Abstract:
An internal clock generator that modulates a high-frequency clock signal to a low-frequency signal to transmit the low-frequency signal if a transmission line for transmitting the high-frequency clock signal is long, and then restores the transmitted low-frequency signal to the high-frequency signal. The internal clock generator includes a first signal generation unit for receiving a first signal having a first frequency and generating a second signal having a second frequency that is lower than the first frequency, and a second signal generation unit for receiving the second signal and generating a third signal having a frequency equal to the first frequency. Here, the third signal is used as a signal for controlling an operating time point of an internal circuit of a synchronous memory device.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the invention  
         [0002]     The present invention relates to an internal clock generator used in a high-speed semiconductor device.  
         [0003]     2. Description of the Prior Art  
         [0004]     Generally, a synchronous semiconductor device controls an internal operation of the semiconductor device using an internal clock that is synchronized with an external clock. A representative synchronous semiconductor device may be a synchronous memory device (hereinafter referred to as a memory device) such as an SDRAM, DDR, SDRAM, etc.  
         [0005]     Recently, with the development of technology, the operating frequency of a memory device is greatly being heightened and the size of the memory device is gradually increasing as well. Typically, the fact that the operating frequency of the memory device is increasing means that the frequency of the internal clock is also being heightened. Here, as a moving distance (i.e., transmission distance) of the internal clock is increasing, an RC loading of a transmission line is also increasing. However, in the case in which the loading of the transmission line is increasing, a problem may occur in transmitting a high-frequency signal.  
         [0006]     Hereinafter, the conventional method of transmitting an internal clock will be explained with reference to the accompanying drawings.  
         [0007]      FIG. 1  is a block diagram of a general memory device that uses the internal clock.  
         [0008]     Referring to  FIG. 1 , a clock buffer  101  receives an external clock signal of an SSTL level or a TTL level. A clock generator  102  receives an output signal of the clock buffer  101 , and generates an internal clock ICLK.  
         [0009]     The internal clock ICLK is applied to a column control unit  11 , a row control unit  12 , a command control unit  13 , a data control unit  14 , etc., in the memory device, and controls the operating timing of the memory device. Here, the column control device  11  is a circuit for controlling a column operation of the memory device, and the row control unit  12  is a circuit for controlling a row operation of the memory device. Also, the command control unit  13  is a circuit for controlling a command such as an active, read, write, precharge, etc., and the data control unit  14  is an input/output (I/O) unit of the memory device.  
         [0010]      FIG. 2  is a view illustrating an example of a conventional clock generator  102  used in the memory device of  FIG. 1 .  
         [0011]     As illustrated in  FIG. 2 , a clock generator causes no trouble in the case in which a period of an input signal IN is longer than a delay time of a delay unit  21 .  
         [0012]     However, if the period of the input signal IN is shorter than the delay time of the delay unit  21 , the memory device may malfunction. Specifically, if the input signal IN is a high-frequency signal over 500 MHz, it is difficult to apply such a clock generator to a high-speed memory device.  
         [0013]     Additionally, in the case in which a signal transmission line is long, this may cause a problem in that the loading of the transmission line becomes extremely great and thus the high-frequency signal is not properly transferred.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an internal clock generator that modulates a high-frequency clock signal to a low-frequency signal to transmit the low-frequency signal if a transmission line for transmitting the high-frequency clock signal is long, and then restores the transmitted low-frequency signal to the high-frequency signal.  
         [0015]     In a first embodiment of the present invention, there is provided an internal clock generator comprising a first signal generation unit for receiving a first signal having a first frequency and generating a second signal having a second frequency that is lower than the first frequency, and a second signal generation unit for receiving the second signal and generating a third signal having a frequency equal to the first frequency.  
         [0016]     Here, the third signal is used as a signal for controlling an operating time point of an internal circuit of a synchronous memory device.  
         [0017]     In a second embodiment of the present invention, there is provided an internal clock generator comprising a frequency modulation unit for receiving a first clock signal having a first frequency and outputting second and third clock signal having a second frequency, and a clock generation unit for receiving the second and third clock signals and outputting a fourth clock signal having the first frequency.  
         [0018]     Here, if the first frequency is fo, the second frequency becomes fo/2.  
