MULTIPLE SOURCE CLOCK ENCODED COMMUNICATIONS ERROR DETECTION CIRCUIT

Abstract: Error detection circuit for detecting errors in a clock signal recovered from a self-cloking digital data signal. A first flip-flop (12) is alternately set and reset by the recovered clock signal (RF1) and by a transmitted clock signal (BFX) of supposedly equal frequency. The output of the first flip-flop (12) is sensed by a second flip-flop (14) at the end of a predetermined time period, as determined by the transmitted clock signal, for determining whether or not a transition in the recovered clock signal occurred within the predetermined time period. If no such transition occurred within the predetermined time period, the second flip-flop (14) outputs an error signal. The circuit in accordance with the present invention makes possible rapid detection of an error in the recovered clock signal (RF1).

Description: -i- . MULTIPLE SOURCE CLOCK ENCODED COMMUNICATIONS ERROR DETECTION CIRCUIT Technical Field This invention relates to digital data trans- itting systems, and, more particularly, to error detection circuits for detecting errors in a clock sig¬ nal recovered from a self-clocking digital data signal. Background Art In the transmission of a self-clocking digital data stream, it is usually essential to the proper oper¬ ation of digital apparatus that the clocking signal be correctly extracted. When a derived clock signal is improperly generated from the input self-clocking digi¬ tal data stream, it is highly desirable to detect such an error as quickly as possible in order to halt the operation of the apparatus and to effect recovery pro¬ cedures, such as a- retransmission of the message. Many techniques have been implemented to detect transmission errors such as parity check or cyclic redundancy check; however, these techniques are more adapted to detection of data errors. Techniques for detecting recovered clock signals errors have been heretofore implemented. Such previous techniques have generally required approximately four bit times or more before an error in the recovered clock can be detected. Disclosure of the Invention It is an object of the invention to provide a circuit for rapidly detecting an error in a clock signal recovered from a self-clocking digital data signal. According to the invention there is provided a circuit for detecting an error in a clock signal recovered from a self-clocking digital data signal characterized by: detecting means for detecting a tran¬ sition in the voltage level of the clock signal, to provide a transition signal indicating whether or not said transition has occurred within a predetermined time period; and sensing means for sensing said transition signal to provide an error signal if no transition has occurred within said predetermined time period. Brief Description of the Drawings A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a simplified block diagram of a system interface receiver showing the inclusion of an error detection circuit; Fig. 2 is a logic diagram of the preferred embodiment of the error detection circuit; Fig. 3 is a logic diagram of the preferred embodiment of a last low frequency generator circuit utilized in the clock recovery circuit of the system interface receiver; Fig. 4 is a logic diagram of the circuit for "the generation of the recovered clock signal; Fig. 5 is a timing diagram of specified sig¬ nals utilized as an example for showing the detection of an error condition within one-half bit time by the cir¬ cuit of the present invention; and Fig. 6 is a timing diagram of specified sig¬ nals utilized as an example for showing the detection of an error condition in approximately one and one-half bit time by the circuit of the present invention. Best Mode for Carrying Out the Invention In the preferred embodiment, an error detection circuit of the present invention is included in a system interface receiver which forms part of an interface adapter connected between a processor-memory bus and a bit serial link. Referring to Fig. 1, a system interface re¬ ceiver 1 is shown. The system interface receiver 1 _OMP accepts a raw serial data input signal from an I/O link 2. A signal generator 4 generates three digital signals and their respective complement (or bar) signals. These digital signals are derived from the raw serial data input signal which is a combination of the data and clock signals, the raw serial data signal being of the Manchester encoded or diphase encoded type. The digital signals comprise a bit serial data signal (BSD) which is a demodulated shaped version of the raw serial data input signal using regeneration techniques well-known in the art, a quarter bit time signal (QBT) -which is a version of BSD delayed by a quarter of a bit time, a three quarter bit time signal (TQBT) which is a version of BSD delayed by three quarters of the bit time, and the respective complement or inverted signals BSD, QBT, and TQBT. Techniques for generating the digital signals and their respective complement signals are well-known in the art; hence, the signal generator 4 will not be discussed further herein. These digital data signals are coupled to a data recovery circuit 6 and a clock recovery circuit 8. The data recovery circuit 6 decodes the encoded data to binary, deserializes the data to a byte format, byte aligns, and transmits the byte data to the OUTPUT. The clock recovery circuit 8 recovers the clock signal from the self-clocking digital data input signal. The clock recovery circuit 8 utilized in the system interface receiver 1 is the same as that described in European Patent Application No. 82900433.2 and reference may be made to said application for a detailed explanation of the operation of the circuit 8. The recovered clock signal RF1 and a data input signal LLF from the clock recovery circuit 8 are coupled to the data recovery circuit 6 and to. an error detection circuit 10 in accordance with the present invention. The data input signal LLF, which indicates that the self-clocking digital data input signal