JPH08180695A - Semiconductor memory - Google Patents

Abstract

PURPOSE: To increase the speed of access in a NAND type mask ROM by precharging the word line of the next address based on the result of judgement about the difference between the address data of consecutive timing. CONSTITUTION: External address data is alternately taken into two systems of address registers of an address buffer 2 by clocks CKA, CKB and sorted to be sent to a column decoder 4 and a row decoder 3 respectively where data is successively read. When the next address judging circuit 10 judges that the present address data AD is different from the next address data NAD, it sends a precharge enable signal to the column and row decoders for precharge 11, 14 to select and precharge only the bit line and word line of the next address. Thus, the precharge of unnecessary lines is eliminated to reduce power consumption and to enable fast access.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device such as a mask ROM for storing data in a nonvolatile manner.

[0002]

2. Description of the Related Art A mask ROM is a memory cell array in which data is stored in a nonvolatile manner, an address buffer for fetching an external address for selecting data in the memory cell array, and bit line selection and word line selection of the memory cell array by the fetched address. It is composed of a decoder for performing, a sense amplifier for reading bit line data, and the like. The memory cell array stores data by setting a large number of memory MOS transistors arranged in an array to, for example, a depression (D) type or an enhancement (E) type according to data. The format of the memory cell array is
There are NOR type and NAND type.

In the mask ROM, while a certain bit line data is being read, the non-selected bit line is separated from the sense amplifier. At this time, if the non-selected bit line is discharged and the potential drops to the ground level, it takes time to raise the bit line to a predetermined level when the bit line is next selected. This delay in charging the bit line prevents high speed access of the mask ROM. In order to enable high-speed access to the mask ROM, a precharge method of charging the non-selected bit line to a predetermined level in preparation for the next access is effective (for example, Japanese Patent Laid-Open No. 5-14428).
(See No. 4).

There is a similar problem in word line selection. For example, in the case of a NAND memory cell, data reading is performed with the selected word line at 0 V and the remaining unselected word lines at VDD. Since the word line is connected to the gates of many memory cells, the load is large, and VDD to O
It takes time to drop to V. In the case of an E-type memory transistor, the threshold value is actually about 0.8V, so that there is a delay until the selected word line drops from VDD to 0.8V. In order to solve the delay in the word line, for example, it has been proposed that only the driving power supply for the word line is set to a low power supply of 3V.

[0005]

The method of precharging all the non-selected bit lines in order to realize the high-speed access of the mask ROM has a problem that the current consumption increases due to the precharge. In addition, the method of lowering the word line drive power supply against the delay of the potential transition of the word line is
Not so effective in reducing access time. Further NAND
In case of type ROM, if the word line power supply is lowered, a large number of unselected memories MO connected to the selected memory MOS transistors
Since the ON resistance of the S-transistor becomes large as a result, the data read performance deteriorates.

An object of the present invention is to provide a semiconductor memory device capable of high speed access by an improved word line bias system.

[0007]

According to the present invention, a memory cell array in which data is stored in a non-volatile manner, an address buffer for fetching an external address for selecting data in the memory cell array, and a bit of the memory cell array according to the fetched address. In a semiconductor memory device having a decoder for performing line selection and word line selection, and a sense amplifier for reading bit line data, the address buffer has at least two clocks with input terminals commonly connected and time-divisional address fetching. A determination unit that has a synchronous address register and determines whether or not the address data of successive timings held in the two address registers of the address buffer is different, and an address based on the determination result of this determination unit While reading the data Next is characterized in that a word line precharge means for selectively precharging only the word line address to be accessed.

In the present invention, preferably, the determining means detects a mismatch of address data held by the two-system address registers and generates a precharge enable signal, and the precharge
It is constituted by a transfer means which passes through the next address data among the address data held by the two systems of address registers under the control of the enable signal. The word line precharge means controls a precharge enable signal to decode the next address data, and a precharge row decoder selects one word line by the decode output and applies a predetermined bias to the selected word line. And a row selector for precharging.

