Refresh control for semiconductor memory device

In the operation cycle, memory chip 200 initiates a refresh operation in sync with an ATD signal indicating change of address after a refresh timing signal RFTM has been issued. In snooze mode (low power consumption mode), a refresh operation is initiated in response to generation of a refresh timing signal RFTM, regardless of the ATD signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to refresh control in a semiconductor memory device.

2. Description of the Related Art

Commonly used semiconductor memory devices include DRAM and SRAM. It is common knowledge that while DRAM offers higher capacity at lower price than SRAM, it requires refresh operations. SRAM, on the other hand, while easier to use due to the lack of a need for refresh operations, is more expensive and has lower capacity than DRAM.

Virtual SRAM (known as VSRAM or PSRAM) is a semiconductor memory device that offers those advantages of both DRAM and SRAM. Virtual SRAM has a memory cell array composed of dynamic memory cells identical to those in DRAM, and also houses a refresh controller allowing refresh operations to be performed internally. Thus, external devices connected to virtual SRAM (such as a CPU) can access (i.e., read or write data) virtual SRAM without being aware of refresh operations. This feature of virtual SRAM is known as refresh transparency.

Certain types of virtual SRAM can assume a plurality of operating modes, such as a normal operating mode and a power saving mode. The issue of how internal refresh operations should be performed in virtual SRAM in each of these operating modes has not been given adequate consideration. This problem is not limited to so-called virtual SRAM, and is a problem common to all types of dynamic semiconductor memory devices that have an internal refresh timer and refresh controller.

SUMMARY OF THE INVENTION

An object of the present invention is to enable refresh operations to be carried out optimally in each of a plurality of available operating modes of a semiconductor memory device.

In order attain at least part of the above and related objects of the present invention, there is provided a semiconductor memory device having a plurality of operating modes. The semiconductor memory device comprises a memory cell array having dynamic memory cells, a refresh controller, an operating mode input terminal, address input terminals, and an address transition detecting circuit. The refresh controller have a refresh timer for generating a refresh timing signal used to determine timing for performing refresh operations on the memory cell array. The refresh controller performs refresh operations on the memory cell array in response at least to the refresh timing signal. The operating mode input terminal receives from an external device an operating mode signal specifying one of the plurality of operating modes of the semiconductor memory device. The address input terminals receives a multiple-bit address signal from the external device. The address transition detecting circuit detects changes in the multiple-bit address signal to generate an address transition signal.

The refresh controller performs internal refresh operations in two refresh modes. In an event that the operating mode signal specifies a first operating mode, the refresh controller performs an internal refresh operation according to a first refresh mode wherein a refresh operation on the memory cell array is initiated in sync with the address transition signal after the refresh timing signal has been generated. In an event that the operating mode signal specifies a second operating mode, on the other hand, the refresh controller performs an internal refresh operation according to a second refresh mode wherein a refresh operation on the memory cell array is initiated in response to generation of the refresh timing signal, regardless of the address transition signal.

In first operating mode, an internal refresh operation is initiated in sync with the address transition signal, thereby facilitating arbitration of access operations by external devices and internal refresh operations by the refresh controller. In second operating mode, an internal refresh operation is initiated in response to generation of a refresh timing signal, regardless of the address transition signal, thereby enabling internal refresh operations even in the absence of address input, for example. Thus, refresh operations in this semiconductor memory device are performed in suitable ways for each of the plurality of operating modes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of the invention are described in the following order.

A. Summary of memory chip pinout and operating modes

B. Overall circuit configuration

C. Internal configuration and operation of refresh controller

D. Whole chip refresh operation

E. Application in an electronic device

A: Summary of Memory Chip Pinout and Operating Modes

FIG. 1 is an illustrative diagram showing the pinout of a memory chip 300 pertaining to an embodiment of the invention. Memory chip 300 has the following terminals.

In the following discussion the same symbols are used to denote both the terminals and the signals. A preceding a terminal name (or signal name) denotes negative logic. While a plurality of address input terminals A 0 -A 19 and I/O data terminals IO 0 -IO 15 are provided, these are depicted in simplified form in FIG. 1 .

Memory chip 300 is configured to function as a virtual SRAM (VSRAM) so as to enable access in substantially the same manner as ordinary asynchronous SRAM. Memory chip 300 has an integrated refresh controller that includes a refresh timer 70 . Herein, data read and write operations by external devices shall be referred to as external access and refresh operations performed by the internal refresh controller shall be referred to as internal refresh operations or simply refresh operations.

Within memory chip 300 is provided an address transition detecting circuit 110 for detecting any change of one bit or more in I/O address A 0 -A 19 . As will be described later, the internal refresh controller controls external access operations and internal refresh operations in sync with an address transition signal generated by ATD circuit 110 . Hereinafter the address transition detecting circuit 110 shall be referred as the ATD circuit and the address transition signal as the ATD signal.

Chip select signal CS and snooze signal ZZ are used to control the operating mode of chip 300 . FIG. 2 shows the operating modes of memory chip 300 associated with different levels of chip select signal CS and snooze signal ZZ. Herein, H level refers to 1 level of a binary signal and L level refers to 0 level.

