Memory Device

The present invention relates to a memory device composed of semiconductor elements and, more particularly, to a refreshing system for a dynamic type memory device using insulated-gate field-effect transisitors.

Dynamic type random access memories (hereinafter referred to as RAMs) are widely used in many fields. Recently, a RAM designated "MK 4816" and employing an improved refresing system was announced by MOSTEK co. of U.S.A. The latter device allows an internal refresh in which a refresh operation for memory cells is internally performed without receiving any refresh address information from the outside thereof by using a refresh control signal in addition to the conventional external refresh which requires refresh address information from the outside. This RAM achieves high-flexibility in control and its application.

RAMs employing multi-address input system in which row address information and column address information are incorporated respectively synchronism with row address strobe signal RAS and column address strobe singal CAS are substantially standard for dynamic RAMs with large capacity. A multi-address input type RAM is disclosed in U.S. Pat. No. 3,969,706 issued to R. J. Proebsting et al.

However, the multi-address input type RAM has some limitations between the strobe signals and the refresh operation, and such limitations have prevented the multi-address type RAM from incorporating the internal refresh of the "MK 4816" therein.

It is an object of the present invention to provide a dynamic RAM of a two-clock, multi-address input type, which has an internal refresh system.

It is another object of the present invention to provide a RAM provided with an internal refresh system used when a refresh control input is enabled during an active period of a row strobe signal.

A dynamic RAM according to the present invention comprises a first terminal for receiving a row strobe signal, a second terminal for receiving a column strobe signal, a set of terminals for receiving address input signals, a third terminal for receiving a refresh control signal, first means responsive to the row strobe signal for incorporating the address input signals as row address information, second means responsive to the column strobe signal for incorporating the address input signals as column address information, a memory cell array including a plurality of memory cells arranged in rows and columns, refresh means for operatively refreshing the memory cells on a row line designated by the row address information, refresh address means for designating a refresh address, third means for producing a first confirmation signal when the active operation of the refresh means ends, fourth means for producing a second confirmation signal when the reset precharge state ends, fifth means for latching the refresh control signal when the row strobe signal is present, a first control means responsive to the second confirmation signal and the latched refresh control signal for introducing refresh operation based on the refresh address means, and second control means responsive to the first confirmation signal for introducing the reset precharge state in the refresh means.

The present invention is summarized as follows: In a dynamic random access memory having as input signals a row strobe signal, a column strobe signal, address input, a write control signal, and a refresh control signal, having a function that, first by enabling the row strobe signal to incorporate the address input as row address information, a circuit involved automaticallly refreshes data stored in all the memory cells on a row line designated by the address input at this time, and then by enabling the column strobe signal, the circuit automatically transfers data between a selected memory cell on the column line designated by the address input at this time and a data input/output circuit, and having a refresh address counter in which when a refresh control signal is enabled, the internal refresh is automatically performed, and at the end of the active operation, a first confirmation signal is produced from an internal circuit; and at the end of a reset precharge operation, a second confirmation signal is produced from the internal circuit. When the refresh control signal is enabled during the active period of the row strobe signal, it is latched by the internal circuit. When the second confirmation signal becomes an active level signal, the internal circuit receives the latched signal to automatically to effect the internal refresh. When the internal refresh ends and the first confirmation signal becomes an active level signal, the circuit automatically enters the reset precharge period and subsequently the reset precharge state is continued.

FIGS. 1 to 4 show sets of waveforms useful in explaning an internal refresh circuit system by applying a RFSH clock thereto during an active period in a conventional dynamic RAM with CE and OE as a basic clock which allows an internal refresh;

FIG. 5 is waveforms useful in explaining the internal refresh system according to the present invention which allows the internal refresh by applying a RFSH clock thereto during an active period of a RAS in a dynamic RAM of the two-clock, multi-address input system with RAS and CAS as basic clocks;

FIG. 6 shows a block diagram of a RAM circuit of the two-clock, multi-address system using the internal refresh system according to the present invention;

FIGS. 7 to 15 show circuit diagrams of the embodiments of the RAM of the two-clock, multi-address system using the system according to the present invention; and

FIG. 16 shows a timing diagrams for explaining the operations mainly relating to the present invention in the circuit shown in FIGS. 7 to 15.

