Network synchronization unit for a telephone exchange

Abstract: In a telephone exchange apparatus which accommodates digital communication lines, and employs a slave synchronization system in which a clock is extracted from a signal received from the network. and an exchange switch is operated in synchronism with the extracted clock so as to prevent data omission caused by a difference between operation frequencies of a network and an exchange apparatus, if there is a phase difference between a pre-switching clock and a post-switching clock when the extracted clocks are switched upon changing of communication lines, the post-switching clock is delayed by the phase difference between the two clocks so as to cause the phase of the post-switching clock to coincide with the phase of the pre-switching clock.

Description: BACKGROUND OF THE INVENTION [FIELD OF THE INVENTION] The present invention relates to a telephone exchange apparatus having a means for generating a clock synchronous with a network. [PRIOR ART] As is well known, for example, a telephone exchange apparatus for accommodating a digital communication line normally employs a slave synchronization system so as to prevent data omission due to a difference in operation frequency between a network and the exchange apparatus. In this system, a clock is extracted from a signal received from the network, and an exchange switch is operated in synchronism with the extracted clock. Such a system is known from JP-A-54 101 610. The conventional telephone exchange apparatus uses a phase controlled oscillator for generating an exchange switch operation clock synchronous with the clock extracted from a received signal. Fig. 8 shows a case wherein a DPLL is used as the conventional phase controlled oscillator. In Fig. 8, reference numeral 30 denotes a phase comparator; 31, a counter; 32, an increment/decrement circuit; 33, a frequency-dividing counter; 34, a clock extracted from a network; 35, a phase comparison DPLL output clock; 36, a phase comparison result signal (for selecting an up- or down-counting operation of the counter 31); 37, a counter clock; 38, a counter carry-out signal; 39, a counter borrow signal; 40, a PLL reference clock; 41, an output after a clock operation of the PLL reference clock; and 42, a PLL output clock used in an operation of the exchange apparatus. In the conventional phase controlled oscillator with the above arrangement, an operation executed when an 8-kHz frame synchronization timing clock is extracted from a network, and a clock of 32.768 MHz is used as a PLL reference clock will be described below. As the PLL output clock 42, and the phase comparison PLL output clock 35, clocks obtained by 2 n -frequency-dividing the PLL reference clock are output. As the phase comparison PLL output clock 35, an 8-kHz clock which is the same as the clock 34 extracted from the network is output so as to compare phases with the 8-kHz clock extracted from the network. Phases are compared at each rising timing of the two clocks. As a result of comparison, if it is determined that the phase of the clock 34 extracted from the network advances from the clock 35, the value of the counter 31 is incremented by one. When the counter value has reached a preset value, one clock is inserted in the PLL reference clock 40 by the increment/decrement circuit 32. This operation is repeated until a phase difference is eliminated. Thus, the clock slaving to the operation clock of the network can be supplied to the telephone exchange apparatus. On the other hand, as a result of comparison, if it is determined that the phase of the clock 34 extracted from the network is delayed from the clock 35, the value of the counter 31 is decremented by one. When the counter value has reached a preset value, one clock is removed from the PLL reference clock 40 by the increment/decrement circuit 32. This operation is repeated until a phase difference is eliminated. Thus, the clock slaving to the operation clock of the network can be supplied to the telephone exchange apparatus. However, in an actual telephone exchange apparatus, it is unusual that the number of clocks extracted from the network input to the phase controlled oscillator is only one, as described above. More specifically, a plurality of digital communication lines are normally accommodated in the exchange apparatus, and when clocks can only be extracted from the accommodated digital communication lines during communications, which clock extracted from the lines is to be used as the clock extracted from the network input to the phase controlled oscillator must be controlled in accordance with the communication conditions. Fig. 9 shows an arrangement of the conventional telephone exchange apparatus having the above-mentioned phase controlled oscillator. In Fig. 9, reference numeral 43 denotes communication lines connected to the network; a line interface for interfacing with the network; 45, a network synchronization unit; 46, a central control unit; 47, an exchange switch; 48, a memory; 49, an extension interface unit; 50, telephone sets connected to the extension interface unit 49; 51, clocks extracted from the lines; 52, signals indicating line conditions; 53, an exchange switch operation clock; 54, a highway; and 55, an address bus/data bus. Fig. 10 shows the first detailed arrangement of the conventional network synchronization unit 45. In Fig. 10, reference numeral 56 denotes a selector; 57, a priority encoder; 58, a flip-flop; 