Patent Publication Number: US-6989700-B2

Title: Delay locked loop in semiconductor memory device and its clock locking method

Description:
FIELD OF INVENTION 
   The present invention relates to a delay locked loop (DLL); and, more particularly, to a DLL capable of being initialized stably in response to a reset signal. 
   DESCRIPTION OF PRIOR ART 
   Generally, a clock signal of a system or a circuit is used as a reference for synchronizing an execution timing and guaranteeing an error-free high speed operation. 
   When an external clock signal is inputted to a semiconductor memory device to be used as an internal clock signal, a clock skew is generated between the external clock signal and the internal clock signal because the external clock signal is delayed while it is inputted to the semiconductor memory device. Therefore, various devices have been introduced to synchronize the internal clock signal with the external clock signal. 
   For example, a phase locked loop (PLL) and a delay locked loop (DLL) have been developed in order to synchronize the internal clock signal with the external clock signal. 
   However, since the DLL is less influenced by a noise than the PLL, the DLL is widely used in a synchronous semiconductor memory such as a synchronous dynamic random access memory (SDRAM) or a double data rate (DDR) SDRAM. 
     FIG. 1  is a block diagram showing a conventional DLL included in a conventional DDR SDRAM. 
   As shown, the conventional DLL includes a clock buffer unit  101 , a clock divider  102 , a phase comparator  103 , a delay controller  104 , a delay line unit  105 , a dummy delay line unit  106 , a delay model  107  and an output buffer  108 . 
   The clock buffer unit  101  receives an external clock signal CLK and an inverted signal of the external clock signal CLK, i.e., an external clock bar signal /CLK, to generate a rising edge clock signal rclk and a falling edge clock signal fclk by buffering the external clock signal CLK and the external clock bar signal /CLK. 
   A clock divider  102  receives the rising edge clock signal rclk to generate a reference clock signal ref and a delay monitoring clock signal dly — in by dividing the rising edge clock signal rclk by N. Herein, the N is a natural number. 
   The phase comparator  103  receives the reference clock signal ref and a feed-backed clock signal fb — clk outputted from the delay model  107  and compares a rising edge of the feed-back clock signal fb — clk with that of an inverted signal of the reference clock signal ref, i.e., a reference clock bar signal /ref, for outputting the comparison result to the delay controller  104 . 
   The delay controller  104  controls delay amounts of the delay line unit  105  and the dummy delay line unit  106  based on the comparison result of the phase comparator  103 . 
   The delay line unit  105  receives the rising and falling edge clock signals rclk and fclk to delay the received signals for a predetermined delay time. Herein, as above mentioned, the predetermined delay time is controlled by the delay controller  104  based on the comparison result of the phase comparator  103 . 
   Likewise, the dummy delay line unit  106  receives the delay monitoring clock signal dly — in and delays the delay monitoring clock signal for a predetermined delay time. Herein, as above mentioned, the predetermined delay time is controlled by the delay controller  104  based on the comparison result of the phase comparator  103 . A structure of the dummy delay line unit  106  is the same as that of the delay line unit  105 , but the dummy delay line unit  106  consumes less power than the delay line unit  105  because the dummy delay line unit  106  receives a clock-divided signal, i.e., the delay monitoring clock signal dly — in. 
   The delay model  107  delays an output signal of the dummy delay line unit  106  to output the feed-backed clock signal fb — clk. Herein, a delay amount of the delay model  107  is the same as a delay amount generated while the external clock signal CLK is passed through the conventional DLL to be outputted by the output buffer  108 . 
   The output buffer  108  outputs data in synchronization with outputted clock signals from the delay line unit  105 . 
     FIG. 2  is a timing diagram showing an operation of the conventional DLL. 
   If the feed-backed clock signal fb — clk and the reference clock signal ref are inputted to the phase comparator  103 , the phase comparator  103  compares a rising edge of the feed-backed clock signal fb — clk with that of the reference clock bar signal /ref. As above mentioned, based on the comparison result of the phase comparator  103 , delay amount of the delay line unit  105  and the dummy delay line unit  106  are controlled. 