The explanation to be given proceed on the assumption that all transistors used are N channel metal-oxide-semiconductor transistors (hereinafter referred to as MOST) which are most typical in the insulated-gate field-effect transistors, and that a high level is logical "1" and a low level is a logical "0". It shall be understood that N-channel MOSTs can be substituted by P-channel MOSTs or another type insulated-gate field-effect transistors.

With reference to FIGS. 1 to 4, the operation of RAM "MK 4816" will be described.

The RAM employs as the input clock signals a chip enable signal CE, an output enable signal OE, a write control signal WE, and refresh control signal RFSH, and an input/output common terminal for data input and output. These signals CE, OE, WE and RFSH take their active state respectively when they are in low level.

In operation, when the chip enable signal CE as a basic clock shifts from high level to low level and the RAM enters an active period, an address input information at that time point is held (latched) and the contents of memory cells on a selected word (row) line are refreshed at the same time. Then data of a memory cell of the selected column and on the selected word line is transferred to a data bus and then is amplified by an output amplifier. The output enable signal OE controls whether the read out data is made to appear at the input/ouptut common terminal or not. When the signal OE is set to low level, the output data is obtained at the input/output common terminal (read cycle). While the data is written into a selected memory cell by shifting the signal WE to a low level to be enabled during the active period. At this time, input data to be written must be placed at the input/output common terminal. For this reason, the signal OE must be at its inactive level, i.e. at high level (write cycle). The reason for this is that if the signal OE is enabled i.e. low level, the read signal appears at the input/output common terminal, and it competes with the write input data. Therefore, in a read modify write cycle, after the signal OE is enabled i.e. low level in the RAM to read out the data, the signal OE is disabled, i.e. high level to be reset, and then the signal WE is turned to its enabled level, i.e. low level to write the data.

Consider now the refresh operation in question of the RAM referring to the drawings. The refresh is divided into the following operation modes.

When the signal RFSH is kept at high level (inactive level) the activation of the signal CE refreshes memory cells on a word line designated by address input information provided from the outside, as in the case of the conventional dynamic RAM.

FIG. 1 shows an operation state of a case where before the read operation is completed in the read cycle, the signal RFSH is turned to its active level. Upon the active level of the signal OE, the read data appears at the input/output common terminal and after the completion of the read operation is confirmed, the whole memory circuit automatically shifts to a reset precharge period in the operation phase. The signal RFSH at an active level is latched. After the reset precharge is completed, the refresh cycle automatically starts. Then, the memory cells on the word line designated by a refresh address counter included in the RAM are refreshed. At the time of the refresh completion, whole memory circuit shifts again to the reset precharge period while the content of the refresh address counter is incremented.

FIG. 2 shows another operation state in which the signal RFSH is turned to its active level after the read-out operation is completed.

With an adequate time elapse after the signal OE is turned to its active level, the read data appears at the input/output common terminal while the whole memory circuit enters the reset precharge period and the operation has been completed substantially. At this time, if the signal RFSH is turned to its active level (low level), the memory circuit immediately starts the refresh cycle as in the operation shown in FIG. 1 and returns to the reset precharge state after the refresh cycle is completed. FIG. 3 illustrates an operation state where the signal RFSH is turned to active level in the early write cycle. In the early write cycle, the circuit automatically enters the reset precharge period after the write operation is completed. The refresh operation caused by the active level of the signal RFSH before the completion of the write operation and after an appropriate time since the completion thereof, is similar as those of FIGS. 1 and 2.

FIG. 4 shows a case that the signal RFSH is turned to its active level in the late write cycle. Also in this case, when the signal WE is at the active level and the write operation is completed, the memory circuit automatically enters the reset precharge period and the refresh operation is similarly performed as in the cases of FIGS. 1 and 2, if the active level of the signal RFSH occurs at a proper time position.

