APPARATUS FOR SKEW COMPENSATING SIGNALS

Abstract: A clock signal is transmitted to nodes (13) of each of several interconnected circuits (16) through a separate adjustable delay circuit (18), the time delay of each delay circuit being adjusted so that the clock signal arrives at each node (13) at the same time, thereby synchronizing operation of the separate integrated circuits one to another. Each delay circuit (18) comprises a set of signal delay elements which can be selectively switched into the clock signal path so that the clock signal delay may be adjusted by adjusting the number of signal delay elements in the clock signal path. Each signal delay element itself has a unit delay adjustable in proportion to an applied control voltage generated by a delay element monitor (30). The delay element monitor (30) measures the unit delay in relation to the period of a stable reference clock and adjusts the delay of each delay element as necessary to ensure that the unit delay remains constant.

Description: The present invention relates to an apparatus for skew compensating pulse signals having sequences of pulses transmitted to separate nodes of a circuit. Operations of separate parts of a typical large scale digital logic circuit are synchronized by one or more clock signals transmitted to many nodes in the circuit. For such a circuit to operate properly each pulse of a clock signal should arrive at each node within a time window that is substantially smaller than the period of the clock in order to avoid loss of synchronization. It is advantageous to implement a large digital logic circuit as a single integrated circuit because the various nodes in the circuit to which a clock signal may be transmitted are close together and clock signal pulses arrive at each node in the circuit at about the same time regardless of where the clock signal generator is located. Thus the frequency of operation of a single integrated circuit is usually not limited by variation in arrival time of clock signal pulses (i.e., "clock skew") at various nodes within the circuit. However, when a digital circuit is too large to be implemented as a single integrated circuit, it is necessary to partition the circuit into two or more integrated circuits interconnected by signal conductors which may be relatively long with respect to signal paths within each integrated circuit. In such case, a clock signal pulse originating in one integrated circuit may arrive at various nodes in other integrated circuits at substantially different times due to variation in signal delay of the signal paths the clock pulse must follow to reach each integrated circuit. Loss of synchronization due to clock skew can be avoided by reducing the frequency of the clock signal, but that reduces the speed of circuit operation. Partitioned circuits may also be designed so that each integrated circuit uses a separate clock and so that separate integrated circuits communicate asynchronously. But asynchronous communication between portions of a circuit involves time consuming handshaking protocols, and the time needed to perform handshaking reduces the rate at which data can be exchanged. It is known from EP-A-0 091 375 to deskew timing signals supplied by an automatic test equipment to separate nodes of a circuit to be tested. The equipment comprises a plurality of delay circuits, the variable delay of which is controlled by a test system computer in order to simultaneously impose test signals on various nodes of the circuit to be tested. From GB-A-2 045 561 a delay circuit is known for producing output signals delayed from corresponding input signals by an adjustable delay time. The variable delay circuit consists of a chain of logic gates each being coupled to a multiplexer for selecting the output of any chosen one of the gates in accordance with a first control signal applied to the multiplexer. the logic gates are connected to a variable voltage supply arranged to vary the threshold voltage at which the gates become enabled. Varying the threshold voltage permits adjustment of the intrinsic delay of the gates. It is an object of the invention to provide an apparatus for skew compensating pulse signals having improved delay time stability. The invention relates to an apparatus for skew compensating pulse signals having sequences of pulses transmitted to separate nodes of a circuit, comprising: a plurality of delay circuits coupled to receive and variably delay the pulse signals transmitted to the nodes; and means for generating a plurality of first delay control signals, each of the first delay control signals being applied to a first control signal input of a separate one of said delay circuits for determining the amount of pulse signal