59, a clock phase difference absorbing unit; 60, a phase controlled oscillator; 61, a switching timing generator; 65, an output frequency-divided clock; 66, a synchronization clock; and 67, an input clock to the phase controlled oscillator. The operation of the conventional first network synchronization unit 45 will be described below with reference to Figs. 9 and 10. In this case, a clock switching section will be mainly described. The line interface unit 44 extracts a timing from a received signal from the network, and generates 8-kHz clocks CLK0 to CLKn by the PLL. The unit 44 outputs the clocks CLK0 to CLKn to the network synchronization unit 45. Suffixes 0 to n attached to the signal names correspond to lines from which the clocks are extracted, and in the following description, n = 8. In a communication state with the network, the 8-kHz clock to be output is synchronized with the timing of the network. However, in a non-communication state, a clock is generated by an independent quartz oscillator, or the like, and is not always synchronized with the timing of the network. The line interface unit 44 outputs signals COM0 to COMn indicating line conditions to the network synchronization unit 45. The line interface unit 44 outputs, as the signals COM0 to COMn, signals at logic "0" in a communication state, and signals at logic "1" in a non-communication state. Note that suffixes 0 to n added to the signal names represent line numbers, and in the following description, n = 8. The line condition signals COM0 to COM8 are input to the priority encoder 57 (Fig. 10) of the network synchronization unit 45. The priority encoder 57 converts the smallest line number of those of the lines at logic "0" into 3-bit binary data, and outputs the binary data to the flip-flop 58. For example, when the signals COM2, COM5, and COM8 are at logic "0", 3-bit data "010" is output. When all the line condition signals are at logic "1", i.e., when no communications are performed at all, 3-bit data "000" is output. In this case, the clock CLK0 is selected and output. The clock CLK0 is a clock of a predetermined line number "0", and the clock extracted from this line is selected. The output from the priority encoder 57 is temporarily latched by the flip-flop 58, and is then input to the selector 56 having an 8-bit input. The selector 56 selects a clock to be output to the phase controlled oscillator 60 from the eight 8-kHz clocks 51 supplied from the line interface unit 44. For example, when the signals COM2, COM5, and COM8 are at logic "0", since the output from the priority encoder is 3-bit data "010", the clock CLK2 is selected. When the second line 2 starts a communication, the signal COM2 goes to logic "1", and the output from the priority encoder 57 is changed to 3-bit data "101". The selector 56 selects the clock CLK5 according to the output from the encoder 57, and the clock CLK5 is input to the phase controlled oscillator 60. When the conventional clock phase difference absorbing unit 59 outputs the clock signal 63 input from the selector 56 to the phase controlled oscillator 60, it merely outputs it in synchronism with the synchronization clock signal 66 from the phase controlled oscillator 60, and can merely absorb a phase difference from the internal clock of the phase controlled oscillator 60. With the above-mentioned operations, a switching operation is performed, so that a clock extracted from the line in communication can be input to the phase controlled oscillator 60. Fig. 11 shows the second detailed arrangement of the conventional network synchronization unit 45. Note that other arrangements are the same as those shown in Fig. 9. In Fig. 11, reference numeral 70 denotes a clock phase difference absorbing unit, which has delay circuits corresponding to input clocks. Reference numeral 71 denotes a flip-flop; 72, a selector; 73, a phase controlled oscillator; and 74, a switching timing generator. Reference numeral 75 denotes a selector control signal; 76, a selected extracted clock; 77, a switching timing signal; 78, an output frequency-divided clock; and 79, an output frequency-divided clock. The operation of the conventional second network synchronization unit 45 will be described below with reference to Figs. 9 and 11. In this case, a clock switching section will be mainly described. The operation of the line interface unit 44 is the same as that described above. 8-kHz clocks output from the line interface unit 44 are input to the phase difference absorbing unit 70 shown in Fig. 11. The outputs from the phase difference absorbing unit 70 are input to the selector 72, and are switched by the selector 72 in accordance with the selector control signal (clock switching control signal 75) from the flip-flop 71 which latches the signals 52 (COM0 to COMn) output from the line interface unit 44 and indicating line conditions (communication conditions). The clock phase difference absorbing unit 70 is a circuit for eliminating the influence of a phase difference among clocks extracted from the lines. The timings of the respective lines have phase differences due to delay time differences in respective line transmission paths. For this reason, the respective clocks are optimally delayed, so that phases of the clocks input to the phase controlled oscillator 73 are locked, thereby preventing a phase difference before and after a clock switching operation. However, the first prior art suffers from the following drawbacks since the means