Semiconductor memory circuit with improved timing and delay control for data read out
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor memory circuit, and particularly to a static type semiconductor memory circuit controlled by a chip select signal. 
     Static type memory circuits have been utilized in various fields as high speed memories. 
     A static memory is controlled by a chip select signal (CS) between an active state for achieving a reading or writing operation and an inactive state in which the memory is set at a stand-by mode in order to implement a high speed operation with a low power consumption. The chip select signal CS is received by a clock input buffer of the memory and the clock input buffer generates an internal control signal for actually controlling peripheral circuits such as addressing circuits and an output circuit included in the memory. 
     When the chip selection signal is brought into its active level, e.g. low level, the internal control signal is set at an active level, e.g. low level so that the peripheral circuits are enabled for a reading or writing operation. While, when the chip selection signal is changed into its inactive state, the internal control signal is also set at its inactive level (e.g. high level) so that the peripheral circuits are disenabled and set at a stand-by mode. 
     In order to achieve a high-speed operation, it is necessary to generate the active level of the internal control signal quickly as soon as the chip select signal is changed to the active level from the inactive level. Therefore, the clock input buffer receiving the chip select signal has been designed to generate the active level of the internal control signal rapidly in response to the change of the chip select signal from the inactive level to the active level, as compared to the case where the clock input buffer generates the inactive level of the internal control signal in response to the change of the chip select signal from the active level to the inactive level. 
     Namely, in the case where the active level and the inactive level of the internal control signal are a low level and a high level, respectively, a lower potential side drive transistor for pulling down the output of the clock input buffer is designed to have a large current ability while a higher potential side drive transistor for pulling-up the output of the clock input buffer is designed to have a small current ability. 
     Accordingly, the clock input buffer generates the active level of the internal control signal rapidly to enable the peripheral circuit. However, the clock input buffer generates the inactive level of the input control signal with a relatively large delay time from a time when the chip select signal has changed from the active level to the inactive level, and therefore the peripheral circuits are still enabled for the period corresponding to the above delay time even after the chip select signal has changed to the inactive level. Thus, the memory inevitably produces an output signal after the chip select signal has changed to the inactive level. 
     In general, an output terminal of a memory circuit is connected to a bus line to which another functional circuit is connected, and therefore, the above phenomenon that the memory produces a read out signal after the chip select signal thereto has changed to the inactive level makes the effective speed of the memory slow and the utilization ratio of the bus line is also lowered. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a memory circuit which can operate at a high speed as a whole. 
     It is another object of the present invention to provide a static memory circuit which can set an output circuit at a high impedance state with a reduced delay time after a change of a chip select signal to its inactive level without increasing access time. 
     The memory circuit according to the present invention comprises an array of memory cells, a selection circuit for selecting a memory cell or cells to be accessed in accordance with address signals, an output circuit for producing an output signal in accordance with data from the selected memory cell, and a control circuit for generating a control signal for enabling the selection circuit and the output circuit in response to an external control signal, and is featured in that the control circuit includes a first buffer circuit for generating an active level of a first control signal a first delay time after a change of the external control signal from its inactive level, to an active level and for generating an inactive level of a first control signal a second delay time after a change of the external control signal from its active level to an inactive level, a second buffer circuit for generating an active level of a second control signal a third delay time after a change of the external control signal from the inactive level to the active level, and an inactive level of a second control signal a fourth delay time after a change of the external control signal from the active level to the inactive level, the first delay time being shorter than the third delay time, the fourth delay time being shorter than the second delay time, first control means responsive to the first control signal for enabling the selection circuit, and second control means responsive to the second control signal for enabling the output circuit. According to the present invention, the output circuit is controlled by the second control signal which is immediately deactivated as soon as the external control signal is changed to the inactive level, and hence the output circuit is immediately disenabled to make an output floating period shorter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a memory circuit according to a prior art; 
     FIG. 2 is a timing chart showing an operation of the memory of FIG. 1; 
     FIG. 3 is a schematic block diagram of a memory circuit according to one embodiment of the present invention; 
     FIG. 4 is a timing chart showing an operation of the memory of FIG. 3; 
     FIG. 5 is a circuit diagram showing the clock input buffer employed in the memory of FIG. 3; 
     FIG. 6 is a schematic block diagram showing a part of the row address buffer; and 
     FIG. 7 is a schematic circuit diagram showing major part of the memory of FIG. 3: 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a major structure of a memory according to a prior art. 
     A plurality of static type memory cells are arranged in rows and columns with word lines in rows and digit lines in columns (not shown) in a memory cell array 13. Row address signals ARl to ARn are applied to a row address buffer 11 which develops their true and complementary signals. A row decoder 12 receives the true and complementary address signals from the row address buffer 11 to select one of the word lines in the rows of the memory array 13. 
     A plurality of column address signals ACl to ACm are applied to a column address buffer 16 and their true and complementary signals are generated by the column address buffer 16. A column decoder 15 selects sense amplifiers to be connected to a read data line RB in a sense amplifier and switch block 14 or digit lines to be connected to a write data line WB via the block 14. An output circuit 17 has an input connected to the read data line RB and an output connected to a data input/output terminal I/O and generates a read out signal to the I/O terminal in a read mode. An input circuit 18 operatively outputs a write signal to the input data line WB in response to an input signal applied to the I/O terminal in a write mode. 