delay provided by the delay circuit, each first delay control signal being set so that pulses of separate output signals of the delay circuits corresponding to the pulse signals arrive at the nodes substantially at the same time. The apparatus is characterized in that each delay circuit comprises first unit delay elements adapted for varying the unit delay of the delay circuit in accordance with a second delay control signal applied in common to a second control input of each delay circuit; the apparatus further comprising: means for generating a periodic reference clock signal having a constant period; a first oscillator comprising second unit delay elements similar to said first unit delay elements for producing a first periodic output signal of a period determined and varying in accordance with said second delay control signal; and means for producing said second delay control signal with a magnitude determined in accordance with a comparison of the period of said reference clock signal with the period of said first periodic output signal for maintaining the unit delays constant. In accordance with the invention, a clock signal is transmitted to nodes of each of several interconnected synchronous integrated circuits through separate adjustable delay circuits, and the time delay of each delay circuit is adjusted so that the clock signal arrives at each node at the same time, thereby synchronizing operation of the separate integrated circuits one to another. Each delay circuit comprises a set of signal delay elements which can be selectively switched in series with the clock signal path whereby the clock signal delay may be adjusted by adjusting the number of signal delay elements in the clock signal path. Each signal delay element itself has a unit delay which is adjustable in proportion to an applied control voltage generated by a delay element monitor. The delay element monitor measures the unit delay in relation to the period of a reference clock and adjusts the delay of each delay element as necessary to ensure that the unit delay remains at a constant predetermined value despite changes in ambient temperature or aging of circuit components. The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation of the invention, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. In the drawings : FIG. 1 is a block diagram of a skew compensation circuit in accordance with the present invention; FIG. 2 is a block diagram of a delay circuit of FIG. 1; FIG. 3 is a schematic diagram of a delay element; FIG. 4 is a timing diagram illustrating operation of the delay element of FIG. 3; FIG. 5 is a schematic diagram of the fine delay circuit of FIG. 2; FIG. 6 is a schematic diagram of the coarse delay circuit of FIG. 2; FIG. 7 is a schematic diagram of the delay element monitor of FIG. 1; FIG. 8 is a schematic diagram of the insertion delay monitor of FIG. 1; FIG. 9 is a combination block and schematic diagram of a phase generator of FIG. 1; and FIGS. 10 and 11 are schematic diagrams of phase generator elements of FIG. 9. Detailed Description With reference to FIG. 1, a skew compensation circuit 10 in accordance with the present invention is adapted to transmit a clock signal (CLOCK) generated by a master clock generator 12 to nodes 13 within each of a plurality of separate integrated circuits 16. The CLOCK signal is transmitted to each node 13 via separate adjustable delay circuits 18 and transmission lines 19, and each delay circuit 18 is adjusted to delay the CLOCK signal so that each pulse of the CLOCK signal arrives at each node 13 substantially at the same time. A phase generator 14 implemented in each integrated circuit 16 monitors the clock signal arriving at node 13 and produces one or more clock phase signals used to control the timing of the integrated circuit 16 in which it is implemented. Clock phase signals are produced sequentially at regular intervals after receipt of the first pulse of the CLOCK signal following deassertion of a RESET1 signal applied as input to the phase generator, and following receipt of every CLOCK signal pulse thereafter. Since the operation of each integrated circuit 16 is synchronized to the same master CLOCK signal, and since each CLOCK signal pulse arrives at each integrated circuit at the same time, the separate integrated circuits 16 may operate together in a synchronous fashion and exchange data with each other without need for asynchronous communication channels. The master clock generator 12 is suitably a resettable phase-locked loop circuit producing N output CLOCK signal pulses in response to every pulse of an input reference clock signal (REFCLK). The REFCLK signal may be produced, for example, by a crystal-controlled oscillator 20 having