Apparatus and method for edge based duty cycle conversion

[0002] The present invention relates generally to integrated circuits and more particularly to an apparatus for duty cycle conversion.

[0003] Some integrated circuits decrease their associated delay times by doubling their data rates. Typically, to support data rate doubling a single phase clock input is split into true and complement clocks by a clock generator. These two internal clocks allow data sampling to occur on both the rising and falling edges of the single phase clock input. FIG. 1 illustrates a prior art Double Data Rate input Receiver (DRR) 26, which receives true and complement clocks from Clock Generator 20. From the single phase clock, the CLK signal, Clock Generator 20 generates the internal true and complement clock signals, which are labeled CLKL and CLKB, respectively. To promote satisfactory receive timing margins, the CLKL and CLKB signals should be 180° apart in phase. A Duty Cycle Correction Circuit may be used to force the CLKL and CLKB signals into this relationship.

[0004] FIG. 2 illustrates a prior art Duty Cycle Correction Circuit 30 coupled to a Clock Generator 20. The complementary clock signals, CLKL and CLKB, are input to the Duty Cycle Correction Circuit 30. Working in concert, Duty Cycle Correction Circuit 30 and Adjustor Circuit 40 set the duty cycle of the CLK signal such that the two cross points of the CLKL and CLKB signals are separated by half the cycle time. In contrast, DRR 26 is sensitive only to the rising edges of the CLKL and CLKB signals; for optimal timing, these rising edges should be separated in phase by 180 degrees. The DCCV and DCCVB signals allow Adjustor Circuit 40 to modify the duty cycle of the CLK signal. Measuring the differences between the CLKL and CLKB signals, the Duty Cycle Correction Circuit 30 is sensitive to the cross points of the CLKL and CLKB signals. In contrast, DRR 26 is sensitive only to the rising edges of the CLKL and CLKB signals. Thus, DRR 26 and Duty Cycle Correction Circuit (DCCC) 30 use different definitions of duty cycle.

[0005] FIG. 3 plots the CLKL and CLKB signals and indicates their duty cycles using a number of definitions. Generally, duty cycle is defined as a signal's high time divided by the sum of the signal's high and low time; i.e., the total cycle time. No disagreement exists as to what constitutes a signal's total cycle time; however, there are a number of competing definitions of cycle high time. In FIG. 3 “t₁” indicates the high time of the CLKL signal as the time between when the rising edge of the CLKL signal crosses the falling edge of the CLKB signal and when the falling edge of the CLKL signal intersects the rising edge of the CLKB signal. Thus, t₁represents the cross-point high time to which the DCCC 30 is sensitive. In FIG. 3 “t₂” indicates the high time of the CLKL signal as the time from when the voltage of the rising edge of the CLKL signal exceeds a selected threshold level to when the voltage of the rising edge of the CLKB signal exceeds the selected threshold level. Thus, t₂represents the rising edge high time to which the DRR 26 is sensitive. Within an integrated circuit using a DRR 26 and a Duty Cycle Correction Circuit 30 even a small difference between the cross-point high time and the rising edge high time substantially impacts receive time margins. Thus, a need exists for a converter circuit to generate a pair of signals whose cross-point duty cycle is equal to the midpoint duty cycle of the CLKL and CLKB signals. Inserted between a DRR 26 and a Duty Cycle Correction Circuit 30, such a converter circuit would improve receive time margins.

[0006] The apparatus of the present invention generates a pair of output signals whose cross-point duty cycle is substantially equal to the edge duty cycle of a pair of input signals. The apparatus of the present invention includes an edge detector and a signal generator. The edge detector detects and indicates a first transition of a first input signal and a second transition of a second input signal. The signal generator takes the outputs of the edge detector and generates a first output signal and a second output signal offset from the first output signal. The signal generator does so by forcing a first time delay between adjacent cross-over points of the first and second output signals to be substantially equal to a second time delay between the first transition and the second transition.

[0007] Additional features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which:

[0008] FIG. 1 illustrates a prior art Double Data Rate input Receiver (DRR).

[0009] FIG. 2 illustrates a prior art Duty Cycle Correction Circuit coupled to a DRR.

[0010] FIG. 3 plots the clock signals input to the DRR of FIG. 1 and indicates their duty cycles using a number of definitions.

[0011] FIG. 4 illustrates an integrated circuit including the Duty Cycle Converter of the present invention.

[0012] FIG. 5 is a timing diagram of the input and output signals associated with the Duty Cycle Converter of FIG. 4.

[0013] FIG. 6 illustrates in block diagram form an embodiment of the Duty Cycle Converter.

[0014] FIG. 7 illustrates an embodiment of the Edge Detector of the Duty Cycle Converter.

[0015] FIG. 8 illustrates an embodiment of the Signal Generator of FIG. 6.