When chip select signal CS is L level (active) and snooze signal ZZ is H level, a read/write operation cycle (hereinafter referred to simply as operation cycle or read/write cycle ) is performed. Reading of data from and writing of data to chip 300 are performed during the read/write cycle.

When chip select signal CS and snooze signal ZZ are both H level, a standby cycle is performed. In the standby cycle all word lines are in the inactive state. However, word lines indicated by refresh addresses are activated during internal refresh operations.

When chip select signal CS is H level (inactive) and snooze signal ZZ is L level, memory chip 300 goes into snooze mode (also termed power down mode ). In snooze mode all circuits not needed for refresh operations are halted. Since power consumption in snooze mode is very low, it is suited to backup of data held in memory.

Refresh operations are performed according to first refresh mode in the operation cycle and refresh cycle, and according to second refresh mode in snooze mode. In first refresh mode, refresh operations are initiated in sync with the ATD signal after the refresh timer 70 has generated a refresh timing signal. In second refresh mode, refresh operations are initiated instantaneously when the refresh timer 70 generates a refresh timing signal. Since refresh operations in second refresh mode are not synchronized with the ATD signal, no address input is needed. In this way the memory chip 300 performs refresh operations according to refresh modes suitable for each of the three operating modes. Refresh operations in the two refresh modes will be described in greater detail later.

As will be apparent from the preceding description, chip select signal CS and snooze signal ZZ act as operating mode signals specifying an operating mode for the semiconductor memory device in the present invention.

Signals other than the three signals CLK, CS, ZZ described above are substantially identical to those used in ordinary memory chips. Address A 0 -A 19 enables 20-bit megaword addressing. I/O data IO 0 -IO 15 represent 16-bit word data. That is, one value of address A 0 -A 19 corresponds to 16 bits (1 word), enabling 16 bits of I/O data IO 0 -IO 15 to be input or output at any one time.

In the operation cycle, when the write enable signal WE goes to L level a write cycle is performed, and when it goes to H level a read cycle is performed. When output enable signal OE goes to L level, output by I/O terminals IO 0 -IO 15 is enabled. Lower byte enable signal LB and upper byte enable signal UB are control signals for performing a read or write of only one byte, either the lower byte or the upper byte of a word ( 16 bits). For example, when lower byte enable signal LB is set to L level and upper byte enable signal UB is set to H level, only the lower 8 bits of a word will be read or written. The power supply terminals are not shown in FIG. 1 .

FIGS. 3 ( a )- 3 ( h ) are timing charts summarizing operation of the memory chip 300 of the Embodiment. Which of the three states in FIG. 2 (operation, standby, or snooze) the device is in is determined in response to changes in the chip select signal CS and snooze signal ZZ. The first three cycles shown in FIG. 3 make up an operation cycle. In the operation cycle, either a read (read cycle) or write (write cycle) is performed depending on the level of write enable signal WE. One cycle period Tc of ATD signal (i.e. the shortest period of change in address A 0 -A 19 ) is the same as the cycle time (also termed cycle period ) of memory chip 300 . Cycle time Tc is set in the range of about 50 ns to about 100 ns, for example.

In the fourth cycle in FIGS. 3 ( a )- 3 ( h ), chip select signal CS rises to H level whereupon standby cycle is initiated. In the fifth cycle, snooze signal ZZ additionally drops to L level, whereupon memory chip 300 enters snooze mode.

B. Overall Circuit Configuration

FIG. 4 is a block diagram showing the internal configuration of memory chip 300 . Memory chip 300 comprises a data I/O buffer 10 , a memory array 20 , and an address buffer 60 . Memory array 20 is divided into four blocks 20 A- 20 D. The first block 20 A has a memory cell subarray 22 A, a row decoder 24 A, and a column decoder 26 A. The other blocks 20 B 20 D are analogous in configuration. As the configuration of blocks 20 A 20 D is the same, the following discussion shall focus principally on the first block 20 A and its related circuitry.

The arrangement of a block 20 A is analogous to that of an typical DRAM memory cell array. Subarray 22 A includes a plurality of 1-transistor/1-capacitor type memory cells in a matrix array. Each memory cell is connected to a word line and a pair of bit lines. A precharge circuit, sense amp, etc. (not shown) are also provided in subarray 22 A. Row decoder 24 A selects and activates one of the plurality of word lines in subarray 22 A. Column decoder 26 A simultaneously selects bit line pairs equivalent to one word (16 bits) from among the plurality of bit line pairs in subarray 22 A. Thus, input of a single address A 0 -A 19 by an external device enables simultaneous access of 16 bits (1 word) of data in one block.

Address buffer 60 shown in the bottom left corner of FIG. 4 inputs a 20-bit address A 0 -A 19 . In sync with the ATD signal supplied by ATD circuit 110 , address A 0 -A 19 is latched in address buffer 60 and supplied to other circuits. The lower 2-bit address A 0 -A 1 is used as a block address for selecting one of the four blocks 20 A- 20 D. The next 6-bit address A 2 -A 7 above block address A 0 -A 1 is used as the column address, and the remaining 12-bit address A 8 -A 19 is used as the row address. Thus, one of the four blocks 20 A- 20 D is selected by means of block address A 0 -A 1 , and from the selected block one word (16 bits) of data is selected by means of column address A 2 -A 7 and row address A 8 -A 19 . The selected word of data is either read or written via data I/O buffer 10 .