a highly stable frequency. The master clock generator 12 also has an input for a RESET2 signal which, when asserted, drives the CLOCK output of the clock generator high, and prevents the generator from generating CLOCK signal pulses. A startup control circuit 34 asserts the RESET2 signal to reset the master clock generator 12 on receipt of an externally generated START signal. At the same time, the startup control circuit 34 asserts the RESET1 signal that resets each phase generator. The startup control circuit 34 then deasserts the RESET1 signal to enable phase generator operation and thereafter deasserts the RESET2 signal to enable master clock generator operation. The startup control circuit 34 suitably comprises two counters 36 and 38 for counting pulses of the REFCLK signal, each count being reset by the START signal. Counter 36 asserts the RESET2 signal and counter 38 asserts the RESET1 signal upon receipt of the START signal. Counter 36 deasserts the RESET2 signal after it has counted a predetermined number of REFCLK signal pulses, and counter 38 deasserts the RESET1 signal after it has counted a lower number of REFCLK signal pulses. The amount of CLOCK signal delay provided by each delay circuit 18 is determined by five-bit control data C0-C4 separately supplied to each delay circuit through a shift register 24 which shifts in each bit of an externally generated serial DATA input in response to each pulse of an externally generated LOADCLK signal pulse. The delay of each delay circuit 18, as set by data C0-C4, is stabilized by a pair of analog control signals VCON.DE and VCON.INS respectively produced by a delay element monitor 30 and an insertion delay monitor 32 as described in detail hereinbelow. FIG. 2 shows a block diagram of a typical delay circuit 18 of FIG. 1 including an input buffer 40 for buffering the input CLOCK signal, a sequence of adjustable delay circuits (insertion delay circuit 42, fine delay circuit 44 and coarse delay circuit 46) for successively delaying the output of buffer 40 by adjustable delay times, and an output buffer 48 for buffering the output of coarse delay circuit 46 so as to produce the delayed CLOCK signal output of delay circuit 18. The delay of coarse delay circuit 46 may be adjusted to mT+I₁ seconds, where m is an integer from 0-3, T is a fixed unit delay time and I₁ is a constant "insertion" delay, the minimum delay of circuit 46. The selection of m is determined by the values of bits C3 and C4 of the control input data C0-C4 shown in FIG. 1. The fine delay circuit 44 may be adjusted for a delay of (n/8)T + I₂ seconds where n is an integer from 0 to 7 determined by the value of control data bits CO-C2, T is the fixed unit delay time, and I₂ is the constant insertion delay of circuit 44. The delay (D ins ) of the insertion delay circuit 42 may be continuously adjusted over its full range in accordance with the magnitude of the VCON.INS control signal output of insertion delay monitor 32 of FIG. 1. The unit delay T of the fine and coarse delay circuits 44, 46 is controlled in accordance with the magnitude of the VCON.DE signal produced by the delay element monitor 30 of FIG. 1. Given that buffers 40 and 48 have delays of D₁ and D₂ respectively, the total delay T tot of delay circuit 18 is the sum of the delays provided by its constituent devices 40-48: T tot = D₁ + D ins + (mT + I₁) + (nT/8 + I₂) + D₂. [1] Rearranging terms of equation [1]: T tot = [m+(n/8)]T + T ins [2] where T ins = (D₁ + D₂ + I₁ + I₂ + D ins ). [3] The first term of equation [2] shows that the delay of circuit 18 may be adjusted to 32 levels in steps of T/8 seconds depending on the various combinations of values of m and n as determined by control data C0-C4. The second term of equation [2], T ins , is the total insertion delay of delay circuit 18, the minimum delay that it provides when m = n = 0. In order to ensure that the delay T tot of the circuit remains constant once the values of m and n are selected, it is necessary that both T and T ins remain constant. As mentioned hereinabove, the value of T is controlled by the VCON.DE output signal of delay element monitor 30 of FIG. 1 and the value of the D ins component of T ins of equation [3] is controlled by the VCON.INS output signal of insertion delay monitor 32 of FIG. 1. As described in detail hereinbelow, delay element monitor 30 is a phase-locked loop circuit that continuously monitors the unit delay T and adjusts VCON.DE so that T remains constant. Similarly insertion delay monitor 32 continuously monitors T ins and adjusts VCON.INS to ensure that T ins remains constant. The insertion delay circuit 42 is implemented by an adjustable delay element depicted in schematic diagram form in FIG. 3. In the preferred