Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K5/00—Manipulating of pulses not covered by one of the other main groups of this subclass
  • H03K5/156—Arrangements in which a continuous pulse train is transformed into a train having a desired pattern
  • H03K5/1565—Arrangements in which a continuous pulse train is transformed into a train having a desired pattern the output pulses having a constant duty cycle
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K5/00—Manipulating of pulses not covered by one of the other main groups of this subclass
  • H03K5/15—Arrangements in which pulses are delivered at different times at several outputs, i.e. pulse distributors
  • H03K5/151—Arrangements in which pulses are delivered at different times at several outputs, i.e. pulse distributors with two complementary outputs
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K5/00—Manipulating of pulses not covered by one of the other main groups of this subclass
  • H03K5/153—Arrangements in which a pulse is delivered at the instant when a predetermined characteristic of an input signal is present or at a fixed time interval after this instant
  • H03K5/1534—Transition or edge detectors
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K5/00—Manipulating of pulses not covered by one of the other main groups of this subclass
  • H03K2005/00013—Delay, i.e. output pulse is delayed after input pulse and pulse length of output pulse is dependent on pulse length of input pulse
  • H03K2005/00019—Variable delay

Definitions

• the clock signal synchronizes communication between different parts within the circuit and between different circuits. Circuits designed to operate with the clock signal may respond at the rising or falling edge of the clock signal.
• Single data-rate (SDR) memory interfaces normally utilize only either the rising or falling edge to transfer data.
• Double data-rate (DDR) memory interfaces transfer data on both the rising edge and falling edge of the clock signal.
• the data transfer rate in a DDR memory interface is twice as fast as the transfer rate of a regular SDR memory interface.
• a duty cycle is the fraction of time that the clock or system is in an "active" state.
• a substantially symmetrical duty cycle is required for most high speed applications as asymmetric duty cycles will make the synchronization of all the clocks in a system more difficult.
• a symmetrical duty cycle or a 50% duty cycle means each clock period has equal high and low periods. In other words, a clock signal with 50% duty cycle spends half the clock period at logic 1 and the other half at logic 0.
• Embodiments of the present invention include circuits and a method for correcting duty cycle distortions in an integrated circuit (IC).
• IC integrated circuit
• an IC with a corrector circuit includes a splitter circuit that receives a clock signal and splits the clock signal into two different clock signals.
• one of the clock signals is an inverted version of the other.
• Each of the clock signals is coupled to a delay circuit.
• Each of the delay circuits generates a delayed version of the corresponding clock signal.
• a corrector circuit is coupled to both the delayed versions of the clock signals to generate a clock output signal based on the delayed versions of the clock signals.
• the generated clock output signal has a corrected duty cycle.
• a duty cycle correction circuit includes a latch and pulse generator circuits.
• One pulse generator circuit is coupled to receive a first clock signal.
• a first pulse signal is generated by the pulse generator circuit based on the first clock signal.
• Another pulse generator circuit is coupled to receive a second clock signal.
• a second pulse signal is generated by the pulse generator circuit based on the second clock signal.
• the latch receives the two pulse signals and generates a clock output signal with a corrected duty cycle based on the two pulse signals.
• a method of operating an IC includes splitting a clock signal into two different clock signals. A delay is applied to each of the two clock signals to generate delayed versions of the two clock signals. Two pulse signals are generated based on each of the delayed versions of the two clock signals. The generated pulse signals transition from one logic level to another based on the transition of the delayed version of the corresponding clock signal. A clock output with a corrected duty cycle is generated based on the first and second pulse signals.
• Figure 1 shows an exemplary waveform of a clock signal with an uneven duty cycle and an exemplary waveform of a clock signal with an even duty cycle.
• Figure 2A shown to be illustrative and not limiting, shows an exemplary circuit with duty corrector circuits for a memory interface in an IC as one embodiment in accordance with the present invention.
• FIG. 2B shown to be illustrative and not limiting, shows a memory interface read path circuit as one embodiment in accordance with the present invention.
• FIG. 3A shown to be illustrative and not limiting, shows a more detailed representation of a leveling block as one embodiment in accordance with the present invention.
• FIG. 3B shown to be illustrative and not limiting, shows a more detailed representation of a delay chain as one embodiment in accordance with the present invention.
• Figure 4 shown to be illustrative and not limiting, shows a more detailed representation of a corrector circuit as one embodiment in accordance with the present invention.
