Abstract:
An address selection circuit and method selects an internal address from a plurality of input address signals and avoids unnecessary signal transitions to reduce current consumption. A latch coupled to the outputs of first and second transfer gates latches either the first or second of the respective input address signals as the internal address and maintains the logic level of the input address signal even after enable signals for each of the transfer gates becomes inactive to prevent unnecessary transitions in the internal address.

Description:
TECHNICAL FIELD 
     The present invention relates to memory devices, and more particularly to address selection circuitry having lower current consumption during an internal address selection operation. 
     BACKGROUND OF THE INVENTION 
     Dynamic Random Access Memories (“DRAMs”) are commonly used in a variety of electronic devices, such as computers. A high level block diagram of a typical DRAM is shown in FIG.  1 . The DRAM shown in FIG. 1 is a synchronous dynamic random access memory (“SDRAM”)  100 , although the principles described herein are applicable to any memory device having a need for internal address selection circuitry. The SDRAM  100  includes an address register  104  that receives row addresses and column addresses on a multiplexed address bus. The address bus is generally coupled to a memory controller (not shown) which provides the row and column addresses. 
     Typically, a row address is initially received by the address register  104  and applied to a row address multiplexer  108 . The row address multiplexer  108  couples the row address to a number of components associated with memory banks  130   a-d , depending upon the state of a bank address bit forming part of the row address. Associated with each of the memory banks  130   a-d  is a respective row address latch/decoder  120   a-d  which stores the row address and applies various signals to its respective array  130   a-d  as a function of the stored row address. The row address multiplexer  108  also couples row addresses to the row address latches  120   a-d  for the purpose of refreshing the memory cells in the arrays  130   a-d . The row addresses are generated for refresh purposes by a refresh counter  110 . As will be described in greater detail below, the row address multiplexer  108  includes address selection circuitry  109  that selects between providing the external address received from the address register  104  and the address received from the refresh counter  110  to the row address latch/decoder  120   a-d  as an internal address. 
     After the row address has been applied to the address register  104  and stored in one of the row address latches/decoder  120   a-d , a column address is applied to the address register  104 . The address register  104  couples the column address to a column address latch  112  where the address is stored. Column decoders  124   a-d  receive the column addresses from the column address latch  112 , and apply various signals to associated column circuitry  128  and respective sense amplifiers for the respective arrays  130   a-d . Data to be read from the arrays  130   a-d  are coupled to the column circuitry  128  and subsequently coupled to a data output register  142 , which applies the data to a data bus  150 . Data to be written to one of the arrays  130   a-d  are coupled from the data bus  150  through a data input register  154  to the column circuitry  128 , where the data are transferred to the arrays  130   a-d.    
     The above-described operation of the SDRAM  100  is controlled by control logic  102  responsive to high level command signals received on a control bus. These high level command signals, which are typically generated by a memory controller (not shown), often include a clock enable signal CKE*, a clock signal CLK, a chip select signal CS*, a write enable signal WE*, a row address strobe signal RAS*, and a column address strobe signal CAS*, where the “*” designates the signal as active low. The control logic  102  generates a sequence of command signals responsive to the high level command signals to carry out various memory functions and program memory modes designated by each of the high level command signals, such as memory read, memory write, refresh operations, standby mode, and the like. For example, driving the RAS* and CAS* inputs low with CKE* high will cause the SDRAM  100  to enter a self-refresh mode. In the self-refresh mode, the control logic  102  causes the memory cells corresponding to the address provided by the refresh counter  110  in the arrays  120   a-d  to be periodically refreshed. These command signals, and the manner in which they accomplish their respective functions, are conventional. Therefore, in the interest of brevity, a further explanation of these control signals will be omitted. 
     As mentioned previously, in the memory device  100 , the row address multiplexer  108  is commanded by the control logic  102  to provide as an internal row address to the row address latches/decoders  120   a-d  either an external address latched by the address register  104  or a CBR address provided by the refresh counter  110 . Where the memory device  100  is operating in a normal mode, the row address multiplexer  108  provides the external address to the row address latches/decoders  120   a-d  for accessing the corresponding row of the memory arrays  120   a-d . The CBR addresses, on the other hand, are provided by the refresh counter  110  to the row address latches/decoders  120   a-d  to facilitate auto-refresh and self-refresh operations, which are well known in the art. 
     FIG. 2 illustrates a known design of an address selector  200  that is included in the address selection circuitry  109 . As illustrated in FIG. 2, the address selector  200  provides one bit of the row address that is provided to the row address latches/decoders  120   a-d . Thus, the address selection circuitry  109  typically includes multiple address selectors  200  to provide appropriate number of bits of the row address to the row address latches/decoders  120   a-d . The multiple address selectors  200  operate identically, and consequently, the description of the address selector  200  provided herein is applicable to the other address selectors as well. 