When the signal RFSH is at the active level during the period that the signal CE is at the inactive level, the circuit enters the refresh operation immediately after the reset precharge is completed. Following the refresh operation, the circuit returns to the reset precharge state. Similarly, memory cells on the word line designated by the refresh address counter are refreshed and at the time point that the refresh of the memory cells are completed, the content of the refresh counter is incremented.

When the signal RFSH is kept at low level (active level) for a long period (20 .mu.s or more), so-called auto-refresh is performed at a relatively long period (15 .mu.s). During this time period, the input signals other than the signal RFSH are all rejected by the memory circuit and the shift of the signal RFSH to a high level causes AUTO-REFRESH to end. This is effective particularly for a POWER DOWN operation (battery backup) and a SINGLE STEP operation of a microprocessor.

The refresh operation of the memory "MK4816" is as described above. When this refresh operation is applied to the multi-address input system which has two clock signals, that is a row strobe signal RAS and a column strobe signal CAS, and address signals are inputted in a multiple manner into a RAM as previously described, the following problems arise. The two-clock, multi-address system is substantially a standard system in the MOS dynamic RAM with a large capacity, for example, 4K, 16K and 64K. When the signal RAS is at its active level (low level), the address information at that time point is latched as a row address information and the contents of all the memory cells on the word line specified by the row address information are refreshed. When the signal CAS is at its active level (low level), the address information at that time is latched as a column address information, and a selected memory cell of the column designated is coupled with a data input/output circuit. Under this condition, the read or the write operation is performed. Therefore, the signal RAS and CAS are not related to the signals CE and OE respectively by one to one correspondence. The external refresh of MODE (1) in this system is exactly the same as the conventional one. MODE (3) corresponds to a case that the signal RFSH is at the active level during the reset period of the inactive level of the signal RAS. This operation is relatively easily performed since it is realized by providing the refresh cycle in the reset precharge state. The refresh mode due to MODE (4) is a refresh mode that the signal RFSH is kept at a low level for a long period and all the input signals other than the signal RFSH are inhibited from inputting into the RAM whereby the refresh of the RAM is automatically performed under control of the internal circuit alone. Accordingly, this refresh mode is also possible in the two-clock, multi-address input system in question. The problem resides in the application of the refresh mode due to "MODE (2)" into the two-clock, multi-address input system. As previously stated, in the MK4816, upon the active level of the signal OE or WE, the data is read out or after the completion of the write operation is confirmed, the circuit is automatically shifted to the reset precharge phase, and following the completion thereof the internal refresh operation is introduced. MODE (2) refresh operation corresponds to a case that the signal RFSH is turned to its active level during the active level period of the signal RAS in the read or write cycle. In the refresh operation, after the completion of the enabled access operation, the reset precharge is automatically performed and then the refresh cycle must be performed. In the case of the MK4816, the output data appears as a result of the active level of the signal OE in the read cycle. In the write cycle, since a single terminal (the input/output common terminal) is used for the data input and output, it is impossible to turn the signal OE at the active level during the write operation. For this reason, the completion of the read operation may be confirmed referring to the time point that the signal OE is turned to the active level. The completion of the write operation may be confirmed referring to the time point that the signal WE is made the active level. In the two-clock, multi-address input system, both the signals RAS and CAS must be made the active level for the read cycle and the write cycle, so that the distinction thereof as in the case of the MK4816 is impossible. Particularly, the distinction between the read cycle and the late write cycle can not be made until the signal WE is made the active level. Accordingly, it is impossible to apply the refresh mode of the MK4816 system to the internal refresh for the signal RFSH enabled during the active period of the RAS.