[0016] FIG. 9 illustrates schematically the embodiment of the Signal Generator of FIG. 8.

[0017] FIG. 10 illustrates another embodiment of Edge Detector of FIG. 6.

[0018] FIG. 11 illustrates an embodiment of a Duty Cycle Converter that is sensitive to negative polarity transitions.

[0019] FIG. 12 is a timing diagram of the input and output signals associated with the Duty Cycle Converter of FIG. 11.

[0020] FIG. 4 illustrates an Integrated Circuit 50 including the Duty Cycle Converter 60 of the present invention. Clock Generator 20 generates a true clock, the CLKL signal on line 22, and a complement clock, the CLKB signal on line 24, from a single phase clock, the CLK signal on line 21. Duty Cycle Converter 60, Duty Cycle Correction Circuit 30 and Adjustor Circuit 40 cooperate to bring to 180° the phase difference between the CLKL and CLKB signals, which are used to clock DRR 26. Duty Cycle Converter 60 receives the CLKL and CLKB signals and determines when their selected polarity transitions occur. The selected polarity transitions may be positive or negative. The time delay between the selected polarity transitions of the CLKL and CLKB signals are indicative of their selected polarity duty cycle. Duty Cycle Converter 60 uses these selected polarity transitions to generate a pair of output signals, the CCLKL and CCLKB signals, whose cross-points are substantially equal to the difference between the selected edge transitions of the CLKL and CLKB signals. Thus, Duty Cycle Converter 60 enables Duty Cycle Correction Circuit 30 and Adjustor Circuit 40 to offset the CLKB signal by 180° from the CLKL signal, thereby improving receive timing margins within Integrated Circuit 50. Receive timing margins may be improved by as much as 50 picoseconds using Duty Cycle Converter 60.

[0021] A. A Duty Cycle Converter Sensitive to Positive Polarity Transitions

[0022] FIG. 5 is a timing diagram of the input and output signals associated with a Duty Cycle Converter 60asensitive to positive polarity transitions; i.e., rising edges. FIG. 5 shows the CLKL signal 23 on line 22 and the CLKB signal 25 on line 24. The Figure also shows the CCLKL signal 63 output on line 62 and the CCLKB signal 65 on line 64. FIG. 5 illustrates that the time t₃between the rising edge midpoints of the CLKL signal and the CLKB signal equals the time t₄between adjacent cross-points of CCLKL signal 63 and CCLKB signal 65.

[0023] FIG. 6 illustrates in block diagram form a Duty Cycle Converter 60asensitive to positive polarity transitions. Duty Cycle Converter 60 includes Edge Detector 66aand Signal Generator 68a.Edge Detector 66areceives as inputs the CLKL signal on line 22 and the CLKB signal on line 24. Edge Detector 66adetects the midpoint of the rising edges of the both the CLKL and CLKB signals and generates output signals indicating these events, which are coupled to Signal Generator 68a.Edge Detector 66aindicates the midpoint of the rising edge of the CLKL signal via the TrueRiseDetect signal on line 80. FIG. 5 illustrates the TrueRiseDetect signal as Waveform 81 Waveform 81 indicates the rising edge of the CLKL signal with pulse 85. Referring again to FIG. 6, Edge Detector 66aindicates the midpoint of the CLKB signal with the ComplementRiseDetect signal on line 82. FIG. 5 illustrates the ComplementRiseDetect signal via Waveform 83, which indicates the rising edge of the CLKB signal with pulse 87.

[0024] While an embodiment of Edge Detector 66 that is sensitive to the midpoints of rising edges of the CLKL and CLKB signals has been discussed, other embodiments are possible. Another embodiment of Edge Detector 66 sensitive to falling, rather than rising, edges of the CLKL and CLKB signals will be discussed below with respect to FIGS. 11-12. In yet another embodiment, Edge Detector 66 may be sensitive to voltage levels other than the midpoints of the CLKL and CLKB signals, for example, such as voltage levels representing 20% or 80% of the maximum voltage level of the CLKL and CLKB signals.

[0025] Referring to FIG. 6, Signal Generator 68atakes the TrueRiseDetect and ComplementRiseDetect signals and generates the CCLKL signal on line 62 and the CCLKB signal on line 64. Signal Generator 68aresponds to the active state of the TrueRiseDetect signal by forcing the CCLKL signal to an active state and the CCLKB signal to an inactive state. FIG. 5 represents the active state as a high voltage level and the inactive state as a low voltage level; however, other voltage levels may be used to represent the active and inactive states consistent with the present invention.

[0026] Referring once again to FIG. 6, Signal Generator 68aresponds to the active state of the ComplementRiseDetect signal by forcing the CCLKL signal to an inactive state and the CCLKB to an active state. By forcing both the CCLKL and CCLKB signals to transition between states at the same time, Signal Generator 68 forces the cross-point duty cycle of these signals to equal the rising edge duty cycle of the CLKL and the CLKB signals.

