Patent Publication Number: US-4320525-A

Title: Self synchronizing clock derivation circuit for double frequency encoded digital data

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is hereby made to a utility application entitled CLOCK DERIVATION CIRCUIT FOR DOUBLE FREQUENCY ENCODED SERIAL DIGITAL DATA, by the same inventor as the instant application, filed Oct. 19, 1979, and assigned Ser. No. 086,268. 
     BACKGROUND OF THE INVENTION 
     Serial digital data is frequently transmitted using a self-clocking encoding. The basic idea behind these self-clocking codes is to guarantee that a certain minimum number of digital &#34;events&#34; (pulses or transitions in level) occur within the digital waveform. These &#34;guaranteed&#34; events can then be used to establish and reconstruct frequency and phase of the clocking signals which were used to encode and transmit the data. 
     &#34;Double frequency&#34; encoding is the simplest of the self-clocked encodings and has many variants; two of these variants are known as &#34;frequency modulated&#34; (FM) and &#34;biphase&#34;. FM and biphase encodings differ slightly, but have as a common characteristic the guarantee that at least one, but no more than two digital events occur per bit cell in the digital waveform. A bit cell is that portion of the waveform which is generated by one cycle of the basic digitalclock and contains a single binary 0 or binary 1. The presence or absence of the second (non-guaranteed) event depends upon the data being encoded. 
     Examples of FM and biphase encoding are illustrated in FIGS. 1 and 2, respectively. In the given example of FM (FIG. 1), pulses are the events of interest. In the given example of biphase, transitions of level are the events of interest. 
     In FM encoding (FIG. 1), a binary 1 is represented by two consecutive events occurring one-half bit cell apart, with a bit cell defined as the time between two guaranteed transitions. Binary 0 is represented by a single event at the beginning of a bit cell. 
     In biphase encoding (FIG. 2), the guaranteed event is a transition in level occurring in the middle of each bit cell. A binary 1 is distinguished from a binary 0 by the relative phase of the waveform surrounding the guaranteed transition. Thus, in the example shown in FIG. 2, a binary 1 is represented by  and a binary 0 is represented by  . Transitions occurring between the guaranteed transitions are required whenever two consecutive bit cells carry the same binary data value. 
     As will be obvious to those skilled in the art, an FM pulse event waveform may be converted to an equivalent FM transition-event waveform by using the FM pulse event waveform to toggle a flip-flop, and then using an output of the flip-flop as the transition-event waveform. All biphase waveforms are of the transition-event type since the phase of the waveform is crucial to the successful recovery of the encoded data. Therefore, the preferred embodiment of the present invention is designed to be driven by transition-event waveforms only. 
     Recovering data from a double frequency encoding depends on the ability to recognize the guaranteed transitions and to synchronize some timing circuit with these recognized guaranteed transitions. The output of the timing circuit can then be used, along with some additional logic, to extract data from the waveform. In the prior art, various timing circuits have been used for this purpose. Two commonly used techniques are briefly discussed below. 
     In the first prior art technique, monostable multivibrators triggered by the guaranteed transitions define a &#34;window&#34; during an interval when a non-guaranteed transition may occur. The pulses produced by the monostables are then used to extract data from the waveform. Technique 1 has the advantage of simplicity, but suffers from wide variations in timing common with monostables. Further, technique 1 is quite susceptible to spurious operation induced by noise in the incoming waveform and is limited to usage with relatively low frequency signals. 
     In the second prior art technique, a variable frequency oscillator is synchronized to the incoming frequency by means of a phase-locked loop. The output of the oscillator is then used to extract data from the incoming waveform. Technique 2 eliminates the timing tolerance problem associated with technique 1 and is much less prone to noise induced failure. However, technique 2 is substantially more complex than technique 1. 
     In certain applications where units containing circuits exemplified by techniques 1 or 2 are placed in series, such as in a ring network, the clocking signals derived from the incoming waveform are used to construct an outgoing waveform for transmission to the next unit downline. In such a configuration, &#34;jitter&#34; (viz., timing uncertainty) in the output waveform must be minimized since the jitter may cascade and be amplified by passage through several units in series. Phase locked loops, such as those of technique 2, always exhibit frequency jitter. The amount of jitter is traded off against capture time (time to go from initial &#34;unlocked&#34; state to &#34;locked&#34;) and lock range (input frequency range within which the phase-locked loop will eventually enter the locked condition). Since less jitter means less tracking ability and hence greater sensitivity to component value tolerances, technique 2 may be unsuitable for applications in which many units, each containing the extraction circuit, are cascaded in series. Using technique 1 it is possible to design a circuit which exhibits no jitter beyond that present in the incoming waveform, but one that suffers from the aforementioned drawbacks. 