[0009]

According to the present invention, of the many unselected word lines, only the word line selected by the next address is precharged. In such a selective word line precharge, in a semiconductor memory device of a type in which an address buffer has two systems of clock synchronous address registers, it is determined whether address data at successive timings held in these address registers is different. It becomes possible by doing. Further, according to the present invention, particularly in the case of the NAND type, high-speed access can be performed without degrading the data read characteristic, as compared with the conventional method of applying a low power supply potential to all the non-selected word lines.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the overall structure of a clock synchronous mask ROM according to an embodiment of the present invention. Mask ROM
As a basic configuration of the memory cell array, a memory cell array 1 for storing data in a nonvolatile manner, an address buffer 2 for fetching an external address for reading the data, a row decoder 3 for selecting a word line of the memory cell array 1, a column decoder for selecting a bit line 4, a column selector 5, a sense amplifier 6 for detecting read data from the memory cell array 1, and an output buffer 7. In this embodiment, the memory cell array 1 is of NAND type.

The address buffer 2, which will be described in detail later, is a clock-synchronized type having two systems of address registers and fetching external addresses A0, A1, ..., A15 by clock synchronization. In this embodiment, for convenience, a case where A0 to A8 are used as column addresses and A9 to A15 are used as row addresses will be described. In order to control the operation of the address buffer 2, there is provided a clock generation circuit 8 for generating necessary internal clocks CKA, CKB, A and B (these waveforms are shown in FIG. 10) from the external clock CK. .

In this embodiment, in order to selectively precharge only the bit line to be accessed next among the non-selected bit lines, a column decoder 4 and a column selector 5 are separately provided around the memory cell array 1. A precharge column decoder 11 and a precharge column selector 12 are provided, and a precharge bias circuit 13 that applies a bias to the bit line selected by the precharge column selector 12 is provided. Similarly, in order to selectively precharge only a word line to be selected next among the unselected word lines to a predetermined level, a row decoder 3 for precharge and a row selector 15 for precharge are provided separately from the row decoder 3. Is provided.

In order to select and precharge one of the unselected bit lines and unselected word lines, it is necessary to know the address to be accessed next. Therefore, a next address determination circuit 10 is provided which compares address data at successive timings held by two systems of address registers in the address buffer 2 to determine the difference and outputs the next address. Based on the determination result of the next address determination circuit 10, the precharge column decoder 11 and the precharge row decoder 1
4 means that one bit line and one word line are selected respectively.

FIG. 2 shows a specific structure of the address buffer 2. As shown in the figure, for each external address terminal, two systems of clock synchronous address registers 21a,
21b is provided with its input terminals commonly connected to the input circuit 2. Selectors 23 and 24 are provided to select the address data AD at the preceding timing and the next address data NAD at the next timing from the address data at the consecutive timings held in the address registers 21a and 21b. Has been. Further, from the two address registers 21a and 21b, determination address data ADA and ADB are taken out in order to determine the difference between the held address data. Note that, as the address data AD, complementary data are usually output in pairs, but they are simply represented by one here.

FIG. 3 shows the address registers 21a and 21b.
And the selectors 23 and 24 are shown more specifically. The address registers 21a and 21b are CMOS latch circuits combining clocked CMOS inverters. One address register 21a is driven by an internal clock CKA and its inverted clock / CKA, and the other address register 21b is an internal clock C.
KA and / CKA are driven by internal clocks CKB and / CKB which are 180 ° out of phase with each other. Therefore, these address registers 21a and 21b alternately take in and hold external addresses.

The selector 23 alternately drives the two clocks A and B, which are half-divided clocks of the external clock CK and have opposite phases A and B, in order to alternately take out the data of the two address registers 21a and 21b. It is composed of a inverter. The selector 23 alternately fetches the data held in the address registers 21a and 21b, thereby selecting the address data AD at the timing that always precedes the continuous timing. The other selector 24 is composed of two clocked inverters similarly driven by clocks A and B, and has address data (next address data) at a timing which always follows in continuous timing.
NAD will be selected.

Address data AD (AD0, AD1, ..., AD15) obtained from the above address buffer 2
Indicates that the column address AD0 to AD8 is the column decoder 4
Then, the row addresses AD9 to AD15 are respectively sent to the row decoder 3, and bit line selection and word line selection are performed according to the normal operation.

