Abstract:
The present invention relates to a semiconductor circuit; and, more particularly, to a duty cycle correction circuit (hereinafter referred to as “DCC”). Furthermore, the present invention relates to an open-loop digital DCC. The duty cycle correction circuit according to the present invention includes: a delayer for delaying an input clock signal and for generating a plurality of delayed clock; a phase comparator for comparing the input clock signal with the plurality of delayed clock signals; a multiplexer for selecting one out of the delayed clock signals in response to an output signal of the phase comparator and for inverting the selected delay clock signals; and a phase combiner for combining the clock signal from the multiplexer and the input clock signal. Accordingly, the digital DCC according to the present invention is of an open loop without any DLL, the duty correction can be made within five clock periods after power-up.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor circuit; and, more particularly, to a duty cycle correction circuit (hereinafter referred to as “DCC”). Furthermore, the present invention relates to an open-loop digital DCC.  
       BACKGROUND OF THE INVENTION  
       [0002]     Generally, a clock signal has been used as a reference signal in a clock synchronization system and also has been used to guarantee high-speed operation of the system without an error. When a clock signal from an external circuit is used for system synchronization, a delay time (clock skew) is generated in an internal circuit. To compensate for this clock skew, a clock synchronizing circuits, such as a delay locked loop and a phase locked loop, have been used.  
         [0003]     On the other hand, in a DDR SDRAM (Double Data Rate Synchronous DRAM) to perform data input and output operations at rising and falling edges of a high-speed clock signal, it is very important to maintain a duty ratio of 50% for the effective margin of input/output data. Accordingly, a duty cycle correction circuit has been used for generating a clock signal in the constant duty ratio of 50%, irrespective of the duty ratio of the external clock signal, and for compensating for a duty variation of an undesired clock signal.  
         [0004]     The duty cycle correction circuit can be classified into an analogue DCC and a digital DCC or into a closed-loop DCC and an open-loop DCC. The analogue DCC is typically implemented by the closed-loop DCC.  
         [0005]      FIG. 1  is a schematic diagram illustrating a conventional analogue closed-loop DCC. As shown in  FIG. 1 , the conventional analogue closed-loop DCC includes a DLL circuit  10  to synchronize a phase of an internal clock signal with that of an external clock signal, a DCC amplifier  12  to amplify an output signal from the DLL  10 , and a DCC integrator  14  to receive and integrate DLL clock signals dll_clk from the DCC amplifier  12  and then to provide information about a duty ratio to the DCC amplifier  12 .  
         [0006]     This analogue closed-loop DCC has merits in that the precision is very high and it is not susceptible to PVT fluctuation; however, it has a shortage in that it takes a lot of time to obtain a desired clock signal of the duty ratio of 50%. Most analogue DCCs have the integrator which stores information in a capacitor (Se Jun Kim et al., “A Low jitter, Fast recoverable, Fully analog DLL using Tracking ADC For High Speed and Low Stand-by power DDR I/O interface”, in Symposium on VLSI Circuits Digest of Technical Papers, 2003). However, in a power-down mode, the information can be lost by a leakage current in the capacitor and therefore lots of time is required to perform a normal operation at the time of restart.  
         [0007]     Although the digital DCCs including a DLL can solve the above problem of the analogue DCC by using a digital code, most of the digital DCCs need lots of time until a normal operation is performed after power-up (Tatsuya Matano et al., ‘A 1-Gb/s/pin 512-Mb DDRII SDRAM using a slew-rate-controlled output buffer’ in Symposium on VLSI Circuits Digest of Technical Papers, 2002).  
       SUMMARY OF THE INVENTION  
       [0008]     It is, therefore, a primary object of the present invention to provide a to a duty cycle correction circuit capable of reducing an initial operation time by excluding a DLL.  
