Abstract:
An input circuit for a semiconductor memory device is disclosed. The input circuit controlling transmission paths for data having passed through a data input buffer by using a 1-clock shifted block column address is provided. In particular, a data input apparatus improving a data processing speed by advancing an operation time point of a data bus writer is provided.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an input circuit of a memory device, and more particularly to an input circuit for a memory device, which improves a data processing speed by controlling transmission paths for data having passed through a data input buffer in response to an input of a block address.  
         [0003]     2. Description of the Prior Art  
         [0004]     The data processing speed of a semiconductor memory device is gradually accelerating. Moreover, with the development of a DDR SDRAM capable of accessing two data in one clock, the data processing speed of the memory device accelerates further. In particular, a processing for input data is one of issues important for improving the data processing speed of the memory device.  
         [0005]      FIG. 1  is a block diagram showing the data input circuit of a conventional memory device. Specifically, the memory device disclosed in the present specification denotes a DDR SDRAM, a DDR 2  SDRAM (next generation memory device), etc.  
         [0006]     As shown in  FIG. 1 , the conventional data input circuit includes data buffers  101  and  102 , an input multiplexer  103 , data bus writers  105  and  106 , block writers  107  and  108 , and an input selection signal generation circuit  104  for controlling the operations of the data bus writers  105  and  106 .  
         [0007]     For convenience of description,  FIG. 1  shows only two data buffers  101  and  102 . However, when the memory device has a data input/output structure of ×16, the number of the data buffers is 16. Accordingly, it is noted that 14 data buffers exist in addition to the data buffers  101  and  102  shown in  FIG. 1 .  
         [0008]     The basic operation of each element is as follows.  
         [0009]     The data buffers  101  and  102  controlled by a control signal Din clk receive corresponding data D 0  and D 1  respectively, and output data D 0 _ 1  and D 1   13    1 . Herein, the control signal Din clk is a signal (or clock) generated by the number of times of BL/2 after a write command and denotes a signal generated in synchronization with the rising edge of a first DQS signal.  
         [0010]     The input multiplexer  103  is a circuit for determining transmission paths of the data D 0 _ 1  and D 1 _ 1 . Herein, the reason for determining the transmission paths of the data is because the memory device having the data input/output structure of ×16 type may be used in a data input/output structure of ×8 type as the situation requires.  
         [0011]     For instance, when a data pin of the memory device is set to ×16, it is assumed that 16 bit data are applied to the memory device. In such a case, the data Do_ 1  are sent to the data bus writer  105  along a solid line and the data D 1 _ 1  are sent to the data bus writer  106  along a solid line. Other data D 2 _ 1 , . . . , D 15 _ 1  are sent to data bus writers along solid lines.  
         [0012]     In a state in which the data pin of the memory device is set to ×16, if is assumed that 8 bit data are applied to the memory device, 8 used buffers of 16 buffers are necessary and the other 8 buffers are unnecessary.  
         [0013]     Meanwhile, even though the data pass through the data buffers  101  and  102 , it is necessary to determine the data bus writer, to which the data are to be sent, by the input multiplexer  103 . For instance, the data D 0 _ 1  having passed through the data buffer  101  are sent to one of the two data bus writers  105  and  106  by the input multiplexer  103 . Herein, when data having the number of bits smaller than the predetermined number of bits are applied, the input multiplexer  103  includes a function of determining the transmission paths of the data.  
         [0014]     The data bus writers  105  and  106  send the data transmitted from the input multiplexer  103  to global input lines gio 0  and gio 1 . When the memory device operates in ×16 type, the data bus writers send the data transmitted from the input multiplexer to the global input lines. Further, when the memory device operates in ×8 type, it is necessary to maintain the output terminal of a data bus writer, to which data are not inputted, in an initialization state or precharge state.  
         [0015]     The block writers  107  and  108  send the data to memory blocks through local input lines lio 0  and lio 1 . Herein, the memory block signifies an area subdivided in a memory bank and the memory bank includes a plurality of memory blocks.  