     OBJECT OF THE INVENTION 
     It is the general object of the present invention to overcome these and other drawbacks of the prior art by providing an improved clock derivation circuit for double frequency encoded serial digital data. 
     It is another object of the present invention to provide an improved clock derivation circuit that automatically synchronizes itself upon receipt of an incoming double frequency encoded waveform. 
     It is a further object of the present invention to provide an improved clock derivation circuit which automatically resynchronizes itself after a burst of noise occurs on a received double frequency encoded waveform. 
     These and other objects, features and advantages of the present invention will become more apparent from the description of the preferred embodiment of the present invention when read in conjunction with the drawings contained herewith. 
     SUMMARY OF THE INVENTION 
     The foregoing objects of the present invention are achieved by providing a circuit for accurately reconstructing the timing information encoded within digital waveforms of the double frequency class, the circuit including facilities for automatically synchronizing itself at start up time or after a burst of noise on an incoming waveform. 
     The incoming double frequency encoded transition-event waveform is fed directly into a delay line which generates three versions of the incoming waveform, each delayed by one-quarter, one-half or three-quarters of the incoming waveforms bit cell time T, respectively. Transitions in the incoming waveform are detected by comparing the one-quarter T dealy line output signal with the incoming waveform by means of a first exclusive OR gate, thus providing a positive pulse of one-quarter T duration for each transition of level in the incoming waveform. A second exclusive OR gate performs an identical comparison on the incoming waveform delayed by one-half T, the second exclusive OR gate receiving its inputs from the one-half T and three-quarters T delayed versions of the incoming waveform. The output of the second exclusive OR gate is therefore identical to the output of the first exclusive OR gate, but delayed by one-half T. 
     A composite clock is derived by NORing together the outputs of the first and second exclusive OR gates, the output of the NOR gate thus providing two positive going transitions per bit cell spaced approximately one-half T apart. The composite clock formed at the output of the NOR gate is used to clock a positive edge triggered D-type flip-flop. As a result, the flip-flop is clocked every one-half T. 
     The Q and Q outputs of the flip-flop are then gated with the outputs of the first and second exclusive OR gates, respectively, as inputs to first and second NAND gates, respectively, to produce the desired clocking signals at the outputs of the first and second NAND gates, respectively. 
     The clocking signals derived at the output of the first and second NAND gates are one-quarter T negative pulses, the first derived clocking signal commencing at each guaranteed transition in the incoming waveform, the second derived clocking signal commencing one-half T after each guaranteed transition in the incoming waveform. In order to assure that the toggle state of the flip-flop is synchronized with the incoming waveform such that the output of the first or second exclusive OR gate is being selected by the first or second NAND gate at the proper time, respectively, the incoming waveform is additionally compared with the identical incoming waveform delayed by one-half T by means of a third exclusive OR gate. The output of the third exclusive OR gate is inputted to a third NAND gate along with the flip-flop&#39;s Q output. The output of the third NAND gate is fed back as the D input to the flip-flop. This effectively assures that the flip-flop toggles to the correct state when it is clocked. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an example showing data encoded as an FM pulse event waveform. 
     FIG. 2 is an example showing data encoded as a biphase transition-event waveform. 
     FIG. 3 is a schematic drawing of the preferred embodiment of the clock extraction circuit of the present invention. 
     FIG. 4 is a timing diagram showing the input waveform, a subset of the internal signals generated and the clocks derived by the clock extraction circuit of FIG. 3. 
     FIG. 5 is a timing diagram showing the input waveform and the internal signals generated to restore proper synchronization to the clock derivation circuit of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows the clock derivation circuit utilized in the preferred embodiment of the present invention. The circuit shown extracts two clock signals S12, S13 from a double frequency encoded transition-event waveform provided as the input S1. The preferred embodiment of the present invention is designed to handle waveforms having a frequency of 1 MHz. However, those skilled in the art will realize that the present invention may be readily modified to handle different frequency waveforms and will be most beneficial when used with waveforms having a frequency equal to or greater than 1 MHz. The clocks S12, S13 derived are negative pulses of 1/4 T duration, where T is the length of a bit cell in the encoded waveform (in the preferred embodiment, T is 1 mirosecond). The two clocks S12, S13 are 1/2 T out of phase with each other, giving available clock edges at 0T, 1/4 T, 1/2 T and 3/4 T in each bit cell. 
     The basic timing element used in the preferred embodiment is a digital delay line 10 with three taps S2, S3, S4. These taps S2, S3, S4 are for delays of 1/4 T, 1/2 T and 3/4 T respectively. The incoming waveform S1 is sent directly into the delay line 10. This results in the generation of three delayed versions S2, S3, S4 of the waveform S1 at the output of delay line 10 (FIG. 4). In the preferred embodiment of the present invention, a Technitrol TTLDL750C digital delay line having a total delay of 3/4 T, or 750 nanoseconds, is utilized to correspond to the frequency of input waveform S1. 