         [0009]     In accordance with the present invention, there is provided a duty cycle correction circuit comprising: a delay means for delaying an input clock signal and for generating a plurality of delayed clock signals which are out of phase with each other; a phase comparison means for comparing the input clock signal with the plurality of delayed clock signals; a multiplexing means for selecting one out of the delayed clock signals in response to an output signal of the phase comparison means and for inverting the selected delay clock signals; and a phase combination means for combining the clock signal from the multiplexing means and the input clock signal.  
         [0010]     Preferably, the delay means includes a delay chain having a plurality of unit delayers which are in series coupled to each other.  
         [0011]     Preferably, the phase comparison means receives the plurality of delayed clock signals and includes a plurality of D flip-flop circuits which receive the plurality of delayed clock signals, respectively.  
         [0012]     In the above, the multiplexing means includes: a controller for generating a plurality of switching control signals in response to output signals from the phase comparison means; a switching unit for selectively outputting one out of the delay clock signals in response to the switching control signals; and a phase inverting unit INV 1  for inverting an output signal from the switching unit.  
         [0013]     In the multiplexing means, the controller includes a plurality of control units, wherein the each of the control units is correspondent to each of the D flip-flop circuits in the from the phase comparison means, and wherein the control units receives an output signal from the D flip-flop circuit, which is positioned at the N-th stage in the phase comparison means, and a previous output signal from the D flip-flop circuit, which is positioned at the (N−1)-th stage in the phase comparison means, and outputs a control signal for a next stage and a switching control signal in response to a control signal from a previous stage.  
         [0014]     Further, the switching unit includes a plurality of switches in order to selectively output one from the delayed clock signals in response to the switching control signals from the control units. Also, the phase inverting unit includes an inverter for inverting the delayed clock signals in response to the switching control signals from the control units.  
         [0015]     Preferably, the control units includes: a first inverter to invert an output signal from the D flip-flop circuit, which is positioned at the (N−1)-th stage in the phase comparison means; a second inverter to invert an output signal from the D flip-flop circuit, which is positioned at the N-th stage in the phase comparison means; a third inverter to invert a control signal from the control unit at an (N−1)-th stage; a NOR gate to receive an output signal from the first inverter, the output signal from the D flip-flop circuit, which is positioned at the N-th stage in the phase comparison means and the output signal from the D flip-flop circuit, which is positioned at the N-th stage in the phase comparison means; a first NAND gate to receive an output signal from the inverter and the output signal from the D flip-flop circuit, which is positioned at the (N−1)-th stage in the phase comparison means; and a second NAND gate to receive output signals from the NAND gate and the inverter and to output a control signal for another control unit at a next stage.  
         [0016]     In the above, the delay chain has an amount of delay time which is correspondent to 60% of a period of the input clock signal and the phase combination means includes a digital phase combiner having two inverters of which two output terminals have a short circuit.  
         [0017]     Also, in accordance with the present invention, there is provided a duty cycle correction circuit comprising: a delay chain for delaying an input clock signal and for generating a plurality of delayed clock signals which are out of phase with each other, wherein the delay chain includes a plurality of unit delayers; a plurality of D flip-flop circuits for receiving the delayed clock signals from the unit delayers and the input clock signal as an input data; a plurality of control units, each of which receives an output signal from the D flip-flop circuit at a corresponding stage and an output signal from the D flip-flop circuits at a previous stage and generates a control signal and a switching control signal in response to a control signal from another control unit at a previous stage; a plurality of switches for selectively outputting one from the delayed clock signals in response to the switching signal from the control unit; and an inverter for inverting the selected delay clock signal; and a phase combination means for combining an clock signal from the inverter and the input clock signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0019]      FIG. 1  is a schematic diagram illustrating a conventional analogue closed-loop DCC;  
         [0020]      FIG. 2  is a schematic diagram illustrating an open-loop DCC in accordance with the present invention;  
         [0021]      FIG. 3  is a block diagram illustrating a control unit in  FIG. 2 ;  
         [0022]      FIG. 4  is a timing diagram illustrating phase relations between an input clock signal and a delayed clock signal;  
         [0023]      FIG. 5  is a timing diagram of the open-loop DCC in FIG.  2 ; and  
         [0024]      FIG. 6  is a plot showing an output duty error in the open-loop DCC in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Hereinafter, with reference to the accompanying drawings, a preferred embodiment of the present invention will be explained in detail.  