such as the priority encoder 57, the selector 56, and the like are used as means for switching clock signals to be input to the phase controlled oscillator 60. (1) In order to obtain a stable output from the phase controlled oscillator 60, it is preferable that a clock input to the phase controlled oscillator 60 is also stable. For this purpose, the switching frequency of clocks is preferably as small as possible. In the conventional arrangement, however, even when a given line maintains a communication state for a long period of time, if a line having a smaller number than the given line starts or ends a communication, the clock is unconditionally switched, and a stable clock cannot be input. (2) When all the lines are in a non-communication state, a clock from a predetermined line is selected. the lines must be accommodated in turn from a predetermined line position in a predetermined order. For this reason, it is troublesome to connect lines to the exchange apparatus, and a connection error may occur. In the second prior art, since all the clocks extracted from the lines are input to the phase difference absorbing unit 70, delay circuits corresponding in number to the lines are undesirably required. In particular, this poses a serious problem in a telephone exchange apparatus which accommodates a large numbers of lines. Furthermore, EP-A-0 242 117 discloses a distributed timing control arrangement that dynamically controls the distribution of timing information in a distributed digital communication system. A reference timing signal is distributed from a reference master node to all other nodes in the system. Each node is connected by digital transmission links to at least one other node, and each node receives timing signals from all of the other links to which it is connected. Each node selects one of these signals as its timing reference. Hence, it becomes possible that each node is controlled by the timing signal from the master node over exactly that path involving the least intermediate nodes. For this purpose, each node is provided with several link interfaces, a digital switching or transmission system, a timing source selector switch, and a clock signal generator. Hence, as disclosed in EP-A-0 242 117, in this prior art arrangement, in case that a signal source is unreliable, timing control of a node is switched to a different source of clock signals; in addition, a timing reference signal is selected based on so-called TPN (timing priority number) information. This TPN information indicates how many nodes exist between a communication apparatus and a master node. Thus, a clock signal or TPN information is constantly received from a plurality of communication lines, i.e. a clock signal is received without consideration of whether a communication is currently performed or completed. Therefore, selection of a clock source is not necessary for the prior art apparatus according to EP-A-0 242 117, and moreover, whether or not a communication is completed has no relationship whatsoever for selecting a clock source. As a result, in this prior art digital communication apparatus, if a digital communication line starts or ends a communication, the clock may be unconditionally switched. Thus, a stable clock cannot be output. In addition, due to the unconditional clock switching, data errors may occur, and in particular data omission may be caused if the clock is switched while communication over one or more of the digital communication lines proceeds. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a digital communication apparatus and a synchronizing method of such a digital communication apparatus with which the reliability of digital transmission can be improved. According to the present invention this object is achieved by a digital communication apparatus according to the new independent claim 1. According to the present invention this object is also achieved by a synchronizing method of a digital communication apparatus according to the new independent claim 6. Thus, according to the present invention, the selection means is controlled according to the line condition, i.e. whether a given digital communication line is in a communication state or not. In detail, one of the plurality of digital communication lines is selected in order to provide clocks to the exchange means or unit. Another digital communication line is not selected until the communication via the one of the plurality of digital communication lines ends. In addition, clocks are provided from the plurality of digital communication lines via which communication is performed. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a telephone exchange apparatus which incorporates a network synchronization unit according to an embodiment of the present invention; Fig. 2 is a detailed block diagram of the network synchronization circuit of this embodiment; Fig. 3 is a detailed block diagram of a clock phase difference absorbing unit of this embodiment; Fig. 4 is a timing chart showing phase difference controlled oscillator input signals without a phase difference absorbing unit; Fig. 5 is an operation timing chart of the phase difference absorbing unit of this embodiment; Fig. 6 is a detailed block diagram of a clock phase difference absorbing unit according to the second embodiment of the present invention; Fig. 7 is an operation timing chart of the phase difference absorbing unit of the second embodiment; Fig. 8 is a block diagram of a conventional