programmable duty cycle generator structure and operating method may comprise individual or combined features, method steps or aspects as mentioned above or below with respect to exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiments of structures and methods relating to programmable duty cycle generation are described with reference to the enclosed drawings. 
         FIG. 1  shows a schematic diagram of an embodiment of a programmable duty cycle generator  10 ; 
         FIG. 2  shows a schematic diagram of an alternate embodiment of a programmable duty cycle generator  10 ; 
         FIG. 3  shows a timing diagram depicting the waveforms produced by the programmable duty cycle generator circuitry; 
         FIG. 4  is a flow chart depicting a method  100  performed by the duty cycle generator  10  of FIGS.  1  and  10 ′ of  FIG. 2 ; 
         FIG. 5  shows a flow chart detailing method  250  of operating the period integrator circuits  25  and  25 ′ of  FIGS. 1 and 2 ; 
         FIG. 6  shows pulse shaper methodology  300  performed by elements forming closed-loop pulse-shaping system  75  of  FIGS. 1 and 2 ; 
         FIG. 7  shows a programmable duty cycle generator  400  of a further embodiment; 
         FIG. 8  illustrates is a flow chart depicting a method  500  performed by the duty cycle generator  400  of  FIG. 7 ; 
         FIG. 9  shows saw-tooth conversion methodology  600  employed by the duty cycle generator of  FIG. 7  for generating the Sawtooth waveform representative of input CLK frequency; and 
         FIG. 10  shows pulse shaper methodology  700  performed by programmable duty cycle generator  400  of a further embodiment. 
     
    
    
     Like or functionally like elements in the drawings have been allotted the same reference characters, if not otherwise indicated. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of an embodiment of a programmable duty cycle generator  10 . 
     An electronic signal generator  12  provides a clock (CLK) signal  15  at a predetermined clock period T per  (CLK period), to a periodic signal integrator circuit  25 . In the generator  10 , periodic signal integrator circuit  25  is of a divide-by-2 architecture such that its output signal ON time equals input CLK period, i.e., a 50% duty-cycle divider architecture for period. In an embodiment, periodic signal integrator  25  includes a first integrator element  24  (Integrator 1) implementing saw tooth generator/Integrator schemes to create the saw type waveforms of the incoming CLK frequency that, in conjunction with Sample-hold circuit  28 , Digital-to-Analog Conversion circuit DAC  20  and other circuit elements forming a pulse-shaping system  75 , is used to create an output signal  50  of a desired duty cycle. 
     In one embodiment, the input periodic CLK signal  15  is first converted by a divide-by-2 circuit  16  providing an output periodic CLKby2 signal  17  having an output ON time equal to CLK time period T per . That is, the provided divider output periodic CLKby2 signal ON time duration plus its OFF time duration is equal to 2×CLK&#39;s Tper. This CLKby2 signal  17  is integrated in the voltage domain by first integrator element  24  to provide a linearly rising voltage signal i1out for the time length of Tper. This integration is done each CLK cycle for real time tracking of input CLK period. Use of NOR gate reset element  27  ensures that the integration always starts from ground reference for each successive time period. The maximum voltage of the linearly rising voltage signal, i1out, is sampled and held to generate a reference voltage V3. Successive time periods can be isolated by the use of divide-by-2 circuit  16 . In one embodiment, the Divide-by-2 architecture need not have a dependency that its output ON time equals CLK period. The ON and OFF times of divider output are “averaged” in voltage domain to provide a reference for Tper. 
     As shown in  FIG. 3 , in this embodiment, the divide-by-2 circuit  16  provides “50% duty cycle” signal  17  as a way of tracking each period of input CLK—the divider&#39;s output ON time will now correspond to a full time period of CLK, and so does its OFF time. This CLK frequency is used as the ON and/or OFF time pulse to trigger the integration. Hence it operates as a time tracker to demarcate when a full time period has gone by, and when a next time period begins, etc. 
     Alternatively, any mechanism that can detect rising edges (or falling edges) of CLK may be used to trigger integration from this rising (or falling) edge to the next rising (or falling) edge before resetting the integrator, may be used. Hence, the Integrator circuit  24  (and other integrator circuits/saw tooth converter circuits described with respect to  FIGS. 1 ,  2  and  7 ) are able to function off edge detections (instead of pulse ON or OFF times that the div2 element provides). Such a front end mechanism can be implemented in many ways using digital gates as long as they enable the integrator  24  to integrate from one edge to the next before resetting. 