         [0026]     Referring to  FIG. 2 , there is provided an open-loop DCC in accordance with the present invention. The open-loop DCC includes a delay block  100 , a phase comparison block  200 , a multiplexing block  300  and a phase combination block  400 .  
         [0027]     The delay block  100  delays an input clock signal CLKIN and generates a plurality of delay clock signals Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) from the delayed input clock signal using a plurality of delay blocks. The phase comparison block  200  compares the input clock signal CLKIN with each of the plurality of delay clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ). The multiplexing block  300  receives the plurality of delay clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) and outputs an inverted signal by selecting and inverting one from the plurality of delay clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) in response to output signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) from the phase comparison block  200 . The phase combination block  400  combines a clock signal from the multiplexing block  300  and the input clock signal CLKIN.  
         [0028]     The delay block  100  is a delay chain having a plurality of unit delayers  110 . The unit delayer  110  can be implemented by two inverters which are in series coupled to each other and the unit delayer  110  has a unit delay time (I) of 170 ps. Also, fifteen (15) unit delayers can be employed in order that the input clock signal CLKIN operates up to 60% in the duty ratio at 250 MHz (tCK=4 ns).  
         [0029]     The phase comparison block  200  includes a plurality of D flip-flop circuits  210  which receives the plurality of delay clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ). The multiplexing block  300  includes: a controller  310  for generating a plurality of switching control signals (R 1 , R 2 , . . . , Rn, . . . ) in response to output signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) from the phase comparison block  200 ; a switching unit  320  for selectively outputting one out of the plurality of delay clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ) in response to the switching control signals (R 1 , R 2 , . . . , Rn, . . . ); and a phase inverting unit INV 1  for inverting an output signal from the switching unit  320 . Also, a phase blender of which two output terminals have a short circuit can be employed as the phase combination block  400 .  
         [0030]     On the other hand, the controller  310  includes a plurality of control units  312 . Each of the control units  312  receives the output signal Qn from the D flip-flop circuit  210 , which is positioned at the N-th stage in the phase comparison block  200 , and the previous output signal Qn− 1  from the D flip-flop circuit  210 , which is positioned at the (N−1)-th stage in the phase comparison block  200 , and outputs a control signal CTR_n for the next stage and the switching control signal Rn in response to a control signal (CTR[n− 1 ]) from the previous stage. The switching unit  320  includes a plurality of switches (sw 1 , sw 2 , . . . , swn, . . . ) in order to selectively output one from the delayed clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n] . . . ).  
         [0031]      FIG. 3  is a block diagram illustrating a control unit in  FIG. 2 . As shown in  FIG. 3 , each of the control units  312  includes: an inverter INV 2  to invert the output signal Qn− 1  from the D flip-flop circuit  210 , which is positioned at the (N−1)-th stage in the phase comparison block  200 ; an inverter INV 3  to invert the output signal Qn from the D flip-flop circuit  210 , which is positioned at the N-th stage in the phase comparison block  200 ; an inverter INV 4  to invert the control signal (CTR[n− 1 ]) from the previous stage ((N−1)-th control unit); a NOR gate NOR 1  to receive an output signal from the inverter INV 2 , the output signal Q[n] from the N-th stage and the control signal (CTR[n− 1 ]); a NAND gate NAND 1  to receive an output signal from the inverter INV 3  and the output signal Q[n− 1 ] from the (N-1)-th stage; and a NAND gate NAND 2  to receive output signals from the NAND gate NAND 1  and the inverter INV 4  and to output the control signal CTR [n] for other control units  312  at the next stage.  