embodiment of the invention, the CLOCK signal and the VCON.INS control signal are differential signals as shown in FIG. 3. The differential input CLOCK signal is applied across the bases of an emitter-coupled transistor pair Q1, Q2 and also as input to a buffer 50 which delays the input CLOCK signal by a small amount. The output of buffer 50 is applied across the bases of another emitter-coupled transistor pair Q3, Q4. The collectors of transistors Q1 and Q3 drive the base of an output buffer transistor Q6 and the collectors of transistors Q2 and Q4 drive the base of another output buffer transistor Q5. The bases of transistors Q5 and Q6 are coupled to a positive voltage source Vcc through resistors R1 and R2, respectively, and the emitters of transistors Q5 and Q6 are coupled to ground through matching current sources 52 and 54, respectively. The emitters of transistors Q1 and Q2 are tied to the collector of a transistor Q7 and the emitters of transistors Q3 and Q4 are tied to the collector of a transistor Q8. The emitters of transistors Q7 and Q8 are coupled to a current source 56 through resistors R3 and R4, respectively. The VCON.INS control signal is applied across the bases of transistors Q7 and Q8. The delayed output CLOCK signal appears across the emitters of transistors Q5 and Q6. VCON.INS controls the relative proportion of the current output of current source 56 that is transmitted to the emitters of transistors Q1 and Q2 or to the emitters of transistors Q3 and Q4. Transistors Q1 and Q2 form a differential amplifier which amplifies the input CLOCK at the bases of transistors Q1 and Q2 to produce a first output differential current signal at their collectors. The gain of the Q1,Q2 amplifier is determined by the proportion of current from current source 56 supplied to the transistor emitters through transistor Q7. Similarly, transistors Q3 and Q4 form a differential amplifier which amplifies the buffer 50 output signal at the bases of transistors Q3 and Q4 to produce a second output differential current signal at the transistor collectors. The gain of the Q3,Q4 amplifier is determined by the proportion of current from current source 56 supplied to the transistor emitters through transistor Q8. On the rising edge of an input CLOCK signal pulse, transistor Q1 immediately begins to pull up the base of transistor Q6 and transistor Q2 begins to pull down the base of transistor Q5, thereby causing the output CLOCK signal voltage across the bases of transistors Q5 and Q6 to begin to go high. However, various capacitances in transistors Q1, Q2, Q5 and Q6 prevent the output CLOCK signal from rising abruptly. Buffer 50 has an inherent delay, and some time after the rising edge of the input CLOCK signal arrives at buffer 50, buffer 50 drives its output signal high, thereby causing transistor Q3 to begin pulling up the base of transistor Q6 and causing transistor Q4 to begin pulling down the base of transistor Q5. FIG. 4 is a timing diagram showing the way in which the output CLOCK signal changes from a minimum negative voltage VMIN to a maximum positive voltage VMAX after the rising edge of the input CLOCK signal arrives at the delay element at time T0. If VCON.INS is large and positive, then substantially all of the current output of current source 56 is directed to the emitters of transistors Q1 and Q2; transistors Q3 and Q4 are off and do not supply any current to the bases of transistors Q5 and Q6. In such case the amplitude of the delayed output CLOCK signal increases quickly as shown by curve 58A. Alternatively when VCON.INS is large and negative, transistors Q3 and Q4 are on and transistors Q1 and Q2 are off, and the output CLOCK signal voltage changes only in response to the delayed output of buffer 50. In such case the output CLOCK signal voltage rise is delayed by the delay time of buffer 50 as illustrated by curve 58D. By inspection of curves 58A and 58D, it can be seen that the output CLOCK signal rises above a threshold voltage VTH at time T1 when transistors Q1 and Q2 convey all of the current of current source 56 and rises above VTH at time T2 when transistors Q3 and Q4 convey all of the current of current source 56. If VTH is the threshold voltage at which the output CLOCK signal is considered to change state, then the "delay" of the delay element of FIG. 3 is the time at which the output CLOCK signal rises above the threshold voltage. Curve 58B shows the rise in output CLOCK signal voltage when transistors Q1 and Q2 carry about 80% of the current and transistors Q3 and Q4 carry about 20% of the current. Curve 58C shows the rise in output CLOCK signal voltage when transistors Q1 and Q2 carry about 20% of the current and transistors Q3 and Q4 carry about 80% of