Abstract:
We describe a semiconductor memory device including a memory cell array and a storage device to store access data. The memory cell array is accessed responsive to the access data. The memory cell array access is determined by the access data stored in the storage device. The memory cell array is accessed according to access data only if necessary, drastically reducing power dissipation.

Description:
[0001]     This application claims priority from Korean Patent Application No. 2004-55638, filed on Jul. 16, 2004, the contents of which we incorporate by reference.  
       FIELD  
       [0002]     This invention generally relates to semiconductor devices and, more specifically, to a semiconductor memory device including a circuit for storing access data.  
       BACKGROUND  
       [0003]     Semiconductor memory devices have been widely used as main memories in computers, embedded memories and cache memories in microprocessors. Semiconductor memory devices are classified into RAM (Random Access Memory) and ROM (Read Only Memory) devices.  
         [0004]     A RAM device is a memory device capable of storing written data and reading the stored data. A RAM device is a volatile memory that does not keep stored data when power is off. Typical examples of RAM devices are Dynamic RAM (DRAM) and Static RAM (SRAM).  
         [0005]     A ROM device is only capable of reading stored data. A ROM device is a non-volatile memory that keeps stored data even if the power is off. ROM devices are classified as Programmable ROMS (PROM) and One Time Programmable ROMS (OT-PROM). PROM devices, in turn, are classified as Erasable PROM (EPROM) and Electrically EPROMS (EEPROM). An example of nonvolatile ROM devices is a Flash memory. Flash memories are capable of high integration.  
         [0006]     Typical semiconductor memory devices comprise a memory cell array, a row decoder, a column decoder, and a sense amplifier circuit. The memory cell array has several memory cells arranged in a matrix with word and bit lines. The row decoder receives a row address to select the word line. The column decoder receives a column address to select a bit line. The sense amplifier circuit senses and amplifies a voltage from the bit line to read data of the selected memory cell.  
         [0007]     In the semiconductor memory device with the above-mentioned structure, if the word line is active, several memory cells are accessed. In certain situations, even if the word line is active, it is undesirable to access all the memory cells connected to the word line.  
         [0008]     An example of this situation is a branch target buffer used in branch prediction. The branch target buffer is generally an SRAM. An address of a branch command and a target address are stored in a memory cell array. The branch target buffer accesses the memory cells connected to the word line by activating the selected word line for taken branches as well as not-taken branches. Where branches are taken, the branch target buffer accesses the memory cell array to read the stored target address and then fetches a command of the pertinent target address. Where branches are not taken, the branch target buffer does not fetch the command of the target address. But if branches are not taken, the branch target buffer unnecessarily accesses the memory cell array, leading to wasted power.  
         [0009]     As portable devices (e.g., portable personal computers, cell phones, personal digital assistants (PDA) and so forth) employing semiconductor memory devices become more widespread, power consumption reductions are actively developed. As power supplied to operate portable devices becomes more and more reduced, there is an increasing need to reduce power consumption. Since needlessly accessed memory cell arrays waste power, its elimination is desirable.  
       SUMMARY  
       [0010]     We describe a semiconductor memory device including a memory cell array and a storage device to store access data. The memory cell array is accessed responsive to the access data.  
         [0011]     The storage device is connected to word lines of the memory cell array and the storage device includes memory cells that each store 1-bit data. A decoder receives an address to select a word line and to provide a word line voltage to the selected word line. A logic gate to provide a word line voltage supplied from the decoder to the memory cell array responsive to the access data.  
         [0012]     The logic gate is an AND gate that receives the word line voltage and the access data. To provide the word line voltage supplied from the decoder to the memory cell array responsive to the access data stored and an operating mode.  
         [0013]     The logic gate comprises an OR gate to receive the access data and the operating mode and an AND gate to receive the word line voltage and an output of the OR gate. The logic gate provides the word line voltage to the memory cell array regardless of the access data when the operating mode is a write mode. The memory cell array is a SRAM memory cell array.  
         [0014]     The storage device is connected to a word line of the SRAM memory cell array, respectively, and is a SRAM cell to store 1-bit data. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is block diagram of an embodiment of a semiconductor memory device.  
         [0016]      FIG. 2  is a circuit diagram showing the word line gating circuit shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0017]     Exemplary embodiments will be described more fully with reference to the accompanying drawings.  
         [0018]      FIG. 1  is a block diagram showing an embodiment of a semiconductor memory device. The semiconductor device  1  includes a memory cell array  100 , a word line gate circuit  200 , a decoder  300 , and a sense amplifier circuit  400 .  
         [0019]     The memory cell array  100  can be variously embodied according to a type of cell. For instance, a DRAM cell includes a capacitor coupled to a transistor for storing data. The transistor functions as a switch. An SRAM cell includes two PMOS transistors and four NMOS transistors. A plurality of memory cells exist (DRAM, SRAM, and so forth) that are connected to a word line and a bit line arranged in a matrix in the memory cell array  100 .  
         [0020]     The decoder  300  receives an address ADDR to select a word line and then applies a word line voltage to the selected word line WL.  