         [0032]      FIG. 4  is a timing diagram illustrating phase relations between an input clock signal and a delayed clock signal and  FIG. 5  is a timing diagram of the open-loop DCC in  FIG. 2 .  
         [0033]     Referring to  FIG. 4 , the D flip-flop circuits  210  in the phase comparison block  200  decides whether the input clock signal CLKIN leads or lags behind the delayed clock signals (Φ[ 1 ], Φ[ 2 ], . . . , ΦD[n], . . . ), by comparing a falling edge of the input clock signal CLKIN with a rising edge of the delayed clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ). That is, the input clock signal CLKIN is inputted into the delay block  100  and the input clock signal CLKIN is gradually delayed by the delay unit in the delay block  100  with the generation of the delayed clock signals (Φ[ 1 ], Φ[ 2 ], . . . , Φ[n], . . . ). As a result, a rising edge of the N-th delayed clock signal Φ[n] lags behind the falling edge of the input clock signal CLKIN so that the falling edge of the input clock signal CLKIN is positioned between a rising edge of the (N−1)-th delayed clock signal Φ[n− 1 ] and the rising edge of the N-th delayed clock signal Φ[n].  
         [0034]     Accordingly, an output signal Q[n] of the N-th D flip-flop circuit is different from the output signal Q[n- 1 ] of the (N−1)-th D flip-flop circuit and the controller  310  in the multiplexing block  300  turns off all the switches except for a N-th switch SWn using such the difference between output signals of the D flip-flop circuits. Therefore, an output signal of the switching unit  320  is to be a delayed clock signal Φ[n] from the N-th unit delayer  110  and this delayed clock signal Φ[n] is inverted by the inverter INV 2 .  
         [0035]     On the other hand, the phase combination block  400  combines the inverted delay clock signal /Φ[n] (when the Φ[n] is activated, there is in a low level) and the input clock signal CLKIN and outputs an output clock signal CLKOUT having an intermediate phase between the falling and rising edges of the two clock signals.  
         [0036]     Referring to  FIG. 5 , the falling edge of the input clock signal CLKIN and the falling edge of the inverted delay clock signal/Φ[n] from the inverter INV 1  are out of phase by (A+B)−T and the rising edge of the input clock signal CLKIN and the rising edge of the inverted delay clock signal/Φ[n] from the inverter INV 1  are out of phase by A−B, where T is a period of the input clock signal CLKIN, A is a low section of the input clock signal CLKIN and B is a total amount of delay time of the corresponding delay clock signal Φ[n], n*τ.  
         [0037]     The output clock signal CLKOUT caused by the phase combination has a high level section of T/2 and a low level section of T/2 as follows: 
 
high level section: (( A+B )− T )/2+( T−A )+( A−B )/2= T/ 2 
 
low level section: ( A−B )/2+( T−A )+(( A+B )− T )/2= T/ 2 
 
         [0038]     As a result, the output clock signal CLKOUT has a duty ratio of 50%.  
         [0039]      FIG. 6  is a plot showing an output duty error in the open-loop DCC in  FIG. 2  and this simulation is carried out by HSPICE. Referring to  FIG. 6 , 50% duty output is obtained on the input clock signal CLKIN having the duty ratio of 40% to 50% and bandwidth of 250 MHz (tCK=4 ns) to 600 MHz (tCK=1.7n).  
         [0040]     As illustrated above, the digital DCC according to the present invention reduces an initial operation time. That is, the digital DCC according to the present invention is of an open loop without any DLL, the duty correction can be made within five clock periods after power-up.  
         [0041]     The present application contains subject matter related to Korean patent application No. 2004-11135, filed in the Korean Patent Office on Feb. 19, 2004, the entire contents of which being incorporated herein by reference.  
         [0042]     Although the preferred embodiments of the invention have been disclosed 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. For example, the present invention illustrates the phase comparison block using the D flip-flop; however, other types of the flip-flops may be available to implement the phase comparison block.