   If an operational frequency of the conventional DLL is low, at an initial state, a rising edge of the feed-backed clock signal fb — clk leads a rising edge of the reference clock bar signal /ref by a time period t 1 . Therefore, the feed-backed clock signal fb — clk should be delayed for the time period t 1  to be synchronized with a rising edge of the reference clock bar signal /ref and, thus, the delay controller  104  increases delay amounts of the delay line unit  105  and the dummy delay line unit  106 . 
   On the other hand, if the operational frequency of the conventional DLL is high, at the initial state, a rising edge of the feed-backed clock signal fb — clk lags behind a rising edge of the reference clock bar signal /ref by a time period t 2 . Therefore, delay amounts of the delay line unit  105  and the dummy delay line unit  106  should be decreased by a time period t 2 . 
   However, at the initial state, delay amounts of the delay line unit  105  and the dummy delay line unit  106  are respectively set to be the minimum delay amount. Therefore, rising edges of the reference clock bar signal /ref and the feed-backed clock signal fb — clk can not be synchronized. That is, the time period t 2  between rising edges of the reference clock bar signal /ref and the feed-backed clock signal fb — clk can not be compensated by controlling delay amounts of the delay line unit  105  and the dummy delay line unit  106 . As a result, because of a clock skew which is not compensable, the compensable the conventional DLL can not generate a delay locked clock signal. 
   In addition, the conventional DLL is reset by a reset signal inputted from an external chipset. The reset signal is inputted to the clock divider  102  and resets the clock divider  102  and the delay controller  104 . 
   However, since a pulse width of the reset signal is narrow, an operation for resetting the conventional DLL may not be performed stably. That is, if the reset signal is inputted to the conventional DLL, the phase comparator  103  should not be operated because the reference clock bar signal /ref and the feed-backed clock signal fb — clk inputted to the phase comparator  103  are not generated. However, if the feed-backed clock signal fb — clk may be generated due to the narrow pulse width of the reset signal, the phase comparator  103  may be operated abnormally. 
   In addition, the conventional DLL includes the clock divider  102  for providing clock signals, i.e., the delay monitoring clock signal dly — in and the reference clock signal ref. The delay monitoring clock signal dly — in and the reference clock signal ref are respectively inputted to the dummy delay line  105  and the phase comparator  103 . Since the reference clock signal ref is a divided signal of the rising edge clock signal rclk, a frequency of the reference clock signal ref is lower than that of the rising edge clock signal rclk. Therefore, a frequency of performing the phase comparison operation of the phase comparator  103  is decreased. The above mentioned operation of the phase comparator  103  may be suitable at a low operational frequency. However, the conventional DLL is not suitable for a semiconductor memory device operated at a high operational frequency because at the high operational frequency, the phase comparison operation should be more frequently performed. 
   SUMMARY OF INVENTION 
   It is, therefore, an object of the present invention to provide a DLL capable of preventing a clock skew, which is not compensable, from being generated, which is stably operated at a high operational frequency and performs a reset operation stably. 
   In accordance with an aspect of the present invention, there is provided a delay locked loop (DLL) for generating a delay locked clock signal, including: a comparator enable signal generator for generating a comparator enable signal in response to a reset signal and a plurality of clock divided signals; a semi locking detector for generating a semi locking detection signal in response to the comparator enable signal; a phase comparator enabled by the comparator enable signal for receiving a rising edge clock signal and a feed-backed clock signal in order to compare phases of the rising edge clock signal and the feed-backed clock signal and output the comparison result; and a DLL generator for generating the delay locked clock signal in response to the comparison result, wherein the comparator enable signal is generated by enlarging a pulse width of the reset signal by a predetermined amount. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram showing a conventional DLL included in a conventional DDR SDRAM; 
       FIG. 2  is a timing diagram showing an operation of a conventional DLL shown in  FIG. 1 ; 
       FIG. 3  is block diagram showing a DLL in accordance with a preferred embodiment of the present invention; 
       FIG. 4  is a schematic circuit diagram showing a clock divider shown in  FIG. 3 ; 
       FIG. 5  is a schematic circuit diagram showing a comparator enable signal generator shown in  FIG. 3 ; 
       FIG. 6  is a schematic circuit diagram showing a semi locking detector shown in  FIG. 3 ; 
       FIG. 7  is a schematic circuit diagram showing a phase comparator and a delay controller shown in  FIG. 3 ; 
       FIG. 8  is a timing diagram showing an operation of a phase comparator shown in  FIG. 7 ; and 
       FIG. 9  is a timing diagram showing an operation of a DLL shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF INVENTION 
   Hereinafter, a delay locked loop (DLL) for use in a semiconductor memory device in accordance with the present invention will be described in detail referring to the accompanying drawings. 