     As further shown in  FIG. 1 , the CLKby2  17  besides being provided to a clock signal input of first integrator  24  (Integrator 1) for Integrator 1 integration (or time to voltage conversion). CLKby2 signal  17  is additionally received as one input at a logic gate, e.g., a NOR gate logic circuit element  27 , providing an output reset signal to a reset input at Integrator 1 for resetting integration operations to the ground reference at each cycle. In other words, when CLKby2 is high (ON time), the integrator is in integration mode (or non-reset mode). When CLKby2 is low (OFF time), the integrator is in reset mode after sampling is done and before the next rising edge of CLKby2. Other circuits besides the NOR circuit may be configured to receive the CLKby2 signal for reset functionality every period. For an Integrator reset, NOR circuit element  27  further receives a “sclk” signal  23  relating to the time base for sample-hold operations of the Integrator 1 output i1out signal  21  as will be described. That is, as shown in  FIGS. 1 ,  2  and  3 , the arrival of the falling edge of CLKby2 signal  17  signals that the “i1out” signal  21  has finished integrating and that its voltage can now be sampled. Hence “sclk” is a sample signal  23  that may be a falling edge based pulse that can be used to sample the voltage of i1out signal  21  at sample and hold circuit. The sclk signal is hence used to give the consent to resetting the integrator after a delay equal or more than the time it takes the sample-hold 28 to sample. 
     Furthermore, Integrator 1 receives an input supply voltage V2, CLKby2 and reset input signals and generates i1out signal (e.g., a signal waveform having a saw rise profile) after every rising edge of CLKby2 for the duration of its ON time each cycle. Output signal i1out is received as input to sample and hold (Sample-hold) amplifier circuit  28 . Sample-hold circuit  28  further receives the sampling clock signal sclk  23  generated from the clk used at Integrator 1 providing time base for Sample-Hold circuit sampling operations. 
     The Integrator 1&#39;s charge-up slope is fixed irrespective of frequency change. Thus the maximum voltage V3 at the end of each T per  is unique (e.g., V3(f)) for a given CLK frequency (f=frequency). Integrator 1 is implemented using Resistive-Capacitive elements (RC) and provides gain such that its output voltage (V3) roof for a chosen CLK frequency range is less than its supply voltage (i.e., V3&lt;V2). 
     The V3 output (of the S-H circuit) is used as the power supply input to the DAC  20 . For any given Tper, the integrator charges from 0 v to Vper volts. Then Vper gets sampled as V3 at the arrival of sclk. Vper can be different for a different frequency and PVT. The V3 is a supply reference input to the DAC  20 . Thus, referring to the above timing of the waveforms shown in  FIG. 3 , V3 is the sampled i1out signal  21 , i.e., V3&#39;s voltage value is equal to i1out&#39;s voltage value at the sample point. 
     The DAC  20  translates user-programmed duty cycle bits  30 , i.e., bits Dcyset&lt;1:x&gt;, to create another reference voltage  35  which is a measure of a required duty cycle or refdcy_v. The output duty cycle can be programmed in steps of (½ x )·Tper. For example if a 6 bit DAC is used, the output duty cycle can be adjusted in 64 steps starting with 1.56% to 178.4% in steps of 1.56%. 
     In generator  10 , sample-hold circuit  28  samples V3 before the falling edge of i1out. This ensures the maximum voltage is sampled at the end of the integration duration. In one embodiment, this is performed using an operational amplifier (Op-amp) based structure (not shown) that uses the sampled V3 as reference and provides output voltage (also V3) but with sufficient current drive. This V3 signal is used as the supply (V3) for the DAC  20  with linear characteristics matching the integrator slope. So V3 is a voltage domain measure of time period of input CLK. 
     The sampling signal sclk is a pulse of certain duration dependent on a minimum time needed to perform a sample operation by the Sample-Hold circuit. It is generated after the integration duration time is complete. It can be triggered and generated off the falling edge of the signal being integrated. 
     The DAC (Digital to Analog converter)  20  performs converting digital bits  30  to an analog voltage  35  providing a reference voltage significant of required output duty cycle—that portion (time) of T per  that needs to be ON. The structure of DAC is as generally known in the art and different architectures are contemplated. 