To the one block 20 A are connected, in series, a row predecoder 30 A, a block controller 40 A, and a refresh request signal generating circuit 50 A. The other blocks 20 B- 20 D are of the same configuration. Within memory chip 300 are additionally provided a refresh timer 70 , a refresh counter controller 90 , a refresh counter 100 , and an ATD circuit 110 .

Refresh timer 70 is a circuit that generates a refresh timing signal RFTM according to a predetermined refresh cycle. Refresh timer 70 is composed of a ring oscillator, for example. The refresh cycle is set to about 32 s, for example.

ATD circuit 110 detects whether there is any change of one bit or more in an I/O address A 0 -A 19 supplied by an external device; if a change is detected, it generates an ATD signal. FIG. 5 is a block diagram showing the internal configuration of the ATD circuit 110 . ATD circuit 110 comprises 20 transition detection circuits (corresponding to the number of bits of the I/O address A 0 -A 19 ) 111 and a 20-input OR gate 118 . Each transition detection circuit 111 has an inverter 112 , two pulse generating circuits 113 , 114 , and an OR gate 115 . One-shot multivibrators may be used for the pulse generating circuits 113 , 114 .

The first pulse generating circuit 113 generates a single pulse having a predetermined pulse width in response to the rising edge of address bit A 0 . Inverter 112 and the second pulse generating circuit 114 generate a single pulse having a predetermined pulse width in response to the falling edge of address bit A 0 . Accordingly, OR gate 115 outputs a single pulse for each of rising and falling edges of address bit A 0 . The procedure is the same for the other address bits A 1 -A 19 .

20-input OR gate 118 has as input the outputs of the 20 transition detection circuits. Thus, if there is any change in level in one or more of the bits of the 20-bit I/O address A 0 -A 19 , a pulsed ATD signal is output from the OR gate 118 . As shown in FIG. 4 , this ATD signal is supplied to an address buffer 60 and refresh request signal generating circuits 50 A- 50 D.

Refresh request signal generating circuits 50 A- 50 D generate refresh request signals RFREQ 0 -RFREQ 3 for each block 20 A- 20 D in response to a refresh timing signal RTFM supplied by refresh timer 70 . These refresh request signals RFREQ 0 -RFREQ 3 are provided respectively to the corresponding block controllers 40 A- 40 D.

Block controllers 40 A- 40 D are supplied with refresh request signals RFREQ 0 -RFREQ 3 as well as with a block address A 0 -A 1 supplied by an external device. Refresh request signals RFREQ 0 -RFREQ 3 indicate that refresh operations should be initiated in the four blocks 20 A- 20 D. In the operation cycle, the block address A 0 -A 1 indicates for which of the four blocks 20 A- 20 D external access is requested. In response to signals RFREQ 0 -RFREQ 3 and block address A 0 -A 1 , block controllers 40 A- 40 D arbitrates external access and internal refresh operations for the four blocks. Specifically, the arbitration is exercised by setting the levels of external access signals EX 0 - EX 3 and refresh signals RF 0 - RF 3 .

In response to the signal levels of external access signals EX 0 - EX 3 and refresh signals RF 0 - RF 3 , row predecoders 30 A- 30 D each select either a row address A 8 -A 19 input from the external device or a refresh address RFA 8 -RFA 19 supplied by refresh counter 100 , and supply it to a row decoder 24 A- 24 D. Address selection is performed independently by each row predecoder. For example, in the event that a request for external access to first block 20 A is made at the same time as a refresh request, the first row predecoder 30 A selects row address A 8 -A 19 and supplies it to the first block 20 A, while the other row predecoders 30 B- 30 D select refresh address RFA 8 -RFA 19 and supply it to the corresponding blocks 20 B- 20 D.

The configuration and operation of refresh request signal generating circuits 50 A- 50 D, block controllers 40 A- 40 D, and row predecoders 30 A- 30 D will be discussed later.

Refresh counter controller 90 detects whether refresh operations for the same given refresh address have been completed in all four blocks 20 A- 20 D. As will be described later, detection is accomplished by checking for changes in the levels of the four refresh request signals RFREQ 0 -RFREQ 3 . When refresh operations in all four blocks 20 A- 20 D have been completed the refresh counter controller 90 supplies a countup signal CNTUP to refresh counter 100 . In response to countup signal CNTUP, refresh counter 100 then increments by 1 the value of refresh address RFA 8 -RFA 19 .

In addition to the circuits depicted in FIG. 4 , memory chip 300 also has a controller for controlling the operating mode of circuits in the chip according to chip select signal CS and snooze signal ZZ, and another controller for controlling I/O status according to the enable signals WE, OE, LB, and UB; however, for convenience these have been omitted in FIG. 4 .

