Abstract:
A method and apparatus for masking data written to a memory device that reduces the effective write cycle time of the memory device is disclosed. Firing of the column selects is preempted, thereby masking data to be written to a memory device. By pre-empting the column selects, the margin required for disabling a write driver can be eliminated, thereby reducing the effective write cycle. Additionally, data masking can be performed on a per-byte basis by associating independent column selects with each data byte on multi-byte wide devices, e.g., ×16 or ×32.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to semiconductor memory devices and, more particularly, to data masking circuits and data masking methods for semiconductor memory devices.  
           [0003]    2. Description of the Related Art  
           [0004]    Electronic equipment and electronic-based systems require some form of high-speed memory devices for storing and retrieving information. While the type of such memory devices vary, semiconductor memory devices are most commonly used in memory applications requiring implementation in relatively small areas. Within this class of semiconductor memory devices, random access memory (RAM) is one of the common types. A RAM incorporates an array of individual memory cells. A user may execute both read and write operations on the memory cells of a RAM. A typical example of a RAM is a dynamic random access memory (DRAM), as is well known in the art.  
           [0005]    To allow a DRAM to operate at high speed, “synchronous” DRAMS, also referred to a SDRAMs, have been developed. A synchronous DRAM can receive a system clock that is synchronous with the processing speed of the overall system. The internal circuitry of the SDRAM can be operated in such a manner as to accomplish read/write operations in synchronism with the system clock.  
           [0006]    SDRAMs include Single Data Rate (SDR) SDRAMs and Double Data Rate (DDR) SDRAMs. In a SDR SDRAM, data can be input and output at either only the rising edge or only the falling edge of a clock signal. In a DDR SDRAM, data can be input and output at both the rising and falling edges of the clock signal. Therefore, the DDR SDRAM can have a data bandwidth which is twice the clock frequency.  
           [0007]    It is also known to use a data input/output mask signal applied externally to the memory device to mask output data from the memory device during a read operation and to mask input data to the memory device during a write operation. For example, situations occur when it is desired to send a data stream to a memory device, but it is also desired that some of the data stored in the memory device remain the same. A data mask can be used to block some of the data in the data stream from reaching the individual memory cells that should remain undisturbed.  
           [0008]    [0008]FIG. 1 illustrates in block diagram form a portion of a conventional memory device  20  in which data write masking is used. The memory depicted illustrates a single bank (BANK0) of a 64 Meg SDRAM. BANK 0  memory array  22  includes memory cells arranged in rows and columns for storing data. Command decoder  24 , included in control logic  26 , receives control signals from a command bus CMD to place control logic  26  in a particular operation sequence. Control logic  26  controls the various circuitry of SDRAM  20  based on decoded commands such as reads or writes from or to memory bank  22 . A specific address for which a read or write command is to occur is provided to address register  28 , which provides the address to row-address multiplexer  30  and column-address counter  32 . Row address multiplexer  30  provides a row address to row decoder  34 , which decodes the row address and activates one of the lines corresponding to the row address in BANK0  22  for a read or write transfer operation. Column address counter  32  provides a column address to column decoder  36 , which activates the I/O gating  38  of the column corresponding to the column address. Data being written to the memory  20  is input on data lines (DQ) via the input/output datapath logic circuit  40 , driven by write drivers  42  and passed to the I/O gates  38  for writing to the array  22 . During a read operation, data from the array  22  is passed through the I/O gates  38  to read latch  44  to datapath logic circuit  40  and output on the data lines (DQ).  
           [0009]    Conventional data masking during a write operation is accomplished by sending a mask control signal (DM) through the datapath logic circuit  40  to the write drivers  42  at the same time the data stream is being routed through the write drivers  42 . This mask control signal causes the write driver  42  to go “tri-state” or high impedance, blocking the data stream&#39;s path to the I/O gates  38 . As illustrated in FIG. 1, each write driver  42  drives 8 bits of data (D 0 -D 7 , D 8 -D 15 , D 16 -D 23 , D 24 -D 32 , respectively) for a total of 32 bits or 4 bytes. Four data mask signals are provided (DM 0 , DM 1 , DM 2 , DM 3 ), one for each group of 8 bits or byte.  