Along with the selected address data AD, the determination address data ADA and ADB of the two systems of address data held in the address buffer 2 and the next address data NAD are sent to the next address determination circuit 10. The next address determination circuit 10 determines whether or not the address data at successive timings are different from each other and determines the next address data N.
It performs AD transfer control and is configured as shown in FIGS.

FIG. 4 shows the EOR column 41 and the sum of the outputs of the EOR column 41 as logic gate means for detecting the mismatch of the determination address data ADA and ADB for each bit of the column address in the column address side determination circuit section. An OR gate 42 is provided. As a result, the precharge enable signal PE (BL) for the bit line which becomes "H" is issued only when the addresses at successive timings are different. Also, this precharge enable signal PE
A transfer circuit 43 is provided which passes through the next address data NAD output from the address buffer 2 and transfers it to the precharge column decoder 11 only when addresses at consecutive timings controlled by (BL) are different. There is.

The transfer circuit 43 uses the next address data NAD.
And a delay circuit 44 for giving a predetermined delay to the precharge enable signal PE (BL) of "H".
It has an AND gate 45 that passes only when. Delay circuit 4
4 is provided to allow the next address data NAD to pass through after the precharge enable signal PE (BL) is determined.

FIG. 5 shows the determination circuit section on the row address side.
The basic configuration is the same as that of FIG. 4, and an EOR column 51, an OR gate 52 that sums the outputs of the EOR column 51 to generate a precharge enable signal PE (WL) of a word line, and the precharge enable signal. It has a transfer circuit 53 that allows the next address data NAD to pass through by PE (WL).

The precharge column decoder 11 for precharging the bit line of the next address based on the output of the next address determination circuit 10 has a decoder body similar to the normal column decoder 4 as shown in FIG. 61
And a CMOS gate 62 for taking out the decoded outputs PS0, PS1, ... Only when the precharge enable signal PE (BL) is "H".

Similarly, the precharge row decoder 14 for precharging the word line of the next address based on the output of the next address determination circuit 10 is similar to the normal row decoder 3 as shown in FIG. Decoder main body 71
And a CMOS gate 72 which takes out its decoded output only when the precharge enable signal PE (WL) is "H". In addition, a delay circuit 73 is provided in the precharge enable signal PE (WL) fetch section. This is because the timing of the word line precharge by the precharge row decoder 14 is slightly delayed with respect to the word line selection by the row decoder 3 and, as a result, the precharge speed is increased while ensuring the data read characteristic. The specific operation will be described later.

FIG. 8 shows a specific circuit configuration around the bit line selection section. The column selector 5 selects the bit line BL of the memory cell array 1 based on the output of the column decoder 4 and connects it to the sense amplifier 6. As shown in the figure, a select transistor for selecting a group of bit lines and a group within the group. It is composed of a select transistor for selecting one bit line. The configuration of the precharge column selector 12 is similar, and the bit line selected by the next address among the non-selected bit lines is selected based on the output of the precharge column decoder 14. Then, the bit line selected by this next address is precharged by the precharge bias circuit 13.
The precharge bias circuit 13 has the same configuration as the sense amplifier 6.

FIG. 9 shows a specific circuit configuration around the word line selection section. One NAND cell of the memory cell array 1 includes, for example, eight NMOS transistors M as illustrated.
, M7, which are connected to the bit line BL via the select transistors S1 and S2. The memory MOS transistors M0, M1, ... Are set to D type or E type by a mask program. Similarly, each gate of a large number of NAND type cells formed in common serves as a word line WL, and select transistors S1 and S
The gates of 2 are also commonly connected in the horizontal direction and the select line SL is connected.
1 and SL2, these word lines and select lines are selected by the row decoder 3.

At the output section of the row decoder 3, buffer circuits 91, which are different between the select line selecting section and the word line selecting section,
92 is provided. The buffer circuit 91 of the select line selection unit has a delay circuit 94 in parallel with the buffer body 93 and an NMOS transistor 95 for lowering the output level by its delayed output. this is,
Initially, the potential of the selected select line is raised by the buffer body 93, and in the stable state, for example, 3 lower than VDD.
This is to keep it at about V. The buffer circuit 92 of the word line selection unit is composed of only the buffer main body in order to set the unselected word line to VDD.