The principle of the internal refresh system according to the invention will be described referring to FIG. 5. The signal RFSH may be enabled at any time position in the active period of the signal RAS and is latched in an internal circuit of a RAM. The signal RFSH remains latched during an active period T1 of the signal RAS and until the signal RAS is turned to the inactive level to be reset. Then the whole RAM circuit enters a reset precharge period T2 and a confirmation signal representing a confirmation of the end of the reset precharge operation is produced. Upon the generation of the confirmation signal, the circuit responds to the signal RFSH latched to automatically enter an internal refresh period T3, and to refresh memory cells on a word line corresponding to a row address specified by a refresh address counter. When the internal refresh operation is complete, the circuit automatically enters reset precharge period T4 where the content of the refresh address counter is incremented and the circuit prepares for the next internal refresh operation. Accordingly, the circuit waits for the next activation (active level) of the RFSH or RAS while it is in the reset precharge state. In the internal refresh by enabling the signal RFSH during the reset precharge (inactive level) period of the signal RAS, immediately following the activation of the signal RFSH, the internal refresh is performed immediately following confirmation of the end of the reset precharge operation, and the circuit automatically enters the reset precharge period after the internal refresh is completed.

A circuit structure of the RAM due to the two-clock, multi-address system (multi-address type RAM), which employs the internal refresh system according to the invention, is illustrated in FIG. 6. The way of inputting the signal RFSH classifies the operation of the multi-address type RAM into the following four modes:

(1) Cycle operation when the signal RFSH is in inactive state

(2) Activation of the signal RFSH during the active period of the signal RAS

(3) Activation of the signal RFSH during the reset period of the RAS

(4) Activation of the RFSH for a long time period

The operation mode of (1) is a normal circuit operation. In the (1) mode, upon the activation of the signal RAS, a RAS series clock generating circuit 115 operates to produce a series of internal control signals. In response to one of the internal control signals, the address input 119 at that time point is latched by a row address inverter buffer 118 as a row address, and the output of the row address inverter buffer 118 is determined. Then, a row decoder 116 performs the selecting operation of a word (a row) line in a memory cell matrix 101. Finally, memory cells in the memory matrix 101 on the word line specified by the row address input are refreshed (external refresh).

When the column address strobe signal CAS is enabled later than the activation of the signal RAS, a CAS series clock generating circuit 108 produces a series of control signals. The address input 119 at that time point is latched as a column address by a column address inverter buffer 104 in response to one of the CAS series control signals, and the output of a column address inverter buffer 104 is determined. Then a column decoder 103 performs the selecting operation of a memory cell belonging to the selected word line in the memory matrix 101, and a selected memory cell on the column selected is coupled with a data input/output bus DB through a sense amplifier, and a data input/output (I/O) gate portion 102.

In the read cycle, read data from the selected memory cell appears at an output terminal DATA OUT, through a data output buffer 105. In the write cycle, following the start of the CAS series clock generating circuit 108, a write clock generating circuit 107 starts to operate in response to the activation of the write enable signal WE. The output of a data input buffer 106 is enabled in accordance with a level of a signal at a data input terminal DATA IN at this time, so that data is written into a selected memory cell.

The operation mode of (2) is an internal refresh mode enabling the present invention to be effective. During the active period of the signal RAS, if the signal RFSH is enabled, the signal RFSH is immediately latched in an internal refresh control clock generating circuit 109. During the active period of the signal RAS, terminated after the signal RFSH is enabled, the active operation of the cycle is continued while the internal refresh operation is not performed. That is, the latch operation of the signal RFSH is independently performed in the refresh control clock generating circuit 109. When the RAS is turned to the inactive level to be reset, RAS series clock generating circuit 115 is reset and the CAS series clock generating circuit 108 and the write clock generating circuit 107 are in turn reset. As a result, the whole circuit shifts its operation to the reset precharge period. Upon the completion of the reset precharge operation, precharge confirmation signal PEND representing the confirmation of the completion is produced by a precharge detection circuit 122. At this time, based on the latched signal RFSH, the internal refresh control clock generating circuit 109 enables a cycle signal for the internal refresh and applies it to a RAS input buffer control logic unit 114. In response to the cycle signal (paths R1-R4), a RAS series clock generating circuit 115 operates while at the same time the CAS series clock generating circuit 108 and the write clock generating circuit 107 are inhibited from operating by a signal path R4 from the internal refresh control clock generating circuit 109. The output signal RCOUT from the refresh address counter 113 is transferred to the row decoder 116 through a multiplexer 117 thereby to refresh memory cells on the word line designated. When the refreshing operation is completed, a refresh confirmation signal AEND is produced by a refresh detection circuit 121, so that the cycle signal of the internal refresh is reset and the content of the refresh address counter 113 is incremented for the next interval refresh. In this connection, since the precharge and the refresh operations are controlled by a series of timing signals including precharge timing signals and enable timing signals produced by the RAS series clock generator circuit 115, the precharge detection circuit 122 and the refresh detection circuit 121 operate based on the timing signals produced by the circuit 115. Through the RAS input buffer control logic unit 114 under the control of the circuit 112, the RAS series clock generator 115 enters the reset precharge period. Subsequently, this state is kept up.