     Referring to FIG. 4, the incoming waveform S1 is shown with asterisks above the guaranteed transitions. The non-guaranteed transitions in S1 are shown with an uncertainty interval since the exact position of the non-guaranteed transitions is determined by the characteristics of the delay line in the preceding unit (assuming a cascade arrangement of units utilizing the present invention). The guaranteed transitions on the outgoing waveform from a preceding unit may be generated by the leading edge of one of the derived clock signals S12. As will be seen, this leading edge is derived directly from the guaranteed transition of the incoming waveform. Thus, the guaranteed transitions on the outgoing waveform are derived from the guaranteed transitions of the incoming signal without any dependence on the timing element 10 within the unit. Therefore, the guaranteed transitions are &#34;precise&#34; in the sense that the interval between these guaranteed transitions is constant throughout the chain of cascaded units. To the contrary, the center (non-guaranteed) transitions are derived from timing signal S13. Timing signal S13 is derived from signals out of the delay line 10. Thus, the exact position of a center (non-guaranteed) transition relative to its neighboring guaranteed transitions is subject to an uncertainity interval determined by the timing tolerance of the delay line 10. On the assumption that units containing the present invention are cascaded, the incoming signal S1 will also have an uncertainity interval surrounding center transitions, as shown in FIG. 4. 
     The preferred embodiment of the present invention (FIG. 3) is implemented using the following integrated circuit types: 
     Exclusive OR gates 12, 14, 24--74LS86 
     NOR gate 16--74LS02 
     Flip-flop 18 (&#34;D&#34;--type, positive edge triggered)--74LS74 
     NAND gates 20, 22, 26--74LS00 
     However, those skilled in the art will realize that various substitutions in the components used may be made without departing from the spirit of the present invention. 
     The operation of the clock derivation circuit will now be explained in detail. 
     As previously mentioned, the input waveform S1 to the clock derivation circuit is a double frequency encoded transition-event waveform S1(FIG. 4) with the guaranteed transitions identified by asterisks. Transitions in input waveform S1 are detected by comparing the output of the first tap S2 of the delay line 10 with S1. The comparison is performed by exclusive OR gate 12, whose output is low whenever both of its inputs are the same and high when they are different. The resulting signal S5 (FIG. 4) provides a positive pulse of 1/4 T duration for each transition of level in S1. Some of these positive pulses in S5 are &#34;fuzzy&#34; (having uncertainty intervals) because they derive from the fuzzy center transition. Another exclusive OR gate 14 performs an analogous function on the incoming signal S1 delayed by 1/2 T. That is, the inputs S1,S2 to exclusive OR gate 12 are out of phase by 1/4 T as are the inputs S3, S4 to exclusive OR gate 14. Thus, the output S7 of exclusive OR gate 14 is identical to the output S5 of exclusive OR gate 12 delayed by 1/2 T. 
     The output S5 of exclusive OR gate 12 has a &#34;clean&#34; (i.e., not fuzzy) pulse for each guaranteed transition in input waveform S1, and so has one clean pulse per bit cell commencing with each guaranteed transition in S1 and lasting 1/4 T. Since S7 is simply S5 delayed, S7 has one clean pulse per bit cell commencing at 1/2 T after each guaranteed transition and lasting until 3/4 T. 
     The clocks which are to be derived are S12 and S13. Clock pulse S12 is a 1/4 T negative pulse commencing with each guaranteed transition. Clock pulse S13 is a 1/4 T negative pulse commencing 1/2 T after each guaranteed transition. In order to complete the task of deriving clocks S12 and S13, the clean pulses must be isolated out of the S5 and S7 waveforms. The means for isolating out just the clean pulses is provided by flip-flop 18. In normal operation (i.e., the incoming waveform S1 is in sync. and there is no noise on S1), flip-flop 18 simply toggles (changes state) every 1/2 T (FIG. 4). Its true and false outputs S9 and S10, respectively, are then gated with S5 and S7 respectively through NAND gates 20 and 22, respectively, to produce the desired clocking signals S12 and S13, respectively. 
     In order for flip-flop 18 to change state, it must be clocked. Flip-flop 18 is triggered by a positive going transition at its clock input. In order for flip-flop 18 to change state every 1/2 T, a composite clock for flip-flop 18 is derived by NORing together S5 and S7 via NOR gate 16 to produce S8 (FIG. 4). Each of these signals S5, S7 has at least one pulse per bit time T, and S7 is S5 delayed 1/2 T. Furthermore, if S5 has a second pulse (the fuzzy one), it overlaps the clean pulse in S7 and vice versa. Therefore, the composite clock S8 has exactly two pulses per bit cell T, and more particularly S8 has exactly two positive going transitions per bit cell T. These positive going transitions in S8 are spaced approximately 1/2 T apart and thus flip-flop 18 is clocked every 1/2 T. 
