Apparatus for controlling outputs of read data in a semiconductor memory device
FIELD OF THE INVENTION 
     This invention relates to an apparatus for controlling outputs of read data in a semiconductor memory device, and more particularly to a semiconductor memory device with a plurality of output circuits which supply read data to an external circuit. 
     BACKGROUND OF THE INVENTION 
     A conventional semiconductor memory device comprises a plurality of output circuits which supply read data to an external circuit in accordance with a chip selecting signal, an address change detecting signal and an output controlling signal, an output controlling circuit which controls output operations of the output circuits by supplying the output controlling signal to the output circuits, and an address change detecting circuit which detects an address change to supply information of the address change to the output controlling circuit and the output circuits. 
     In operation, the semiconductor memory device is selected to operate, when the low-active chip selecting signal turns to a low level. When the address change is detected in this state, the address change detecting circuit supplies an address change detecting signal to the output controlling circuit, which in turn supplies an output controlling signal to the output circuits. Thus, discharge of the output terminals is carried out in the output circuits, simultaneously. After that, the read data are supplied through the the output circuits to the external circuit. 
     However, the conventional apparatus for controlling outputs of read data in a semiconductor memory device has a disadvantage in that noise may be produced when a large amount of current flows in the simultaneous discharge of the output circuits, if many of the output terminals thereof are in a high level state. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an apparatus for controlling outputs of read data in a semiconductor memory device which produces no noise during the discharge of the output terminals of the output circuits even in the condition that the number of the output terminals of a high level state is large. 
     According to the invention, an apparatus for controlling outputs of read data in a semiconductor memory device comprises, 
     a plurality of output circuits for supplying read data which are read from a memory to an external circuit; and 
     means for controlling the output circuits to discharge charge of the output circuits sequentially with a predetermined time interval, the charge being accumulated by read data of a high level formerly supplied to the external circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with appended drawings, wherein: 
     FIG. 1 is a circuit diagram showing a conventional apparatus for controlling outputs of read data in a semiconductor memory device, 
     FIG. 2 is a timing chart showing operations the conventional apparatus for controlling outputs of read data in a semiconductor memory device, 
     FIG. 3 is a circuit diagram showing an apparatus for controlling outputs of read data in a semiconductor memory device in a preferred embodiment according to the invention, and 
     FIG. 4 is a timing chart showing operations the apparatus for controlling outputs of read data in a semiconductor memory device in the preferred embodiment according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before describing an apparatus for controlling outputs of read data in a semiconductor memory device according to the invention, the conventional apparatus for controlling outputs of read data in a semiconductor memory device briefly described before will be explained in conjunction with FIGS. 1 and 2. 
     FIG. 1 shows the conventional apparatus for controlling outputs of read data in a semiconductor memory device. The apparatus for controlling outputs of read data in a semiconductor memory device comprises a plurality of output circuits 10 1 , 10 2 , ˜, 10 n  which are supplied with read data from a memory, an output controlling circuit 20 which controls output operation of the output circuits 10 1 , 10 2 , ˜, 10 n , and an address change detecting circuit 30 which supplies an address change detecting signal ADC to the output controlling circuit 20. 
     Every circuit of the output circuits 10 1 , 10 2 , ˜, 10 n  comprises inverters 11, 12 and 13, NOR circuits 14 and 15, an OR circuit 16, a P-MOS transistor 17, and N-MOS transistors 18 and 19. The inverter 11 receives a corresponding bit RD  k  (k=1, 2, . . . , n) of read data to invert a logic level of the input thereof. The inverter 12 receives the output of the inverter 11 to invert a logic level thereof. The NOR circuit 14 produces a NOR logic output from the output of the inverter 11 and a chip selecting signal CS. The NOR circuit 15 produces a NOR logic output from the output of the inverter 12 and the chip selecting signal CS. The inverter 13 inverts a logic level of the output of the NOR circuit 14. The OR circuit 16 produces an OR logic output from the output of the inverter 13 and the address change detecting signal ADC of the address change detecting circuit 30. The P-MOS transistor 17 is connected at a source terminal to a power supply V cc , and at a gate terminal to the output of the OR circuit 16. The P-MOS transistor 17 and the N-MOS transistors 18 and 19 are connected with one another at drain terminals to an output terminal which produces a corresponding output signal EX k  (k=1, 2, . . . , n). The N-MOS transistors 18 and 19 are connected together at source terminals to ground. The N-MOS transistor 18 is connected at a gate terminal to the output of the NOR circuit 15. The N-MOS transistor 19 is connected at a gate terminal to the output controlling circuit 20. 
     The output controlling circuit 20 comprises an inverter 21 which inverts a logic level of the chip selecting signal CS, and an AND circuit 22 which produces an output controlling signal ADC o  from the output of the inverter 21 and the address change detecting signal ADC. The output controlling signal ADC o  is supplied to the gate terminals of the N-MOS transistors 19. 
     The address change detecting circuit 30 receives an address signal AD to produce the address change detecting signal ADC as a one shot pulse when the address signal changes. 
     FIG. 2 shows operations of the conventional semiconductor memory device. 
     The semiconductor memory device is selected to operate, when the chip selecting signal CS turns to a low level at the time t 1  to remain at a low level. At this state, when the address signal AD changes, the address change detecting circuit 30 detects the change thereof to produce the address change detecting signal ADC of a high level at the time t 2 . The signal ADC remains at a high level during a period between the times t 2  and t 4 , so that the output controlling signal ADC o  of the output controlling circuit 20 turns to a high level. As a result, the N-MOS transistor 19 turns to an ON state to carry out discharge of the output terminals of the output circuits 10 1 , 10 2 ,˜, 10 n . At this time, noises may be produced because a large amount of current flows when many of the output terminals thereof are in a high level and because discharge of the output terminals thereof are carried out simultaneously. 