When the signal RFSH is enabled during the reset period of the RAS in the operation mode (3), the internal refresh control clock generating circuit 109 immediately operates and, in response to the precharge confirmation signal of the end of the reset precharge operation, the cycle signal of the internal refresh is enabled. As in the case of the mode (2), the RAS series clock generating circuit 115 operates through the RAS input buffer control logic unit 114 to refresh memory cells on the word line designated by the refresh address counter 113. After the operation of the refresh is completed, the cycle signal (R1-R4) of the internal refresh is reset and the refresh address counter 113 is incremented, so that the whole system enters the reset precharge period. The operation mode (4) corresponds to a case that the signal RFSH is enabled for a predetermined time period or more and, in this mode, "AUTO REFRESH" is performed. In this case, an output with a fixed period of an oscillator circuit 111 is necessary which is commonly used for a substrate bias voltage generating circuit 110, normally. When the signal RFSH enabled and a predetermined time is elapses, the cycle signals (R1-R4) for the AUTO REFRESH are enabled to operate the RAS series clock generating circuit 115, through the RAS input buffer control logic unit 114. As a result, memory cells on the word line specified by the refresh address counter 113 are refreshed. During a period that the signal RFSH is enabled, the RAS input buffer control logic unit 114 rejects the inputting thereto but accepts only the cycle signal (R1) of the AUTO REFRESH which occurs periodically. Accordingly, the cycle signal (R1) of AUTO REFRESH superposes the operation of the whole circuit, so that the increment of the refresh address counter 113 and the reset precharge operation are kept up after its completion so long as the signal RFSH is kept low level.

The explanation of the multi-address type RAM will be further described in detail referring to FIGS. 7 to 15 illustrating detailed circuit diagrams of the RAM and FIG. 16 illustrating a timing diagram useful in explaining the operation of the embodiment. The explanation will proceed with relation to the operation modes (1) to (4), for ease of explanation. In the cycle operation, where the signal RFSH of the (1) is inactive, the timing signals to control the internal refresh such as RF1, RFAA1, RFFPP1, RFAA2, RFPP2, AR0 and AR0 which are produced from the circuit 109 in FIG. 6 are all in low level in FIG. 7. A node 1 of the RAS input buffer control logic unit 114 becomes low in level (V.sub.DD -threshold voltage), so that the level of RAS appears at node 2 as it is. When the signal RAS is enabled, the RAS series clock generating circuit 115 operates with an interrelation as shown in FIG. 7. The enable timing signals such as RAS, RAS0, RAS1, . . . SE3 successively rise thereby to refresh the memory cell on the selected word line. Following the signal RAS, the signal CAS is enabled, so that the selected memory cell is coupled to the data input/output circuit (106 or 105). As a result, the contents of the memory cell is read out or input data is written into the memory cell in accordance with the signal WE. For effecting the internal refresh to be given, logically necessary are signals for confirming the end of the active operation (refresh) and the end of the reset precharge operation. To this end, the refresh detection circuit 121 produces the active operation (read and early write) end confirmation signal AEND in response to the final enable timing SE3 produced by the circuit 115. The precharge detection circuit 122 produces the reset precharge operation and confirmation signal PEND in response to the final precharge timing XP3. Detailed structures of the refresh detection circuit 121 and the precharge detection circuit 122 are shown in FIG. 8.