     Programmable resistor dividers are used as the DAC in one example architecture. The number of digital states that can be created from bits Dcyset[1:x]  30  are first identified, which then translate to a unique reference voltage (refdcy_v) signal  35  between 0 and V3 volts that represents the indirect measure of required duty cycle to be achieved. The more the DAC&#39;s linearity matches the Integrators&#39; linearity across PVT, the more accurate the output duty cycle (signal  50 ) will be with respect to the required duty cycle needed. 
     In generator  10  of  FIG. 1 , a closed-loop pulse shaper  75  creates an output pulse  50  with the correct (programmed) duty cycle. This is accomplished using a pulse rising transition pulse  45  triggered off any one chosen edge of incoming CLK  15 . In one embodiment, output pulse  50  generation is accomplished with the set function of an SR latch. As shown, an edge-based pulse generator circuit  40  receives CLK input clock  15  providing up_OUT pulse  45  (output rising transition based on an CLK signal edge). This output rising transition pulse  45  is provided as input to a set function of an SR latch  55  generating output pulse  50 . Integrator 2  60  performs an integration of this pulse  50  after its output rising transition is started. The Integrator 2  60  is configured to have the same rising slope characteristics as the Integrator 1 (i.e., Integrator 1 and 2 of  FIG. 1  are matched with respect to RC time constants to have matching rise slopes and reduce mismatch of time period to voltage (t→v) conversion. 
     Comparator device  70  performs a real-time comparison of the output  65  of the second integrator  60  with the analog reference refdcy_v  35 . Once the output value equals or exceeds refdcy_v, the falling edge of the same output pulse  50  is forced. This is affected as drop_OUT signal  72  of comparator  70 . Thus falling edge of output pulse  50  is accomplished with the reset function of the SR latch  55  receiving drop_OUT signal  72 . The integrator  60  is also reset to ground reference in the mean time, e.g., via OR circuit element  80 , before the arrival of the next cycle of OUT&#39;s rising transition  45 . The OUT signal  50  then has the required duty cycle. 
     More particularly, in view of the signal timing diagram of  FIG. 3 , the ON_v signal  65  is of a saw like profile is input to the (on +&#39; ive terminal of comparator  70 ) while signal wave is refdcy_v signal  35 . When ON_v signal  65  exceeds refdcy_v signal  35  (on −&#39;ive terminal of comparator  70 ), the comparator will create the rising drop_OUT signal  72  which will trigger a reset of the SR latch  55 , hence creating the falling transition of OUT signal  50 . The 0→1 rising drop_OUT signal is additionally input to the OR gate  80 , making its output a logic “1”, hence initiating the reset function of the integrator2  60  and forcing the integrator2&#39;s output (which is ON_v) start to decay. Once it goes below the refdcy_v value, the drop_OUT signal created by comparator will be a falling transition from 1→0. Hence the delay of the elements from the above reaction determines the pulse width of drop_OUT signal. 
     It is understood that the reset signal of integrator2 element  60  is to be completely asserted until the next rising edge of CLK (edge converted as the signal UP_out  45 ). 
     Thus, the 1→0 transition of the OUT signal  50  now also is input to the OR gate  80  at the inverter terminal input, hence forcing the OR gate output to be 1. This keeps integrator2 in reset mode, even after the other input (drop_OUT  72 ) of the OR gate goes low. Thus integrator2 will stay in reset mode until the set (“S”) input of SR latch  55  gets asserted by UP_out  45 . 
     In an example implementation, an incoming CLK frequency (CLK) is equal to 1 GHz, with a dutycycle (On vs. off time) of 90%. With signal refdcy_v  35  being 0.35 v and signal V3 being 0.875 v, the output duty cycle of signal  50  is 0.35 v/0.875 v=0.4 (representing a required 40% duty cycle). Output duty cycle of the 50 of the programmable duty cycle generator  10  in the example was about 40% as shown in the timing diagram of the various waveforms as shown in  FIG. 3 . 