phase difference controlled oscillator; Fig. 9 is a block diagram showing a conventional telephone exchange apparatus; Fig. 10 is a block diagram showing a conventional network synchronization unit; and Fig. 11 is a block diagram showing the second detailed arrangement of the conventional network synchronization unit. Figs. 12A and 12B are flowcharts showing examples of a switching control table stored in the memory of this embodiment. DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described in detail below with reference to the accompanying drawings. [First Embodiment] Fig. 1 is a block diagram showing an arrangement of a telephone exchange apparatus incorporating a network synchronization unit according to an embodiment of the present invention. In Fig. 1, reference numeral 1 denotes communication lines; 2, a line interface unit; 3, a network synchronization unit; 4, a central control unit; 5, an exchange switch; 6, a memory; 7, an extension interface unit; 8, telephone sets; 9, clocks extracted from the respective lines; 10, a clock switching control signal; 11, an exchange switch operation clock; 12, a highway; and 13, a data bus/address bus. Fig. 2 shows the detailed arrangement of the network synchronization unit 3 of this embodiment shown in Fig. 1. In Fig. 2, reference numeral 14 denotes a selector; 15, a flip-flop; 16, a clock switching timing generator; 17, a clock phase difference absorbing unit; 18, a phase controlled oscillator; and 19, an output frequency-divided clock, which is an 8-kHz clock signal in this embodiment. Reference numeral 20 denotes a selector control signal; 21, an extracted clock selected by the selector 14; 22, a switching timing signal; 23, a synchronization clock; and 24, a phase controlled oscillator input clock. Fig. 3 shows the detailed arrangement of the clock phase difference absorbing unit of this embodiment shown in Fig. 2. In Fig. 3, reference numeral 25 denotes a flip-flop; 26, a phase comparator; 27, a delay circuit; 28, a shift register; and 29, a selector. The operation of this embodiment comprising the above-mentioned arrangement will be described below with reference to the timing charts of Figs. 4 and 5. The line interface unit 2 extracts a timing from a received signal from the network, and generates 8-kHz clocks CLK0 to CLK8 by a PLL. In a communication state with the network, the 8-kHz clock to be output is synchronized with the timing of the network. However, in a non-communication state, a clock is generated by an independent quartz oscillator, or the like, and is not always synchronized with the timing of the network. The line interface unit 2 includes a register 2a indicating a communication condition, and the central control unit 4 reads out condition data set in the register to detect a line condition (communication condition). The 8-kHz clock signals output from the line interface unit 2 are input to the selector 14 (Fig. 2) of the network synchronization unit 3. The selector 14 selects and outputs one of the input clock signals in accordance with the clock switching control signal 10 output from an output port incorporated in the central control unit 4. The clock switching control signal 10 is output in accordance with a switching control table which is realized as firmware in various patterns according to communication conditions and is stored in the memory 6. The central control unit 4 properly reads the register 2a of the line interface unit 2 to read out the line condition, and outputs an optimal clock switching control signal 10 from its output port in accordance with the switching control table stored in the memory 6 in quick response to a recognized change in communication condition, thus minimizing the number of times of switching operations. Figs. 12A and 12B show examples of a switching control table stored in the memory 6. The flow charts shown in Figs. 12A and 12B are executed by the central control unit 4. In Fig. 12A, if it is determined in step S1 that all the lines are ready, the flow advances to step S2. In step S2, a value '111' as data for selecting a free-running clock is sent to the network synchronization unit 3. The network synchronization unit 3 supplies a free-running clock (about 8 kHz) to the exchange switch 5 on the basis of data = '111' from the central control unit 4. The free-running clock is used by the exchange switch 5 in a stand by mode of the apparatus. In step S3, it is checked if a communication is started. If it is determined in step S3 that the communication is started, the flow advances to step S4. In step S4, a line which is switched to a busy state is selected to extract a clock. When a plurality of lines are switched to a busy state at the same time, a line having the youngest number is selected. It is checked in step S5 if a communication of the line selected in step S4 is ended. If Y in step S5, the flow advances to step S6. In step S6, it is checked if communications of all the connected lines are ended. If it is determined in step S6 that the communications of all the connected lines are not ended, the flow returns to step S4, and a line having the youngest number is selected from the lines in communication so as to extract a clock. If it is determined in step S6 that the communications of all the connected lines are ended, the flow returns to step S1, and a standby state is set. In an arrangement for selecting a line according to a conventional hardware logic, if a line having a younger number than that of