The precharge row selector 15 driven by the output of the precharge row decoder 14 is an N for lowering the potential of the word line WL to a predetermined level.
It is composed of a MOS transistor 96. That is, N
In the case of an AND type ROM, when reading data, the selected word line is set to 0V and the remaining unselected word lines are set to VDD. Of the unselected word lines, the potential of the next accessed word line is about 3V in advance. It is the word line precharge in this embodiment that is lowered to. This precharge potential is applied to the buffer circuit 92 connected to one end of the word line,
N of the precharge row selector 15 connected to the other end
It is determined by the voltage division by the ON resistance of the MOS transistor 96, and may be set to a value as low as possible within a range in which an unselected memory MOS transistor can be kept in an ON state required for data reading.

The operation of the NAND type mask ROM configured as above will be described below. FIG. 10 is a timing chart of the basic operation. External address as shown,
Is supplied, the address data is alternately fetched into the two systems of address registers 21a and 21b of the address buffer 2 by the internal clocks CKA and CKB having opposite phases. The fetched address data is alternately selected by the selector 23 and is sent to the column decoder 4 and the row decoder 3 according to a normal operation, and the data is read sequentially.

As described above, the address buffer 2 has address data of two consecutive timings. As described with reference to FIG. 2, the address data AD to be accessed now is accessed next by the selector 24. Expected next address data NAD is selected and taken out. Then, when the next address determination circuit 10 determines that the address to be accessed is different from the next address, the bit line precharge enable signal PE (BL) and the word line precharge enable signal PE (WL ) Is output, and the next address data N
AD is passed through and sent to the precharge column decoder 11 and the precharge row decoder 14, respectively.

Then, while a certain bit line and word line are selected and data is being read, only the bit line of the next address among the unselected bit lines is precharged by the bias circuit 13, and similarly the unselected word line is selected. Of the lines, only the word line of the next address is selected by the precharge row selector 15 and precharged. The bit line precharge is set to a potential of about 3V where it should normally be 0V in the non-selected state. The precharge of the word line lowers the place where it should normally be at the power supply VDD level to 3V.

Here, the precharge of the word line of the next address is delayed by the delay circuit 73 shown in FIG. 7 for a predetermined time from the word line selection by the row decoder 3. The meaning will be described with reference to FIG. As shown in FIG. 11A, it is assumed that the row decoder 3 operates and the selected word line falls at timing t0 and the remaining unselected word lines rise. If the precharge of the word of the next address is started at the same timing t0, as is clear from the description of FIG.
At the same time, it is selected by the precharge row selector 15.

That is, at this time, the word line of the next address is charged from one end by the row decoder 3 and simultaneously discharged from the other end by the row selector 15. Therefore, as shown by the broken line in FIG. 11B, the rise is delayed and it takes time to reach 3V. On the other hand, if the delay circuit 73 delays the start of precharging until the timing t1, as shown by the solid line in FIG. 11 (b), initially the row decoder 3 only charges VDD, and therefore FIG. 11 (a). It exhibits the same rising characteristics as the other non-selected word lines shown in FIG. As a result, the high-speed read characteristic is not hindered even though the potential of the non-selected word line VDD is originally set to 3V. Further, the time required to reach the final precharge potential of 3V is shortened.

Further, when the word line precharged to 3V is selected next, it will be discharged to 0V. At this time, as is apparent from FIG. 9, the delay circuit 73 delays the timing when the transistor 96 of the precharge row selector 15 is turned off. Therefore,
Since the discharge from the other end continues for the time of the delay circuit 73 simultaneously with the discharge of the selected word line by the row decoder 3, the time until the precharge operation is stopped becomes 0V as compared with the case where the precharge operation is stopped at the same time as the selection of the word line. It will be shortened.