     FIG. 3  is block diagram showing a delay locked loop (DLL) in accordance with a preferred embodiment of the present invention. 
   As shown, the DLL includes a clock buffer unit  301 , a clock divider  302 , a comparator enable signal generator  303 , a semi locking detector  304 , a phase comparator  305 , a delay controller  306 , a delay line unit  307 , a delay model  308  and an output buffer  309 . 
   The clock buffer unit  301  receives an external clock signal CLK and an inverted signal of the external clock signal CLK, i.e., an external clock bar signal /CLK in order to generate a rising edge clock signal rclk and a falling edge clock signal fclk by buffering the external clock signal CLK and the external clock bar signal /CLK. 
   The clock divider  302  receives the rising edge clock signal rclk to generate a first to a third clock divided signals clk — v 4   — p 1 , clk — v 4   — p 2  and clk — v 8 . The first and the second clock divided signals clk — v 4   — p 1  and clk — v 4   — p 2  are generated by dividing the rising edge clock signal rclk by 4; the third clock divided signals clk — v 8  is generated by dividing the rising edge clock signal rclk by 8. 
   Herein, each clock cycle of the first and the second clock divided signals clk — v 4   — p 1  and clk — v 4   — p 2  is equal to four clock cycles of the external clock signal CLK. The first and the second clock divided signals clk — v 4   — p 1  and clk — v 4   — p 2  are in a logic high level during one clock cycle of the external clock signal CLK; and are in a logic low level during three clock cycles of the external clock signal CLK. Likewise, a clock cycle of the third clock divided signal clk — v 8  is equal to eight clock cycles of the external clock signal CLK. The third clock divided signal clk — v 8  is in a logic high level during one clock cycle of the external clock signal CLK; and is in a logic low level during seven clock signals of the external clock signal CLK. 
   The comparator enable signal generator  303  receives the second and the third clock divided signals clk — v 4   — p 2  and clk — v 8  in order to generate a comparator enable signal cmp — en. 
   The semi locking detector  304  receives the second clock divided signal clk — v 4   — p 2 , the comparator enable signal cmp — en and a comparison signal out 1  outputted from the phase comparator  305  in order to generate a semi locking detection signal semi — lock. 
   The phase comparator  305  receives a feed-backed clock signal outputted from the delay model  308 , the rising edge clock signal rclk and the semi locking detection signal semi — lock for generating the comparison signal out 1  and a plurality of shift right control signals and shift left control signals, i.e., a first shift right control signal sr 1 , a second shit right control signal sr 2 , a first shift left control signal s 11  and a second shit left control signal s 12 . 
   The delay controller  306  controls a delay amount of the delay line unit  307  based on the first and the second shift right control signals sr 1  and sr 2  and the first and the second shift left signals s 11  s 12 . 
   The delay line unit  307  delays the rising edge clock signal rclk and the falling edge clock signal fclk for a predetermined delay time in order to generate delay locked clock signals, i.e., a delay locked rising edge clock signal rclk — d 11  and a delay locked falling edge clock signal fclk — d 11 . Herein, as above mentioned, the predetermined delay time is determined by the delay controller  306  based on the first and the second shift right control signals sr 1  and sr 2  and the first and the second shift left signals s 11  s 12 . 
   The delay model  308  delays the delay locked rising edge clock signal rclk — d 11  in order to output the delayed delay locked rising edge clock signal rclk — d 11  as the feed-backed clock signal fb — clk. Herein, a delay amount of the delay model  308  is the same as a delay amount generated while the external clock signal CLK is passed through the conventional DLL until it is outputted by the output buffer  108 . 
   The output buffer  309  outputs data in synchronization with the delay locked rising and falling edge clock signals rclk — d 11  and fclk — d 11 . 
     FIG. 4  is a schematic circuit diagram showing the clock divider  302  shown in  FIG. 3 . 
   As shown, the clock divider  302  includes a first to a third D-type flip-flops  411  to  413  and a plurality of logic gates, i.e., a first to a sixth inverters I 1  to I 6  and a first to a third NAND gates ND 1  to ND 3 . 