a line which is currently used in clock extraction is used, the line having the younger number is selected for clock extraction. Therefore, the switching frequency of lines becomes very high. As compared to this arrangement, according to the embodiment shown in Fig. 12A, since lines are selected in a software manner, once a line is selected, the selected line can be held until a communication is ended. In this manner, the switching frequency of lines can be decreased, and clocks can be stably supplied. Fig. 12B is a flow chart as a partial modification of Fig. 12A. In steps S1 to S6 in Fig. 12B, the same control operations as in steps S1 to S6 in Fig. 12A are executed. Step S7 is executed when a communication of a line which is selected first from a state wherein all the lines are ready. More specifically, step S7 is executed when the second and subsequent line selections are performed from a state wherein all the lines are ready. In step S7, the oldest busy line is selected from lines in use so as to extract a clock. The reason why the oldest busy line is selected will be explained below. The oldest busy line has been subjected to many phase synchronization operations with clocks of other lines. Therefore, the phase synchronization operation with clocks of other lines of this line is almost close to its limit. Thus, the oldest busy line itself is selected for clock extraction, so that the selected line need not be subjected to phase synchronization. In this manner, according to the flow chart shown in Fig. 12A, the switching frequency of clocks can be decreased, and clocks can be stably supplied. According to the flow chart shown in Fig. 12B, since the oldest busy line is selected in the second and subsequent line selections for clock extraction, disconnection of a communication caused by a phase asynchronization state with clocks of other lines can be prevented. The clock switching control signal 10 is temporarily latched by the flip-flop 15 to be synchronized with an operation timing of the clock phase difference absorbing unit 17 (to be described later) after it is input to the selector 14 of the network synchronization unit 3. The timings of the respective lines have phase differences due to delay time differences in respective line transmission paths. The clock phase difference absorbing unit 17 is a circuit for eliminating the influence of a phase difference between clocks when the clocks extracted from the lines are switched. Fig. 4 shows the input clock 21 to the clock phase difference absorbing unit 17 when the phase of a pre-switching post-switching clock is different from that of a clock in the selector 14. The phase controlled oscillator 18 of this embodiment has the same arrangement as that of the conventional one shown in Fig. 8 described above. For this reason, when the clock signal including a phase difference shown in Fig. 4 is directly input to the phase controlled oscillator 18, an increment/decrement circuit is operated under the above-mentioned control, and one clock is inserted in the PLL reference clock. In order to effectively prevent such a phenomenon, according to this embodiment, control is made such that the clock input to the phase controlled oscillator 18 does not suffer from a change in phase upon switching by the following operation of the clock phase difference absorbing unit 17. The operation of the clock phase difference absorbing unit 17 of this embodiment will be described below with reference to Fig. 5. Note that reference symbols a to g in Fig. 5 correspond to output signals a to g in Fig. 3. In order to cause the phase of an extracted clock b from a switched line to coincide with the phase of a pre-switching clock a , the clocks a and b are latched by the flip-flop 25 in response to the synchronization clock 23 having a higher frequency than the extracted clocks, and output from the phase controlled oscillator 18. In this manner, the clocks are synchronized with the synchronization clock 23. In the conventional apparatus, the synchronized signal is directly output to the phase controlled oscillator. However, in this embodiment, the following control is made. The phase comparator 26 compares a phase difference between the pre-switching clock a and the post-switching clock b, which are latched by the flip-flop 25, and outputs a comparison result c indicating a time corresponding to the phase difference. The delay circuit 27 outputs a clock d obtained by delaying the post-switching clock b by the comparison result signal c from the phase comparator 26 to the shift register 28 and the selector 29. At this time, since the two clocks a and b are synchronized with the synchronization clock 23, only one clock can be easily delayed using, e.g., the shift register. In order to prevent an asynchronous state of clocks caused by a phenomenon shown in Fig. 4 while the phase comparator compares the phases of the two clocks a and b, the selector 29 outputs a clock e obtained by delaying the post-switching clock b by the shift register 28 to the phase controlled oscillator 18 during a phase comparison period. A timing signal for switching whether the clock is input to the phase controlled oscillator 18 with or without going through the shift register 28 is generated by the clock switching timing generator 16 so as not to switch the clocks at their leading or trailing edges. In this manner, even when a signal from a line is changed, the phases of pre- and post-switching clock signals are compared, the