       FIG. 2  shows a schematic diagram of an alternate embodiment of a programmable duty cycle generator  10 ′. Like elements shown in  FIG. 2  correspond to those of  FIG. 1 . In one respect generator  10 ′ replaces the periodic signal integrator  25  of  FIG. 1  with periodic signal integrator  25 ′ including a first integrator element  24 ′ (Integrator 1A) and second integrator element  24 ″. The first integrator element  24 ′ (Integrator 1A) functions identically as the first integrator element  24  (Integrator 1) of  FIG. 1 . It is preferred that Integrator 1A, 1B, and 2 of  FIG. 2  are matched with respect to RC time constants to have matching rise slopes and reduce mismatch of input CLK period time to voltage (t→v) conversion. In the embodiment of  FIG. 2 , programmable duty cycle generator  10 ′ implements an averaging technique: The Div-by-2 architecture with 50% dutycycle output as shown in  FIG. 1  is not necessary as, in this generator embodiment, the divider circuit output&#39;s ON duration time is not necessarily equal to CLK&#39;s Tper, and similarly, the divider circuit output&#39;s OFF duration is not necessarily equal to CLK&#39;s Tper. In this embodiment, the two signals (divider circuit output&#39;s ON time and OFF time) are averaged to represent the full Tper. Hence the V3a voltage (voltage representation of divider output&#39;s ON time) and V3b (voltage representation of divider output&#39;s OFF time) are averaged. 
     Thus, while the divider circuit  16  is shown in  FIG. 2  as a divide by 2 component, it does not necessarily require a div-by-2 device. However, the divider circuit  16  is shown in  FIG. 2  as further generating a complement CLKby2 signal  17 ′ (the complement of the CLKby2 signal  17 ) that is provided to a clock signal input of second integrator (Integrator 1B)  24 ″ for clock signal integration thereof CLKby2 signal  17 ′ is additionally received as one input at an NOR circuit element  27 ′ providing an output reset signal to a reset input at Integrator 1B for resetting integration operations to the ground reference at each cycle (period). In other words, when CLKby2 is high (ON time), the Integrator 1A is in integration mode (or non-reset mode) while the Integrator 1B is in reset mode after sampling is done and before the next rising edge of the complement of CLKby2 signal  17 ′. When CLKby2 is low (OFF time), the Integrator 1A is in reset mode after sampling is done and before the next rising edge of CLKby2 while the Integrator1B is in integration mode (or non-reset mode). The NOR circuit may include a variety of other circuit elements and configurations as known in the art. The NOR circuit element  27 ′ further receives the sclk_b signal for resetting the Integrator 1B. 
     Integrator 1B particularly receives an input supply voltage V2, CLKby2 signal  17 ′ and reset input signals and generates i2out signal (e.g., a signal waveform having a saw rise profile) after every rising edge of CLKby2 for the duration of its ON time each cycle. Respective output signals i1out (generated by Integrator 1A) and i2out (generated by Integrator 1B) are received substantially simultaneously at respective sample and hold (Sample-hold) amplifier circuits  28 ′ and  28 ″. Each sample-hold circuit further receives a sampling clock signal, sclk, generated from the clk at the Integrators 1A, 1B providing timing control for sampling operations as described with respect to Sample and Hold operations of the generator  10  of  FIG. 1 . Thus, in this embodiment, a sampling of i1out occurs after the falling edge or CLKby2; hence sclk is a falling edge triggered pulse; and, a sampling of i2out occurs after the rising of CLKby2, hence sclk_b is a rising edge triggered pulse. The sclk (and sclk_b) signals are hence used to give the consent to resetting the integrators 1A (and 1B) respectively after a delay equal or more than the time it takes the sample-hold  28 ′ (and  28 ″) to sample. 
     In the embodiment of  FIG. 2 , each Integrator 1A and Integrator 1B generates a respective voltage domain measure V3A, V3B—voltage domain measure V3A being the ON time of the divider (divideby2) clock output (input of Integrator 1A) and voltage domain measure V3B being the OFF times of the divider (divideby2) clock output (input of Integrator 1B). An averaging circuit element  29  receives both V3A, V3B and generates from the V3A and V3B voltage measures an average V3 signal  31 ′ that is used to supply the DAC  20 . In one embodiment, the averager  29  is a DC averaging circuit to bring out the common mode value. One example is a form such as: 
     V3A connected to Resistor R1 (not shown) connected to V3 connected to Resistor R1 connected to V3B. 
       FIG. 4  is a flow chart depicting a method  100  performed by the duty cycle generator  10  of  FIG. 1  and the duty cycle generator  10 ′ of  FIG. 2 . 