Of the circuits depicted in FIG. 4 , circuitry other than the memory cell array 20 , data I/O buffer 10 , address buffer 60 , and ATD circuit (i.e., 30 A- 30 D, 40 A- 40 D, 50 A- 50 D, 70 , 90 and 100 ) as a whole act as the refresh controller in the present invention. In particular, the circuitry composed of row predecoders 30 A- 30 D, block controllers 40 A- 40 D, and refresh request signal generating circuits 50 A- 50 D has the function of arbitration circuitry for arbitrating internal refresh and external access operations.

C. Internal Configuration and Operation of Refresh Controller

FIG. 6 is a block diagram showing the internal configuration of refresh request signal generating circuit 50 A and block controller 40 A. The other refresh request signal generating circuits 50 B- 50 D and other block controllers 40 B- 40 D have the same configuration.

Refresh request signal generating circuit 50 A comprises an inverter 52 , a NAND gate 54 , a pulse generating circuit 55 , two latching circuits 56 , 68 and an AND gate 57 .

One of the input terminals of NAND gate 54 has snooze signal ZZ as input, while the other input terminal has as input the ATD signal which has been inverted by inverter 52 . The output Q 54 of NAND gate 54 is provided as input to AND gate 57 .

Pulse generating circuit 55 generates a pulse signal Q 55 in response to the rising edge of refresh timing signal RFTM. This pulse generating circuit 55 consists, for example, of a one-shot multivibrator. Pulse signal Q 55 is presented to the set input terminal of first latch 56 . The output Q 56 of first latch 56 is input to AND gate 57 together with the output Q 54 of NAND gate 54 . The output Q 57 of AND gate 57 is supplied to the set input terminal of the second latch 58 . The output Q 58 of second latch 58 is supplied as a refresh request signal RFREQ 0 to block controller 40 A, and is also input to the reset input terminal of first latch 56 .

Block controller 40 A comprises an external access signal generating circuit 42 , a refresh signal generating circuit 44 , and a pulse generating circuit 46 . External access signal generating circuit 42 has as inputs the chip select signal CS, block address A 0 -A 1 , and the ATD signal. The refresh signal generating circuit 44 has as inputs CS, A 0 -A 1 , and ATD, as well as the refresh request signal RFREQ 0 .

At the rising edge of the ATD signal, external access signal generating circuit 42 decides if there is a request for external access to the block 20 A associated with block controller 40 A. This decision is made on the basis of chip select signal CS and block address A 0 -A 1 . Specifically, when chip select signal CS is L level (active) and block address A 0 -A 1 is 00 , it is decided that there is a request for external access to block 20 A. At this time external access signal generating circuit 42 sets the external access signal EX 0 to L level (active). In the absence of a request for external access to block 20 A, external access signal generating circuit 42 sets the external access signal EX 0 to inactive (H level).

Like external access signal generating circuit 42 , refresh signal generating circuit 44 decides whether there is a request for external access to the block 20 A. In the absence of a request for external access to block 20 A, if there is a refresh request, refresh signal generating circuit 44 sets refresh signal RF 0 to L level (active). In the absence of both a request for external access and a refresh request, refresh signal RF 0 is set to H level (inactive). When refresh signal RF 0 becomes active, a refresh operation in block 20 A is initiated.

In the event of a request for external access to block 20 A, refresh signal RF 0 is set to H level (inactive) even if there is a refresh request. Refresh signal RF 0 is subsequently held at H level until external access of block 20 A is completed, and once external access is complete it is set to L level (active). There are two instances of completion of external access of block 20 A: a first instance wherein there is a request for external access to a block other than block 20 A, and a second instance wherein chip select signal CS goes to H level (inactive) and standby mode is assumed. At each rising edge of the ATD signal, refresh signal generating circuit 44 checks which of these two instances has occurred, and at the point in time at which either has occurred, sets the refresh signal RF 0 to L level (active). In this way, at the point in time that refresh signal RF 0 becomes active, the refresh operation in block 20 A is initiated.

Pulse generating circuit 46 generates a short-pulse reset signal RESETO in response to the rising edge of refresh signal RF 0 . This pulse generating circuit 46 consists, for example, of a one-shot multivibrator. Reset signal RESETO is presented to the reset input terminal of second latch 58 .

FIGS. 7 ( a )- 7 ( m ) are timing charts showing initiation of a refresh operation in the standby cycle. At time t 1 , a refresh timing signal RFTM (FIG. 7 ( f )) supplied by refresh timer 70 rises to H level, whereby refresh request signal generating circuit 50 A is notified of the need to initiate a refresh operation. When refresh timing signal RFTM rises to H level at time t 1 , a pulse signal Q 55 (FIG. 7 ( g )) is generated, in response to which the output Q 56 (FIG. 7 ( h )) of first latch 56 also rises to H level.

Subsequently, address A 0 -A 19 (FIG. 7 ( d )) changes and an ATD signal is generated (FIG. 7 ( a )). As a general rule, the I/O address A 0 -A 19 does not change during the standby cycle. However, as noted with reference to FIG. 2 , the standby cycle employs a first refresh mode wherein refresh operations are synchronized with the ATD signal. In order to cause the ATD signal, an external device periodically changes at least one address bit (A 0 , for example) during the standby cycle, and internal refresh operations are performed accordingly. In preferred practice, the period for the change in the address bit will be no more than one half of the refresh period stipulated by the refresh timing signal RFTM. The reason is that if the address bit changes at an interval equivalent to one half or less of the refresh period, the ATD signal will be generated at least once while the refresh timing signal RFTM is H level.