   The first and the second D-type flip-flops  411  and  412  receive the rising edge clock signal rclk through their clock input terminals. A data terminal of the first D-type flip-flop receives an output signal of the fifth inverter I 5  and outputs a first D-type flip-flop output signal dff 1 . Herein, an input terminal of the fifth inverter is connected to a output terminal of the second D-type flip-flop  412  and a clock input terminal of the third D-type flip-flop  413 . 
   A data terminal of the second D-type flip-flop  412  receives the first D-type flip-flop output signal dff 1  to output a second D-type flip-flop output signal dff 2 . 
   A data terminal of the third D-type flip-flop  413  receives a sixth inverter I 6  whose input terminal is connected to a output terminal of the third D-type flip-flop  413 . The output terminal of the third D-type flip-flop  413  outputs a third D-type flip-flop output signal dff 3 . 
   The first inverter I 1  inverts the first D-type flip-flop output signal dff 1 , and the first NAND gate ND 1  performs a logic NAND operation to an output signal of the first inverter I 1  and the second D-type flip-flop dff 2 . The second inverter I 2  inverts an output signal of the first NAND gate ND 1  to output the second clock divided signal clk — v 4   — p 2 . 
   The second NAND gate ND 2  performs a logic NAND operation to the first and the second D-type flip-flop output signals dff 1  and dff 2 , and the third inverter I 3  inverts an output signal of the second NAND gate ND 2  to output the first clock divided signal clk — v 4   — p 1 . 
   Likewise, the third NAND gate ND 3  performs a logic NAND operation to the third D-type flip-flop dff 3  and the first clock divided signal clk — v 4   — p 1 , and the fourth inverter I 4  inverts an output signal of the third NAND gate ND 3  to output the third clock divided signal clk — v 8 . 
     FIG. 5  is a schematic circuit diagram showing the comparator enable signal generator  303  shown in  FIG. 3 . 
   The comparator enable signal generator  303  generates the comparator enable signal cmp — en by enlarging a pulse width of a reset signal rst. 
   The comparator enable signal cmp — en is used for the DLL not to be abnormally operated due to a remaining clock signal after the reset signal rst is inputted to the DLL. Herein, the remaining clock signal includes any clock signal of the DLL which is still activated after the reset signal rest is inputted to the DLL. Therefore, a pulse width of the comparator enable signal cmp — en is required to be equal to a delay time generated while a clock signal inputted to the delay line unit  307  is passed through the delay model  308  and the phase comparator  305 . 
   As shown, in detail, the comparator enable signal generator  303  includes a first latch  501 , a second latch  502 , a seventh to an eleventh inverters I 7  to I 11 , a first transfer gate TR 1 , a second transfer gate TR 2  and a first p-channel metal oxide semiconductor (PMOS) transistor MP 1 . 
   If the reset signal rst is inputted to the seventh inverter I 7 , the first PMOS transistor MP 1  is turned-on, and, thus, the comparator enable signal cmp — en becomes in a logic low level. 
   Thereafter, if the second divided signal clk — v 4   — p 2  turns on the first transfer gate TR 1 , an output signal of the first latch  501  becomes in a logic high level. Thereafter, the second transfer gate TR 2  is turned on by the third divided signal clk — v 8 , and an output signal of the second latch  502  becomes in a logic high level. As a result, the comparator enable signal cmp — en becomes in a logic high level. 
     FIG. 6  is a schematic circuit diagram showing the semi locking detector  304  shown in  FIG. 3 . 
   As shown, the semi locking detector  304  includes a third latch  601 , a fourth NAND gate ND 4 , a second PMOS transistor MP 2 , a first n-channel metal oxide semiconductor (NMOS) transistor MN 1 , a second NMOS transistor MN 2  and a twelfth to a fourteenth inverters I 12  to I 14 . 
   If the comparator enable signal cmp — en is in a logic low level, the second PMOS transistor MP 2  is turned on and then, the semi locking detection signal semi — lock becomes in a logic low level. Thereafter, if the comparator enable signal cmp — en becomes in a logic low level, the second PMOS transistor MP 2  is turned off. Therefore, the semi locking detection signal semi — lock is controlled by the comparison signal out 1 . 