         [0016]     The input selection signal generation circuit  104  receives a 2-clock shifted block column address and a control signal clk Din and outputs signals for controlling the operations of the data bus writers  105  and  106 . Herein, the 2-clock shifted block column address is a two-clock delayed signal than an input column address inputted by a write command as shown in  FIG. 2 . That is, the 2-clock shifted block column address is an address for selecting the specific block of the memory bank. The control signal clk Din is a clock signal generated by the number of times of BL/2 after a two-clock delay after the write command. That is, as shown in  FIG. 2 , the control signal clk Din is a clock signal generated in synchronization with the rising edge of a clock clk at a time point t 3 .  
         [0017]      FIG. 2  is a waveform view illustrating the operation of the circuit of  FIG. 1 . In  FIG. 2 , the clock clk denotes a clock signal applied to the memory device and the control signal Din clk is a signal for controlling the data buffers  101  and  102 . Further, the data D 0 _ 2  denotes data outputted from the input multiplexer  103  and the control signal clk Din is a two-clock delayed clock signal after the write command. The 2-clock shifted block column address is a signal two-clock delayed than a column address inputted in synchronization with the same clock as the write command input.  
         [0018]     In the operation of the memory device, the input selection signal generation circuit  104  enables the data bus writers  105  and  106  when both the 2-clock shifted block column address and the control signal clk Din are in high level.  
         [0019]     However, in the prior art, after the 2-clock shifted block column address has been generated, the control signal clk Din is generated after a predetermined period of time passes. That is, after the 2-clock shifted block column address has been generated, the control signal clk Din is generated with a predetermined time margin. Therefore, in the prior art, the operation time of the data bus writer is delayed by the time margin, so that a data transmission speed slows.  
       SUMMARY OF THE INVENTION  
       [0020]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide an input circuit capable of improving a data processing speed by accelerating the operation time point of a data bus writer.  
         [0021]     It is another object of the present invention to provide an input circuit capable of improving a data processing speed by shifting a block column address inputted in a write command by one clock and using the shifted block column address.  
         [0022]     In order to achieve the above objects, according to one aspect of the present invention, there is provided an input circuit for a memory device operating in synchronism with a clock signal comprising: data buffer part for receiving data applied from an external of the input circuit; input multiplexer part for receiving the data passed through the data buffer part; data bus writer part receiving the data passed through the input multiplexer part and outputting the data to global input/output lines of the memory device; and input selection signal generation circuit outputting a signal to control the operation of the data bus writer part, wherein the output signal of the input selection signal generation circuit is activated when both a first control signal generated after a write command is applied and then the clock signal is toggled n times and a second control signal generated after a write command is applied and then the clock signal is toggled n−1 times are enabled.  
         [0023]     In the present invention, the first control signal is generated in synchronism with a rising edge of the n-th clock signal generated after the write command is applied, and the second control signal is generated in synchronism with a rising edge of the (n−1)-th clock signal generated after the write command is applied.  
         [0024]     In the present invention, the second control signal is generated by shifting by 1tCK a block column address inputted to the memory device when the write command is applied.  
         [0025]     In order to achieve the above objects, according to one aspect of the present invention, there is provided an input circuit for a memory device comprising: 2N data buffers for receiving data applied from an external of the input circuit; N input multiplexers; 2N data bus writers; N block column address shifters; and N input selection signal generation circuits, Wherein each pair of data buffer of the 2N data buffers is connected to each of the N input multiplexers, each of the N input multiplexers is connected to said each pair of data bus writers of the 2N data bus writers, and each of the N input selection signal generation circuits controls an operation of said each pair of data bus writers of the 2N data bus writers.  
         [0026]     In the present invention, the input selection signal generation circuit allows the i th  and the i+1 th  data bus writer to be enabled in a predetermined case, and the input selection signal generation circuit allows a data bus writer receiving the third data of the i th  and the i+1 th  data bus writer to be enabled in a predetermined case.  
         [0027]     In order to achieve the above objects, according to one aspect of the present invention, there is provided an input circuit of a memory device comprising: a plurality of data buffers for inputting data applied from outside; an input multiplexer being connected to two or more data buffers, for multiplexing output data of the data buffers; a block column address shifter for outputting a block column address one-clock delayed than a column address; an input selection signal generation circuit for inputting the block column address and a control signal generated in a write operation; and a data bus writer being connected to an output terminal of the input multiplexer, for operating in response to an output signal of the input selection signal generation circuit.  