At time t 2 of the next rising edge of the ATD signal, the output Q 56 (FIG. 7 ( h )) of first latch 56 is maintained at H level. Accordingly, the output Q 57 (FIG. 7 ( i )) of AND gate 57 rises to H level in response to the change in the output Q 54 of NAND gate 54 , setting the second latch 58 . As a result, the output of second latch 58 , namely refresh request signal RFREQ 0 (FIG. 7 ( j )) rises to H level.

Further, since both of chip select signal CS and snooze signal ZZ are at H level at time t 2 , a standby cycle is performed subsequent to time t 2 so no external access is performed. Thus, external access signal generating circuit 42 maintains external access signal EX 0 (FIG. 7 ( k )) at H level (inactive).

In response to refresh request signal RFREQ 0 , refresh signal generating circuit 44 sets refresh signal RF 0 (FIG. 7 ( l )) to L level (active). Once enough time for the refresh operation has subsequently elapsed, refresh signal generating circuit 44 brings refresh signal RF 0 back up to H level (inactive). In response to the rising edge of refresh signal RF 0 , pulse generating circuit 46 generates a reset signal RESET 0 pulse (FIG. 7 ( m )). This reset signal RESET 0 is presented to the reset input terminal of second latch 58 , so refresh request signal RFREQ 0 returns to L level (inactive) in response to the reset signal RESET 0 pulse.

Since refresh request signal RFREQ 0 is also input to the reset input terminal of first latch 56 , this latch 56 resets in response to the rise in refresh request signal RFREQ 0 (FIG. 7 ( h )).

FIGS. 8 ( a )- 8 ( m ) are timing charts for initiation of a refresh operation in the operation cycle. Since data write and read operations are enabled in the operation cycle, address A 0 -A 19 changes frequently. Accordingly, ATD signal pulses are generated frequently in response to changes in address A 0 -A 19 .

When refresh timing signal RFTM (FIG. 8 ( f )) rises to H level at time t 11 , refresh request signal RFREQ 0 (FIG. 8 ( j )) rises to H level at the time t 12 of the next rising edge of the ATD signal. Operation up to this point is analogous to that in the standby cycle depicted in FIGS. 7 ( a )- 7 ( m ).

In the example of FIGS. 8 ( a )- 8 ( m ), chip select signal CS is L level at time t 12 , and thus the operation cycle is performed subsequent to time t 12 . Here, the block address BA ( A 0 -A 1 ) value is 00 and external access to first block 20 A is requested. Accordingly, external access signal generating circuit 42 ( FIG. 6 ) sets external access signal EX 0 (FIG. 8 ( k )) to L level (active), while refresh signal generating circuit 44 maintains refresh signal RF 0 (FIG. 8 ( l )) at H level (inactive) for a while.

At the time t 13 of the next rising edge of the ATD signal, since external access of block 20 A is ongoing, there is no change in the level of external access signal EX 0 or of refresh request signal RFREQ 0 . At the time t 14 of the next rising edge, block address BA changes to a value 01 indicating external access to second block 20 B. Thus, in the cycle subsequent to time t 14 the external access signal EX 0 for first block 20 A goes to H level (inactive), and refresh request signal RFREQ 0 goes to L level (active). As a result, in the cycle subsequent to time t 14 the refresh operation is performed on the first block 20 A. The refresh operation for the entire chip will be described later.

As noted, in standby cycle or operation cycle, once notified by means of refresh timing signal RFTM of the need to perform a refresh operation, a refresh request signal RFREQ 0 is generated in sync with the ATD signal, in response to which a refresh operation is initiated.

FIGS. 9 ( a )- 9 ( m ) are timing charts showing initiation of a refresh operation in snooze mode. In snooze mode, snooze signal ZZ is held at L level so the output Q 54 of NAND gate 54 is held at H level (FIG. 9 ( e )). Accordingly, refresh timing signal RFTM (FIG. 9 ( f )) rises at time t 21 , and in response the output Q 56 (FIG. 9 ( h )) of first latch 56 also rises, whereupon the output Q 57 (FIG. 9 ( i )) of AND gate 57 rises to H level. The output of the second latch 58 , i.e., refresh request signal RFREQ 0 (FIG. 9 ( j )) also rises to H level in response thereto. Since external access is not performed in snooze mode, refresh signal RF 0 always goes to L level (active) when refresh request signal RFREQ 0 goes to H level, and the refresh operation is initiated immediately. Operation subsequent to this point in time are the same as in FIGS. 7 ( a )- 7 ( m ).

As noted, in snooze mode, once notified by means of refresh timing signal RFTM of the need to perform a refresh operation, a refresh operation is initiated immediately on the four blocks 20 A- 20 D. Thus, in snooze mode refresh operations are enabled solely by means of circuits within memory chip 300 , without the need for an address A 0 -A 19 or the ATD signal.