     The address selector  200  provides either the external address or the CBR address as the internal address to the row address latch/decoder  120  based on the row address activation signal ARA. Specifically, where the ARA signal is HIGH, the external address is provided, and where the ARA signal is LOW, the CBR address is provided instead. The generation of the ARA signal is typically performed by circuitry (not shown) under the command of the control logic  102  and in response to the mode of operation of the memory device  100 . 
     As is commonly known in the art, the address bus of the memory device  100  is multiplexed between row and column addresses, consequently, the ARA signal is strobed to latch the row address before the column address becomes valid. Thus, after the external row address is latched by the row address latches/decoders  120   a-d , the ARA signal returns to an inactive state, the multiplexer  202  switches to providing the CBR address as the internal address. In essence, the row address multiplexer  108  provides the CBR address to the row address latches/decoder  120   a-d  as a default when the ARA signal is not active. 
     It is generally accepted that it is more desirable to have lower power consuming devices, including in particular, memory devices. To this end, circuits included in devices have been designed to be more power efficient. From this perspective, the address selector  200  may not be a desirable alternative because its design includes inherent power inefficiencies. 
     An example of such inefficiencies will be discussed with respect to FIG.  3 . As previously discussed, the address selector  200  provides the CBR address to the row address latches/decoders  120   a-d  as a default, or more specifically, when the ARA signal returns to an inactive state after latching an external row address. As shown in FIG. 3, at a time T 0  a memory array access operation is initiated by activating a bank and row of memory. The ARA signal becomes active to set the multiplexer  202  (FIG. 2) to provide the clocked external row address as the internal address for activation of the row corresponding thereto. At a time T 1 , when the ARA signal becomes inactive after latching the external row address, the multiplexer is set to the default state and provides the CBR address to the row address latches/decoders. At a time T 2 , the ARA signal becomes active again to latch an external row address corresponding to the row of memory that is to be deactivated, thus, completing the memory array access operation. Consequently, the multiplexer  202  is again set to provide the external address to the row address latches/decoders  120   a-d . When the ARA signal again becomes inactive at a time T 3  following the latching of the external row address, the multiplexer  202  switches back to the default state of providing the CBR address as the internal address. 
     The power inefficiencies related to the address selector  200  result from the multiple transitions of the internal address that occur each time the ARA signal returns to an inactive state. It is well known that with CMOS circuitry, which is generally extensively incorporated in the design of integrated circuits, such as memory device  100  of FIG. 1, current is consumed or dissipated when transitions from one logic state to another occur. Where multiple circuits are involved, such as in the address selection circuitry of the row address multiplexer  108 , buffer circuitry driving the internal address bus, and latch and decoder circuitry of the row address latches/decoders  120   a-d , the cumulative current consumption during a logic transition may be significant. Thus, minimizing the number of unnecessary transitions the circuitry must make will generally reduce power consumption. 
     As illustrated in FIG. 3, the ARA signal is typically strobed twice in a memory access cycle, resulting in four transitions of the internal address. As mentioned previously, the transitions occurring at the times T 1  and T 3  merely result from the address selector  200  returning to the default state of providing the CBR address as the internal address, although the CBR address is not used during the memory access operation. Consequently, the transitions at the times T 1  and T 3  cause the address circuitry involved to unnecessarily consume current. In the worst case, the external row address latched while the ARA signal is strobed and the CBR address are complementary, causing all bits of the internal address to transition. Moreover, the current consumption of the memory device  100  having address selection circuitry  109  that includes the address selection circuit  200  becomes unpredictable because the quantity of current consumed as a result of transitions of the internal address is related to the particular external address latched and the particular CBR address provided by the refresh counter  110 . As this changes frequently, it becomes difficult to accurately predict the power consumption of the memory device  100 . 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an address selection circuit and method that reduces current consumption by avoiding unnecessary transitions in an internal address signal. The address selection circuit includes a first transfer gate having an input to which a first address signal is applied and an output to which the input is coupled in response to an active first control signal applied to a control terminal of the first transfer gate. The address selection circuit further includes a second transfer gate having an input to which a second address signal is applied and an output to which the input is coupled in response to an active second control signal applied to a control terminal of the second transfer gate. A latch coupled to the outputs of the first and second transfer gates is also included in the address selection circuit to latch the either the first or second of the input address signals as the internal address. The latch maintains the logic level thereof even after the enable signal becomes inactive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional memory device. 
     FIG. 2 is a schematic drawing of an address selection circuit. 
     FIG. 3 is a timing diagram of various signals related to the address selection circuit of FIG.  2 . 
     FIG. 4 is a schematic drawing of an address selection circuit according to an embodiment of the present invention. 
     FIG. 5 is a timing diagram of various signals related to the address selection circuit of FIG.  4 . 