is still active, will be des¬ cribed in detail hereinafter. The LLF signal indicates, by a change of state, when a low frequency of the BSD signal has occurred within a bit time. A transmit clock signal BFX, inputted to the system interface receiver 1, is also coupled to the error detection circuit. Upon detecting an error, the error detection circuit 10 out¬ puts an error signal, ERR. A logic diagram of the error detection circuit 10 is shown in Fig. 2. The circuit of the preferred embodiment comprises a first flip-flop 12 and a second flip-flop 14, both flip-flops being of the D-type flip- flops which are clocked on the falling edges, the oper¬ ation of D-type flip-flops being well understood by those skilled in the art. Both the first and second flip-flops 12, 14 have the clock input (C) connected to the transmit clock signal (BFX) which is inputted to the system interface receiver. The set input(S) of the first flip-flop 12 is connected to the recovered clock signal (RFl), and the D input is tied to ground or the logic zero level. The Q output, or true output, of the first flip-flop 12 is inputted to a NOR-gate 16. NOR- gate 16 also has an LLF signal connected to a second input and the recovered clock signal (RFl) connected to a third input. The output of NOR gate 16 is connected to the D input of the second flip-flop 14. The Q output of flip-flop 14 is the error signal (ERR) . When ERR is high, an error in the clock has been detected. Basically, the error detection circuit 10 samples the recovered clock with respect to the transmit clock. If between two samples the recovered clock signal (RFl) has not changed, then an error condition exists. The operation of the error detection circuit 10 compares the recovered clock signal (RFl) to a reference or absolute clock signal (transmit clock BFX), of sup¬ posedly equal frequency. If between any two consecutive negat ve-going transitional edges of the absolute clock source the recovered clock is active and does not have a transition, an error is detected, i.e., the recovered O ■ v-^- W V clock period is too long to be valid. Hence, the period of transmit clock (BFX) is the predetermined period in which the recovered clock signal (RFl) is sensed. The absolute clock signal, transmit clock (BFX), used by the error detection circuit 10, is the transmit clocking signal of the system interface transmitter (not shown), which forms part of the interface adapter mentioned above, the various system interface transmitters within the system having their individual transmit clock sig- nals derived from their own crystals which are of sub¬ stantially equal frequency. The comparison of the two clocks is essentially done by first flip-flop 12. Transmit clock BFX, input to the clock input of first flip-flop 12, is trying to propagate a low digital signal through to the NOR-gate 16. At the same time, the recovered clock signal RFl is input to the set terminal of the first flip-flop 12. Thus, first flip-flop 12 is being set and reset by the action of these two clock signals muc like a switch. If a clock pulse is missing from the recovered clock, first flip-flop 12 will fail to set. On the next fal¬ ling edge of the absolute clock source, transmit clock signal BFX, first flip-flop 12 will go low, that is, the Q output of first flip-flop 12 will be low partially qualifying NOR-gate 16. The second input to NOR-gate 16 containing the RFl signal will likewise be low still partially qualifying NOR-gate 16. The last input, the LLF signal, for qualifying NOR-gate 16 will be low totally qualifying NOR-gate 16. That will result in the D input to second flip-flop 14 being high. On the next trailing edge of the BFX signal, second flip-flop 14 will be set high, causing the error signal ERR to be generated. The LLF signal is derived by the last low frequency generator circuit which is part of the clock recovery circuit 8 and will be described in conjunction with Fig. 3. The last low frequency generator circuit combines the BSD and BSD signals with the QBT and TQBT signals and their complement .signals simultaneously while the recovered clock, RFl, is a logic one (high) . The last low frequency generator circuit of the pre¬ ferred embodiment is shown in Fig. 3. An LLF latch 31 is set and reset in accordance with the following logic equations: LLF = RFl • (BSD • QBT • TQBT) LLF = RFl • (BSD • QBT • TQBT) When the signals BSD, QBT are a logic one (high) , and RFl is a logic zero (low), the output of NOR-gate 32 is a logic zero (or low) . Inverter 33 will invert the output of NOR-gate 32 resulting in a logic one input to NOR-gate 34. When the TQBT signal is low, the output of NOR-gate 34 is low, the LLF latch 31 being reset, i.e. LLF = 0, due to LLF being high. At the same time, the QBT and BSD inputs to NOR-gate 36 are low, and the RFl signal still is low, which results in. the output of NOR- gate 36 being a logic one; hence, the output of inverter 37 and the input to NOR-gate 38 is low. Since TQBT is low, TQBT is high, yielding a low output from NOR-gate 38. This results in the output of NOR-gate 38 going high, causing LLF to go low and LLF to go high, thereby setting LLF latch 31. As can be seen from the above condition, NOR-gates 32 and 34 together with inverter 33 yield the resultant AND function for the TQBT, QBT and BSD signals for setting LLF latch 31, and NOR-gates 36 and 38 together with inverter 37 give the AND function for the TQBT, QBT, and BSD signals for resetting the LLF latch 31, the setting and resetting of LLF latch 31 being performed in accordance with the above equations. The recovered clock signal RFl is generated by the setting and resetting of a latch 81, shown in Fig. 4, in accordance with the logic equations set forth in Table 1. A setting and resetting logic 82 implements set and reset signals defined in Table 1. A more de¬ tailed explanation to the operation of the circuit of Fig. 4 may be had by referring to the aforementioned European application. The operation of the error detection circuit 10 may best be understood with the aid