         [0021]     A word line gating circuit  200  is located between the memory cell array  100  and the decoder  300 . The word line gating circuit  200  is a storage device for storing access data. The access data determines whether the memory cell array  100  is accessed or not. The word line gating circuit  200  may include a memory cell  210  and a logic gate  220  as shown in more detail in  FIG. 2 .  
         [0022]     The memory cell  210  stores access data. The memory cell  210  is any storage device including DRAM, SRAM, and the like. The memory cell  210  is connected to a plurality of word lines provided to the memory cell array  100 . In an embodiment, the memory cell  210  stores 1-bit access data.  
         [0023]     The logic gate  220  provides a word line voltage to the memory cell array  100  responsive to access data. For instance, if the access data stored in the memory cell  210  is “1”, the logic gate  220  provides the word line voltage applied from the decoder  300  to the memory cell array  100 . If the access data is “0”, on the other hand, the logic gate  220  does not provide the word line voltage to the memory cell array  100 , thereby avoiding unnecessary wasteful power cell accesses.  
         [0024]     The logic gate  220  can be embodied so as to provide the word line voltage to the memory cell array  100  responsive to access data as well as an operating mode. That is, if an operating mode is a write mode according to a control signal (e.g., write) in  FIG. 1 , the logic gate provides the word line voltage to the memory cell array regardless of access.  
         [0025]     The sense amplifier circuit  400  is connected through bit lines BL, /BL, Bit, and /Bit to the memory cell array  100  and the memory cell  210 . The sense amplifier circuit  400  senses and amplifies a voltage of the bit lines BL, /BL, Bit, and /Bit.  FIG. 1  shows only a couple of bit lines but many more may be included as is well known. The semiconductor memory device is not limited to this embodiment and applicable where one memory cell is connected to one bit line (e.g., a flash memory).  
         [0026]      FIG. 2  is a circuit diagram showing a preferred embodiment of the word line gating circuit  200  shown in  FIG. 1 . Referring to  FIG. 2 , the word line gating circuit  200  is connected to the decoder  300  (see  FIG. 1 ) by the word line WL and is connected to the memory cell array  110  by a word line Gated WL. In addition, the word line gating circuit  200  is connected to the sense amplifier circuit  400  (see  FIG. 1 ) by a couple of bit lines Bit and /Bit. The circuit  200  includes the memory cell  210  and the logic gate  200 .  
         [0027]     In  FIG. 2 , the memory cell  210  is a SRAM cell that stores 1-bit data. The SRAM cell  210  is a general SRAM cell, including two PMOS transistors P 1  and P 2 , and four NMOS transistors N 1 ˜N 4 . Access data is stored in the 1-bit SRAM cell  210 . The memory cell array  110  is accessed responsive to the access data. In an embodiment, the memory cell  210  is a SRAM cell, the memory cell array  110  is a memory cell array of a SRAM memory and includes a plurality of SRAM cells connected to the word line Gated WL. If the memory cell  210  is a DRAM cell, the memory cell array  110  is constituted with a plurality of DRAM cells connected to the word line Gated WL.  
         [0028]     The logic gate  220  is includes one AND gate G 1  and one OR gate G 2 . The AND gate G 1  has two input terminals and one output terminal. One input terminal is connected to a word line WL, and the other input terminal is connected to an output terminal of the OR gate G 2 . The OR gate G 2  has two input terminals and one output terminal. One input terminal is connected to an output terminal of the SRAM cell  210 , the other input terminal receives a control signal (e.g., write).  
         [0029]     In the word line gating circuit  200  where the access data stored in the SRAM cell  210  is “1”, the output of the OR gate G 2  is always “1”. As a result, if the word line WL is active, the Gated WL wordline is active. Where access data is “1”, if the word line WL becomes active, the memory cell array  110  is accessed. Where the control signal (Write) is not active, if data stored in the SRAM cell  210  is “0”, an output of the OR gate G 2  becomes “0”. Thus, the output of the AND gate G 1  becomes “0” so that a voltage applied to the word line WL is not provided to the word line Gated WL. Consequently, the memory cell array  110  is not accessed.  
         [0030]     On the other hand, where the control signal (Write) becomes active, an output of the OR gate G 2  is “1”. As a result, the voltage applied to the word line WL is provided to the word line Gated WL regardless of the access data stored in the SRAM cell  210 .  
         [0031]     The semiconductor memory device according to the present invention has the word line gating circuit  200  for storing access information in a word line that is respectively connected to the memory cell array  100 . Where the access data stored in the word line gating circuit  200  is “0”, the memory cell array is not accessed. However, since there is a request to perform a write operation regardless of the access data during the write operation, the memory cell array is accessed.  
         [0032]     In accordance with the semiconductor memory device of the present invention, if access information stored in the word line gating circuit is “0”, the memory cell array is not accessed. Therefore, it is possible to reduce power consumption caused by unnecessarily accessing the memory cell array. In addition, now that access information is stored in each of the word lines connected to the memory cell array, power consumption of the semiconductor memory device can be reduced by only controlling the word line without a complex control circuit and a large amount of delay time.  
         [0033]     As previously mentioned, it is possible to dramatically reduce power consumption of semiconductor devices by including means for storing access information in each of word lines that is connected to the memory cell array.  
         [0034]     Changes can be made to the invention in light of the above detailed description. The terms used here should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.