     A clock input buffer 20 receives a chip select signal CS which controls the memory between an active state and an inactive state, and generates an internal control CS&#39;. The internal control signal CS&#39; is applied to the row address buffer 11 and the column address buffer 16 and to the respective inputs of NOR gates 21 and 22. 
     A read/write control signal R/W is applied to a control input buffer 19 which generates a low level of read enable signal RE when R/W is at low in level and a low level of write enable signal WE when R/W is at high in level. The signal RE is synchronized with the internal control signal CS&#39; by the NOR gate 21 to generate a read execution signal RE&#39; for enabling the output circuit 17 in a read mode. The signal WE is synchronized with the internal control signal CS&#39; by the NOR gate 22 and when both of WE and CS&#39; are at low in levels, a high level of a write execution signal WE&#39; for enabling the input circuit 18 is generated. 
     In the above memory, the chip select signal CS works as the basic control signal to the memory, and the internal control signal CS&#39; actually controls the peripheral circuits. 
     Referring to FIG. 2, an operation of the memory of FIG. 1 is explained. 
     At a time t 1 , the signal CS is changed from a high (inactive) level to a low (active) level. The signal CS&#39; is changed from a high (inactive) level to a low (active) level at a time t 2  which is delayed by a period T 1  from the time t 1 . The period T 1  is determined by a response time of the buffer 20 for the low level input of CS. While the signal R/W is set at a low to designate a read mode and the signal RE is changed to a low level prior to the time t 1 . 
     Therefore, at the time t 2  , the high (active) level of RE&#39; is also generated. Thus, the peripheral circuits such as the buffers 11, 16 and the output circuit 17 are enabled to commence actually a read operation, and at a time t 3 , a valid data is produced to the I/O terminal. 
     Then, at a time t 4 , the signal CS is changed from the low level to the high level, and hence the internal control signal S&#39; is also changed to the high (inactive) level from the low level at a time t 5  which is after t 4  by a period T 2 . The period T 2  is longer than the period T 1 . The period T 2  corresponds to a response time of the buffer 20 to the change of CS from the low level to the high level. In response to this, the signal RE&#39; is changed from the high level to the low level and the peripheral circuits are disenabled and the terminal I/O is rendered at a high impedance state (Hi-Z) at a time t 6 . 
     In FIG. 2, a period T AC  from t 1  to t 3  is corresponds to an access time of the memory, and a period T HZ  from t 4  to t 6  is an output floating time during which the memory still outputs the valid data although the signal CS is already set to the high level. 
     Since the response time T 1  of the buffer 20 is designed short in order to make T AC  short, the response time T 2  of the buffer is sacrificed for the purpose of short T AC , the data output is undesirably produced at the I/O during a relatively long period of T HZ . This in fact lowers the effective operation speed of the memory and also lowers the utilization rate of the bus line connected to the terminal I/O. FIG. 3 shows one embodiment of the present invention. 
     This embodiment is achieved by inverters 20&#39;-1 to 20&#39;-3 in place of the buffer 20 in FIG. 1. 
     The inverter 20&#39;-3 works as an inverted buffer to generate an inverted signal CS&#39; with respect to CS at a high speed. A response time of the inverter 20&#39;-3 to the change of a low level input to a high level input is almost the same as that of a high level input to a low level input. The inverter 20&#39;-1 develops a low level output (CS&#39;) in response to a high level of CS&#39; at a high but it develops a high level output (CS&#39;) in response to a low level of CS&#39; at a relatively low speed. To the contrary, the inverter 20&#39;-2 has the opposite delay response characteristics with respect to the inverter 20&#39;-1. Namely, the inverter 20&#39;-2 develops a high level output (CS&#34;) in response to a low level input (CS&#39;) at a high speed while it develops a low level output (CS&#34;) in response to a high level input (CS&#39;) at a relatively low speed. The output CS&#39; of the inverter 20&#39;-1 is applied to the peripheral circuits such as the row address buffer 11 and the column address buffer 16 other than the output circuit 17 and the input circuit 18. The output CS&#34; of the inverter 20&#34;-2 is used to synchronize the signal RE and the signal WE by the NOR circuits 21 and 22 and the outputs RE&#34; and WE&#34; are applied to the output circuit 17 and the input circuit 18 for enabling them, respectively. 
     Examples of detailed structures of the inverters 20&#39;-1 to 20&#39;-3 are shown in FIG. 5. 
     The inverter 20&#39;-3 is composed of a P-channel field effect transistor (FET) QP1 and an N-channel FET QN1. The ratio WP1/LP1 of a channel width WP1 to a channel length LP1 of FET QP1 and the ratio WN1/LN1 of a channel width WN1 to a channel length LN1 are selected approximately the same. Namely, [WP1/LP1]≃[WN1/LN1]is provided. 
