Abstract:
Circuitry and methods allow selected memory word lines (WLs) to be deactivated without using a global deactivate signal. All active WLs do not therefore have to be deactivated simultaneously, which can cause voltage at a common deactivate node to rise undesirably. This undesirable voltage rise can adversely affect a system by, for example, inadvertently activating an inactive WL. The invention advantageously limits the voltage fluctuation at the common deactivate node.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/759,388, filed Jan. 15, 2004, which claims priority to Japanese patent application No. JP2003-411053, filed Dec. 9, 2003, which are hereby incorporated by reference herein in their entireties. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to integrated circuit memories. More particularly, this invention relates to the deactivation of selected word lines in dynamic random access memories (DRAMs).  
         [0003]     A DRAM is a form of semiconductor random access memory (RAM) commonly used as main memory in computers and other electronic systems. DRAMs store information in integrated circuits called cells, which hold one bit of information each. Cells are typically grouped into one-dimensional arrays called words.  
         [0004]     In certain DRAM architectures, selection of a word for reading or writing occurs as follows: the word to be activated is determined by decoding an address. The selected word line (WL) is then connected to a common node with a sufficiently positive voltage, typically referred to as the VCCP node. Similarly, deactivation involves connecting the WL to a common node with a negative or sufficiently low voltage. This node may be ground or a special purpose node referred to herein as VWLN. As transistor sizes continue to shrink and transistor leakage current becomes more of a problem, the VWLN node is preferred over ground.  
         [0005]     More than one WL may be activated or deactivated simultaneously for testing or other purposes. A problem sometimes arises when deactivating multiple WLs. The positive electrical charge residing on a previously activated word line discharges into the VWLN node, pulling the node&#39;s voltage up. In some cases, VWLN node voltage may increase enough to inadvertently activate WLs intended to remain deactivated, thus adversely affecting data integrity throughout a system.  
         [0006]     Many systems rely on a single pre-charge signal that is shared across all WLs for WL deactivation. This sharing forces all WLs to be deactivated simultaneously. As a result, it is often impractical to activate more than a few WLs at a time, because their simultaneous deactivation could cause a substantial change in VWLN node voltage.  
         [0007]     In view of the foregoing, it would be desirable to provide circuitry and methods that reduce the voltage fluctuation at the VWLN node, thus allowing more WLs to be active at the same time.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of this invention to provide circuitry and methods that reduce the voltage fluctuation at the VWLN node, thus allowing more WLs to be active at the same time.  
         [0009]     In accordance with this invention, circuitry and methods are provided that deactivate selected WLs individually or in small selectable numbers, substantially reducing voltage fluctuation at the VWLN node. All activated WLs can still be deactivated in a short period of time, but their cumulative effect on VWLN node voltage is reduced, because the deactivation of all WLs is no longer simultaneous. That is, the number of simultaneously deactivated WLs can be controlled.  
         [0010]     In a preferred embodiment of the invention, each WL is activated and deactivated by the output of a respective row address latch circuit. The latch circuit&#39;s inputs include an ACTIVE signal shared by all WLs and an address bit specific to each WL. By pulsing the ACTIVE signal while the address bit is active, a specific WL&#39;s activation status (i.e., active or inactive) can be toggled. This allows selected WLs to be deactivated, advantageously avoiding the simultaneous deactivation of all active WLs.  
         [0011]     The invention therefore advantageously reduces voltage fluctuation at the VWLN node when deactivating multiple WLs, thus permitting more WLs to be active concurrently. This is particularly useful in reducing test time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:  
         [0013]      FIG. 1  is a circuit diagram of typical DRAM cells;  
         [0014]      FIG. 2  is a block diagram of a representative WL activation/deactivation architecture;  
         [0015]      FIG. 3  is a circuit diagram of a typical row address latch;  
         [0016]      FIG. 4  is a timing diagram of input and output signals of the row address latch of  FIG. 3 ;  
         [0017]      FIG. 5  is a circuit diagram of an exemplary embodiment of a row address latch according to the invention;  
         [0018]      FIG. 6  is a timing diagram of input and output signals of the row address latch of  FIG. 5  according to the invention;  
         [0019]      FIG. 7  is a block diagram of an alternative WL activation/deactivation architecture that incorporates the invention; and  
         [0020]      FIG. 8  is a block diagram of a system that incorporates the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     DRAMs are, in their simplest form, arrays of cells each including a capacitor for holding a charge and a transistor acting as a switch for accessing the charge held in the capacitor. DRAM arrays are typically arranged in columns and rows.  FIG. 1  shows four DRAM cells  102   a, b, c, d  (four cells are shown merely for illustrative purposes). Each row of cells  104   a  and  104   b  is called a word. The transistor of each cell in a word is connected to a shared WL  106   a  or  106   b . WLs  106   a  and  106   b  control the ON/OFF state of transistors  108   a, b, c, d , which allow information to be read from or written to capacitors  110   a, b, c, d . The information to be read or written is transferred via bit lines  112   a, b . When a WL is activated, it drives the coupled transistors conductive (i.e., turns the transistors ON).  