Next, with reference to FIGS. 9 to 12, detailed structure of the internal refresh control clock generator circuit 109 will be described. In FIG. 9, when the signal RFSH is enabled, the signal RF firstly rises to the V.sub.DD (power voltage) level and then the signal RFP shifts to a low level and further the signal RF1 rises to the level V.sub.DD. Those three timing signals exhibit level change in synchronism with the signal RFSH. The waveforms illustrating the operation of the mode (2) performed when the internal refresh system according to the present invention are illustrated in FIG. 16. In the figure, A designates an active period, P a reset precharge period, R an internal refresh period, and INC a time point that the refresh address counter is incremented. When the signal RFSH is enabled during the active period of RAS, in the circuit shown in FIG. 10 the signal RF1 rises and the signal at node 37 rises. As shown in FIG. 7, signals RASR and XPR are synchronized with the signal RAS and are an active timing signal and a reset precharge timing signal, which are isolated from the part of the internal refresh control clock generating circuit (109) in FIG. 9. A signal at the node 37 rises only during the active period of the RAS. A buffer circuit comprised of MOSTs Q.sub.80 to Q.sub.89 responds to the rise of the signal at the node 37, so that a signal at a node 42 rises to reach V.sub.DD level. At the time that the level of the node 42 rises, a signal at node 40 shifts to ground potential and the MOST Q.sub.86 become nonconductive. And the node 42 is isolated from the signal RF1 and it is kept at the V.sub.DD level during the remaining active period of RAS. A MOST Q.sub.90 charges a node 43 to a (V.sub.DD -threshold voltage) level. At this time, a node 46 also has the same level by a signal RASR. With a MOST Q.sub.94 having a much larger current ability than a MOST Q.sub.93, the signal RFAA1 is kept at low level. When the signal RAS is reset and enters the reset precharge period, the signal RAS and RASO in FIG. 7 shift to low level, so that the reset precharge operation starts. At the same time, the signal RASR shifts to low level and the signal XPR rises to the V.sub.DD level. The node 42 shifts to ground potential by the XPR while the node 43 is still kept at the (V.sub.DD -threshold voltage) dynamically. Upon the completion of the reset precharge operation, the confirmation signal PEND is issued and the node 46 shifts to low level, so that the MOST Q.sub.94 becomes nonconductive the signal RFAA1 rises to reach the V.sub.DD level. In FIG. 7, the MOST Q.sub.3 conducts to shift the node 1 to low level so that the MOST Q.sub.7 is nonconductive to isolate the signal RAS from the node 2. The MOST Q.sub.9 also conducts and the node 2 shifts to ground potential, and further the signal RAS rises so that the RAS series clock generating circuit 115 enters the active period. The row decoder 116 is constructed as shown in FIG. 15 and provided with the multiplexer 117 so that either the output of the address inverter buffer 118 or the output of the refresh address counter 113 serves as a decoder input. Upon rise of the signal RFAA1, a node B2 becomes ground potential, the MOST QQ.sub.5 becomes nonconductive so that the address inverter output and the decoder input are isolated from one another, while at the same time the output from the refresh address counter 113 is transferred through the MOST QQ.sub.6 to the decoder input. As a result, the memory cells on the selected word line are refreshed. As shown in the same figure, the rise of the signal RFAA1 restricts an output CAS of the first stage of the CAS series clock generator circuit 108 to be low level, so that the CAS series clock generating circuit 108 can not operate and only the refresh operation at the row side is performed. When the refresh operation completes and the signal AEND rises in level, the signal at the node 48 in FIG. 10 follows the rise of the signal AEND, so that MOSTs Q.sub.91 and Q.sub.95 conduct to shifts the signal RFAA1 to ground potential. In response to a change of the RFAA1 to the low level, the refresh address counter 113, a first and a second stages CNT1 and CNT2 which are shown in FIG. 14, is caused to increment while at the same time it causes the signal RFPP1 of the circuit 109 shown in FIG. 11 to rise in level. In FIG. 7, the node 2, while being isolated from the RAS, is raised to the (V.sub.DD -threshold voltage) level by the MOST Q.sub.8, the signal RAS shifts to low level, and the RAS series clock generating circuit 115 enters the reset precharge period. Upon the completion of the reset precharge operation, the circuit operation for enabling the signal RFSH during the active period of RAS is completed.