     As shown in  FIG. 4 , method  100  includes at  101  receiving at the integrator circuit the clock waveform (or desired periodic electronic signal) to be duty cycle corrected. In the embodiment depicted in  FIG. 1  and timing diagram of  FIG. 3 , this represents clock waveform  15  being received at the divider such that the divided clock waveform  17  is provided at the clk input of the period integrator  25  of the generator  10  for integration measurement. Concurrently in time or simultaneously, at  102 , the desired waveforms duty cycle setting as represented as Dcyset&lt;1:X&gt; bits  30  are input to the DAC circuit  20  of the generator  10 . Continuing at  103 , the Integrator 1 device  24  performs an integration of the divided input clk signal as described in greater detail herein with respect to the method  250  of  FIG. 5 . Integrator 1 outputs a signal i1out  21  as shown in  FIGS. 1 and 3 . This i1out is provided to the input of the sample and hold circuit  28 . In the alternative embodiment on  FIG. 2 , Integrator 1a outputs a signal i1out  21 ′ and Integrator 1b outputs a signal i2out  21 ″ as shown in  FIGS. 2 and 3 . This i1out  21 ′ and is i2out  21 ″ are provided to the input of the sample and hold circuits  28 ′ and  28 ″ respectively. At  105 , the output of Sample and hold circuit provides the V3 voltage supply level (this would be a dc average for the alternate embodiment shown in  FIG. 2 ) to be input to the DAC  20  as shown in the generator  10 . In  FIG. 2 , at  107 , based on the desired duty cycle bit setting  30  and the V3 voltage at DAC input, the DAC performs the digital to analog conversion of the V3 signal to obtain the reference voltage of the required duty cycle measure refdcy_v voltage  35 . This DAC processing is performed at  107  in  FIG. 2  and waveforms shown in  FIG. 4 . In this manner, there is obtained the voltage domain measures of the ON and OFF times of divider output (t→v). The average, V3 signal  31  is used to supply the DAC  20 . Finally, at  109 , pulse shaper circuit elements forming a close loop system  75  using second integrator  60  and comparator  70  provides the desired output (clock) waveform  50  as shown in  FIGS. 1 and 3 . Generator circuit  10  of  FIG. 1  employs a pulse shaper methodology  300  described herein with respect to  FIG. 6 . 
       FIG. 5  shows a flow chart detailing method  250  of operating the periodic integrator circuits  25  and  25 ′ of  FIGS. 1 and 2 . At  253 , clock signal of an original duty cycle is input to divide by 2 circuit which changes the frequency of the clock for input to an Integrator 1 in the embodiment of  FIG. 1  (Integrator 1A and 1B in the embodiment of  FIG. 2 ). At  256 , the integrator performs an integration of the ON time duration, representative as a transformation of the clk on-time period to a voltage (i.e., t→v). Then at  258 ,  FIG. 5 , the maximum voltage is sampled at the end of the integration duration and the periodic integration process ends. 
     With respect to the implementation of Integrators 1A and 1B in embodiment of  FIG. 2 ,  FIG. 5  shows a simultaneous parallel integration process performed where Integrator 1A performs steps  253 ,  256  and  258  using CLKby2 signal  17  and simultaneously Integrator 1B performs identical steps  254 ,  257  and  259  using complement CLKby2 signal  17 ′ to provide the DC average value V3 signal  31 ′. 
       FIG. 6  shows pulse shaper methodology  300  performed by elements forming closed-loop output pulse shaping system  75  of  FIGS. 1 and 2 . First there is performed selecting an edge (rise or fall) of CLK and using this reference edge to create a rising edge signal of output signal OUT. The OUT&#39;s rising transition generated on that referenced CLK edge triggers the Integrator 2 to integrate OUT signal at  307 . An iterative process is initiated at  303  including first the arrival of the CLK reference edge to trigger the SR latch element and at the same time the rising edge transition to trigger the integration of the OUT&#39;s ON time duration, i.e., ON_v output of Integrator 2. Then, a determination is made at  309  as to whether the ON_v signal (On) time becomes greater than the measure of the DAC output, i.e., whether ON_v&gt;refdcy_v. As shown, the integrator 2 integrates at  309  until the instant Integrator 2 output (measure of the output duty cycle) becomes greater than the programmed refdcy_v reference level. At such time the Integrator 2 output (measure of the output duty cycle) is greater than the refdcy_v level, a falling transition of the generator OUT signal  50  is created and drop_out signal  72  is created. Simultaneously, at  311 , the Integrator 2 is reset using drop_out signal  72  and a logic element  80 , e.g., a NOR gate or like equivalent. The Integrator 2 output signal ON_v  65  goes to 0. Then, the process returns to step  303  which depicts the detecting the selected edge of the CLK signal of the next period from which the generator output clock ON time is measured. 