[0027] FIG. 7 illustrates an embodiment of Duty Cycle Converter 60athat realizes Edge Detector 66aas two Positive Edge Detectors 67aand 67b.Positive Edge Detector 67arecognizes the midpoint of the CLKL signal and in response generates the TrueRiseDetect signal on line 80. Positive Edge Detector 67brecognizes the midpoint of the CLKB signal and in response generates the ComplementRiseDetect signal on line 82.

[0028] FIG. 8 illustrates in greater detail the embodiment of Edge Detector 66aof FIG. 7. In the illustrated embodiment, each Positive Edge Detector 67aand 67bincludes a logical NAND gate 100 and a group of serially coupled Inverters 102. Within Positive Edge Detector 67a,one input to logical NAND gate 100ais coupled directly to the CLKL signal. The CLKL signal is also input to the group of serially coupled Inverters 102a,the output of which is coupled to the other input of logical NAND gate 100a.Observe that prior to a digital low to high transition, serially coupled Inverters 102aapply a digital high value to one input node of logical NAND gate 100aTherefore, the output of logical NAND gate 100awill go low when a digital high signal is received at the other input node of logical NAND gate 100a.Thus, Positive Edge Detector 100aresponds to a selected point on the rising edge of the CLKL signal by pulsing low the TrueRiseDetect signal on line 80. The location of the selected point on the rising edge is a function of the threshold voltage, V_THof logical NAND gate 100a.Thus, control of V_THallows the selected point of the rising edge to be set at any desired percentage of the CLKL signal. The duration of the low pulse of TrueRiseDetect signal is determined by the number of inverters included within the group of serially coupled Inverters 102a.The total delay produced by the group of serially coupled Inverters 102ashould be sufficient to cause Signal Generator 68 to change state. Positive Edge Detector 67boperates in a similar fashion, and is preferably matched, to Positive Edge Detector 67a.

[0029] In the embodiment of FIG. 8, Signal Generator 68ais realized as a Set-Bar Reset-Bar (SBRB) Flip-Flop 68a.FIG. 9 illustrates schematically SBRB Flip-Flop 68a,which includes a pair of logical NAND gates 110 and 112, coupled together in the classic Flip-Flop configuration. One input of logical NAND gate 110 is coupled to the TrueRiseDetect signal on line 80, while the other input of logical NAND gate 110 is coupled to the output of logical NAND gate 112, on line 64. One input of logical NAND gate 112 is coupled to the ComplementRiseDetect signal on line 82, while the other input of the logical NAND gate 112 is coupled to the output of logical NAND gate 110. FIG. 5 illustrates the operation of SBRB Flip-Flip 68a,plotting both its input signals 81 and 83, and its output signals 63 and 65.

[0030] FIG. 10 illustrates another embodiment of Edge Detector 66a,which, in addition to Positive Edge Detectors 67aand 67b,includes Matching Input Circuits 69aand 69b.Inserted between an input signal and a Positive Edge Detector 67, each Matching Input Circuit 69 helps match the loads driven by the CLKL and CLKB signals, as well as matching the switching thresholds. Each Matching Input Circuit 69aand 69bis an identical DRR, which includes a D Flip-Flop (D-FF) 120 and an Inverter 122, coupled between the Q output and the D input of D-FF 120.

[0031] B. A Duty Cycle Converter Sensitive to Negative Polarity Transitions

[0032] FIG. 11 illustrates schematically Duty Cycle Converter 60b,which is sensitive to negative polarity transitions; i.e., falling edges. Duty Cycle Converter 60bincludes Edge Detector 66band Signal Generator 68b.Edge Detector 66bis realized as two Negative Edge Detectors 130aand 130b.Negative Edge Detector 130arecognizes the midpoint of the falling CLKL signal and in response generates the TrueRiseDetect signal on line 136. Negative Edge Detector 130brecognizes the midpoint of the falling CLKB signal and in response generates the ComplementRiseDetect signal on line 138. Each Negative Edge Detector 130 is realized as a group of serially coupled inverters an a logical NOR gate, coupled together in the same configuration as used in the Positive Edge Detectors. Signal Generator 68bis realized a pair of logical NOR gates 150 & 152, coupled together in the classic Flip-Flop configuration.

[0033] FIG. 12 is a timing diagram of the input and output signals associated with a Duty Cycle Converter 60b.FIG. 12 shows the CLKL signal 160 and the CLKB signal 162. The Figure also shows the CCLKL signal 164 and the CCLKB signal 166. Observe that the time t₅between the falling edges of the CLKL signal 160 and CLKB signal 162 equals the time t₆between adjacent cross-points of CCLKL signal 164 and CCLKB signal 166.

[0034] While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.