     Since the outputs S9, S10 of flip-flop 18 are being used to isolate the clean (i.e., not fuzzy) pulses on S5, S6 which correspond to the negative pulses on S12, S13 respectively, closer attention must be given to the fuzziness of clocking signal S8. S8 is the complement of the logical OR of S5 and S7. Thus, S8 must be low as long as either S5 or S7 is high. Referring now to FIG. 4, and more particularly to an instance of a clean pulse occurring on S5 coincident with a fuzzy pulse on S7, it will be seen that the S7 pulse may begin before or after the S5 pulse and may end before or after the S5 pulse. Further, as will be obvious to those skilled in the art, the timing tolerance associated with the delay is not so large as to allow the S7 pulse to end before the S8 pulse begins. Thus, S8 goes low no later than S5 goes high (disregarding gate propagation delay) and goes high no earlier than S5 goes low. This is illustrated in FIG. 4 by the halving of the uncertainty interval. This same logic applies to the case of a clean pulse on S7 coinciding with a fuzzy pulse of S5. Any pulse occurring on S5 without a coincident pulse on S7, or vice versa, must be a clean pulse since a clean pulse occurs every 1/2 T on either S5 or S7. 
     The implication from the discussion of the last paragraph is that the positive going edges on S8 always occur no earlier than the end of the clean pulse on either S5 or S7. Thus, since flip-flop 18 is triggered by positive going edges on S8, flip-flop 18 changes state after the end of the clean pulse on either S5 or S7. Since the outputs S9 and S10 of flip-flop 18 are being used to isolate the clean pulses on S5 and S7, the implication is that an entire clean pulse from S5 or S7 is allowed through NAND gates 20 or 22, respectively, before flip-flop 18 changes state to select the next clean pulse from S7 or S5 respectively. Additionally, by the same logic, a positive edge on S8 may occur no earlier than the end of a fuzzy pulse on S5 or S7, and thus flip-flop 18 changes state at the later of the end of the pulse on S5 or the end of the pulse on S7. Consequently, when flip-flop 18 changes state to enable NAND gate 22 rather than NAND gate 20 or vice versa, it is guaranteed that no trailing part of a pulse on S5 or S7 will be allowed through NAND gate 20 or NAND gate 22, respectively. 
     The discussion thus far has assumed that the state of flip-flop 18 is properly synchronized to the incoming waveform S1 such that S5 or S7 is being selected by NAND gate 20 or NAND gate 22 at the proper time. However, it is possible at start up time or after a burst of noise occurs on the incoming signal S1 that the state of flip-flop 18 will not be properly synchronized. The present invention deals with such a condition by automatically restoring flip-flop 18 to proper synchronization. 
     The timing diagram of FIG. 5 illustrates the case of flip-flop 18 beginning in the wrong state and being restored to proper operation by the present invention. The incoming waveform S1 appearing in FIG. 5 is identical with that shown in FIG. 4 and discussed previously. Still referring to FIG. 5, exclusive OR gate 24 compares the incoming waveform S1 with S3, S3 being identical to S1 delayed by 1/2 T. Under circumstances where the incoming waveform S1 has a transition between each pair of guaranteed transitions, the signals S1, S3 entering exclusive OR gate 24 are always of opposite phase (except for brief periods corresponding to the uncertainty intervals occurring at center transitions). Thus, under circumstances where the incoming waveform S1 has a transition between a pair of guaranteed transitions, the output S6 of exclusive OR gate 24 is always high except for possible brief negative pulses at 1/2 T and T in each cycle. Under these conditions, with S6 high at 1/4 T and 3/4 T, NAND gate 26 simply complements the true output S9 of flip-flop 18 to cause flip-flop 18 to toggle when flip-flop 18 is triggered. 
     If, however, there is ever a &#34;missing&#34; center transition, inputs S1 and S3 to exclusive OR 24 will be the same between 1/2 T and T in that cycle, and hence S6 will be low between 1/2 T and T, forcing S11 high between 1/2 T and T. When flip-flop 18 is triggered at 3/4 T, it will be set to a value of 1 (true output high), thus enabling NAND gate 20 to pass the next pulse on S5, which will be a clean pulse. From this point on flip-flop 18 is in proper synchronization. Additional missing center transitions in S1 will again set flip-flop 18 to 1 (true output high), but that is the state it would have toggled to anyway. 
     Having shown and described the preferred embodiment of the present invention, those skilled in the art will realize that various omissions, substitutions and changes in forms and details may be made without departing from the spirit of the present invention. It is the intention therefore, to be limited only as indicated by the scope of the following claims.