     In sum, there is “integrated” the entire CLK input signal period by the “period integrator” blocks (which repeats every cycle) from a minimum (“min”) to a maximum (“max”) value. Thus, a linear min to max value in voltage is generated equivalent to a full time period of CLKIN. In one embodiment, a div2 circuit approach is used with a 50% duty cycle architecture (its ON time is equal to its OFF time) where the divider&#39;s circuit output signal ON time represents a full CLK time period. That duration is linearized by integration like a sawtooth&#39;s linear rise. The linear DAC generates a reference voltage (signifying required duty cycle based on the user-programmed input bits) that lies between the min and max value of the linear rise signal. The created closed loop  75  implementing a comparator is such that generator output signal OUT&#39;s rising edge happens at the time of the “min” voltage is generated. The same OUT signal is integrated real-time. The comparator forces a falling edge at a time when the saw tooth&#39;s rise voltage equals the reference voltage (measure of desired duty cycle on time per period) and additionally resets the Integrator 2, i.e., the positive terminal of comparator  70  constantly ramps and resets every cycle. 
       FIG. 7  illustrates an alternate embodiment of a programmable duty cycle generator  400  with like elements indicated as in the generator  10  of  FIG. 1 . In the programmable duty cycle generator  400 , the integrator element  425  includes a saw tooth converter element  424  in place of Divider2 and Integrator 1 elements of the prior embodiments. In circuit  400  of  FIG. 7 , the generator is feed forward based (unlike feedback based in the prior embodiments), i.e., the method and circuit structure includes a feed forward based correction pertaining to a voltage reference output pulse creation with respect to incoming time period using a feed forward path for pulse shaping. 
     In circuit  400  of  FIG. 7 , the saw-tooth converter element  424  is a Sine or Square to Saw tooth converter  424  employed to convert the incoming clock signal to a conventional saw wave with linear rise and negligible fall. Hence, the time period of incoming clock signal is equivalent to the Saw output&#39;s rising time. A DAC (as in the prior embodiments) is configured to be supply driven of the maximum Saw voltage: either by sampling or direct set (if known based on saw converter&#39;s architecture). User programmed duty-cycle bits are converted by the DAC (as in the prior embodiments) to now provide a reference duty-cycle voltage (refdcy_v) that is a linear function of its supply. 
     The feed forward mechanics is such that the CLK edge creates an output pulse rising transition as well as it triggers the saw converter&#39;s rising transition. This may be accomplished with the set function of an SR latch  455 . The saw-tooth converter output is forwarded and compared to the refdcy_v. Once the saw-tooth converter output value equals or exceeds refdcy_v, the falling edge of the same output pulse is forced. This can typically be accomplished with the reset function of the SR latch  455 . The OUT signal  450  then will have the required duty cycle. 
     More particularly,  FIG. 7  shows the generated Saw tooth waveform saw-out signal  421  which is linear and has a negligible fall delay. The saw-out signal&#39;s output slope is constant irrespective of input frequency changes. So maximum output voltage at the end of Tper is, for example, V3 (V3&lt;V2), where V2 is the supply of the Saw converter. So V3 is a voltage domain measure of time period of the input CLK. 
     This V3 voltage is sampled just before the falling edge of Saw_out. This can be performed using operation amplifier sample-hold techniques that uses the sampled V3 as vref and provides output voltage also V3 but with sufficient current drive. This is used to generate the supply for a DAC with linear characteristics matching the saw converter slope. So V3 is a voltage domain measure of CLK (input) time period. 
     The sampling “sclk” signal is a pulse of certain duration dependent on minimum time needed to sample by the Sample-Hold circuit. It is generated after the saw output reaches maximum voltage and before the saw falling transition occurs. It can be triggered and generated off a time advanced version of the saw-falling edge. 
     The DAC (Digital to Analog converter) element  420  that converts digital bits to an analog voltage is used to provide reference voltage significant of required output dutycycle—that portion of Tper that needs to be ON time. The structure of DAC may comprise any well-known architecture, e.g., programmable resistor dividers. The number of digital states that can be created from bits Dcyset[1:x] are first identified, which then translate to a unique reference voltage (refdcy_v) between 0 and DAC&#39;s supply that represents the indirect measure of required duty cycle to be achieved. 