         [0028]     In the present invention, the data buffers operate in response to an input of a second control signal generated in synchronization with a DQS signal.  
         [0029]     In the present invention, the block column address shifter comprises: a first transmitter for synchronizing an input of an address latched by a clock signal, which is applied from outside, with a first pulse signal generated in a write command or a read command, and transmitting the synchronized signal; a second transmitter for transmitting a signal outputted from the first transmitter in response to an input of an internal clock synchronized with an external clock; a third transmitter for transmitting a signal outputted from the second transmitter in response to an input of a second pulse signal generated after one clock after a write command; a delay unit for delaying a signal outputted from the third transmitter; and an output unit for inputting an optical signal and an output signal of the delay unit and outputting a 1-clock shifted block column address.  
         [0030]     In the present invention, the block column address shifter comprises: a first transmitter for synchronizing an input of an address latched by a clock signal, which is applied from the external, with a first pulse signal generated in a write command or a read command, and transmitting the synchronized signal; a second transmitter for transmitting a signal outputted from the first transmitter in response to an input of an internal clock synchronized with an external clock; a third transmitter for transmitting a signal outputted from the second transmitter in response to an input of a second pulse signal generated after one clock after a write command; a delay unit for delaying a signal outputted from the third transmitter; an output unit for inputting an optical signal and an output signal of the delay unit and outputting a 1-clock shifted block column address; and a fourth transmitter for transmitting an input of the latched address to the third transmitter in response to an input of a third pulse signal generated in a read command.  
         [0031]     In the present invention, the input selection signal generation circuit comprises: a first decoder for inputting a first 1-clock shifted block column address and a first option signal; a second decoder for inputting a second 1-clock shifted block column address and a second option signal; a first output unit for inputting output signals of the first decoder and the second decoder and the control signal and outputting a first driving signal; a second output unit for inputting the first 1-clock shifted block column address, the output signal of the second decoder and the control signal, and outputting a second driving signal; a third output unit for inputting the second 1-clock shifted block column address, the output signal of the first decoder and the control signal, and outputting a third driving signal; and a fourth output unit for inputting the first 1-clock shifted block column address, the second 1-clock shifted block column address and the control signal, and outputting a fourth driving signal.  
         [0032]     In the present invention, the data bus writer comprises: a first data bus writer being connected to a first output terminal of the input multiplexer to output data to a first global input line; and a second data bus writer being connected to a second output terminal of the input multiplexer to output data to a second global input line.  
         [0033]     In the present invention further comprises: a first block writer being connected to the first data bus writer to output data to a first local data line; and a second block writer being connected to the second data bus writer to output data to a second local data line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0035]      FIG. 1  is a block diagram showing the data input circuit of a conventional memory device;  
         [0036]      FIG. 2  is a waveform view illustrating the operation of the circuit of  FIG. 1 ;  
         [0037]      FIG. 3  is a block diagram of a data input circuit according to the present invention;  
         [0038]      FIG. 4  is a circuit diagram showing the Yb shifter of  FIG. 3  according to one embodiment of the present invention;  
         [0039]      FIG. 5  is a circuit diagram showing the Yb shifter of  FIG. 3  according to another embodiment of the present invention;  
         [0040]      FIG. 6  is a circuit diagram showing the input selection signal generation circuit of  FIG. 3  according to an embodiment of the present invention; and  
         [0041]      FIG. 7  is a waveform view illustrating the operation of the circuit of  FIG. 3  according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0043]      FIG. 3  is a block diagram showing a data input circuit according to the present invention. Hereinafter, a ×16 type in which the number of data buffers is 16 will be described.  
         [0044]     The data input circuit of a memory device according to the present invention includes data buffers  301  and  302 , an input multiplexer  303 , a Yb shifter  309 , an input selection signal generation circuit  304 , data bus writers  305  and  306 , and block writers  307  and  308 . The data buffers  301  and  302  input data D 0  and D 1  applied from the external of the data input circuit, and the input multiplexer  303  is connected to the data buffers  301  and  302  and multiplexes the output data D 0 _ 1  and D 1 _ 1  of the data buffers  301  and  302 . The Yb shifter  309  is a block column address shifter outputting a block column address one-clock delayed than a column address. The input selection signal generation circuit  304  inputs the block column address and a control signal clk Din generated in a write operation. The data bus writers  305  and  306  are connected to the output terminal of the input multiplexer  303  and operate in response to the output signal of the input selection signal generation circuit  304 . The block writers  307  and  308  output the outputs gio 0  and gio 1  of the data bus writers  305  and  306  to local data lines.  