           [0010]    [0010]FIGS. 2A and 2B are timing diagrams of various signals generated in the memory device  20  during a write operation with data masking. In order to save space, in FIGS. 2A, 2B,  4 A, and  4 B, the data lines (DQ 0 -DQ 31 ) are not individually shown. Instead, each group of data lines corresponding to each byte of data are shown. Thus, XB 0  represents the group of data lines corresponding to the first byte of data (DQ 0 -DQ 7 ), XB 1  represents the data lines corresponding to the second byte of data (DQ 8 -DQ 15 ), XB 2  represents the data lines corresponding to the third byte of data (DQ 16 -DQ 23 ), and XB 3  represents the data lines corresponding to the fourth byte of data (DQ 24 -DQ 31 ). Additionally, several signals are prefixed with “X”, “Y”, or “Z”. These prefixes indicate different points in time, wherein X designates the time at which a memory device is presented with the write command, Y designates the time after the write command and the associated memory address has been decoded, but before the time when the data is written to the memory arrays of the memory device, while Z indicates the time when the memory arrays are written. Thus, the timing diagrams of FIGS. 2A, 2B,  4 A, and  4 B, permit the reader to follow the relationship between the data signals and data mask signals relative to other signals in the memory device as the data travels through the memory device.  
           [0011]    In FIG. 2A, the illustrated memory device  20  is a 32-bit wide (×32) memory undergoing 16-byte write of data bytes B 0 -B 15 . Since the memory device is 32-bit or 4-bytes wide, the memory device accepts 4-bytes per clock cycle for writing on data byte lines XB 0 -XB 3 . In order for the memory device to support per-byte data masking, the memory device must support one data mask line (XDM 0 -XDM 3 ) per data byte line (XB 0 -XB 3 ). At a first clock cycle of the clock CLK, the WRITE command is present on the command bus CMD. Not illustrated, but also present is the address associated with the first data byte B 0 . Present shortly after the write command are the data (B 0 -B 15 ) to be written as well as an associated data mask on data mask lines XDM 0 -XDM 1 .  
           [0012]    Referring now to FIG. 2B, it can be seen that data on signal lines YB 0 -YB 3  and the data mask on data mask signal lines YDM 0 -YDM 3  have been delayed by an identical amount due to the need for the command decoder  24  to decode the write command and the column decoder  36  and row address decoder  34  to decode the address. At this point data is present on the data lines YB 0 -YB 3  can be driven by the write drivers  42  to the I/O gates  38  if the write driver enable lines WD 0 -WD 3  are high. As shown, data which is to be written, for example data B 0 -B 5 , B 8 -B 10 , and B 12 -B 15 , are accompanied by a high write driver enable signal to permit the data to be driven to the I/O gates  38  while the column select signal ZCS 0 -ZCS 3  associated with those bytes are also driven high to activate the proper column in the memory array, thereby permitting the data to be written to the array  22 . On the other hand, when data needs to be masked from writing, for example data B 6 , B 7 , and B 11 , the data mask signal YDM 0 -YDM 3  is high, causing the write driver signal WD 0 -WD 3  to go low, thereby preventing masked data from being driven to the I/O gates  38  and written to the array  22 .  
