Patent Publication Number: US-6340904-B1

Title: Method and apparatus for generating an internal clock signal that is synchronized to an external clock signal

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 08/798,226, filed Feb. 11, 1997, now U.S. Pat. No. 5,940,608. 
    
    
     TECHNICAL FIELD 
     This invention relates to clock circuits for generating a clock signal, and, more particularly, to a clock circuit for generating an internal clock signal for an integrated circuit that is synchronized to an external clock signal despite delays in coupling the external clock signal to the clock circuit. 
     BACKGROUND OF THE INVENTION 
     The preferred embodiment of the invention is specially adapted to solve an increasing problem in high-speed integrated circuits in which an externally applied clock is intended to be registered with other signals present in the integrated circuit. The external clock is frequently applied to a large number of circuits so that their operation can be synchronized to each other. As a result, the signal path to which the external clock signal is applied is capacitively loaded to a far greater degree than signal paths receiving other signals. As a result of this heavy capacitive loading, the external clock signal may be delayed significantly before it reaches the internal circuits in the integrated circuit. This delay may be so significant that the delayed external clock signal fails to be properly registered with other signals. 
     The above-described problem is exemplified by the integrated circuit  10  shown in FIG.  1 . The integrated circuit  10  may be any of a wide variety of digital circuits including DRAMs, SRAMs, bus bridges, etc. that receives an external clock CLK signal and a data signal D, in addition to a large number of other signals which have been omitted for the purpose of brevity and clarity. The clock signal is coupled through a signal path  12  to a number of circuits  14   a,    14   b,    14   n  which use the clock signal for a variety of purposes. Once again, the circuits  14   a-n  can be any of a variety of circuits conventionally used in integrated circuits. The externally applied clock CLK signal is often used to synchronize the entire operation of the integrated circuit  10  and is thus typically routed to a large number of circuit nodes. As a result, the capacitive loading on the signal path  12  is relatively high. In particular, the capacitive loading on the signal path  12  will often be far higher than the capacitive loading on a data path  20  extending from an external terminal D to a far fewer number of signal nodes or to a single node which, in this example, is a NAND gate  22 . As a result, there is relatively little delay of the data signal as it is coupled from the D terminal to the NAND gate  22  compared to the delay of the clock signal as it is coupled to the NAND gate  22  and the other circuits  14   a-n.  Because of this delay, the clock input to the NAND gate  22  is designated a delayed clock CLK-DEL. 
     The operation of the exemplary circuit  10  shown in FIG. 1 is best explained with further reference to the timing diagram of FIG.  2 . As shown in FIG. 2, the leading edge of the external clock CLK signal is aligned with the leading edge of the data signal applied to the D terminal, although the data signal has only a 25% duty cycle. It is common for the data signal to be synchronized to the clock CLK signal before being applied to the integrated circuit  10  because the clock CLK signal may have been used to clock the data out of another integrated circuit (now shown). Primarily because of the capacitive loading of the signal path  12 , the delayed clock CLK-DEL signal coupled to the NAND gate  22  is delayed by one-quarter of a clock period, or 90°, as illustrated by the third waveform of the timing diagram. As a result, by the time the CLK-DEL signal has gone high, the data signal has gone low so that the output OUT signal remains high. Thus, because of the delay of the external clock, the external clock signal is ineffective in clocking the data through the NAND gate  22 . 
     As clock speeds continue to increase, timing tolerances have become increasingly severe. This problem is exacerbated by the increasing complexity in contemporary integrated circuits which require a large number of events to be accurately timed with respect to each other. These timing constraints threaten to create a significant road block to increasing the operating speeds of many conventional integrated circuits. 