     FIG. 7  is a schematic circuit diagram showing the phase comparator  305  and the delay controller  306 . 
   As shown, the phase comparator  305  includes a fourth to a sixth D-type flip-flops  3051  to  3053 , two multiplexers and a plurality of logic gates. The delay controller  306  includes a T-type flip-flop  3061  and a plurality of logic gates. 
   The phase comparator  305  starts to be operated if the comparator enable signal cmp — en is activated as a logic high level. 
   The fourth D-type flip-flop  3051  compares rising edges of the rising edge clock signal rclk and the feed-backed clock signal fb — clk in order to determine shift right or shift left rising edges of the rising edge clock signal rclk and the falling edge clock signal fclk in the delay line unit  307 . The fifth and the sixth D-type flip-flops  3052  and  3053  determine speed of the shift right or the shit left operation. 
   That is, if a rising edge of the rising edge clock signal rclk leads a rising edge of the feed-backed clock signal fb — clk, an output signal of the third D-type flip-flop  3051 , i.e., the comparison signal out 1  becomes in a logic low level. In other words, if a rising edge of the feed-backed clock signal fb — clk lags behind a falling edge of the rising edge clock signal rclk, the comparison signal out 1  becomes in a logic high level. Therefore, even though a rising edge of the feed-backed clock signal fb — clk leads a rising edge of the rising edge clock signal, the delay line unit  307  can delay the feed-backed clock signal fb — clk by delaying the rising edge clock signal rclk until a rising edge of the feed-backed clock signal fb — clk lags behind a falling edge of the rising edge clock signal rclk. 
   Likewise, the fifth D-type flip-flop  3052  compares rising edges of the feed-backed clock signal fb — clk and a delayed rising edge clock signal which is a delay signal of the rising edge clock signal rclk. Also, the sixth D-type flip-flop  3053  compares rising edges of the rising edge clock signal rclk and a delayed feed-backed clock signal which is a delayed signal of the feed-backed clock signal fb — clk. 
     FIG. 8  is a timing diagram showing an operation of the phase comparator  305  shown in  FIG. 7 . 
   As shown in a case  1  and a case  2 , i.e., when a gap between a rising edge of the feed-backed clock signal fb — clk and a rising edge of the rising edge clock signal rclk is smaller than a predetermined length, the second divided clock signal clk — v 4   — p 2  is inputted to the T-type flip-flop  3061  in order to slowly shift a phase of the feed-backed clock signal fb — clk. On the other hand, in a case  2 , a case  3 , a case  4  or a case  5 , i.e., when the gap between a rising edge of the feed-backed clock signal fb — clk and a rising edge of the rising edge clock signal rclk is larger than a predetermined length, the rising edge clock signal rclk is inputted the T-type flip-flop  3061  in order to shift a phase of the feed-backed clock signal fb — clk in a fast speed. 
   Meanwhile, if the semi locking detection signal semi — lock is in a logic low level, the two multiplexers output a power supply voltage VDD and a ground voltage GND. If the semi locking detection signal semi — lock becomes in a logic high level, the two multiplexers transfers output signals of the fourth D-type flip-flop  3051 . 
     FIG. 9  is a timing diagram showing an operation of the DLL. 
   If the reset signal rst is inputted, the comparator enable signal cmp — en becomes in a logic low level. Therefore, the semi locking detector  304 , the delay line unit  307  and the phase comparator  305  are reset while the comparator enable signal cmp — en is in a logic low level, and, thus input signals of the semi locking detector  304 , the delay line unit  307  and the phase comparator  305  are ignored while the comparator enable signal cmp — en is in a logic low level. Thereafter, if the comparator enable signal cmp — en becomes in a logic high level, the semi locking detector  304 , the delay line unit  307  and the phase comparator  305  is operated normally in response to their input signals. 
   As described above, the DLL in accordance with the present invention can perform a phase comparison operation at a higher frequency than that of the conventional DLL and also can prevent a clock skew, which is not compensable, from being generated at an initial state of the DLL. In addition, the DLL can perform a reset operation stably without an error and also can reduce a power consumption because the DLL does not include a dummy delay line. 
   The present application contains subject matter related to Korean patent application No. 2004-14910, filed in the Korean Patent Office on Mar. 5, 2004, the entire contents of which being incorporated herein by reference. 
   While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.