the current. Thus it can be seen that by adjusting the magnitude of VCON.INS, the delay of the delay element may be adjusted to any value between T1 and T2. As illustrated in FIG. 5, fine delay circuit 44 of FIG. 2 is similar to the delay element of FIG. 3 except that buffer 50 of FIG. 3 is replaced with another adjustable delay element 60 similar to the delay element of FIG. 3, and the control signal applied across the bases of transistors Q7 and Q8 is produced by a digital to analog converter (DAC) 62 in accordance with the input data C0-C2. The delay of delay element 60 is controlled by the VCON.DE signal so that it remains constant. The coarse delay circuit 46, shown in more detail in FIG. 6, includes a set of four delay elements 66, 68, 70 and 72 connected in series and a multiplexer 74 which may selectively transmit either the CLOCK signal input to coarse delay circuit 46, or the output of one of delay elements 66, 68, or 70 to buffer 48 of FIG. 2. The output of delay element 72 is not used, but element 72 is provided so that elements 66-70 all have similar output loading. The multiplexer 74 switching state is determined by the C3 and C4 control data bits applied thereto. The delay of each delay element 66-72 is the unit delay T, and the unit delay is held to a constant value by the VCON.DE signal applied as control input to each delay element. Delay elements 66-72 are similar to the delay element shown in schematic diagram form in FIG. 3 except that VCON.DE rather than VCON.INS is applied across the bases of transistors Q7 and Q8. FIG. 7 is a block diagram of the delay element monitor 30 of FIG. 1. A set of delay elements 80, 82, 84, and 86, similar to delay elements 66-72 of FIG. 6, are connected in series with the output signal of element 86 being inverted and applied as input to element 80, thereby forming a ring-type oscillator 91 of period 8T, where T is the unit delay of each element 80-86. The output of each element is applied to a separate input of a 4x1 multiplexer 88, similar to multiplexer 74 of FIG. 6. The two control input bits to multiplexer 88 are tied to a voltage source of logic level "1" so that multiplexer 88 always selects the output of delay element 84. The output of multiplexer 88 is provided as an input to a frequency divider 90, which divides the frequency of its input signal by a factor of K to produce an output signal applied to a phase detector 92. Phase detector 92 compares the output of divider 90 to the reference clock signal REFCLK and produces an output signal which is high or low depending on whether the output signal produced by divider 90 leads or lags REFCLK. The phase detector output signal is filtered by a filter 94 to produce the VCON.DE output signal of the delay element monitor 30. VCON.DE is also applied to the control inputs of delay elements 80-86. Thus the output signal of divider 90 is phase locked to REFCLK, and the duration of the unit delay T of each delay element 80-86 is determined by the frequency dividing ratio K of frequency divider 90 in accordance with the expression T = 8T ref /K [4] where T ref is the period of REFCLK. When REFCLK is produced by a highly stable source such as a crystal oscillator, T is highly stable and not affected by changes in ambient temperature or variation in material or the fabrication process utilized to manufacture integrated circuits employing the delay elements. With reference to FIG. 8, showing the insertion delay monitor 32 of FIG. 1 in more detailed block diagram form, monitor 32 includes another delay circuit 18 identical to the delay circuits 18 of FIGS. 1 and 2 including input and output buffers 40 and 48, an insertion delay circuit 42, fine delay circuit 44 and coarse delay circuit 46. However in the delay circuit 18 of FIG. 8, the output of buffer 48 is negatively fed back to the input of buffer 40 to form an oscillator 101. The output of coarse delay circuit 46 is provided as input to a frequency divider 100, which divides the frequency of its input signal by a factor of M to produce its output signal. The output of frequency divider 100, along with the reference clock REFCLK signal, are applied as inputs to a phase detector 102 similar to phase detector 92 of FIG. 7. The output of phase detector 102 is filtered by a filter 104 to produce the VCON.INS output signal of delay monitor 32. The VCON.INS signal is applied to the control input of insertion delay circuit 42 while the VCON.DE signal is applied to the control inputs of delay circuits 44 and 46. The C0-C4 inputs of fine delay circuit 44 and coarse delay circuit 46 are tied to a "0" logic level source such that the delay of fine delay circuit 44 is its insertion delay I₂ and such that the delay of coarse delay circuit 46 is