FIGS. 10 ( a )- 10 ( m ) are timing charts of operations during a transition from snooze mode to the operation cycle. At time t 21 , a refresh operation has been initiated in snooze mode. During initiation of the refresh operation, behavior of the signals (FIGS. 10 ( e )- 10 ( m )) is the same as in FIGS. 9 ( e )- 9 ( m ). At time t 22 the refresh signal RF 0 (FIG. 10 ( l )) is set to L level (active), and in response a refresh operation on the first block 20 A is initiated.

In this example, chip select signal CS and snooze signal ZZ change at time t 22 , and the operation cycle begins. Since a refresh operation has already been initiated, external access is performed only after the refresh operation is finished. Specifically, external access signal EX 0 (FIG. 10 ( k )) goes to L level (active) after a delay Td from the time t 22 that the operation cycle begins.

The timing for the external access signal EX 0 in the operation cycle is set so as to be optimized by default for situations like that in FIGS. 10 ( a )- 10 ( m ). Specifically, in the operation cycle the external access signal EX 0 level is set such that external access is initiated after a predetermined delay Td from the time that the operation cycle begins (specifically, the time at which the chip select signal CS goes from H level to L level). The external access signals EX 1 - X 3 for the other blocks behave the same way.

In preferred practice, the cycle Tc of the memory chip 300 will be of sufficient duration that external access can be completed within one cycle Tc, even in a situation like that in FIGS. 10 ( a )- 10 ( m ). That is, the duration of cycle Tc will preferably be such that external access can be completed in the operation cycle, even in the event that an internal refresh operation is initiated just at the point in time at which the operation cycle begins. By so doing, external access can be completed within one cycle Tc, even in a situation like that in FIGS. 10 ( a )- 10 ( m ), obviating the need for external devices to be aware of internal refresh operations and allowing them to access the memory chip 300 at any time. This enhances refresh transparency.

FIG. 11 is a block diagram of the internal configuration of row decoder 30 A. Row decoder 30 A comprises two switch & latch circuits 34 , 36 and a decision circuit 38 . The other row decoders 30 B- 30 D have the same configuration.

Decision circuit 38 has as inputs the refresh signal RF 0 and the external access signal EX 0 supplied by block controller 40 A. When external access signal EX 0 is active (L level) the decision circuit 38 sets the latch signal LEX supplied to first switch & latch circuit 34 to active. In response to this latch signal LEX the first switch & latch circuit 34 latches the row address A 8 -A 19 input from the external device and supplies it to row decoder 24 A. At this time the latch signal LRF supplied to the second switch & latch circuit 36 is set to inactive, preventing output from the second switch & latch circuit 36 .

On the other hand, when the refresh signal RF 0 is active (L level), decision circuit 38 sets the latch signal LRF supplied to the second switch & latch circuit 36 to active. In response to this latch signal LRF the second switch & latch circuit 36 latches the refresh address RFA 8 -RFA 19 and supplies it to row decoder 24 A. At this time the latch signal LEX supplied to the first switch & latch circuit 34 is set to inactive, preventing output from the first switch & latch circuit 34 .

Block controller 40 A ( FIG. 6 ) is configured such that the external access signal EX 0 and refresh signal RF 0 for the same given block 20 A are never active at the same time. When both the external access signal EX 0 and refresh signal RF 0 are inactive, row predecoder 30 A does not supply an address to row decoder 24 A.

In this way, in response to the levels of the external access signal EX 0 and refresh signal RF 0 , row predecoder 30 A selects and supplies either a row address A 8 -A 19 supplied by an external device, or a refresh address RFA 8 -RFA 19 . Thus, when there is a request for external access to block 20 A, one word line in block 20 A is activated in response to row address A 8 -A 19 . When, on the other hand, there is no request for external access to block 20 A and a refresh operation is requested, the refresh operation is performed on a plurality of memory cells on one word line in block 20 A in response to refresh address RFA 8 -RFA 19 .

The operations depicted in FIGS. 7 ( a )- 7 ( m ), 8 ( a )- 8 ( m ), 9 ( a )- 9 ( m ), and 10 ( a )- 10 ( m ) are carried out analogously for the other blocks 20 B- 20 D. However, external access is performed only for the one block indicated by block address A 0 -A 1 ; in no event are two or more blocks externally accessed simultaneously. As will be apparent from the following description, however, refresh operations may be performed simultaneously on all four blocks 20 A- 20 D.

D. Whole Chip Refresh Operation

FIGS. 12 ( a )- 12 ( t ) are timing charts showing an overall chip refresh operation in the standby cycle. As noted with regard to FIGS. 7 ( a )- 7 ( m ), when the refresh timing signal RFTM (FIG. 12 ( e )) rises at time t 1 , the refresh request signal RFREQ 0 for block 20 A rises to H level in sync with the rising edge (time t 2 ) of the ATD signal (FIG. 12 ( a )). Refresh request signals RFREQ 1 -RFREQ 3 for the other blocks 20 B- 20 D also rise to H level at this same time. Since external access is not enabled in the standby cycle, the external access signals EX 0 - EX 3 for the four blocks 20 A- 20 D are held at H level (inactive) and the refresh signals RF 0 - 3 are set to L level (active). As a result, in all four blocks 20 A- 20 D, all memory cells on the nth word line indicated by the same given refresh address RFA 8 -RFA 19 (FIG. 12 ( t )) are refreshed.