• Figure 5A shown to be illustrative and not limiting, shows a more detailed representation of a splitter circuit as one embodiment in accordance with the present invention.
• Figure 5B shown to be illustrative and not limiting, shows a more detailed representation of a pulse generator circuit.
• FIG. 5C shown to be illustrative and not limiting, shows a detailed representation of a latch as one embodiment in the present invention.
• Figure 6 shown to be illustrative and not limiting, shows exemplary waveforms that describe the relationship between the clock signal, the inverted version of the clock signal, the set pulse signal, the reset pulse signal and the clock output signal.
• Figure 7 is a simplified flow chart that shows a method flow for operating an IC as another embodiment in accordance with the present invention.
• Embodiments of the present invention include circuits and a method for correcting duty cycle distortions in an integrated circuit (IC).
• IC integrated circuit
• the embodiments described herein provide circuits and techniques to correct duty cycle distortions in an IC to generate a clock signal with approximately 50% duty cycle across all PVT ranges. It should be appreciated that in high speed applications, e.g., high speed memory interfaces like DDR3, a 50%, or close to 50%, duty cycle is required to ensure that the transmission window is wide enough to ensure that data is appropriately captured. In high speed applications, clock signals with an asymmetric duty cycle may make data synchronization more difficult.
• the disclosed embodiments provide a dynamic duty cycle correction circuit that can easily correct duty cycle distortions in clock signals within an IC device.
• FIG. 1 shows exemplary waveform 100 of an asymmetrical clock signal and waveform 110 of a symmetrical clock signal. Waveform 100 has an uneven period of high and low for each clock period. Interval T as shown on waveform 100 is the clock period of the clock signal.
• one clock period refers to the period from one rising/falling edge to the next rising/falling edge.
• the duty cycle parameter specifies the relative duration of the logic high output during each clock period.
• each clock period T has a relatively shorter logic high duration compared to its logic low duration.
• a symmetrical clock signal has a substantially 50% duty cycle.
• each clock period T should have an equal, or almost equal, duration of logic high and logic low outputs.
• Exemplary waveform 110 of Figure 1 shows a symmetrical clock signal.
• Each clock period T on waveform 110 has an even duration of high and low outputs.
• waveform 110 is a waveform of a symmetrical clock signal with a 50% duty cycle.
• Figure 2A shown exemplary circuit 200 with duty corrector circuits for a memory interface in an IC as one embodiment in accordance with the present invention.
• the embodiment of Figure 2A shows the write path of a memory interface in an IC and output pins 250A and 250B may be connected to an external memory module.
• output pins 250A and 250B may represent the data (DQ) pin and the data strobe (DQS) pin, respectively.
• the embodiment of Figure 2A shows leveling block 210 coupled to I/O registers 240 A and 240B through logic block 230.
• Clock signal 202 is coupled to leveling block 210.
• leveling block 210 includes a delay chain that is coupled to receive clock signal 202.
• the delay chain is a plurality of buffers connected in series that generates a plurality of delayed versions of clock signal 202.
• Output 212 of delay locked-loop (DLL) 215 is coupled to leveling block 210.
• DLL 215 is coupled to an enable input of each of the plurality of buffers in leveling block 210, details of which will be explained in later paragraphs.
• leveling block 210 is a current- starved delay chain. DLL 215 outputs and sends a delay setting to leveling block 210 to control the delay of each leveling stage and the delay setting is maintained across different PVT conditions.
• Leveling block 210 generates multiple delayed versions of clock signal 202 as outputs 214. [00028] Referring still to Figure 2A, outputs 214 are connected to corrector circuits
• each of corrector circuits 220 includes a set/reset (SR) latch.
• An output of each of corrector circuits 220 is coupled to selector circuits 235 A and 235B.
• Outputs 237A and 237B are coupled to I/O registers 240A and 240B, respectively.
• I/O registers 240A and 240B include multiple storage circuits or registers connected to various logic elements.
• Outputs 237 A and 237B are connected to the clock terminals of registers 244A - 244H in I/O registers 240 A and 240B, respectively.
• I/O registers 244A - 244H are triggered by edges of the clock output signals from corrector circuits 220.
• output 237 A is connected to the clock terminals of registers 244A - 244D, and select input 243C of selector circuit 242C in I/O register 240A.
• Registers 244A and 244B coupled to logic gate 246A and selector circuit 242A, control enable terminal 249A of output buffer 245A.