         [0019]     Additionally, a rising edge of the second clock signal is in synchronization with a rising edge of the first clock signal, and a rising edge of the third clock signal is in synchronization with a falling edge of the first clock signal.  
         [0020]     It is preferable that a high-level period of the second and third clock signals is equal to a period of the first clock signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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:  
         [0022]      FIG. 1  is a block diagram of a general memory device that uses the internal clock;  
         [0023]      FIG. 2  is an exemplary circuit diagram of a conventional clock generator  102  used in the memory device of  FIG. 1 ;  
         [0024]      FIG. 3  is a block diagram of an internal clock generator according to an embodiment of the present invention;  
         [0025]      FIG. 4  is an exemplary circuit diagram of a frequency modulator as illustrated in  FIG. 3 ;  
         [0026]      FIG. 5  is an exemplary circuit diagram of a clock generator as illustrated in  FIG. 3 ; and  
         [0027]      FIG. 6  is a waveform diagram illustrating clock signals IN, Up_clk, Down_clk and ICLK that appear in the circuits of FIGS.  3  to  5 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted.  
         [0029]      FIG. 3  is a block diagram of an internal clock generator according to an embodiment of the present invention.  
         [0030]     In  FIG. 3 , the construction and operation of a clock buffer  301  that receives an external clock signal CLK are the same as those of the conventional clock buffer. For example, the clock buffer  301  is the same as the clock buffer  101  explained with reference to  FIG. 1 . Also, the period of a clock signal IN output from the clock buffer  301  is the same as that of the external clock signal CLK (See  FIG. 6 ).  
         [0031]     A frequency modulation unit  302  modulates the frequency of the clock signal IN output from the clock buffer  301 . In an embodiment of the present invention, the frequency of output signals Up_clk and Down_clk of the frequency modulation unit  302  is a ½ of the frequency of the clock signal IN. That is, the frequency modulation unit  302  of  FIG. 3  performs the same function as a frequency divider. Here, the clock signal Up_clk rises in synchronization with a rising edge of the clock signal IN, and falls in synchronization with a rising edge of the next clock signal IN (See  FIG. 6 ). Additionally, the clock signal Down_clk rises in synchronization with a falling edge of the clock signal IN, and falls in synchronization with a falling edge of the next clock signal IN (See  FIG. 6 ).  
         [0032]     Clock generation units  35  to  38  receive in common the output signals Up_clk and Down_clk of the frequency modulation unit  302 , and output an internal clock signal ICLK. The clock generation units  35  to  38  correspond to a column control unit  31 , a row control unit  32 , a command decoder  33  and a data control unit  34  etc. in a one-to-one manner because there are differences in distance from frequency unit(differences in operating time points) among the column control unit  31 , row control unit  32 , command decoder  33  and data control unit  34  etc.  
         [0033]     The clock generation units  35  to  38  that have received the clock signals Up_clk and Down_clk combine the clock signals and generate the internal clock signal ICLK having the same frequency as the clock signal IN.  
         [0034]     The frequency modulation unit and the clock generation units as described above are constituent elements of the internal clock generator newly proposed in one embodiment of the present invention.  
         [0035]      FIG. 4  is an exemplary circuit diagram of the frequency modulation unit as illustrated in  FIG. 3 . In  FIG. 3 , a method of increasing twice the period of the input clock signal IN using flip-flops is illustrated. Here, a power-up signal pwrup is a signal for sensing an input of an external power supply to the memory device and initializing internal circuits of the memory device when the external power is supplied to the memory device. Specifically, the power-up signal sets an initial level of the flip-flops.  
         [0036]     In  FIG. 4 , the flip-flop  41  receives the clock signal IN and outputs the clock signal Up_clk having the period that is twice the period of the clock signal IN, and the the flip-flop  42  receives the clock signal IN and outputs the clock signal Down_clk having the period that is twice the period of the clock signal IN. As described above, the clock signal Up_clk rises in synchronization with the rising edge of the clock signal IN and falls in synchronization with the rising edge of the next clock signal, and the clock signal Down_clk rises in synchronization with the falling edge of the clock signal IN and falls in synchronization with the falling edge of the next clock signal IN. For reference, the flip-flop  42  receives an inversed signal of the clock signal IN.  