     The inverter 20&#39;-1 is composed of a P-channel FET QP2 and an N-channel FET QN2. The ratio of [WP2/LP2]of a channel width WP2 to a channel length LP2 of FET QP2 is selected smaller than the ratio WN2/LN2 of a channel width WN2 to a channel length LN2 of FET QN2. Namely, WP2/LP2&lt;WN2/LN2 is satified. [WN2/LN2] is preferably made larger than WP2/LP2 by three times or more. In this embodiment, [WP2/LP2]: [WN2/LN2]=1: 10. 
     The inverter 20&#39;-2 is composed of a P-channel FET QP3 and an N-channel FET QN3. The ratio [WP3/LP3] of a channel width WP3 to a channel length LP3 of FET QP3 is made larger than the ratio WN3/LN3 of a channel width WN3 to a channel length LN3 of FET QN3. [WP3/LP3]is preferably made larger than [WN3/LN3-] by three times or more. In this embodiment, [WP3/LP3]: [WN3/LN3 ]=10:1 is provided. 
     In other words, the conductance or current flowing ability of QN2 is set larger than that of QP2 in the inverter 20&#39;-1, and the conductance or current flowing ability of QP3 is set larger than that of QN3 in the inverter 20&#39;-2. 
     Thus, the inverter 20&#39;-1 responds to the change of CS&#39; from a low level to a high level to change the output CS&#39; from a high level to a low level rapidly. 
     Similarly, the inverter 20&#39;-2 responds to the change of CS&#39; from a high level to a low level to change the output CS&#34; from a low level to a high level rapidly. 
     FIG. 6 shows a part of the row address buffer 11 with respect to one bit input A i . 
     A NOR gate 31 transmits the address input Ai when the signal CS&#39; is at low and inverters 32 and 33 produce complementary and true address signals Ai&#39; and Ai&#39;, respectively. 
     FIG. 7 shows an example of detailed structure of major part of the memory. 
     Each memory cell is made of a flip-flop composed of a pair of load resistors R1 and R2, a pair of N-channel drive FETs QN6 and QN7, and a pair of N-channel transfer gate FETs QN4 and QN5. A word line WLi is connected to gates of FETs QN4 and QN5, and a pair of digit lines Dj and Dj are provided in each column. The sense amp and switch block 14 includes a gated-type differential amplifier composed of N-channel input FETs QN8 and QN9 having gates coupled to the digit lines Dj and Dj and a switch FET QN10 having a gate receiving one of output Yj from the column decoder 15, and a pair of common load P-channel FETs QP4 and QP5 controlled by CS&#39;. 
     The output circuit 17 includes a NOR gate receiving CS&#34; and a complement read data line RB, a NOR gate 42 receiving CS&#34; and a true read data line RB, an inverter 43 and an output inverter composed of a P-channel FET QP6 and an N-channel FET QN11. The NOR gate 42 transmits data on the line RB to a gate of FET QN11 and data on the line RB is transmitted via the NOR gate 41 and inverted by the inverter 43 for driving FET QP6 when CS&#34; is at low in level. When CS&#34; is at high in level, the outputs of the NOR gates 41 and 42 are kept at low, and therefore the gate potentials of FETs QP6 and QNll are a high level and a low level, respectively. Thus, both of FETs QP6 and QN11 are rendered non-conductive so that the terminal I/O is set at a high impedance (floating) state. FIG. 4 shows an operation of the memory of FIG. 3. 
     At a time t 1 , the signal CS is changed from a high level to a low level. Then, the inverter 20&#39;-1 changes its output CS&#39; from a high level to a low level at a time t 2  which is later from t 1  by a relatively short period T 1 . Therefore, the peripheral circuits for achieving selection of memory cells are enabled except the output circuit 17 and the input circuit 18. Then, at a time t 3  which is after t 2 , the inverter 20&#39;-2 changes its output CS&#34; from a high level to a low level and the signal RE&#39; is changed to a high level. Thus, the output circuit 17 is enabled and an output data (DATA) is produced at I/O at a time t 4 . 
     At a time t 5 , the signal CS is reset to a high level. In response to this change of CS, the inverter 20&#39;-2 changes its output CS&#34; at a time t 6  which is shortly after t 5  by a period T&#39; 2 , while the inverter 20&#39;-1 responds to this change of CS after a longer period T 4  has elapsed from t 5 . 
     Thus, in response to the change of CS&#34; at t 6 , the signal RE&#39; is deactivated at a time t 7  so that the output circuit 17 is disenabled. Therefore, the terminal I/O is set at a high-impedance state. 
     Since the response time T 2  &#39; of the inverter 20&#39;-2 is set shorter than that T 4  of the inverter 20&#39;-1, the output circuit 17 is disenabled immediately after the signal CS is reset to a high level. Thus, a short high floating period T&#39; Hz  can be obtained. 
     As has been explained above, the memory according to the present invention can operate at a high speed. 
     Although, in the above embodiment, the signal CS&#39; is applied to a plurality of peripheral circuits other than the output circuit and the input circuit, it is also practical to generate a plurality of sub-control signals each applied to each peripheral circuit in accordance with the signal CS&#39;.