           [0013]    As illustrated in FIG. 2B, the column select lines ZCS 0 -ZCS 3  are fired each time regardless of whether data is to be masked nor not. Between each successive firing of the column selects, there is a time period x at which the column select is off. This time period x is provided to give a margin for the mask to turn on, i.e., to disable the write driver. Additionally, the data lags the firing of the column selects by a period of time y. Thus, the effective cycle time for each write operation to occur can be calculated as follows:  
           Effective write cycle=Minimum write time+x+y   (1)  
           [0014]    The data masking operation described above effectively masks data being written to a DRAM. As processor frequencies have increased, however, additional speed is being demanded of memory. The data masking operation described above is an impediment to faster write operations in ways that affect transparency of the DRAM. For example, the masking operation is an additional operation that must be accomplished by the DRAM. The time required to perform data writes utilizing the data masking as described above limits the speed at which the writes can be performed to the effective cycle time as calculated by Equation 1. This necessarily limits that speed at which the memory device can operate, and thus the speed at which the overall system in which the memory device is located can operate. It is therefore desirous to provide a memory device with a decreased effective cycle time for performing write operations to allow operation at faster speeds.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention alleviates the problems associated with the prior art and provides a method and apparatus for masking data written to a memory device that reduces the effective write cycle time of the memory device. In accordance with the present invention, firing of the column selects is pre-empted, thereby masking data to be written to a memory device. By preempting the column selects, the margin required for disabling a write driver can be eliminated, thereby reducing the effective write cycle. Additionally, data masking can be performed on a per-byte basis by associating independent column selects with each data byte on multi-byte wide devices, e.g.. ×16 or ×32. These and other advantages and features of the invention will become more readily apparent from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The foregoing and other advantages and features of the invention will become more apparent from the detailed description of the preferred embodiments of the invention given below with reference to the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 is a block diagram of a portion of a conventional memory device;  
         [0018]    [0018]FIGS. 2A and 2B are timing diagrams of various signals generated in the memory device of FIG. 1 during a write operation with data masking;  
         [0019]    [0019]FIG. 3 is a block diagram of a portion of a memory device having data masking according to the present invention;  
         [0020]    [0020]FIGS. 4A and 4B are timing diagrams of various signals generated in the memory device of FIG. 3 during a write operation with data masking according to the present invention; and  
         [0021]    [0021]FIG. 5 is a block diagram of a processor system that includes a memory circuit having data masking according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The present invention will be described as set forth in the exemplary embodiments illustrated in FIGS.  3 - 5 . Other embodiments may be utilized and structural or logical changes may be made without departing from the spirit or scope of the present invention. Like items are referred to by like reference numerals.  
         [0023]    In accordance with the present invention, firing of the column selects is preempted to mask data to be written to a memory device, thereby reducing the effective cycle time and allowing operation at faster speeds. FIG. 3 illustrates in block diagram form a portion of a memory device  120  having data masking according to the present invention. FIG. 3 is similar to FIG. 1 except as noted below, and the description of like items will not be repeated here.  
         [0024]    Data masking during a write operation according to the present invention is accomplished by sending a mask control signal (DM 0 -DM 3 ) through the datapath logic circuit  140  to an associated column decoder  36   a - 36   d . The column decoder  36   a - 36   d  receiving an active data mask signal DM 0 -DM 3  will be pre-empted from firing its column select, thus preventing the data stream from passing from the I/O gating  38   a - 38   d  to the memory array  22 .  
         [0025]    [0025]FIGS. 4A and 4B are timing diagrams of various signals within the memory device  120  of the present invention. As before, the illustrated memory device  120  is a 32-bit wide (×32) memory undergoing a 16-byte write of data bytes B 0 -B 15 . Since the memory device is 32-bit or 4-bytes wide, the memory device accepts 4-bytes per clock cycle for writing on data byes lines XB 0 -XB 3 . In order to support per-byte data masking, the memory device must support one data mask line per byte width. Thus, the illustrated memory device includes  4  data mask lines XDM 0 -XDM 3 .  
         [0026]    On a first clock cycle of the clock signal CLK, a write command is presented on command bus CMD, along with an associated address for data B 0  (not illustrated). On the following clock cycle, data to be written (e.g., B 0 -B 3 ) to the memory device  120  appears on data lines XB 0 -XB 3  and the write mask for that data appears on data mask lines XDM 0 -XDM 3 . On each of the following 3 clock cycles additional data and data masks are presented on the data lines XB 0 -XB 3  and data mask lines XDM 0 -XDM 3 .  