         [0045]     For convenience of description,  FIG. 3  shows only two data buffers  301  and  302 . However, since it is assumed that the data input/output structure is the ×16, it is noted that 14 data buffers exist in addition to the data buffers  301  and  302 . The basic structures of the 14 data buffers are identical to those of the data buffers  301  and  302  of  FIG. 3 .  
         [0046]     Further, the basic constructions of the data buffers  301  and  302 , the data bus writers  305  and  306 , and the block writers  307  and  308  of  FIG. 3  are actually identical to those of the data buffers  101  and  102 , the data bus writers  105  and  106 , and the block writers  107  and  108  of  FIG. 1 .  
         [0047]      FIG. 4  is a circuit diagram showing the Yb shifter  309  (the block column address shifter of the data input circuit) according to an embodiment of the present invention.  
         [0048]     The Yb shifter  309  according to the present invention includes a first transmitter  41 , a second transmitter  42 , a third transmitter  43 , a delay unit  44  and an output unit  45 . The first transmitter  41  synchronizes the input of an address ‘eat’ latched by a clock signal, which is applied from the external of the Yb shifter  309 , with a pulse signal cas 6  generated in a write command or a read command, and transmits the synchronized signal. The second transmitter  42  transmits the signal outputted from the first transmitter  41  in response to the input of an internal clock clkp 4  synchronized with an external clock. The third transmitter  43  transmits the signal outputted from the second transmitter  42  in response to the input of a pulse signal cas 6 _wt_lclk generated after one clock after a write command. The delay unit  44  delays the signal outputted from the third transmitter  43 . The output unit  45  inputs an optical signal opt (e.g., a ×16 relating signal in the ×16 type) and the output signal of the delay unit  44  and outputs a 1-clock shifted block column address gay_blcok_wt.  
         [0049]     In the above construction, each of the transmitters  41  to  43  is constructed by a transmission gate and a latch. Further, it is preferred that the delay unit  44  is constructed by a circuit (e.g., an inverter chain) capable of delaying the signal of a node N 1 . Furthermore, the output unit  45  is constructed by a NAND gate for inputting the optical signal opt and the output signal of the delay unit  44 , and an inverter connected to the output terminal of the NAND gate.  
         [0050]     The Yb shifter  309  shown in  FIG. 4  has a constructive characteristic in which a result obtained by one-clock shifting the address eat inputted in the write command is transmitted to the node N 1 .  
         [0051]     Meanwhile, the Yb shifter  309  as shown in  FIG. 4  is a circuit realized by two-clock shifting an address only in a write operation, in consideration of a case in which a 2-clock shifted block column address inputs an address in both a read operation and a write operation in the prior art. Accordingly, a bus in the read operation must be additionally constructed.  
         [0052]     An embodiment for solving such a problem is shown in  FIG. 5 .  
         [0053]      FIG. 5  is a circuit diagram showing an Yb shifter (block column address shifter) according to another embodiment of the present invention.  
         [0054]     Referring to  FIG. 5 , the Yb shifter according to another embodiment of the present invention includes a first transmitter  51 , a second transmitter  52 , a third transmitter  53 , a delay unit  54 , an output unit  55  and a fourth transmitter  56 . The first transmitter  51  synchronizes the input of an address ‘eat’ latched by a clock signal, which is applied from the external of the Yb shifter, with a pulse signal cas 6  generated in a write command or a read command, and transmits the synchronized signal. The second transmitter  52  transmits the signal outputted from the first transmitter  51  in response to the input of an internal clock clkp 4  synchronized with an external clock. The third transmitter  53  transmits the signal outputted from the second transmitter  52  in response to the input of a pulse signal cas 6 _wt_lclk generated after one clock after a write command. The delay unit  54  delays the signal outputted from the third transmitter  53 . The output unit  55  inputs an optical signal (e.g., a ×16 relating signal in the ×16 type) opt and the output signal of the delay unit  54  and outputs a 1-clock shifted block column address gay_blcok_wt. The fourth transmitter  56  transmits the input of the address ‘eat’ to the third transmitter  53  in response to an input of a pulse signal cas 6 _rd generated in a read command.  