     SUMMARY OF THE INVENTION 
     The inventive clock generator is adapted for use in an integrated circuit in which an external clock is coupled to a plurality of internal circuits with significant delays that impair the operation of at least some of the internal circuits. The integrated circuit may be a dynamic random access memory or some other digital circuit. The clock generator uses the delayed external clock signal to generate an internal clock signal that is synchronized to the undelayed external clock signal. The clock generator generates the internal clock signal using a phase-lock loop which includes a phase detector receiving the delayed external clock signal and the internal clock signal. The phase detector determines the difference in phase between the delayed external clock signal and the internal clock signal. This phase comparison is then adjusted by a phase offset corresponding to the difference between the phase of the external clock signal and the phase of the delayed external clock signal. The adjusted phase comparison is then used to control the frequency and phase of the internal clock signal so that the phase of the internal clock signal is substantially the same as the phase of the external clock signal. In addition to the phase detector, the phase-lock loop preferably includes a voltage controlled oscillator (“VCO”) generating the internal clock signal at a frequency determined by a frequency control signal, and a loop filter generating the frequency control signal from a signal corresponding to the adjusted phase comparison. The clock circuit may also include a storage device storing data indicative of one of a plurality of predetermined frequency ranges of the external clock signal. The stored data is then used to cause the VCO to operate in one of a plurality of discrete frequency bands corresponding, respectively, to the predetermined frequency ranges. As a result, the frequency and phase of the internal clock signal need only be controlled responsive to the adjusted phase comparison in a relatively narrow band of frequencies in the frequency range corresponding to the data from the storage device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art integrated circuit in which an external clock signal is excessively delayed while being coupled to a circuit using the external clock signal. 
     FIG. 2 is a timing diagram showing various signals present in the integrated circuit of FIG.  1 . 
     FIG. 3 is a block diagram and schematic of a preferred embodiment of the invention in which an internal clock signal is synchronized to an external clock signal despite significant delays in the external clock being coupled to a clock generator generating the internal clock signal. 
     FIG. 4 is a timing diagram showing various signals present in the integrated circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the invention is exemplified in an integrated circuit  30  shown in FIG.  3 . As explained in detail below, the integrated circuit avoids the external clock delay problems of the prior art by creating an internal clock from the delayed external clock signal. Significantly, the phase of the internal clock signal is offset from the delayed external clock signal so that corresponding portions of the internal clock signal actually occur before the corresponding portions of the delayed external signal. This phase offset corresponds to the delay of the external signal as it is coupled to internal circuits so that the internal clock is substantially synchronized with the external clock. 
     With reference to FIG. 3, the external clock CLK-E signal is applied to the exemplary circuits  14   a-n.  As in the example of FIG. 1, the circuits  14   a-n  may be any of a wide variety of conventional or hereinafter developed circuits such as, for example, circuits commonly found in dynamic random access memories. In face, for purposes of this example, the integrated circuit  30  will be considered to be a dynamic random access memory device. The external clock CLK-E signal is also applied to a phase-lock loop  34  which includes a conventional phase detector  36 , a conventional high gain differential amplifier  38 , a loop filter  40 , and a conventional VCO  42 . The output of the voltage control oscillator is an internal clock CLK-I signal that is fed back to the phase detector  36 . The phase detector  36  compares the phase of the delayed clock CLK-D signal with the phase of the internal clock CLK-I signal and generates a resulting error E signal corresponding to the phase difference. The error E signal is applied through a resistor  50  to a summing junction  52  of the differential amplifier  38 . Also coupled to the summing junction  52  is a negative feedback signal coupled through resistor  56  from the output of the differential amplifier  38 , and an offset voltage V applied through a resistor  58 . The noninverting input of the differential amplifier  38  is coupled to ground through a resistor  60 . 
     As is well known in the art, the differential amplifier  38  generates an output O signal that is proportional to the difference between the error E signal weighted by the ratio of the resistor  56  to the resistor  50  and the offset voltage V weighted by the ratio of the resistor  56  to the resistor  58 . Thus, if the delayed clock CLK-D signal was synchronized to the internal clock CLK-I signal so that the error E signal was 0, the output of the differential amplifier  38  would be equal to the weighted value of the offset voltage V. However, when the output voltage of the differential amplifier  38  was substantially 0, the difference between the phase of the delayed clock CLK-D signal and the phase of the internal clock CLK-I signal would correspond to the weighted value of the offset voltage V. The significance of this characteristic will be subsequently apparent. 
     The output of the differential amplifier  38  is applied to a loop filter  40  which controls the loop dynamics of the phase-lock loop  34 . The design of suitable loop filters  40  is well within the ability of those skilled in the art and will depend upon a variety of operating parameters. 
     The output of the loop filter  40  is applied to a frequency control input of the VCO  42  which generates the internal clock CLK-I signal. The frequency of the internal clock CLK-I signal is determined by the value of the voltage from the loop filter  40 . The VCO  42  also includes a frequency band select signal f 0  which will be explained below but will be ignored for the present. 