its insertion delay I₁. Thus the total delay between the input of buffer 40 and the output of buffer 48 is equal to insertion delay T ins , as defined by equation [3] hereinabove, and the period of oscillation of the output signal of frequency divider 100 is MT ins . Since the frequency divider output signal is phase locked to the period T ref of the reference clock, T ins = T ref /M. [5] Since M is a constant and T ref is highly stable, T ins is highly stable Inasmuch as VCON.INS is applied as the control input to the insertion delay circuit 42 within every delay circuit 18 of FIG. 1, every delay circuit 18 has identical insertion delay T ins . Substituting equations [4] and [5] into equation [2], the total delay of each delay circuit 18 of FIG. 1 is T tot = [8m/K + n/K + 1/M]T ref . [6] From equation [6] it is seen that the total delay T tot of each delay circuit 18 is proportional to the period T ref of the reference clock and the constant of proportionality is determined by a combination of m, n, K and M. K and M are constant frequency divider ratios and m and n are determined by the values of C0-C4. Therefore the delay provided by each delay circuit 18 is as stable as the period of the reference clock which is highly stable. FIG. 9 is a combination block and schematic diagram of a phase generator suitable for use as a phase generator 14 of FIG. 1 adapted to produce N clock phase signals PH1 - PHN, each having a period of NT clock /2 where T clock is the period of the master CLOCK signal, the clock phase signals being phase shifted one from another by T clock /2 seconds. The phase generator 14 comprises a sequence of N phase generator elements 106, each providing a separate one of the clock phase signal outputs PH1 - PHN in response to a combination of three inputs, the RESET1 signal, a prebias signal Vbias (shown as Vbin), and a timing control signal Iin. Each phase generator element 106 also produces a Vbout output signal which is in phase with its phase signal output. The Vbout output signal of each phase generator element 106 is provided as Vbin, the prebias input signal Vbias, to a next phase generator element of the sequence, with the Vbout output of the Nth phase generator element being supplied as Vbin, the Vbias input, to the first phase generator element of the sequence. The RESET1 signal output of counter 38 of FIG. 1 is provided in parallel to the RESET1 input of each phase generator element 106. The CLOCK signal is applied across the bases of an emitter coupled transistor pair Q9, Q10, the emitters of transistors Q9 and Q10 being coupled to ground through a current source 108. The collector of transistor Q9 is connected to the Iin input of the "odd" phase generator elements 106 which produce odd numbered clock phase signals PH1, PH3, ..., PH(N-1), and the collector of transistor Q10 is connected to the Iin input of the "even" phase generator elements which provide even numbered clock phases PH2, PH4,..., PHN. (N is always an even number.) Each phase generator element drives its clock phase signal output and its Vbout output high when the current from current source 108 is supplied to its Iin input, provided, however, that its prebias Vbias input (Vbin) is high at the time. When the input current Iin is terminated, the phase generator element drives its clock phase and Vbout output signals low. As the CLOCK signal oscillates, it alternately switches transistors Q9 and Q10 on, thereby alternately connecting current source 108 to the Iin input terminals of the even and odd clock phase generator elements. When transistor Q9 is on, only a single one of the odd clock phase generator elements drives its output signals high because only one of these elements has a high prebias input signal. Similarly, when transistor Q10 is on, only a single one of the even clock phase generator elements drives its output signals high because only one of these elements has a high prebias input signal. Each time a particular clock phase signal PH1-PHN is asserted, the prebias input to the next phase generator element in the sequence is also asserted, and when the CLOCK signal next changes state, the clock phase signal output of that next phase generator element is asserted. FIG. 10 is a schematic diagram of the phase generator element 106 of FIG. 9 that supplies the PH1 output signal. A three-emitter transistor Q11 provides the PH1 and Vbout signals at two of its emitters, and the third emitter is connected to the base of another transistor Q12. A current source 115 is also connected to the base of transistor Q12, and a current source 117 is connected to the emitter of transistor Q11 that provides the PH1 signal. The Iin input appears at the emitter of transistor Q12, and the collector of