post-switching clock signal is delayed by their phase difference, and the delayed signal is output, so that the pre- and post-switching clocks will not be different from each other. When the clock signal is further switched thereafter, a signal obtained a signal from the line by a given delay time by the delay circuit 27 using the phase comparison result c between the pre- and post-switching clock signals as a reference pre-switching clock signal is used. Fig. 5 is a timing chart schematically showing the above-mentioned operations. The reference symbols a to g in Fig. 5 correspond to those of the signals shown in Fig. 3. In the above description, the number of communication lines connected to the telephone exchange apparatus is eight. However, the number of accommodated communication lines of this embodiment is not limited to this, and the same effect as described above can be obtained by the above arrangement regardless of the number of communication lines. For example, as can be easily understood from the above description, when the telephone exchange apparatus can connect n lines, the number of clock switching control signals 10 output from the central control unit 4 is log2n. Therefore, as the number of accommodated lines n is increased, the effect of this embodiment can be more outstanding. In the above description, the telephone exchange apparatus include one central control unit 4. In a large-scale system, however, it is difficult to control all the operations by one central control unit 4. Therefore, this embodiment also includes a case wherein a plurality of central control units 4 are used. In this case, when the central control unit 4 for accessing a register indicating a communication condition, and for outputting a switching timing signal is arranged in, e.g., the line interface unit 2, the same effect as described above can be expected. In the above description, the apparatus is connected to lines from which clocks can be extracted in only a communication state. However, the clock extraction timing is not limited to this. When the apparatus of the present invention is connected to lines from which clocks can be extracted even in a non-communication state, it can always extract clocks, and can be utilized in processing when supply of clocks is disabled due to a line trouble. Therefore, the same effect as described above can be expected. As described above, according to this embodiment, the following effects can be obtained. (1) The switching frequency of a clock to be input to the phase controlled oscillator 18 can be decreased, and a clock to be generated can be further stabilized. (2) Even when all the lines are in non-communication state, since a line to be selected need not be determined beforehand, limitations upon connection of the lines to the exchange apparatus can be removed. (3) The circuits can be simplified as compared to the prior arts. (4) The number of signal lines input to the network synchronization unit 3 can be decreased from 2n to (n + log2n). [Second Embodiment] In the first embodiment described above, if there is a phase difference between pre- and post-switching clocks upon switching of the clocks, the post-switching clock is delayed by a required time by the delay circuit 27 of the clock phase difference absorbing unit 17 (Fig. 3) so as to lock the phase of the post-switching clock with that of the pre-switching clock. However, the method of locking the phases of the two clocks is not limited to the above-mentioned method, but may be realized by using, e.g., a shift register. The second embodiment of the present invention for delaying a post-switching clock by a shift register will be described hereinafter. In the second embodiment to be described below, the overall arrangement of the telephone exchange apparatus is that shown in Fig. 1, and the overall arrangement of the network synchronization unit 3 is that shown in Fig. 2. In the second embodiment, the clock phase difference absorbing unit 17 of the network synchronization unit 3 can have an arrangement shown in Fig. 6. In Fig. 6, reference numeral 85 denotes a flip-flop; 86, a phase comparator; 87, a counter; 88, a counter load signal; 89, a flip-flop; 90 and 92, shift registers; and 91 and 93, selectors. In the above arrangement, timings of the respective lines in line transmission paths have phase differences based on delay time differences. However, in this embodiment, the clock phase difference absorbing unit 17 shown in Fig. 6 eliminates the influence of the phase difference between the clocks upon switching of the clocks extracted from the lines like in the first embodiment. More specifically, in this embodiment, an input clock to the clock phase difference absorbing unit 17 when the phase of a pre-switching clock is different from that of a post-switching clock in the selector 14 is as shown in Fig. 4. When the clock signal including a phase difference shown in Fig. 4 is directly input to the phase controlled oscillator 18, an increment/decrement circuit is operated under the above-mentioned control, and one clock is inserted in the PLL reference clock. In order to effectively prevent such a phenomenon, according to this embodiment, control is made such that the clock input to the phase controlled oscillator 18 does not suffer from a change in phase upon switching by the following operation of the clock phase difference absorbing unit 17. Fig. 7 is a timing chart schematically showing the above-mentioned