         [0022]      FIG. 2  shows a representative activation/deactivation architecture for a DRAM with four WLs. The two-bit ADDRESS signal specifying which WL to activate is input into an address decoder  202 . Decoder  202  has one output for each WL. It determines which WL is specified by the address and activates the corresponding output. The address decoder&#39;s outputs are then processed by row address latches  204 . Each row address latch has three inputs, the ADDR signal from the decoder, a shared ACTIVE signal, and a shared PRE-CHARGE signal. Each row address latch has one output, which is connected to a specific WL. For example, the top row address latch has output signal W 0 , which is connected to WL 0 . The PRE-CHARGE signal deactivates all WLs in a section of a DRAM chip by connecting them to the VWLN node. The ACTIVE signal is pulsed after the address decoder&#39;s outputs have settled in order to activate only the target WL. During the pulse, each row address latch activates its output W and connects the WL to the VCCP node if its ADDR signal is active. After a WL has been activated, it can ordinarily only be deactivated by asserting the PRE-CHARGE signal.  
         [0023]      FIG. 3  shows a typical row address latch  300 . Latch  300  includes p-type transistor  302 , n-type transistors  304  and  306 , and inverters  308 ,  310 ,  312 , and  314 . Node  301  can be coupled to VCCP or another voltage level. Inputs PRE-CHARGE and ADDR 0  are active low (i.e., activated by a logical 0 signal), while ACTIVE is active high (i.e., activated by a logical 1 signal). When PRE-CHARGE is driven to a logical 0, it activates transistor  302 , which drives W 0  low. Note that after PRE-CHARGE is asserted low, transistor  306  becomes conductive.  
         [0024]     When the ACTIVE signal is a high pulse, it drives transistor  304  conductive. During the pulse, if ADDR 0  is active (i.e., low), it drives output W 0  high and activates the corresponding WL. When W 0  is high, transistor  306  is non-conductive (its input from inverter  312  is low), and subsequent pulses on the ACTIVE input will have no effect on the W 0  output. To deactivate this WL, the PRE-CHARGE signal is asserted low, which also simultaneously deactivates all other WLs in the same section of the DRAM chip. As described above, this can have an adverse effect on other WLs depending on the number of WLs being simultaneously deactivated.  
         [0025]      FIG. 4  shows a timing diagram  400  in which three row address latches of  FIG. 3  are used to respectively activate and deactivate three WLs. Signal CLK is a system clock signal. When PRE-CHARGE is pulsed low at signal transition  402 , all three WLs are deactivated. Signal ACTIVE is then pulsed three times in conjunction with signal ADDRESS at signal transitions  403 - 405 , activating W 0 , W 1 , and W 2  in turn. Finally, when signal PRE-CHARGE is again pulsed low at signal transition  406 , all three WLs are simultaneously deactivated in transition  408 , causing the three associated WLs to be connected to the VWLN node. This simultaneous deactivation can result in a substantial amount of positive charge flowing into the VWLN node, causing the node&#39;s voltage to drift upwards undesirably.  
         [0026]     An exemplary embodiment of the invention is shown in  FIG. 5 . Row address latch circuit  500  includes p-type transistors  502  and  516 , n-type transistors  504  and  506 , inverters  508 ,  510 ,  512 , and  514 , delay element  518 , and logic  520 . Nodes  501  can be coupled to VCCP or another voltage level. Logic  520  controls transistor  516  via signal DEAC and preferably includes NOR gate  522  and inverters  524 ,  526 , and  528 . The SWLD input signal is active high and enables latch circuit  500  to selectively deactivate its WL.  
         [0027]     Circuit  500  responds to a PRE-CHARGE pulse by deactivating its output W 0  in the same fashion as latch circuit  300 . When the ACTIVE signal is pulsed high while signal ADDR 0  is low, output W 0  will be asserted high. Note that during this assertion phase, transistor  516  is non-conductive, because the DEAC signal is a logical 1 when the ACTIVE pulse arrives. When ACTIVE is again pulsed high while SWLD is high and ADDR 0  is low, logic  520  outputs the DEAC signal low, driving transistor  516  conductive and de-asserting W 0 . Thus, the row address latch according to the invention allows a specific WL to be deactivated, without using the PRE-CHARGE signal which is shared across all WLs. Delay element  508  assures that the ACTIVE pulse passes before output signal OPEN becomes high, driving transistor  506  conductive (signal OPEN is also fed to NOR gate  522 ). Note that until the next pulse of the PRE-CHARGE signal resets latch circuit  500 , each time ACTIVE is pulsed high while SWLD is high and ADDR 0  is low, the value of W 0  will toggle (i.e., alternate between a logical 1 and a logical 0).  