• Output buffer 245A is coupled to output pin DQ 250A.
• Registers 244C and 244D are coupled to output buffer 245 A through selector circuits 242C and 242B.
• Input terminal 24 IB of register 244B is coupled to the output of register 244A.
• input terminals 241 A, 241 C and 24 ID may be connected to the core of the IC that may provide either a logic high or a logic low level.
• selector circuits 242A and 242B can be used to bypass registers 244A-244D, logic gate 246A and selector circuit 242C.
• select inputs 243A and 243B are connected to a static value or a configuration random access memory (CRAM) bit to control the output of selector circuits 242A and 242B, respectively.
• input terminals 247A and 247B are coupled to the core the IC.
• input terminals 247A and 247B may be connected to registers in the core of the IC.
• the output of selector circuit 242A enables and disables output buffer 245A and the output of selector circuit 242B carries output data from I/O register 240A to output pin DQ 250A through output buffer 245A.
• I/O register 240B shares similarities with I O register 240A.
• elements shown in I/O register 240A and described above e.g., registers 244E - 244F, input terminals 241E - 241H, input terminals 247D and 247E, select inputs 243D and 243E, selector circuits 242D - 242F and logic gate 246B, are not repeated or described in detail here.
• Selector circuit 235B of logic block 230 selects one of the delayed versions of the clock signal from leveling block 210 as an output signal.
• select input 234B is coupled to a core logic block in the IC.
• the core logic block controls select input 234B and selects different phases during a calibration process in order to determine a suitable phase for the clock signal 202.
• select input 234A of selector circuit 235A selects one of input signals 231 as output 237 A according to a selection of selector circuit 235B.
• selector circuit 235A may select a phase shifted version of the input signal 231 that is selected as output 237B of selector circuit 235B.
• outputs 237A and 237B are substantially 90° out of phase alignment.
• Output 237B is coupled to the clock terminals of registers 244E - 244H in I/O register 240B.
• Outputs of selector circuits 242D and 242E in I/O register 240B are connected to enable terminal 249B and output buffer 245B, respectively.
• Output buffer 245B is connected to output pin 250B.
• output pins 250A and 250B are DQ and DQS pins, respectively, and both DQ and DQS pins are connected to an external memory module.
• DQ and DQS signals are used in DDR memory interfaces.
• DQS is asserted when DQ pins have data to be written to the memory module.
• FIG. 2B shown circuit 290 as one embodiment in accordance with the present invention.
• circuit 290 represents a read path of a memory interface circuit in an IC. As such, well known elements are not described in detail so as to not obscure the present invention.
• pins 280A and 280B are connected to an external memory module and pins 280A and 280B are input pins that read data from the memory module to the IC.
• pin 280A may be a DQS pin and pin 280B may be a DQ pin.
• Pin 280A is coupled to input buffer 265A that receives signals transmitted from pin 280A. Signals from input buffer 265A are transmitted to delay chain 270.
• delay chain 270 includes several buffers connected in series to produce a delayed version, or a phase shifted version, of the input signal.
• Corrector circuit 220 corrects distortions in the phase shifted clock signal to produce a clock signal with a substantially 50% duty cycle.
• the output from delay chain 270 is connected to corrector circuit 220.
• the output from delay chain 270 includes a delayed version of the input signal and an inverted version of the delayed version of the same signal. Details of delay chain 270 and corrector circuit 220, shown in Figures 3B and 4, respectively, will be described in later paragraphs.
• the output of corrector circuit 220 is coupled to one of the input terminals of logic gate 264B and the clock terminal of register 262C.
• the clock terminal of register 262C is an active low terminal, in one embodiment. In other words, in the embodiment of Figure 2B, register 262C is triggered by the falling edges of the received clock signal.
• register 262C outputs a logic high signal and is connected to one of the other input terminals of logic gate 264B.
• Selector circuit 235B selects a DQS enable signal to control registers 262A and 262B.
• the DQS enable signal is also known as a postamble clock signal. It should be appreciated that the postamble clock signal is a clock signal that has been calibrated to achieve correct timing for the DQS enable signal.
• the output of selector circuit 235B is connected to the clock terminals of selector circuits 262A and 262B.
• Selector circuits 262A and 262B are coupled to logic gate 264A to control the clear terminal of register 262C.
• Register 262C is cleared when registers 262A and 262B are enabled.
• registers 262A and 262B, together with logic gate 264A, are enabled only when circuit 290 is reading from a memory module in order to prevent glitches in the circuit when circuit 290 is not reading from the memory module.