operations. Reference symbols a to i in Fig. 7 correspond to those of signals a to i in Fig. 6, respectively. The operation of the clock phase difference absorbing unit of the second embodiment will be described below with reference to Fig. 7. More specifically, in order to cause the phase of a clock b extracted from a switched line to coincide with that of a pre-switching clock a , the two clocks a and b are latched by the flip-flop 85 in response to a clock 83 having a frequency higher than the extracted clocks (e.g., a 64-kHz clock). The phase comparator 86 compares a phase difference between the latched pre-and post-switching clocks a and b, and outputs a comparison result c indicating a time corresponding to the phase difference to the counter 87. The counter 87 is set in a load state for a time corresponding to the phase difference, and starts a counting operation. If the extracted clock is delayed by a count time (an output value from the counter 87 x a time for one period of a counter clock) corresponding to a count value of the counter 87 in the load state, a phase difference between the extracted clock and a clock input to the phase controlled oscillator 18 can be eliminated. For this reason, the count value of the counter 87 is latched by the flip-flop 89, and is output to the selector 91. In order to obtain a clock delayed by a desired delay amount, the shift registers 90 and 91 are used in this embodiment. More specifically, the synchronized extracted clock is shifted by the above-mentioned 64-kHz clock, thereby generating extracted clocks d,..., e having eight different delay amounts. The eight clocks are input to the selector 91. The selector 91 selects one of the eight clocks in accordance with a signal from the flip-flop 89 corresponding to an output value from the counter 87 in the load state, and outputs the selected clock signal f to the shift register 92. As a result, while the lines from which clocks are extracted are not switched, the count value of the counter 87 to be input to the selector 91 is equal to one set in the flip-flop 89 at the immediately preceding switching timing, and the extracted clock f is controlled to always have a predetermined delay amount. The extracted clock is then output to the phase controlled oscillator 18 as an output signal i via the selector 93. However, if the clock f delayed by the predetermined delay amount is directly output to the phase controlled oscillator 18 even when the clocks are switched, a clock switching state shown in Fig. 4 occurs. For this reason, when the clocks are switched, the clock f selected by the selector 91 is input to the shift register 92, and a clock obtained by delaying the clock f to be in phase with the pre-switching clock is selected by the selector 93. Thus, the clock selected by the selector 93 is used as the output signal i. In the above description, as a means for delaying a clock selected from a plurality of clocks extracted from the lines, a shift register is used. However, the delay means is not limited to the shift register, but may be any other means. For example, an analog delay element may be used. In the above description, in order to delay a clock selected from a plurality of clocks extracted from the lines, a signal having a frequency of 64 kHz is used. When a signal having a higher frequency is used, quicker response to a change in frequency can be attained. Furthermore, the frequency of a clock to be extracted from a line is not limited to 8 kHz. The same effect as described above can be obtained even when other frequencies are adopted. Moreover, the number of communication lines connected to the telephone exchange apparatus is eight. However, the number of accommodated communication lines of this embodiment is not limited to this, and the same effect as described above can be obtained by the above arrangement regardless of the number of communication lines. For example, as can be easily understood from the above description, when the telephone exchange apparatus can connect n lines, the number of clock switching control signals 10 output from the central control unit 4 is log2n. Therefore, as the number of accommodated lines n is increased, the effect of this embodiment can be more outstanding. As described above, according to this embodiment, the following effects can be obtained. (1) The switching frequency of a clock to be input to the phase controlled oscillator 18 can be decreased, and a clock to be generated can be further stabilized. (2) Even when all the lines are in non-communication state, since a line to be selected need not be determined beforehand, limitations upon connection of the lines to the exchange apparatus can be removed. (3) The circuits can be simplified as compared to the prior arts. The present invention may be applied to either a system constituted by a plurality of devices, or an apparatus constituted by a single device. The present invention can also be applied to a case wherein processing is attained by supplying a program to a system or an apparatus. As described above, according to the present invention, the circuit arrangement can be simplified. The switching frequency of a clock to be input to the phase controlled oscillator can be decreased, and a clock to be generated can be further stabilized. Even when all the lines are in non-communication state, since a line to be selected need not be determined beforehand, limitations upon connection of the lines to the exchange apparatus can be removed.