         [0055]     In the above construction, each of the transmitters  51 ,  52 ,  53  and  56  is constructed by a transmission gate and a latch (but, the fourth transmitter  56  is constructed by only a transmission gate). Further, it is preferred that the delay unit  54  is constructed by a circuit (e.g., an inverter chain) capable of delaying the signal of a node N 1 . Furthermore, the output unit  55  is constructed by a NAND gate for inputting the optical signal opt and the output signal of the delay unit  44 , and an inverter connected to the output terminal of the NAND gate.  
         [0056]     The Yb shifter as shown in  FIG. 5  is realized, thereby solving the problem in that the bus in the read operation must be additionally constructed.  
         [0057]      FIG. 6  is a circuit diagram showing the input selection signal generation circuit  304  according to an embodiment of the present invention. Specifically,  FIG. 6  shows the input selection signal generation circuit  304  realized on an assumption that four driving signals are necessary for the two data bus writers  305  and  306  of  FIG. 3 .  
         [0058]     Referring to  FIG. 6 , the input selection signal generation circuit  304  according to the present invention includes a first decoder  61 , a second decoder  62 , a first output unit  63 , a second output unit  64 , a third output unit  65  and a fourth output unit  66 . The first decoder  61  inputs a 1-clock shifted block column address gay_blcok_wt_ 11  and an option signal ×16b and the second decoder  62  inputs a 1-clock shifted block column address gay_blcok_wt_ 12  and an option signal ×4. The first output unit  63  inputs the output signals of the first decoder  61  and the second decoder  62  and the control signal clk Din and outputs a first driving signal gay_BC_wt_ 0 . The second output unit  64  inputs the 1-clock shifted block column address gay_blcok_wt_ 11 , the output signal of the second decoder  62 , and the control signal clk Din, and outputs a second driving signal gay_BC_wt_ 1 . The third output unit  65  inputs the 1-clock shifted block column address gay_blcok_wt_ 12 , the output signal of the first decoder  61 , and the control signal clk Din, and outputs a third driving signal gay_BC_wt_ 2 . The fourth output unit  66  inputs the 1-clock shifted block column address gay_blcok_wt_ 11 , the 1-clock shifted block column address gay_blcok_wt_ 12 , and the control signal clk Din, and outputs a fourth driving signal gay_BC_wt_ 3 .  
         [0059]     In the construction of  FIG. 6 , each of the first decoder  61  and the second decoder  62  is constructed by a NAND gate and each of the output units  63  to  66  is constructed by a NAND gate and an inverter.  
         [0060]     Referring to the construction of  FIG. 6 , when the data input/output structure is the ×16 type, since the option signal ×16b is logically in a low level and the option signal ×4 also is logically in a low level, both the 1-clock shifted block column address gay_blcok_wt_ 11  and the 1-clock shifted block column address gay_blcok_wt_ 12  are logically in a high level. Further, the output signals of the first decoder  61  and the second decoder  62  also are logically in a high level. Accordingly, the driving signals gay_BC_wt_ 0  to gay_BC_wt_ 3  operating the data bus writers  305  and  306  of  FIG. 3  are logically in a high level, so that all data bus writers are enabled.  
         [0061]     Meanwhile, when the data input/output structure is the ×8 type, since the option signal ×16b is logically in a high level and the option signal ×4 is logically in a low level, both the 1-clock shifted block column address gay_blcok_wt_ 12  and the output signal of the second decoder  62  are logically in a high level. Further, the 1-clock shifted block column address gay_blcok_wt_ 11  and the output signal of the first decoder  61  have values determined according to the input of the address ‘eat’ latched by the clock signal applied from the external of the Yb shifter  309 . Accordingly, one of the second driving signal gay_BC_wt_ 1  and the fourth driving signal gay_BC_wt_ 3  is enabled, and one of the first driving signal gay_BC_wt_ 0  and the third driving signal gay_BC_wt_ 2  is enabled.  