• the postamble signal from selector circuit 235B gates pin 280A to prevent glitches or false edges in the clock signal.
• pin 280B is connected to input buffer 265B.
• pin 280B is a DQ pin that reads data from an external memory module and transmits that data to input register 285.
• input register 285 includes a few storage elements, or registers, 262D - 262H, that are connected to each other as illustrated. Data or signals received at pin 280B are transmitted through input buffer 265B to registers 262D - 262H.
• Registers 262F, 262G and 262H are controlled by the corrected clock output signal from logic gate 264B. The clock output signal may be connected to delay circuits 280.
• delay circuits 280 are simple delay chains, e.g., two inverters coupled in series, that are used to correct or reduce clock skews in the circuit. As such, delay circuits 280 may be bypassed when the skew is substantially low. It should be appreciated that circuit 290 may be connected to an external memory module that operates with a different clock domain. Therefore, in one embodiment, registers 262D and 262E are used to resynchronize the clock signal from the external memory module with the clock signal within the system in this embodiment.
• FIG. 3A shown to be illustrative and not limiting, shows a more detailed representation of leveling block 210 as one embodiment in accordance with the present invention.
• Clock signal 202 is connected to splitter circuit 310.
• Splitter circuit 310 splits clock input signal 202 to generate two different clock signals 312A and 312B.
• clock signal 312B is an inverted version of clock signal 312A.
• the two clock signals, i.e., 312A and 312B are coupled to leveling delay blocks 210A and 210B.
• Each of leveling delay blocks 21 OA and 210B includes multiple delay buffers 211 connected in series. Each of buffers 211 generates a delayed version of the corresponding clock signals 312A and 312B.
• each of buffers 211 is connected to corrector circuit 220.
• DLL block 215 is connected to each of buffers 211 in leveling delay blocks 21 OA and 210B.
• Each of corrector circuits 220 outputs a clock signal with a corrected duty cycle based on the corresponding delayed clock signals from leveling delay blocks 21 OA and 210B.
• the clock signal generated by each of corrector circuits 220 is coupled to selector circuit 235 A and 235B.
• FIG. 3B shown a more detailed representation of delay chain 270 of Figure 2B as one embodiment in accordance with the present invention.
• DLL block 215 is connected to each of delay chains 270A and 270B.
• Terminal DQSIN 305 is connected to input buffer 265 A of Figure 2B.
• Splitter circuit 310 splits the clock signal received at terminal DQSIN 305 into clock signals 312A and 312B.
• clock signal 312B is an inverted version of clock signal 312A.
• Each of delay chains 270A and 270B includes multiple buffers 211 connected in series.
• corrector circuit 220 generates a clock signal with a corrected duty cycle based on the clock signals from delay chains 270A and 270B.
• the clock signal with the corrected duty cycle is connected to logic gate 264B and register 262C in the embodiment of Figure 2B.
• buffers 211 are shown in leveling blocks 21 OA and 210B of Figure 3A, and delay chains 270A and 270B of Figure 3B, one skilled in the art should appreciate that the required number of buffers may vary across different circuits for different applications. Thus, more or fewer buffers 211 can be used in leveling blocks 21 OA and 210B and delay chains 270A and 270B.
• FIG 4 shown to be illustrative and not limiting, shows a more detailed representation of corrector circuit 220 as one embodiment in accordance with the present invention.
• Corrector circuit 220 includes two pulse generator circuits, 415A and 415B, connected to latch 420.
• Clock signals 405 A and 405B are similar to the output signals from leveling blocks 21 OA, 210B and delay chains 270A and 270B in the embodiments of Figures 3 A and 3B.
• Clock signals 405 A and 405B are connected to pulse generator circuits 415A and 415B, respectively.
• Pulse generator circuits 415A and 415B generate relatively small pulses based on clock signals 405 A and 405B, respectively.
• latch 420 is an SR-latch and pulse generator 415A is connected to the set terminal of latch 420 while pulse generator 415B is connected to the reset terminal of latch 420.
• pulse generators 415A and 415B generate non-overlapping set and reset signals, respectively.
• output 425 of SR-latch 420 is a clock signal with a corrected duty cycle that is generated based on the pulses from pulse generators 415A and 415B.
• FIG. 5A shown a more detailed representation of splitter circuit 310 as one embodiment in accordance with the present invention.