Claims: A digital communication apparatus comprising: connection means (2) for connecting a plurality of digital communication lines (1); exchange means (5) for exchanging communication between said plurality of digital communication lines (1); selection means (4) for selecting one of said plurality of digital communication lines (1); and providing means (3) for providing clocks which are synchronized with clocks from said one of said plurality of digital communication lines selected by said selection means (4) to said exchange means (5); characterized in that clocks (9) are provided from said plurality of digital communication lines (1) via which communication is performed; and in that said selection means (4) are adapted not to select another one of said plurality of digital communication lines (1) once said one of said plurality of digital communication lines (1) has been selected until a communication via said one of said plurality of digital communication lines (1) ends. The apparatus according to claim 1, characterized in that said connection means (2) can connect a maximum of n communication lines (1); and in that said selection means (4) are adapted to read out an algorithm stored in a memory (6) which stores algorithms for performing selection, and performs the selection in accordance with at least log (2n) data bits. The apparatus according to claim 1, characterized by further comprising data input means for inputting data from said plurality of digital communication lines (1) connected by said connection means (2) to said exchange means (5) in synchronism with the clock from said one of said plurality of digital communication lines (1) selected by said selection means (2). The apparatus according to claim 1, characterized in that said selection means (4) are adapted to select another one of said plurality of digital communication lines (1) when the communication via said one of said plurality of digital communication lines (1) ends. The apparatus according to claim 1, characterized in that said selection means (4) are adapted to select another one of said plurality of digital communication lines (1) in accordance with a duration time of communication performed via said plurality of digital communication lines (1) when the communication via said one of said plurality of digital communication lines (1) ends. A synchronizing method of a digital communication apparatus for exchanging communication between a plurality of digital communication lines (1), said method comprising the steps of: selecting one of said plurality of digital communication lines (1); and providing clocks which are synchronized with clocks from said one of said plurality of digital communication lines (1) selected in said selection step to an exchange unit (5); characterized in that clocks are provided from said plurality of digital communication lines (1) via which communication is performed; and in that another one of said plurality of digital communication lines (1) is not selected once said one of said plurality of digital communication lines (1) has been selected until a communication via said one of said plurality of digital communication lines (1) ends. The method according to claim 6, characterized in that a maximum of n communication lines (1) can be connected; an algorithm stored in a memory (6) which stores algorithms for performing selection is read out; and in that the selection is performed in accordance with at least log (2n) data bits. The method according to claim 6, characterized by further comprising a data input step of inputting data from said plurality of digital communication lines (1) to said exchange unit (5) in synchronism with the clock from said one of said plurality of digital communication lines (1) selected in said selection step. The method according to claim 6, characterized in that another one of said plurality of digital communication lines (1) is selected when the communication via said one of said plurality of digital communication lines (1) ends. The method according to claim 6, characterized in that another one of said plurality of digital communication lines (1) is selected in accordance with a duration time of communication performed via said plurality of digital communication lines (1) when the communication via said one of said plurality of digital communication lines (1) ends.

Inventors: IZUMI MICHIHIRO, IZUMI, MICHIHIRO, IZUMI, MICHIHIRO, CANON DAI-ICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H04J3
IPC codes: H04Q11, H04J3