transistor Q12 is connected to a positive voltage source Vcc through a resistor R9. The Vbias (Vbin) input to element 106 is also supplied to the base of transistor Q12. The collector of transistor Q12 is connected to the base of a transistor Q13 and the collector of transistor Q13 is coupled to Vcc through another resistor R10, and coupled to the base of transistor Q11. The emitter of transistor Q13 is tied to the emitter of another transistor Q14, while the collector of transistor Q14 is tied directly to Vcc. The base of transistor Q14 is connected to a reference voltage source Vref. The RESET1 signal is applied across the bases of an emitter coupled transistor pair Q15 and Q16, with the emitters of transistors Q15 and Q16 being connected to a current source 110. The collector of transistor Q15 is tied to the emitters of transistors Q13 and Q14, while the collector of transistor Q16 is tied to the base of transistor Q11. During normal operation, the RESET1 signal is negative so that transistor Q15 is on and transistor Q16 is off. Thus the current from current source 110 passes through either transistor Q13 or transistor Q14 via transistor Q15 depending on which transistor Q13 or Q14 is on and which is off. When the base of transistor Q11 is low, but prebias input signal Vbias is high, transistor Q12 will begin conducting current when the CLOCK signal next switches transistors Q9 and Q10 of FIG. 9 so as to supply current to transistor Q12. The current through the collector-emitter path of transistor Q12 pulls the base of transistor Q13 below Vref causing current from current source 110 to be switched through transistor Q14. The drop in current through resistor R10 pulls up the base of transistor Q11, thereby driving the PH1 and Vbout signals high. The third emitter of transistor Q11 keeps transistor Q12 on even though the transistor Q11 in the preceding phase generator element supplying the prebias input vbias no longer pulls Vbias high. When the CLOCK signal changes state, transistor Q9 of FIG. 9 turns off and current is no longer supplied to transistor Q12. Transistor Q12 turns off, resistor R9 pulls the base of transistor Q13 above Vref, transistor Q13 turns on, and transistor Q14 turns off. As transistor Q13 turns on, it pulls down the base of transistor Q11, driving down PH1. All of the phase generator elements 106 of FIG. 9 are similar to the element illustrated in FIG. 11 except that, as shown in Fig. 10, the collector of transistor Q16 is tied to the base of transistor Q11 in the phase generator element that produces PH1. In all other phase generator elements, the collector of transistor Q16 is tied to Vcc, as shown in Fig. 11. When the RESET1 signal is asserted (driven positive), transistor Q15 in each phase generator element turns off and transistor Q16 turns on. In the first phase generator element (Fig 10) transistor Q11 asserts PH1. But in all other phase generator elements (Fig. 11), since the collector of Q16 is tied to Vcc and not to the base of transistor Q11, transistor Q11 turns off, and PH2-PHN go low. When the RESET1 signal is subsequently deasserted while the CLOCK signal is high, the current from current source 108 of FIG. 9 keeps transistor Q12 on. Thereafter the clock phase signals PH1-PHN are asserted in proper order as the CLOCK signal continues to change state. Thus assertion and deassertion of the RESET1 signal initializes operation of the clock phase generator 14 of FIG. 9 so that PH1 is asserted. The CLOCK signal across the bases of transistors Q9 and Q10 in each phase generator circuit 14 of FIGS. 1 and 9 is applied as input to a buffer 119 that supplies a signal to a test pin 109 on each integrated circuit 16. The test pins are used when calibrating the system. Each pulse of the CLOCK input generates a test signal pulse on the test pin 109. With reference to FIG. 1, the test signals on the test pins of pairs of integrated circuits may be input to a logic gate, for example an XOR gate (not shown), through matched delay transmission lines, and the output of the XOR gate may be monitored to determine if a test signal pulse on one test pin 109 rises substantially before or after the test signal pulse on the other test pin 109. The timing data C0-C4 supplied to each delay circuit 18 may then be adjusted in an iterative fashion to ensure that pulses appear at each test pin 109 at the same time. There has thus been described a clock signal skew compensator for adjustably delaying a clock signal so that it arrives at various nodes in a circuit at the same time. The invention may also be used to adjustably delay pulses of separately generated input signals applied to each delay circuit 18 of FIG. 1 so that they arrive at separate nodes 13 at the same time.