     Then, the read data RD  k  (k=1, 2, . . . , n) is supplied to the input of the inverter 11 of the corresponding circuit of the output circuits 10 1 , 10 2 , ˜, 10 n . When the address change detecting signal ADC turns to a low level at the time t 4 , the N-MOS transistor 19 turns to an OFF state. On the other hand, the chip selecting signal CS is maintained to be a low level, so that the output signals EX 1 , EX 2 , ˜, EX n  corresponding to the read data RD 1 , RD 2 , ˜, RD n  are supplied to the external circuit. That is, the outputs of each of the circuit elements are indicated below, when the read data RD k  (k=1, 2, . . . , n) is &#34;high&#34; in one of the output circuits 10 k  (k=1, 2, ˜, n). 
     
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RD.sub.k                                                                  
   INVERTER                                                               
          INVERTER                                                        
                 NOR NOR INVERTER                                         
                                OR P-MOS                                  
                                        N-MOS                             
                                             N-MOS                        
                                                  EX.sub.k                
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1  0      1      1   0   0      0  ON   OFF  OFF  1                       
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     On the other hand, the outputs of the circuit elements are indicated below, when the read data RD k  is &#34;low&#34;. 
     
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RD.sub.k                                                                  
   INVERTER                                                               
          INVERTER                                                        
                 NOR NOR INVERTER                                         
                                OR P-MOS                                  
                                        N-MOS                             
                                             N-MOS                        
                                                  EX.sub.k                
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0  1      0      0   1   1      1  OFF  ON   OFF  0                       
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     Meanwhile, the output terminals of the output circuits 10 k  become a high impedence state, when the chip selecting signal CS turns to a high level. 
     Next, FIG. 3 shows an apparatus for controlling outputs of read data in a semiconductor memory circuit according to a preferred embodiment of the invention. The apparatus for controlling outputs of read data in a semiconductor memory device comprises a plurality of output circuits 10 1 , 10 2 , ˜, 10 n  which supply read data to an external circuit, an output controlling circuit 20 which controls output operation of the output circuits 10 1 , 10 2 , ˜, 10 n , an address change detecting circuit 30 which supplies an address change detecting signal ADC to the output controlling circuit 20, and a timing controlling circuit 40 which controls the timing of supplying an output controlling signal of the output controlling circuit 20 to the output circuits 10 1 , 10 2 , ˜, 10 n . 
     Every circuit of the output circuits 10 1 , 10 2 , ˜, 10 n  comprises inverters 11, 12 and 13, NOR circuits 14 and 15, an OR circuit 16, a P-MOS transistor 17, and N-MOS transistors 18 and 19. The circuit structure is the same as that in FIG. 1, except that N-MOS transistor 19 is connected at a gate terminal to a corresponding output of the timing controlling circuit 40. 
     The output controlling circuit 20 comprises an inverter 21 which inverts a logic level of the chip selecting signal CS, and an AND circuit 22 which produces an output controlling signal ADC 0  from the output of the inverter 21 and the address change detecting signal ADC. The output controlling signal ADC 0  is supplied to the timing controlling circuit 40. 
     The address change detecting circuit 30 receives an address signal AD to produce the address change detecting signal ADC of a one shot pulse when the address signal changes. 
     The timing controlling circuit 40 comprises a plurality of AND circuits 40 1 , 40 2 ,˜, 40 n . The AND circuit 40 1  produces a timing controlling signal ADC 1  from the output controlling signal ADC 0  received at two input terminals, so that the logic level of the output signal ADC 1  is equal to that of the input signal ADC 0 . The AND circuit 40 2  produces a timing controlling signal ADC 2  from the output signal ADC 0  and the timing controlling signal ADC 1 , and so on, then the AND circuit 40 n  produces a timing controlling signal ADC n  from ADC 0  and ADC n-1 . 
     FIG. 4 shows operation of the semiconductor memory device according to the preferred embodiment according of the invention. 
     The semiconductor memory device is selected to operate, when the chip selecting signal CS turns to a low level at the time t 1  to remain at a low level. At this state, when the address signal AD changes, the address change detecting circuit 30 detects the address change to produce the address change detecting signal ADC of a high level at the time t 2 . The signal ADC remains at a high level during a period between the times t 2  and t 4 , so that the output controlling signal ADC 0  of the output controlling circuit 20 turns to a high level, and the timing controlling signal ADC 1  of the AND circuit 40 1  also turns to a high level at the time t 2 . Then, the timing controlling signal ADC 2  of the AND circuit 40 2  turns to a high level, after an interval of an activation time T 1  of an AND circuit, at the time t 2  +T 1 . In such a manner, the timing controlling signal ADC n  of the AND circuit 40 n  turns to a high level, after an interval of the period T n-1  behind the turning of the timing controlling signal ADC 1  of the AND circuit 40 1  to a high level, so that the N-MOS transistor 19 turns to an ON state one after another at sequential intervals. As a result, in the simultaneous discharge of the output terminals of the output circuits 10 1 , 10 2 , ˜, 10 n , is prevented, so that noises may not be produced. 
     When the address change detecting signal ADC turns to a low level at the time t 4 , the N-MOS transistor 19 turns to an OFF state. On the other hand, the chip selecting signal CS is at a low level, so that the output signals EX 1 , EX 2 , ˜, EX n   corresponding to the read data RD 1 , RD 2 , ˜, RD n  are supplied through the output terminals of the output circuits 10 1 , 10 2 , ˜, 10 n  to the external circuit, as explained in FIGS. 1 and 2. Meanwhile, the output terminals thereof becomes a high impedence state, when the chip selecting signal CS turns to a high level. 
     Although the invention has been described with respect to a specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.