When the refresh operation in the four blocks 20 A- 20 D is completed, the four refresh request signals RFREQ 1 -RFREQ 3 (FIGS. 12 ( f )- 12 ( i )) return to L level. In response to the change in level of refresh request signals RFREQ 1 -RFREQ 3 the refresh counter controller 90 ( FIG. 4 ) generates a countup signal CNTUP (FIG. 12 ( s )).

FIG. 13 is a block diagram of the internal configuration of refresh counter controller 90 . Refresh counter controller 90 comprises a 4-input NOR gate 92 , a NAND gate 94 , a delay circuit 96 , and an inverter 98 . The 4-input NOR gate 92 has as inputs the four refresh request signals RFREQ 1 --RFREQ 3 . The output Q 92 of the 4-input NOR gate 92 is input to one of the input terminals of the NAND gate 94 . Output Q 92 is also input to the other terminal of the NAND gate 94 , after being delayed by the delay circuit 96 and inverted by the inverter 98 . As will be apparent from this arrangement, the countup signal CNTUP output from NAND gate 94 is a pulse signal (FIG. 12 ( s )) that goes to L level after the four refresh request signals RFREQ 1 -RFREQ 3 drop to L level, and kept at L level during a delay interval of delay circuit 96 .

In response to countup signal CNTUP refresh counter 100 increments by 1 the refresh address RFA 8 -RFA 19 (FIG. 12 ( t )). Accordingly, the next refresh operation is performed on the (n 1)th word line.

Since blocks 20 A- 20 D are not externally accessed in the standby cycle, refresh operations may be performed simultaneously on all four blocks 20 A- 20 D.

FIGS. 14 ( a )- 14 ( t ) are timing charts of a whole chip refresh operation during the operation cycle. As noted with regard to FIGS. 8 ( a )- 8 ( m ), when the refresh timing signal RFTM (FIG. 14 ( e )) rises at time t 11 , the refresh request signal RFREQ 0 for block 20 A rises to H level in sync with the next rising edge (time t 12 ) of the ATD signal (FIG. 14 ( a )). Refresh request signals RFREQ 1 -RFREQ 3 for the other blocks 20 B- 20 D also rise to H level at this same time. At time t 12 , the block address A 0 -A 1 value is 00 and there is a request for external access to first block 20 A. Accordingly, external access signal EX 0 (FIG. 14 ( k )) for the first block 20 A is set to L level (active), while refresh signal RF 0 is held at H level (inactive). Operation up to this point is analogous to that in FIGS. 8 ( a )- 8 ( m ).

As regards the other blocks 20 B- 20 D for which external access is not requested at time t 12 , the external access signals EX 1 - EX 3 (FIGS. 14 ( l )- 14 ( n )) are held at H level (inactive) and the refresh signals RF 1 - RF 3 (FIGS. 14 ( p )- 14 ( r )) are set to L level (active). Accordingly, in the cycle beginning at time t 12 external access is performed on the first block 20 A while refresh operations are performed on the other three blocks 20 B- 20 D. Refresh request signals RFREQ 1 -RFREQ 3 for the other three blocks 20 B- 20 D return to L level upon completion of the refresh operation. The refresh request signal RFREQ 0 for block 20 A, in which the refresh operation has been postponed, is maintained at H level.

At the time of the next rising edge (time t 13 ) of the ATD signal, since external access of the first block 20 A is ongoing, block 20 A is not refreshed. Accordingly, refresh request signal RFREQ 0 is maintained at H level.

At the time of the next rising edge of the ATD signal (time t 14 ), block address BA ( A 0 -A 1 ) changes to a value 01 indicating external access to second block 20 B. External access signal generating circuit 42 now sets the external access signal EX 0 (FIG. 14 ( k )) for the first block 20 A to H level (inactive) and sets the refresh signal RF 0 (FIG. 14 ( o )) to L level (active) so that a refresh operation is performed. Thus, in the cycle subsequent to time t 14 only the first block 20 A is refreshed. Upon completion of the refresh operation the refresh request signal RFREQ 0 returns to L level.

When refresh operations in all four blocks 20 A- 20 D are completed and the four refresh request signals RFREQ 0 -RFREQ 3 return to L level, a countup signal CNTUP (FIG. 14 ( s )) is generated and the refresh address RFA 8 -RFA 19 (FIG. 14 ( t )) is incremented by 1.

Let it now be assumed that an external access of a given block 20 A continues for more than one refresh period (one period of refresh timing signal RFTM). In this instance the next refresh timing will occur prior to refresh address RFA 8 -RFA 19 being incremented. In this event the refresh operation for the nth word line will be repeated again for the four blocks 20 A- 20 D by the procedure shown in FIGS. 13 ( a )- 13 ( t ). That is, since the refresh address is incremented only after refresh operations for a given refresh address have been completed for all blocks, all word lines in the four blocks 20 A- 20 D will be refreshed in a reliable manner.