• Splitter circuit 310 includes pass gate 510 and inverter 520.
• pass gate 510 is a complementary metal-oxide-semiconductor (CMOS) pass gate.
• CMOS complementary metal-oxide-semiconductor
• Splitter circuit 310 receives a clock signal, CLKin signal 505, and splits the signal into CLK 512 and CLKB 522 signals.
• CLKin signal 505 is transmitted through pass gate 510 and inverter 520.
• Inverter 520 inverts CLKin signal 505 to generate CLKB signal 522.
• clock signal CLKB 522 is an inverted version of clock signal CLK 512.
• Each of clock signals CLK 512 and CLKB 522 may be connected to a leveling delay block or a delay chain as shown in the embodiments of Figure 3A and 3B, respectively.
• FIG. 5B shown a more detailed representation of pulse generator circuit 415.
• Input signal IN 532 is connected to configurable delay chain circuit 550 in pulse generator circuit 415.
• input signal IN 532 may be connected to either clock signal CLK 512 or clock signal CLKB 522 as shown in Figure 5A.
• Configurable delay chain circuit 550 controls the width of the pulses that are generated by pulse generator circuit 415.
• the signal from configurable delay chain circuit 550 is inverted by inverter 520 and connected to logic gate 560.
• logic gate 560 is an AND gate that receives the inverted signal from configurable delay chain circuit 550 and input signal IN 532.
• Output signal OUT 562 is generated based on a logic state of input signal IN 532 and the inverted signal from configurable delay chain circuit 550.
• output signal OUT 562 is a signal with relatively small pulses.
• Output signal OUT 562 may be connected to a latch.
• output signals from pulse generators 415A and 415B are connected to the set and reset terminals, respectively, of latch 420.
• FIG. 5C shown to be illustrative and not limiting, shows a detailed representation of latch 420 as one embodiment in the present invention.
• latch 420 is an SR-latch and as such, well-known elements of such a latch are not described in detail so as not to obscure the present invention unnecessarily.
• Latch 420 includes two inverters, 520A and 520B, and two NAND gates, 570A and 570B.
• Pass gate 510 is coupled to the output of inverter 520A and an input of NAND gate 570A.
• pass gate 510 is used to equalize the signal paths from input signal S 562A to output OUT 572 and input signal R 562B to output OUT 572.
• Input signal S 562A and input signal R 562B are generated by a pulse generator circuit.
• input signal S 562A is generated by pulse generator 415A while input signal R 562B is generated by pulse generator 415B.
• output OUT 572 of latch 420 is a clock signal with a substantially 50% duty cycle that is generated based on the pulses from input signal S 562A and input signal R 562B.
• a latch is used in this embodiment, one skilled in the art should appreciate that similar logic or storage elements that perform similar functions can be used in place of a latch in this context.
• Figure 6 shown exemplary waveforms 610, 620, 630, 640 and 650 that describe the relationship between the clock input signal, the inverted version of the clock signal, the set pulse signal, the reset pulse signal and the clock output signal in accordance with one embodiment of the invention.
• Waveform 610 is an exemplary waveform of a clock input signal
• waveform 620 is an exemplary waveform of the inverted version of the clock input signal.
• the clock input signal is a phase shifted clock signal with a delay and the inverted version of the clock input signal is an inverted version of the phase shifted clock signal with the delay.
• the clock input signal is a clock output signal from one of buffers 211 in leveling block 21 OA and the inverted version of the clock output signal from one of buffers 211 in leveling block 210B of Figure 3 A.
• the clock input signal as represented by waveform 610 does not have a 50% duty cycle as each clock period, T, does not have an equal period of logic high and logic low level.
• the clock input signal has a substantially longer logic high period in each clock period, T.
• the inverted version of the clock signal as represented by waveform 620 also does not have a 50% duty cycle.
• waveform 630 shows the generated set signal.
• the set signal as represented by waveform 630 is an active-high signal with substantially short active pulses.
• an active-high signal is considered “active" when it is at a logic high level, i.e., logic 1.
• the set signal as represented by waveform 630 is active when waveform 630 is at a logic high state.
• Waveform 640 shows the generated reset signal with relatively short pulses.
• the set signal 630 changes from a logic low level to a logic high level when the clock input signal 610 transitions from a logic low level to a logic high level.
• the reset signal 640 changes from a logic low level to a logic high level when the inverted version of the clock input signal 620 transitions from a logic low level to a logic high level.