of the specific examples shown in Figs. 5 and 6. Referring to Fig. 5, an example is shown in which one-half bit time elapses before detection of the error. During time interval T , the system interface receiver is operating normally. Fig. 5 shows the bit times and the bit serial data signal BSD, the QBT, TQBT, and the BFX input signals to the error detection circuit 10. The recovered clock signal RFl and the generated LLF and LLF signals are also shown. The output signal of second flip-flop 14 (ERR) is also shown along with the output signal of first flip-flop 12 and the output of NOR-gate 16. When RFl is high, the output of flip-flop 12 is also high. The output of NOR-gate 16 is maintained low through most of the period by LLF input being high, the LLF and LLF signals being generated in accordance with the equations defined above. In this.specific example, the error condition starts at time Tl, namely, QBT should be going low at this time. As a result of this error condition, no recovered clock pulse is generated at time Tl, that is, RFl fails to go high at time Tl, as shown by the dotted line. Since RFl is low during the time interval T , NOR-gate 16 is partially qualified, and LLF also being low further partially qualifies NOR- gate 16. The output of first flip-flop 12 is low as a result of having been reset by the BFX negative going signal at time T2 and has failed to be set because of missing clock pulse RFl. As a result, when the transmit clock BFX goes low at time T3, the D input to second flip-flop 14 is now high causing second flip-flop 14 to set, thereby raising the error signal, ERR, at time T3. The interval of time from the start of the error con¬ dition at Tl to the time the error signal ERR is raised at T3 is one-half of a bit time. Referring to Fig. 6 there is shown an example in which the elapsed time before detection of an error is one and one-half bit times. Fig. 6, like Fig. 5, shows the bit time, the input bit serial data (BSD), QBT, TQBT, and the BFX signals. The generated LLF and LLF signals, along with the RFl signal are also shown. The output signal of second flip-flop 14 (ERR) is shown along with the output signal of first flip-flop 12 and the output of NOR-gate 16. Still referring to Fig. 6, the error detection circuit 10 is operating normally during time interval T , as described above. At time Tl, the error condition starts. The output of first flip—flop 12 remains high throughout the period T . Ά. This is because the recovered clock signal is still- high holding the first flip-flop 12 set during time period of the falling edge of BFX just prior in time to time T4. At time T2, the output of first flip-flop 12 will be reset, since the transmit clock pulse is missing. At time T2, RFl and LLF are low, partially qualifying NOR- gate 16. Once the output of first flip-flop 12 goes low, NOR-gate 16 is totally qualified, causing the D input to second flip-flop 14 to be high. At time T3, during the falling edge of the BFX signal, the output of second flip-flop 14 will be set, raising the error signal ERR. The period of time from the start of the OM fe?NA error condition to detection of the error condition at time T3, time interval T , is one and one-half bit times This time interval is the maximum time interval for detection of an error condition for the error detection circuit 10 of the present invention. It will be understood that the error signal may be outputted directly from the system interface receiver 1 or may be used to set a latch before out- putting the error signal to the system. In the latter case, the latch will remain set until the error is recognized and an appropriate recovery action has been taken by the system.

Claims: CLAIMS : 1. A circuit for detecting an error in a clock signal recovered from a self-clocking digital data signal characterized by: detecting means (12, 16) for detecting a transition in the voltage level of the clock signal, to provide a transition signal indicating whether or not said transition has occurred within a predetermined time period; and sensing means (14) for sensing said transition signal to provide an error sig¬ nal if no transition has occurred within said predeter- mined time period. 2. A circuit according to claim 1, charac¬ terized in that said detecting means (12, 16) has an input terminal (C) adapted to receive a reference clock signal for defining said predetermined time period. 3. A circuit according to- claim 2, charac¬ terized in that said detecting means (12, 16) comprises switch means (12), for alternating between a first state and a second state in response to the clock signal and the reference clock signal, respectively; and gate means (16), operatively connected to said switch means (12), for gating the state of said switch means as a function o the clock signal, thereby providing said transition signal. 4. A circuit according to claim 3, charac¬ terized in that said sensing means (14) comprises a flip-flop having an " input (D) operatively connected to said gate means (16), and having a second input (C) arranged to receive said reference clock signal. 5. A circuit according to claim 3, charac¬ terized in that said gate means (16) comprises a logic gate having first, second, and third inputs, said first 5 . ( concluded ) input being operatively connected- to said switch means i (12), said second input being arranged to receive the recovered clock signal, and said third input being arranged to receive a data input signal. 6. A circuit according to any one of claims 3 to 5, characterized in that said switch means (12) comprises a flip-flop having a first input connected to a logic zero level, a second input arranged to receive said reference clock signal, and a third input arranged to receive the recovered clock signal, and further having an output operatively connected to an input terminal of said switch means (12). c?.:

Inventors: COOK DONALD MARVIN, COOK, DONALD, MARVIN
Applicants: NCR CO, NCR CORPORATION
Application year: 1982
Main classification: G06F11
IPC codes: G06F3, G06F11, H04L1