Claims: An apparatus for skew compensating pulse signals having sequences of pulses transmitted to separate nodes (13) of a circuit, comprising: a plurality of delay circuits (18) coupled to receive and variably delay the pulse signals transmitted to the nodes (13); and means (24) for generating a plurality of first delay control signals (C0-C4), each of the first delay control signals (C0-C4) being applied to a first control signal input of a separate one of said delay circuits (18) for determining the amount of pulse signal delay provided by the delay circuit (18), each first delay control signal (C0-C4) being set so that pulses of separate output signals of the delay circuits (18) corresponding to the pulse signals arrive at the nodes (13) substantially at the same time, characterized in that each delay circuit (18) comprises first unit delay elements (60; 66-70) adapted for varying the unit delay of the delay circuit (18) in accordance with a second delay control signal (VCON.DE) applied in common to a second control input of each delay circuit (18); the apparatus further comprising: means (20) for generating a periodic reference clock signal (REFCLK) having a constant period; a first oscillator (91) comprising second unit delay elements (80-86) similar to said first unit delay elements (60; 66-70) for producing a first periodic output signal of a period determined and varying in accordance with said second delay control signal (VCON.DE); and means (88-94) for producing said second delay control signal (VCON.DE) with a magnitude determined in accordance with a comparison of the period of said reference clock signal (REFCLK) with the period of said first periodic output signal for maintaining the unit delays constant. An apparatus as recited in claim 1 wherein each of said delay circuits (18) is characterized by: a plurality of said first unit delay elements (60; 66 - 70), each producing an output signal pulse in response to an input signal pulse, the output signal pulse being delayed from said input signal pulse by the unit delay controlled by the second delay control signal (VCON.DE); and means (62, 74) for interconnecting a selected number of said first unit delay elements (60; 66 - 70) to form a sequence of first unit delay elements (60; 66 - 70) for delaying the pulse signals by at least the sum of unit delays of the selected number of unit delay elements, said number of first unit delay elements (60; 66 - 70) being selected in accordance with the first delay control signal (C0 - C4). An apparatus as recited in claim 2 wherein each of said first unit delay elements is characterized by: means for delaying the input signal pulse to produce a first output signal; first means (Q1, Q2) for amplifying the input signal pulse by a first gain factor to produce a second output signal; second means (Q3, Q4) for amplifying the first output signal by a second gain factor to produce a third output signal; means (Q5, Q6) for producing a fourth output signal as the output signal pulse, the fourth output signal being a sum of the second and third output signals; and means (Q7, Q8) for controlling the first and second gain factors in accordance with the second delay control signal. An apparatus as recited in any one of claims 1 to 3, further characterized by means (32) for generating a third delay control signal (VCON.INS) applied in common to a third control input of each delay circuit (18), the third delay control signal (VCON.INS) being continuously adjusted so that an insertion delay for each delay circuit (18) remains constant. An apparatus as recited in claim 4 wherein the third delay control signal generating means (32) is characterized by: a second oscillator (101) comprising a further delay circuit similar to said delay circuits (18) for producing a second periodic output signal of period determined and varying in accordance with said third delay control signal (VCON.INS); and means (100 - 104) for producing said third delay control signal (VCON.INS) of magnitude determined in accordance with a difference between the period of said reference clock signal (REFCLK) and the period of said second periodic output signal.

Inventors: GREUB, HANS J.
Applicants: TEKTRONIX, INC.
Application year: 1988
Main classification: H03K5
IPC codes: H03K5, H03K3, G06F1, H04L7, H04B7