         [0037]     The frequency modulation unit of  FIG. 4  is exemplary, and it is apparent to those skilled in the art that diverse circuits for dividing the frequency by 2 can be used in place of the frequency modulation unit.  
         [0038]      FIG. 5  is an exemplary circuit diagram of the clock generator as illustrated in  FIG. 3 .  
         [0039]     The clock generator of  FIG. 5  includes a NAND gate  51  for receiving the clock signals Up_clk and Down_clk, an inverter  52  for receiving an output signal of the NAND gate  51 , a NOR gate  53  for receiving the clock signals Up_clk and Down_clk, a NAND gate  54  for receiving an output signal of the inverter  52  and an output signal of the NOR gate  53 , and an inverter  55  for receiving an output signal of the NAND gate  54 . The inverter  55  outputs the internal clock signal ICLK.  
         [0040]      FIG. 6  is a waveform diagram illustrating clock signals IN, Up_clk, Down_clk and ICLK that appear in the circuits of FIGS.  3  to  5 .  
         [0041]     As can be seen in  FIG. 6 , the period of the clock signals Up_clk and Down_clk is twice the period of the clock signal IN, and the period of the internal clock signal ICLK is equal to the period of the clock signal IN.  
         [0042]     Hereinafter, the operation of the internal clock generator according to the present invention that is advantageous in high-frequency operation will be explained in detail with reference to FIGS.  3  to  6 .  
         [0043]     The clock buffer  301  receives the external clock signal CLK of the SSTL level or the TTL level, and output the clock signal IN. Here, the low level of the clock signal IN corresponds to a ground voltage, and the high level thereof corresponds to a driving voltage VCC. That is, the clock signal IN is a clock signal fully swinging between the ground voltage and the driving voltage VCC. Here, the driving voltage VCC is for driving the clock buffer  301 .  
         [0044]     The frequency modulation unit  302  generates and transmits the clock signals Up_clk and Down_clk having the period that is twice the period of the clock signal IN to the clock generation units  35  to  38 . In the case of doubling the period of the high-frequency clock signal IN as in the internal clock generator according to the present invention, a signal distortion is greatly reduced in comparison to the conventional clock generator. Particularly, if the loading of the transmission line is great, the signal distortion can properly be reduced by transmitting the low-frequency signal using the frequency modulation unit.  
         [0045]     The clock generation units  35  to  38  receive the output signals of the frequency modulation unit  302 , generate and provide the internal clock signal ICLK having the same frequency as the operating frequency of the memory device to the column control unit  31 , the row control unit  32 , etc.  
         [0046]     In the present invention, the clock signals Up_clk and Down_clk having the lower frequency than the clock signal IN are generated and transferred to the clock generation units, and then the internal clock signal ICLK is generated. Here, it is preferable to make the transmission distance of the clock signals Up_clk and Down_clk, which reach the clock generation units, longer than the distance of the internal clock signal ICLK which reaches the column control units  31 , etc. By doing so, the signal distortion which may occur during the transmission of the high-frequency signal can be prevented. As described above, the present invention can prevent the signal distortion occurring when the high-frequency signal is transmitted for a long distance through the transmission of the low-frequency signal instead of the high-frequency signal. Particularly, it may be possible that the clock signal transmission is performed using the same circuit as the conventional circuit if the internal transmission line is short, while the clock signal transmission is performed using the circuit proposed according to the present invention if the internal transmission line is long.  
         [0047]     As described above, according to the present invention which is applicable to an ultrahigh-speed memory device, a stable internal clock signal can be generated by transmitting a low-frequency clock signal through a transmission line of a great loading and then generating an internal clock signal having the same frequency as an operating frequency using the transmitted low-frequency signal.  
         [0048]     The present invention is not limited to this, but in the case in which the loading of the transmission line is extremely great, the clock signal having a frequency that is four times or more lower than the original clock signal may be generated and transmitted, and then the transmitted lower-frequency clock signal may be restored to the original clock signal.  
         [0049]     In the case in which the internal clock generator according to the present invention is used, a stable internal clock can be generated even if the operating frequency is greatly high.  
         [0050]     Although preferred embodiments of the present invention have been described for illustrative purposes, 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.