Synchronization failure detection

Abstract: A method, and apparatus implementing that method, for monitoring a synchronization circuit to ensure proper operation thereof. The synchronization circuit is of the type that receives asynchronously occurring input pulses to produce therefrom representa­ tions of the received input pulses, having state tran­ sitions synchronized to the state transitions of a pe­ riodic clock signal. The invention also receives the input pulses and the synchronized representation of those pulses, to ensure that for every input pulse there is provided a synchronized pulse by the synchronization circuit.

Description: The present invention relates generally to digital equipment, and more particularly to a method and apparatus for monitoring a synchronisation circuit to ensure proper operation of the circuit. In many digital systems, it is often necessary to receive asynchronously appearing pulses, and to synchronise those pulses with an internal clock (i.e., a periodic pulse train). An example of a synchronization circuit may be found in U.S. Patent No. 4,700,346. There are occasions, however, when a synchronization circuit will fail to respond to a received input pulse. For example, when the received input pulse is shorter than the sampling period of the synchronizer no synchronized representation will be produced. Also, when the pulse rate of the input signal is higher than the synchronization rate the synchronizer may fail to respond. Of course, the synchronizer itself may fail, causing it to ignore input events. These are all circumstances which could create problems if not detected. In a different field, DE-A-3500896 discloses a system for detecting when a transmission apparatus, such as satellite communication equipment, fails to respond to an input signal. In this case, the transmission apparatus transmits burst-like communication signals on the basis of equivalent inputted burst-like signals. The detection system operates by wave-reforming the input and output signals and executing an exclusive OR operation on both wave-reformed signals. However, this system relies on output and input signals existing at about the same time and is unsuitable for synchronization circuits having a varying time relationship between input and output signals. According to the present invention there is provided in a digital system, having synchronizing means operable to receive an asynchronously occurring input pulse to generate therefrom an output signal in the form of a representation of the input pulse having at least one digital state change synchronized to a transition of a digital clock signal, apparatus for monitoring operation of the synchronizing means to determine that for each received input pulse a corresponding representation is produced, the apparatus comprising: first counting means coupled to note occurrence of the input pulse for generating a first signal indicative of the noted occurrence of the input pulse; second counting means coupled to note the appearance of the output pulse to produce a second signal indicative of the noted appearance of the output pulse; and third circuit means coupled to receive the first and second signals and responsive to occurrence of the input pulse for producing therefrom an output error signal when occurrence of the input signal is without a responsive occurrence of the output signal of the synchronizing means. The present invention may be implemented by an event counter in the form of a pair of flip-flops, each connected to note occurrence of the input signal to the synchronizer, and the resultant output pulse from the synchronizer, Noting the former without the latter upon receipt of a subsequent input pulse generates the error signal. According to the present invention there is also provided in digital apparatus of the type having synchronizing means operable to receive an asynchronously occurring input pulse to generate therefrom a representation of the input pulse having a transition from one digital state to another digital state synchronized to a transition of a periodic digital clock signal, a method for checking for appearance of the representation for each occurrence of the input signal, the method comprising the steps of: counting each occurrence of the input pulse; counting each occurrence of the representation of the input pulse; and producing an error signal when two successive occurrences of the input pulse are not interspersed by generation of the representation. One implementation of the method of the present invention comprises the steps of producing a count for each input pulse, and subtracting from that count each resultant output pulse of the synchronizer. If that count is greater than one (i.e., 2) the error signal is produced. Accordingly, the present invention provides a method, and apparatus for implementing that method, of monitoring the input to, and the output from, a synchronization circuit of the type described above to produce an error signal when two input pulses are received without an interstitial output pulse. A number of advantages are achieved by the present invention. The invention is able to detect occurrence of an input pulse that the synchronization circuit fails to "see," producing an error signal indicative thereof. Further, should the synchronization circuit fail, the invention is capable of detecting that failure. These and other aspects and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description of the invention, which should be taken in conjunction with the accompanying drawings. Fig. 1 is a block diagram of the invention used in conjunction with a synchronizer circuit; and Fig. 2 is a timing diagram to illustrate the operation of the invention. Referring now to the Figures, and in particular Fig. 1, there is shown a synchronization circuit 12 coupled to an event counter, designated generally with the reference numeral 10, and constructed in accordance with the teachings of the present invention. The synchronization circuit 12 operates to receive an input signal (IN) on a signal line 14, producing therefrom, and a clock (CLK) signal, an output signal (OUT) on line 16. The OUT signal is a synchronized representation of the IN signal, having transitions that are synchronized to transitions of the CLK signal that is coupled to the synchronization circuit 12 by signal line 18. Used to monitor proper operation of the synchronization circuit 12, the event counter 10 receives the IN and OUT signals via signal lines 20 and 22 that couple them to the clock (CK) inputs of a pair of flip-flops 24, 26, respectively. As Fig. 1 indicates, the flip-flops 24, 26 are connected in the toggle configuration in that each has its negative or FALSE output ( Q ) connected to its data (D) input. Further, the flip-flops 24, 26 are edge-triggered; that is, they are set to the state of the signal applied to their data (D) inputs at a state transition of the signal received at the CK input (in the present embodiment, a positive-going transition). The positive or TRUE outputs (Q1, Q2) of the flip-flops 24 and 26 are coupled to the inputs of an EXCLUSIVE-OR gate 28, whose output produces a signal A that is coupled to the set (J) input of a J/K flip-flop 30. The reset (K) input of the flip-flop 30 is connected to a LOW or ground G, to configure the flip-flop 30 as a latch. The IN signal at the clock (CK) input of the flip-flop 30, and its true or positive output (Q) forms an ERROR signal, signifying improper operation of the synchronization circuit 12 when HIGH. The flip-flop 30 is also an edge-triggered device. Positive-going state transitions at its CK input (i.e., in the IN signal) will cause the flip-flop to set its Q output to the level (HIGH or LOW) of the signal at its J input. Once set, it cannot be reset in the illustrated configuration (since the K or reset input is permanently LOW) - thus its latch capability. Turning now to Fig. 2, operation of the circuit is as follows: The synchronization circuit 12 may be constructed to synchronize the leading edge of each positive pulse forming the IN signal to the rising or positive-going transitions of the CLK signal. Thus, as indicated in Fig. 2, at the time indicated as T₁, between two rising edges of the CLK signal, a leading edge of the IN signal is received. Assume, at time T₁, the flip-flops 24 and 26 are both in a reset state (i.e., the Q output of each is HIGH). The positive transition 32 of the IN signal will cause the flip-flop 24 to toggle, and the Q1 output to go HIGH. The IN signal is also received by the J/K flip-flop 30. However, when the state transition of the IN signal occurs (at time T₁), both outputs Q1 and Q2 are LOW, so that the signal applied to the set (J) input of the flip-flop 30, via the EXCLUSIVE-OR gate 28, is LOW. Thus, the flip-flop 30 remains in a reset condition. Assume, at time T₂, synchronization is achieved, and the synchronization circuit 12 produces the OUT signal in the form of a synchronized representation of the IN signal, having a leading edge 34 synchronized with a rising state transition of the CLK signal, as Fig. 2 illustrates. The leading edge 34 of the OUT signal operates to toggle the flip-flop 26, causing its output Q2 to go HIGH. Note that the time period between T₁ and T₂ the signal A from the EXCLUSIVE-OR gate 28 was HIGH, but when the output Q2 went HIGH with the rising edge 34 of the OUT signal the signal A disappeared. Thus, upon receipt of the next IN signal pulse 36, which clocks the flip-flop 30, a ZERO or LOW is at the data (D) input of the flip-flop 30, and its Q output remains LOW. However, at time T₅ a narrow pulse 38 is received by the synchronization circuit 12 that causes a problem: The pulse 38 of the input signal IS does not last long enough to allow the synchronization circuit 12 to establish the necessary synchronization. Thus, at time T₅ the flip-flop 24 is toggled to a ONE at its Q1 output. At time T₆, before a synchronized representation can be formed, the IN signal disappears. The output Q2 of flip-flop 26, however, remains LOW because the synchronization circuit 12 was unable to produce the OUT signal, due to the short time period of the pulse 38 of the input signal IN. Thus, when the leading edge of the next pulse 40 of the IN signal occurs at time T₇, the signal A is HIGH and, therefore, flip-flop 30 is set to a ONE, generating the ERROR signal. The error remains latched by the flip-flop 30.