• the width, W, of pulses in the set signal and reset signal as represented by waveforms 630 and 640, respectively, can be adjusted.
• configurable delay chain circuit 550 is used to adjust the width, W, of each of the set 630 and reset 640 pulses.
• the width, W, of each of the set 630 and reset 640 pulses are less than 1 ⁇ 2 period, 1 ⁇ 2 T, of the clock input signal 610.
• waveform 650 shows a generated clock output signal with a substantially 50% duty cycle, i.e., equal periods of high and low logic levels.
• the clock output signal 650 is generated based on the set 630 and reset 640 signals.
• the clock output signal 650 changes from a logic low level to a logic high level when the set signal 630 changes from a logic low level to a logic high level.
• the reset signal 640 changes from a logic low level to a logic high level
• the clock output signal 650 transitions from a logic high level to a logic low level to achieve a substantially 50% duty cycle.
• the set and reset pulses as represented by waveforms 630 and 640, respectively, are used to set and reset latch 420 of Figure 5C.
• Output OUT 572 in Figure 5C represents the clock output signal shown in waveform 650.
• Each clock period, T, of the clock output signal in the embodiment of Figure 6 has a substantially 50% duty cycle.
• the clock output signal has a duty cycle that is not worse than 45/55.
• FIG 7 shown to be illustrative and not limiting, is a simplified flow chart that shows method flow 700 for operating an IC as another embodiment in accordance with the present invention.
• a first clock signal is split in operation 710 to generate second and third clock signals.
• CLKin signal 202 is split into CLK signal 312A and CLKB signal 312B by splitter circuit 310.
• the third clock signal may be an inverted version of the second clock signal, in one embodiment.
• a delay is applied to the second and third clock signals to generate delayed versions of the second and third clock signals in operation 720.
• the delayed versions of the second and third clock signals are phase shifted versions of the second and third clock signals.
• the delayed version of the clock signal is generated by leveling block 210.
• a pulse signal is generated based on the delayed version of the second clock signal in operation 730.
• the pulse signal has relatively small pulses of logic high level and the width of each of the pulses is less than half of the width of the logic high pulse of the second clock signal.
• a configurable delay chain circuit e.g., configurable delay 550 of Figure 5B, may used to adjust the width of each of the pulses.
• the pulse signal is a set pulse signal that is used to set a latch, such as SR-latch 420 as shown in Figure 5C. The pulse signal transitions from a logic low level to a logic high level when the second clock signal transitions from a logic low level to a logic high level.
• waveform 630 shows a representation of the pulse signal.
• the pulse signal generated in operation 740 is a reset pulse signal that transitions from a logic low level to a logic high level when the third clock signal transitions from a logic low level to a logic high level.
• the reset pulse signal is represented by waveform 640 in the illustrative embodiment of Figure 6.
• the reset pulse signal is used to reset latch 420 in the embodiment of Figure 5C.
• a clock output signal is generated in operation 750.
• the clock output signal, generated based on the two pulse signals generated in operations 730 and 740, has a corrected duty cycle.
• the clock output signal is represented by waveform 650.
• the rising edges of the clock output signal occurs substantially contemporaneously with the rising edges of the pulse signal generated in operation 730 while the falling edges of the clock output signal occurs substantially contemporaneously with the rising edges of the pulse signal generated in operation 740.
• the clock output signal is a clock signal with a substantially symmetrical, i.e., close to 50%, duty cycle.
• CMOS complementary metal-oxide-semiconductor
• PAL programmable array logic
• PLA programmable logic array
• FPLA field programmable logic array
• EPLD electrically programmable logic devices
• EEPLD electrically erasable programmable logic device
• LCDA logic cell array
• FPGA field programmable gate array
• ASSP application specific standard product
• ASIC application specific integrated circuit
• the programmable logic device described herein may be part of a data processing system that includes one or more of the following components; a processor; memory; I/O circuitry; and peripheral devices.
• the data processing system can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, digital signal processing, or any suitable other application where the advantage of using programmable or re-programmable logic is desirable.
• the programmable logic device can be used to perform a variety of different logic functions.
• the programmable logic device can be configured as a processor or controller that works in cooperation with a system processor.
• the programmable logic device may also be used as an arbiter for arbitrating access to a shared resource in the data processing system.
• the programmable logic device can be configured as an interface between a processor and one of the other components in the system.
• the programmable logic device may be one of the devices owned by the assignee.

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