         [0027]    The data B 0 -B 15  and the data mask signals are accepted by the memory device  120  and are routed within the memory device. The data makes its way through the write drivers  44  and are driven to the I/O gates  38 , as shown on signal lines YB 0 -YB 3 . The data mask signals are routed within the memory device  120  to the column decoders  36   a - 36   d , as represented by signal lines YDM 0 -YDM 3 . There is one data mask signal per column decoder, and each column decoder is associated with generating the column select signal for one byte of data.  
         [0028]    Each column decoder  36   a - 36   d  decodes the address associated with the data in order to generate a column select signal. The addresses presented to the column decoders are preferably delayed so as to arrive coincident with the data mask signals YDM 0 -YDM 3 . The column decoders  36   a - 36   d  also examine the state of the data masking signals YDM 0 -YDM 3 . Each column decoder  36   a - 36   d  asserts its column select signal ZCS 0 -ZCS 3  only if its associated data masking signal YDM 0 -YDM 3  is not asserted. If a column select signal ZCS 0 -ZCS 3  is asserted, the selected column is turned on thereby permitting data to be written into that column. If a column select signal ZCS 0 -ZCS 3  is not asserted, the data cannot be written to that column, thereby masking the data from being written.  
         [0029]    Thus, as illustrated in FIG. 4B, column select lines are associated with different bytes of the data stream and are fired only for data bytes which are not being masked. Accordingly, since the column selects fire only if a data byte is not being masked, there is no need for any delay between the firing of the column selects to give a margin for the mask to turn on, i.e., to disable the write driver, as in the conventional memory devices. The effective cycle time for each write operation according to the present invention can be calculated as follows:  
         Effective write cycle=Minimum write time+y   (2)  
         [0030]    where y is the time period the data lags the firing of the column select to provide a margin to ensure the next data stream will not write to a previous column. Thus, the effective write cycle time according to the present invention (Equation 2) is reduced by the value of x (from Equation 1) as compared to conventional memory devices, thereby allowing operation at faster speeds.  
         [0031]    In addition, the data masking according to the present invention can be provided as a user selectable option. For example, a Data Mask Enable bit can be provided in a mode register. When the Data Mask Enable bit is set to a “1,” data masking is operational, requiring increased timing parameters for the data masking to occur. When set to a “0,” data masking is disabled, thus allowing decreased timing parameters for the memory device to be used.  
         [0032]    It should be noted that preempting of the firing of the column selects according to the present invention can be done in addition to or instead of preempting the firing of the write drivers as described with respect to FIGS. 1 and 2.  
         [0033]    A typical processor based system that includes memory circuits according to the present invention is illustrated generally at  200  in FIG. 5. A computer system is exemplary of a system having memory circuits. Most conventional computers include memory devices permitting storage of significant amounts of data. The data is accessed during operation of the computers. Other types of dedicated processing systems, e.g., radio systems, television systems, GPS receiver systems, telephones and telephone systems also contain memory devices which can utilize the present invention.  
         [0034]    A processor based system, such as a computer system, for example, generally comprises a central processing unit (CPU)  210 , for example, a microprocessor, that communicates with one or more input/output (I/O) devices  240 ,  250  over a bus  270 . The computer system  200  also includes dynamic random access memory (DRAM)  260 , and, in the case of a computer system may include peripheral devices such as a floppy disk drive  220  and a compact disk (CD) ROM drive  230  which also communicate with CPU  210  over the bus  270 . Data masking by DRAM  260  is preferably performed according to the present invention as previously described with respect to FIGS. 3 and 4. CPU  210  and memory  260  may be integrated on a single chip.  
         [0035]    While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. For example, the principles of the present invention are also applicable to wider or narrower memory devices, such as a 16-bit wide memory device, which would have two independent column decoders and two data mask signal lines. Additions, deletions, substitutions, and other modifications can be made without detracting from the spirit or scope of the present inventor. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.