Claims: In a digital system, having synchronizing means (12) operable to receive an asynchronously occurring input pulse (32) to generate therefrom an output signal (16) in the form of a representation (34) of the input pulse (32) having at least one digital state change (34) synchronized to a transition of a digital clock signal (18), apparatus (10) for monitoring operation of the synchronizing means (12) to determine that for each received input pulse (32) a corresponding representation (34) is produced, the apparatus comprising: first counting means (24) coupled to note occurrence of the input pulse (IN) for generating a first signal (Q1) indicative of the noted occurrence of the input pulse (IN); second counting means (26) coupled to note the appearance of the output pulse (OUT) to produce a second signal (Q2) indicative of the noted appearance of the output pulse (OUT); and third circuit means (28,30) coupled to receive the first (24) and second (26) signals and responsive to occurrence of the input pulse (4) for producing therefrom an output error signal when occurrence of the input signal is without a responsive occurrence of the output signal of the synchronizing means. The apparatus of claim 1, wherein the first counting means (24) operates to count occurrences of input signal (32). The apparatus of claim 2, wherein the first counting means (10) includes a flip-flop (24) connected in toggle configuration and coupled to receive the input signal as a clocking signal, the flip-flop (24) being operable to produce an output signal indicative of occurrence of the input signal. The apparatus of claim 1, wherein the first (24) and second (26) counting means form a counter means (10) coupled to be incremented by the input pulse and decremented by the output pulse, the counter means (10) being operable to produce the first and second signals. A method for checking for appearance of a representation pulse derived from an asynchronously occurring input pulse used in digital apparatus of the type having synchronizing means (12) operable to receive the asynchronously occurring input pulse to generate therefrom the representation pulse with a transition from one digital state to another digital state synchronized to a transition of a periodic digital clock signal, the method comprising the steps of: counting each occurrence of the input pulse; counting each occurrence of the representation of the input pulse; and producing an error signal when two successive occurrences of the input pulse are not interspersed by occurrence of the representation. A method according to claim 5 comprising the steps of producing a count of each input pulse and subtracting from that count each resultant output pulse of the synchronizing means.

Inventors: SANNER MARTIN W, SANNER, MARTIN W.
Applicants: TANDEM COMPUTERS INC, TANDEM COMPUTERS INCORPORATED
Application year: 1988
Main classification: H04L1
IPC codes: H03K5, G06F1, H04L1, H04L7, G06F11