When the RFSH is enabled during the reset period of the RAS in the mode (3), a signal at node 57 in FIG. 12 rises in level following the signal RF1, the signal level at a node 62 is charged to the (V.sub.DD -threshold voltage), and the signal levels at the node 61 and 65 shift to low level. Then, the signal RFAA2 rises to the V.sub.DD level. Following the rise of the signal level at the node 57, the signal level at the node 61 shifts to ground potential. The signal level at the node 65 goes to ground potential at the time that the reset precharge is completed. With the rise of the signal RFAA2, the signal level at the node 2 in FIG. 7 shifts to low level, the signal RAS rises and the RAS series clock generating circuit 115 enters the active period. As in the case of the signal RFAA1, cells on the word line designated by the refresh address counter 113 are refreshed, and the operation is completed to rise the signal AEND. As a result, the signal RFAA2 shifts to ground potential, as shown in FIG. 12. Immediately after this, the signal RFPP2 rises. Upon the level change of the signal RFAA2 to low level, the content of the refresh address counter 113 is incremented while the signal level at the node 1 in FIG. 7 rises to the (V.sub.DD -threshold voltage) level. Then, the signal RAS of high level appears at the node 2 through the MOST Q.sub.7. Accordingly, the RAS series clock generating circuit 115 enters the reset precharge period and the operation ends. At this time, the circuit operation in the case of the mode (3) terminates.

The AUTO REFRESH which is introduced when the signal RFSH of the mode (4) is enabled for a given period of time or longer, is controlled by the circuit shown in FIG. 13. As shown in FIG. 13, the auto refresh timing generator circuit 112 is made of a counter including a plurality stages of master-slave flip-flop circuits 112-0-112-n. During the period of time that the signal RFSH is in high level, the counter is set to the initial condition and it shifts to low level to be enabled. At this time, the output signal from the oscillator circuit 111 is transferred to MOST QHO. The output ARCYC from the final stage 112-n of the counter determines the timing of the AUTO REFRESH and at the initial stage the RFSH shifts to low level and after a given time the ARCRC shifts to low level. At this time, the signal ARO rise and the signal level at the node 2 in FIG. 7 shifts to ground potential. As a result, the RAS series clock generating circuit enters the active period. The refresh operation in this case is quite similar to that of the case of the signal RFAA1 for RFAA2. After the refresh operation completes and the signal AEND rises, the ARO in FIG. 13 shifts to low level, and the signal level at the node 2 rises. Accordingly, the RAS series clock generating circuit 115 enters the reset precharge period. Subsequently, it is left in the reset precharge state until the signal ARCYC changes from high level to low level. So long as the signal RFSH is kept at low level, the RF1 is in high level and the signal level at node 1 in FIG. 7 is in low level. The signal RAS is isolated from the node 2. The operation of the RAS series clock generating circuit 115 is controlled by the signal ARO and the signal ARO. Since the signal ARCYC is a counter responds for the output signal from the oscillator circuit 111, it takes a waveshape with a given cycle during the period the signal RFSH is in low level. The internal refresh automatically continues.

As described above, in the internal refresh circuit according to the invention, when receiving a refresh control signal RFSH enabled during the active period of the row strobe signal RAS, the memory circuit latches it by an internal circuit. Then row strobe signal RAS is reset, and the circuit enters the reset precharge period. The latched state of RFSH is kept until a signal for confirming the end of the reset precharge operation is produced. When the confirmation signal rises, the circuit receives the latch signal to automatically enter the internal refresh period. As a result, memory cells on the word line correponding to the row address designated by the refresh address counter. When the internal refresh operation ends, a confirmation signal is produced and the circuit automatically enters the reset precharge period. Subsequently, the circuit keeps the reset precharge state. The internal refresh is applicable for the refresh control signal which is enabled during the active period of the row strobe signal RAS in the dynamic RAM of the two-clock, multiaddress system. A flexible use of the RAM is attained in the practical use.