         [0062]     Further, when the data input/output structure is a ×4 type, since the option signal ×16b is logically in a high level and the option signal ×4 is logically in a high level, only one of the first to the fourth driving signal gay_BC_wt_ 0  to gay_BC_wt_ 3  is enabled.  
         [0063]     Hereinafter, the operation of the data input circuit of  FIG. 3  according to the present invention will be described in detail with reference to the embodiments shown in FIGS.  4  to  6 .  
         [0064]     First, in the operation of the ×16 type, data applied to each data buffer are applied to each data bus writer along the solid lines of the input multiplexer. Then, the data are applied to the block writer by the control signal clk Din. Accordingly, the basic data transmission path is identical to that of  FIG. 1 .  
         [0065]     Next, in the operation of the ×8 type, it is assumed that data are applied to the data buffer  301  and data are not applied to the data buffer  302 . Further, only the data buffer  301  is enabled by a control signal and the data buffer  302  is disabled.  
         [0066]     A first case: the data D 0 _ 1  outputted from the data buffer  301  can be applied to the data bus writer  305  through a path ‘a’ by the input multiplexer  303 . In such a case, the data D 1 _ 2  of an output terminal to which data are not sent maintain a previous state.  
         [0067]     A second case: the data D 0 _ 1  outputted from the data buffer  301  can be applied to the data bus writer  306  through a path ‘b’ by the input multiplexer  303 . In such a case, the data D 0 _ 2  of the output terminal to which the data are not sent maintain the previous state.  
         [0068]     The data bus writers  305  and  306  receive the output signals D 0 _ 1  and D 0 _ 2  of the input multiplexer  303 .  
         [0069]     The Yb shifter  309  (block column address shifter) outputs a 1-clock shifted block column address Yb. Herein, the 1-clock shifted block column address Yb denotes a signal one-clock delayed after a block column address designating the specific block (i.e., memory block) of the memory bank by the write command has been applied.  
         [0070]     The input selection signal generation circuit  304  receives the 1-clock shifted block column address Yb and the control signal clk Din and outputs the signal operating the operations of the data bus writers  305  and  306 . Herein, the control signal clk Din denotes a signal generated in synchronization with a clock signal after two clocks after the write command.  
         [0071]     In the operation of the ×16 type, the input selection signal generation circuit  304  allows the data bus writers  305  and  306  to be enabled.  
         [0072]     In the operation of the ×8 type, the input selection signal generation circuit  304  selectively allows only one of the data bus writers  305  and  306  to be enabled. That is, the input selection signal generation circuit  304  allows only the data bus writer connected to the path (a or b) selected by the input multiplexer  303  to be enabled.  
         [0073]     The operation after the data bus writer is identical to that of  FIG. 1 .  
         [0074]      FIG. 7  is a waveform view illustrating the operation of the circuit shown in  FIG. 3  according to the present invention.  
         [0075]     As shown in  FIGS. 3 and 7 , the input selection signal generation circuit  304  receives the 1-clock shifted block column address Yb and the control signal clk Din and controls the data bus writers.  
         [0076]     As compared to the conventional circuit described in  FIGS. 1 and 2 , in the prior art, a predetermined time margin is required until the control signal clk Din is generated after the 2-clock shifted block column address has been generated. Therefore, the data processing speed is delayed.  
         [0077]     However, in the present invention, the 1-clock shifted block column address is used, so that the generation time point of generation of the control signal clk Din may be lo earlier than that of the prior art. That is, in the present invention, even though the control signal clk Din is immediately generated after a two-clock delay after the write command, there occurs no any problem. Therefore, the operation time point of the input selection signal generation circuit  304  can be earlier. In the present invention, the ×16 type, the ×8 type and the ×4 type are described. However, the technical scope of the present invention can be applied to various cases including a ×32 type, etc.  
         [0078]     As described above, in the present invention, a 1-clock shifted block column address is used, so that the operation time point of a data bus writer can be advanced, thereby accelerating the data processing speed. Further, in the present invention, a design in which a control signal clk Din can pass a shortest path can be made.  
         [0079]     The preferred embodiment of the present invention has been described for illustrative purposes, and those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.