     In operation, the gain of the phase-lock loop  34  is sufficient so that the frequency of the internal clock CLK-I signal is identical to the frequency of the delayed clock CLK-D signal, and the phase of the internal clock CLK-I signal is offset from the phase of the delayed clock CLK-D signal by a magnitude corresponding to the weighted offset voltage V. In other words, the gain of the phase-lock loop  34  is sufficient so that the VCO  42  will be adjusted so that the output of the differential amplifier  38  approaches 0 volts. As explained above, in order for the output of the differential amplifier  38  to be substantially 0, the weighted value of the error E signal must correspond to the weighted value of the offset voltage V. In order for the error E signal to have a large enough value to correspond to the offset voltage V, there must be a significant phase difference between the delayed clock CLK-D signal and the internal clock CLK-I signal. In operation, the weighted value of the offset voltage V is selected so that the phase difference in the signals applied to the phase detector  36  corresponds to the delay of the external clock CLK-E signal as it is coupled to the external circuitry, i.e., the phase difference between the CLK-E signal and the CLK-D signal. 
     The operation of the phase-lock loop  34  is best explained further with reference to the timing diagram of FIG.  4 . As shown in FIG. 4, the external clock CLK-E signal is delayed by one-quarter clock period, or 90°, as it is coupled from the external terminal to the internal circuits  14   a-n.  Once again, the data signal is applied to the D terminal of the integrated circuit  30  and is coupled to a NAND gate  70 . The NAND gate  70  is gated by the internal clock CLK-I signal from the VCO  42 . Thus, as illustrated in FIG. 4, the internal clock CLK-I signal occurs one-quarter clock period or 90° before the delayed clock CLK-D signal so that the internal clock CLK-I signal is synchronized to the external clock CLK-E signal at the external terminals of the integrated circuit. As a result, the internal clock CLK-I signal is able to clock the entire data D signal through the NAND gate  70 . Thus, the signal OUT at the output of the NAND gate  70  goes low during the entire portion of the data signal D. 
     In the event the frequency of the external clock CLK-E signal is expected to vary significantly, the VCO  42  should be constructed so that it is switched to operate in different frequency bands. By operating in different frequency bands or ranges, it is only necessary for the output of the loop filter  40  to tune the frequency of the internal clock CLK-I signal over a relatively narrow range, thereby minimizing “phase jitter.” Phase jitter occurs from noise on the signal applied to the frequency control input of the VCO  42  from the loop filter  40 . Basically, a larger change in VCO frequency output for a given change in control voltage will result in greater phase jitter when the phase-lock loop  34  is locked. By using a VCO  42  that operates in discrete frequency bands and using he control voltage to tune the frequency of the VCO  42  only within this band, he change in frequency for a given change in the control voltage can be relatively small. Voltage controlled oscillators  42  having these characteristics are conventional and well within the ability of those skilled in the art. The frequency band of the VCO  42  is selected by a data signal from a speed register  74  which contains data indicative of the frequency of the external clock signals CLK-E. The data may be loaded into the speed register  74  through a conventional input device  76 , such as a keyboard. Alternatively, the data may be recorded in the speed register  74  by other means. Preferably, the speed register  74  includes a plurality of storage cells  78   a-f  corresponding to respective allowable frequencies of the external clock CLK-E signal. Only one of the storage cells  78 C contains a bit, i.e., logic “1”, designating its respective frequency as the frequency of the external clock CLK-E signal. 
     The preferred embodiment of the invention  30  illustrated in FIG. 3 is thus able to compensate for the significant delays of the external clock CLK-E signal as it is coupled through the integrated circuit  30 . 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, although the preferred embodiment of the invention uses a phase-lock look, it will be understood that other techniques may be used, including a delay-lock loop or some other means of generating an internal clock signal from the delayed external clock signal in which the phase of the internal clock signal is substantially the same as the external clock signal. Similarly, although the preferred embodiment of the invention has been explained for illustrative purposes as part of a synchronous or aesynchronous dynamic random access memory, it will be understood that it may be used as a part of other integrated circuit devices. Accordingly, the invention is not limited except as by the appended claims.