When a refresh operation is requested in the operation cycle, the refresh operation is delayed only for that block for which external access has been requested, with refresh operations being performed in the usual manner on the remaining three blocks. When external access of block for which external access has been requested is completed, a refresh operation is performed on that block. This procedure offers the following advantages.

A first advantage is that refresh transparency is ensured. As used herein, refresh transparency refers to a lack of delay of external access by the internal refresh operation, as perceived by an external device. That is, in the operation cycle the decision to execute external access or a refresh operation for each block is made in sync with the ATD signal. The refresh operation is completed within one cycle Tc. Accordingly, when external access is requested the external access may always be performed promptly without delay.

A second advantage is that in the event of prolonged external access to memory chip 300 , refresh operations may nonetheless be performed on all blocks provided that the block being externally accessed changes during this time. This second advantage has the effect of further enhancing refresh transparency. The preceding advantages do not require dividing the memory array 20 into four blocks; it is sufficient to divide it into at least two blocks. However, blocks for external access should be switched frequently. This may be achieved by assigning a 2-bit block address A 0 -A 1 that changes frequently. Typically, in a multiple-bit address it tends to be easier to change the lower bits. Accordingly, when assigning addresses identifying a plurality of blocks in a memory array it is typically preferable to assign to the block address the lowest bits of the multiple-bit address.

FIGS. 15 ( a )- 15 ( t ) are timing charts of the refresh operation for the whole chip in snooze mode. As noted with regard to FIGS. 9 ( a )- 9 ( m ), when the refresh timing signal RFTM (FIG. 15 ( e )) rises at time t 21 , the refresh request signals RFREQ 0 -RFREQ 3 for blocks 20 A- 20 D immediately rise to H level. Since external access is not performed in snooze mode, external access signals EX 0 - EX 3 for the four blocks 20 A- 20 D are maintained at H level (inactive), and refresh signals RF 0 - RF 3 drop to L level (active). As a result, memory cells on a given nth word line in blocks 20 A- 20 D are refreshed. Subsequent operation is analogous to that in the standby cycle shown in FIGS. 12 ( a )- 12 ( m ).

Thus, in snooze mode the timing for initiating refresh operations is not synchronized with the ATD signal; rather, the four blocks 20 A- 20 D are refreshed simultaneously as soon as a refresh operation initiation time is indicated by the refresh timing signal RFTM.

As set forth hereinabove, in operating mode the memory chip 300 of this embodiment simultaneously decides external access requests and internal refresh operation requests in sync with the ATD signal (i.e., in sync with changes in address A 0 -A 19 ), so there is no delay in external access. In snooze mode, when there is a refresh operation request by the refresh timing signal RFTM, all blocks 20 A- 20 D are refreshed, enabling refresh operations to be performed in the absence of an address A 0 -A 19 supplied by an external device.

That is, in the preceding embodiment the operation cycle and snooze mode employ different signals (the ATD signal and the refresh timing signal RFTM) for deciding the timing for initiating refresh operations, allowing refresh operations to be performed in a manner suitable for each mode. Specifically, in the operating cycle refresh operations are performed in a manner that ensures refresh transparency, while in snooze mode refresh operations are performed in a reliable manner in low power consumption operating mode.

E. Application in an Electronic Device

FIG. 16 is a perspective view of a mobile phone by way of an exemplary electronic device embodying the semiconductor memory device herein. Mobile phone 600 comprises a body 610 and a cover 620 . Body 610 is provided with a keyboard 612 , a liquid crystal display 614 , a speaker 616 , and an antenna 618 . Cover 620 is provided with a microphone 622 .

FIG. 17 is a block diagram of the electronics of mobile phone 600 . CPU 630 is connected via a bus line (not shown) to keyboard 612 , an LCD driver 632 for driving liquid crystal display 614 , a SRAM 640 , a VSRAM 642 , and an EEPROM 644 .

SRAM 640 is used as a high speed cache memory, for example. VSRAM 642 is used as a working memory for image processing, for example. The memory chip 200 of Embodiment 1 or the memory chip 300 of Embodiment 2 may be used for VSRAM 642 (referred to as virtual SRAM or pseudo SRAM). EEPROM 644 contains various settings for the mobile phone 600 .

When operation of mobile phone 600 is temporarily suspended, VSRAM 642 may be maintained in snooze mode. By so doing, internal refresh operations in VSRAM 642 will be performed automatically, enabling data in VSRAM 642 to be preserved. As the memory chip 300 has relatively large capacity, a further advantage is that large amounts of data such as video data can be held for extended periods.

In the preceding embodiment, refresh operations are performed according to a first refresh mode in the standby cycle; however, refresh operations may be performed according to a second refresh mode in the standby cycle. Alternatively, refresh operations may be performed according to yet another refresh mode different from the first and second refresh modes in the standby cycle.

In the first refresh mode, refresh operations are initiated immediately after generation of the refresh timing signal RFTM. However, an arrangement whereby a refresh operation is initiated after a predetermined time interval has elapsed after generation of the refresh timing signal RFTM is also possible. In general, in second refresh mode it is sufficient for a refresh operation to be initiated in response to generation of a refresh timing signal RFTM, regardless of an address A 0 -A 19 input or ATD signal.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and embodiment only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.