         [0028]      FIG. 6  shows a timing diagram  600  of signals applied to latch circuit  500  according to the invention. Similar to latch circuit  300 , transition  602  of signal PRE-CHARGE deactivates all WLs. Following this deactivation, the ACTIVE signal is pulsed three times in transitions  603 - 605  to activate all three WLs. Advantageously, however, the WLs can each be deactivated without asserting the PRE-CHARGE signal, and the deactivation of each WL need not be simultaneous with the deactivation of other WLs. Rather, the three successive pulses of the ACTIVE signal shown in transitions  606   a ,  607   a , and  608   a  deactivate the three WLs in turn, illustrated in transitions  606   b ,  607   b , and  608   b . This non-simultaneous deactivation results in smaller transient spikes on the VWLN node than that caused by the simultaneous deactivation of all WLs at transition  408  of timing diagram  400 .  
         [0029]     Note that circuit  500  and timing diagram  600  are both merely illustrative. Other latch circuits that toggle the W 0  output independently of the PRE-CHARGE signal can be used. For example, p-channel field effect transistor  516  could be replaced with an n-channel field effect transistor if inverter  528  were removed. Similarly, if the PRE-CHARGE signal were active high, transistor  502  could be replaced with an n-channel field effect transistor. Another possibility would be to force the output W 0  to swing between ground and VCCP, rather than between VWLN and VCCP, by setting the voltage range of inverter  514  appropriately.  
         [0030]     Also, the number of row address latches controlling each WL can vary from that shown. For example,  FIG. 7  shows an activation/deactivation architecture where each WL is controlled by two row address latches. Architecture  700  includes address pre-decoders  702 , row address latches  500 , row address latches  300 , and logical AND gates  706 . In this example, decoding of a four-bit address occurs in two stages. In the first stage, the address&#39; two most significant bits ADDRESS_MSB and its two least significant bits ADDRESS_LSB are separately decoded. Each address pre-decoder  702  outputs one ADDR signal low, and the others high. These outputs are then processed by row address latches  500  or  300  according to the invention. In the final stage of decoding, each WL is tied to the outputs of two row address latches by an AND gate  706 , one latch output for each address pre-decoder  702 . These two latch outputs represent the combination of ADDRESS_MSB and ADDRESS_LSB values that correspond to a particular WL&#39;s address. Thus, a given WL can only be activated if both its corresponding latch outputs are high. Note that, in  FIG. 7 , any given row address latch  500  is tied to more than one WL. Thus, in contrast with the architecture of  FIG. 2 , the architecture shown in  FIG. 7  makes it possible to activate or deactivate multiple WLs with a single ACTIVE pulse. For instance, suppose the four WLs tied to signals W 0 -W 3  were activated. If latch output RA 0  were subsequently driven low, then those four WLs would all be simultaneously deactivated. The number of row address latches could alternatively depend on other considerations as well.  
         [0031]     Timing diagram  600  could also vary, depending on the particular application required. For example, circuit  500  could be used to deactivate only WL 0  and WL 1 , and the PRE-CHARGE signal could have been pulsed to deactivate WL 2 . Other operation sequences are possible, depending on, for example, the task to be performed and the limitations of the hardware.  
         [0032]      FIG. 8  shows a system that incorporates the invention. System  800  includes a plurality of DRAM chips  801 , a processor  880 , a memory controller  882 , input devices  884 , output devices  886 , and optional storage devices  888 . DRAM chips  801  each include one or more latch circuits  500 . Data and control signals are transferred between processor  880  and memory controller  882  via bus  881 . Similarly, data and control signals are transferred between memory controller  882  and DRAM chips  801  via bus  883 . Input devices  884  can include, for example, a keyboard, a mouse, a touch-pad display screen, or any other appropriate device that allows a user to enter information into system  800 . Output devices  886  can include, for example, a video display unit, a printer, or any other appropriate device capable of providing output data to a user. Note that input devices  884  and output devices  886  can alternatively be a single input/output device. Storage devices  888  can include, for example, one or more disk or tape drives.  
         [0033]     Note that the invention is not limited to DRAM chips, but is applicable to other integrated circuit chips having a circuit or group of circuits where the simultaneous activation or deactivation of certain signal lines is undesirable.  
         [0034]     Thus it is seen that circuits and methods are provided to deactivate multiple WLs individually or in small selectable numbers, thus reducing the total number of WLs deactivated simultaneously. One skilled in the art will appreciate that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.