As described above, according to this embodiment, only the bit line to be accessed next is precharged without precharging all of a large number of unselected bit lines. Therefore, as compared with the method of precharging all the non-selected bit lines, useless power consumption is reduced and high-speed access becomes possible. Further, according to this embodiment, only the word line to be accessed next among the many non-selected word lines is precharged, which enables high speed access. Moreover, unlike the conventional method in which a low power supply potential is applied to all non-selected word lines, the non-selected word lines except the next address are driven by VDD, so that the data read characteristic is not deteriorated. Further, of the non-selected word lines, the timing for biasing only the non-selected word line of the next address to 3V is delayed with respect to the word line selection, and initially, like the other non-selected word lines, by the power supply VDD. By driving, high-speed access is possible while ensuring excellent read performance.

Although the mask ROM having the NAND type memory cell has been described in the embodiment, the present invention is not limited to this and can be similarly applied to the one having the NOR type memory cell. For bit line precharge,
The case of the NOR type memory cell is similar to that of the NAND type memory cell. The word line precharge is slightly different. In the NOR type, normally, VDD is applied to the selected word line and 0V is applied to the unselected word line. Therefore, regarding the word line precharge, the range where the memory MOS transistor is not turned on for the word line of the next address among the unselected word lines. A bias higher than 0 V may be applied. In addition to the mask ROM, the present invention further includes EPROM and EEPROM.
Can be similarly applied to.

[0036]

As described above, according to the present invention, in the semiconductor memory device of the type in which the address buffer has two systems of clock synchronous address registers, the address data at successive timings held in these address registers is used. The read performance is deteriorated by determining the difference between the two and precharging only the word line selected by the next address among a large number of unselected word lines based on the determination result. Without
High-speed access can be realized.

[Brief description of drawings]

FIG. 1 shows a configuration of a mask ROM according to an embodiment of the present invention.

FIG. 2 shows a specific configuration of an address buffer of the embodiment.

FIG. 3 shows a more specific configuration of the same address buffer.

FIG. 4 shows a configuration of a column address section in the next address determination circuit of the same embodiment.

FIG. 5 shows a configuration of a row address section in the next address determination circuit of the same embodiment.

FIG. 6 shows a specific configuration of a column decoder of the same embodiment.

FIG. 7 shows a specific configuration of the row decoder of the same embodiment.

FIG. 8 shows a specific configuration around a bit line selection unit in the same embodiment.

FIG. 9 shows a specific configuration around the word line selection unit of the same embodiment.

FIG. 10 shows an operation timing of the embodiment.

FIG. 11 shows how the word line potential changes in the same embodiment.

[Explanation of symbols]

1 ... Memory cell array, 2 ... Address buffer, 3 ... Row decoder, 4 ... Column decoder, 5 ... Column selector, 6 ... Sense amplifier, 7 ... Output buffer, 8 ... Clock generation circuit, 10 ... Next address determination circuit, 11 ... Precharge column decoder, 12 ... Precharge column selector, 13 ... Precharge bias circuit, 14 ... Precharge row decoder, 15 ... Precharge row selector, 21a, 21b ... Address register, 23,
24 ... Selector.

Claims (2)

[Claims] 1. A memory cell array in which data is stored in a nonvolatile manner, an address buffer for fetching an external address for selecting data in the memory cell array, and bit line selection and word line selection of the memory cell array are performed by the fetched address. In a semiconductor memory device having a decoder and a sense amplifier for reading bit line data, the address buffer has at least two systems of clock-synchronized address registers which are commonly connected to input terminals and take in addresses in a time division manner. And determining means for determining the difference between consecutive timings of address data held in the two address registers of the address buffer, and while reading data of an address based on the determination result of this determining means. , Next address to access The semiconductor memory device according to claim only the word line selectively, further comprising a word line precharge unit for precharging. 2. The determination means is controlled by the logic gate means for detecting a mismatch of address data held by the address registers of the two systems and generating a precharge enable signal, and the precharge enable signal. A transfer means for letting through the next address data out of the address data held by the two systems of address registers, and the word line precharge means for precharging
It has a precharge row decoder which is controlled by an enable signal to decode the next address data, and a precharge row selector which selects one word line by the decoded output and gives a predetermined bias thereto. The semiconductor memory device according to claim 1.