       FIG. 8  illustrates is a flow chart depicting a method  500  performed by the duty cycle generator  400  of  FIG. 7 . 
     As shown in  FIG. 8 , method  500  includes at  501  receiving the input clock waveform CLK at the sawtooth converter circuit (or desired periodic electronic signal) to be duty cycle corrected as in the embodiments depicted in  FIGS. 1 and 2 . Continuing at  503 , the Sawtooth converter device  424  performs a conversion of the input CLK signal. Sawtooth converter outputs a signal  421  as shown in  FIG. 7  which is provided to the input of the sample and hold circuit  428 . At  505 , the output of sample and hold circuit  428  provides the V3 voltage supply level to be input to the DAC  420  as shown in the generator  400 . Additionally, if needed, a ground signal “Vlo” is generated for the DAC. 
     That is, as the lowest voltage from where the sawtooth output  421  rises each cycle, that voltage should be the same as the ground potential of the DAC  420  to ensure that DAC voltage scale matches the Saw converter&#39;s min to max voltage scale. If the Saw converter architecture is such that the min voltage is different (e.g., Vlo), then the low point also needs to be sampled as the ground reference of the DAC. 
     Concurrently in time, or simultaneously, at  509 , the desired waveforms duty cycle setting as represented as Dcyset&lt;1:X&gt; bits  30  are input to the DAC circuit  420  of the generator  400 . From received bits  30 , and the V3 voltage at DAC input, the DAC circuit  420  performs the digital to analog conversion of the V3 signal to generate a reference voltage  435  representative of the desired output duty cycle setting as indicated at  511 . 
     Then at  515 , feed forward pulse shaping is performed based on the received reference voltage  435  representative of the desired output duty cycle from  511 , and the Sawtooth waveform  421  voltage level. The employing of pulse shaper methodology  600  is now described herein with respect to  FIG. 10 . 
       FIG. 9  shows saw-tooth conversion methodology  600  employed by the period integrator2  425  of  FIG. 7  for generating the Sawtooth waveform representative of input CLK frequency. At  601 , the sawtooth generator performs an integration of the time period of the input clock using the saw converter&#39;s rise. Then at  603  the max voltage (V3) is generated by sampling the sawtooth waveform at the end of the integration duration. Further, at  605 , the minimum or ground reference voltage Vlo is generated by sampling at the end of integration. 
       FIG. 10  shows pulse shaper methodology  700  performed by programmable duty cycle generator  400  of  FIG. 7 . At  703 , first the delay of the first edge based pulse generator  440  providing up_OUT signal which triggers the latch to form the output signal rising edge and is adjusted or matched to equal or match the delay engendered by the feed forward processing performed by the comparator, the Sawtooth generator, and a second edge based pulse generator element  442 . This may entail first selecting an edge (rise or fall) of input CLK and using this reference edge to create a rising edge signal by edge pulse generator circuit  440 . The referenced input CLK edge triggers the SR latch element  455  such that output signal  450  of the programmable duty cycle generator (i.e., OUT) has a rising transition generated on that SR latch edge trigger to form the OUT&#39;s signal ON time duration as indicated at  705 . Meanwhile, at  707 , a determination is made as to whether the rising saw_out signal  421 &#39;s voltage becomes greater than the reference voltage output from the DAC output, i.e., whether saw_out&gt;refdcy_v. That is, the comparator  460  performs real-time comparing of the saw_out signal  421  with the refdcy_v signal  435 , and when the saw_out signal  421  exceeds the refdcy_v, the falling edge of the same output pulse  450  is forced as indicated at  709  as comparator drop_out signal  472  is generated. The drop_out signal  472  is input to a second edge-based pulse generator  442  which generates a signal to reset the SR latch  455  and create the falling edge of the OUT signal  450 . The timing of the rising and falling edges of output OUT signal  450  complies with the programmed duty cycle. 
     In each of the embodiment described herein, the output signal tracks changes in incoming frequency, and is independent of incoming duty cycle variations. 
     While the disclosure has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Various embodiments of the present disclosure can be employed either alone or in combination with any other embodiment, unless expressly stated otherwise or otherwise clearly incompatible among one another. Accordingly, the disclosure is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the disclosure and the following claims.

Technology Category: 5