     FIG. 6 is a block diagram of a computer system having a memory device including address selection circuitry according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are directed to a method and apparatus for selecting an internal address having reduced current consumption. Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
     FIG. 4 illustrates an address selection circuit  400  according to an embodiment of the present invention. The address selection circuit  400  includes a transfer gate  402  having an input to which one bit of the external address is applied and an output coupled to a latch circuit  420 . The transfer gate  402  couples the external address applied to the latch circuit  420  in response to a HIGH ARA signal. A transfer gate  404  is also included in the address selection circuit  400 . The transfer gate  404  has an input to which one bit of the CBR address is applied and an output coupled to the latch circuit  420 . The output of a two-input NOR gate is coupled to the control terminals of the transfer gate  404 . Thus, the transfer gate  404  couples the CBR address applied to its input to the latch circuit  420  in response to either an auto-refresh command signal AREF_H or a self-refresh command signal SREF_EN having a HIGH logic level. 
     The operation of the address selection circuit  400  will be explained with respect to the timing diagram illustrated in FIG.  5 . As previously discussed with respect to the timing diagram of FIG. 3, the ARA signal is strobed twice during a memory access operation. As illustrated in FIG. 5, the ARA signal is active a first time between times T 0  and T 1  to latch a row address corresponding to the row of memory that will be activated during the memory access operation. The ARA signal is active a second time between times T 2  and T 3  to latch the row address of the row of memory to be deactivated to complete the memory array access operation. 
     In response to the ARA signal becoming HIGH at the time T 0 , the transfer gate  402  couples the external address applied to its input to the latch circuit  420  to be stored. As the ARA signal transitions LOW, the transfer gate  402  merely decouples the external address from the latch circuit  420 . However, because the external address is latched while the ARA signal is active, the latched external address remains valid until either transfer gates  402  or  404  couple a new row address to the latch circuit  420 . Consequently, the internal address provided by the address selection circuit  400  does not transition upon the. ARA signal returning LOW, unlike the conventional address selection circuit  200  (FIG.  2 ). The internal address transitions a second time when the ARA signal strobes for the second time at the time T 2 . Again, as with the first time the ARA signal is strobed, the external address is latched by the latch circuit  420  when the transfer gate  402  is activated by the ARA signal and remains valid after the ARA signal returns to a LOW logic level at a time T 3 . 
     Where the auto-refresh or self-refresh mode is enabled, an active auto-refresh command signal AREF_H or self-refresh command signal SREF_EN is generated and provided to the NOR gate  408 . Either an active AREF_H or SREF_EN signal will enable the transfer gate  404  to couple the CBR address to the latch circuit  420  for provision to the row address latches/decoders as the internal address. After an auto-refresh command is issued and subsequently becomes inactive, the CBR address remains latched until a new address is provided to the latch circuit  420 . In a self-refresh mode, the multiplexer can be made to remain on for the duration, or clocked as the other signals, depending on the state of the self-refresh mode enable signal SREF_EN. It will be appreciated that those of ordinary skill in the art will have a sufficient understanding of the present to generate the ARA signal and the AREF_H and SREF_EN signals in a coordinated fashion such that only one of the transfer gates  402  and  404  are active at a given time to avoid address contention. 
     Comparing the timing diagram of FIG. 4 for the address selection circuit  400  and the timing diagram of FIG. 2 for the conventional address selection circuit  200 , it is apparent that the unnecessary transitions of the internal address that occurred at times T 1  and T 3  for the conventional address selection circuit  200  are avoided by the address selection circuit  400 . Consequently, the current consumption of the address selection circuit  400  is less than that of the conventional address selection circuit  200 . 
     In summary, embodiments of the present invention avoid the unwanted transitions in the internal address by neither providing the CBR address or the external address as a default. Instead, embodiments of the present invention select whether to provide the CBR address or external address through a command signal, and subsequently latching the address that is to be provided as the internal address. The ARA signal continues to be the external row address selection signal, which allows the external address to be latched. However, when the ARA signal becomes inactive, the internal address signal remains valid until receiving a subsequent active ARA signal, or an auto-refresh or self refresh enable signal, which couples the CBR address to the latch circuit. 
     FIG. 6 is a block diagram of a computer system  600  including computing circuitry  602 . The computing circuitry  602  contains a memory  601 , that can be a volatile memory, such as a DRAM, or a non-volatile memory, such as a FLASH memory. The computing circuitry  602  could also contain both a DRAM and FLASH memory. The memory  601  includes charge pump circuitry having oscillator circuitry according to embodiments of the present invention. The computing circuitry  602  performs various computing functions, such as executing specific software to perform specific calculations or tasks. In addition, the computer system  600  includes one or more input devices  604 , such as a keyboard or a mouse, coupled to the computer circuitry  602  to allow an operator to interface with the computer system. Typically, the computer system  600  also includes one or more output devices  606  coupled to the computer circuitry  602 , such output devices typically being a printer or a video terminal. One or more data storage devices  608  are also typically coupled to the computer circuitry  602  to store data or retrieve data from external storage media (not shown). Examples of typical storage devices  608  include hard and floppy disks, tape cassettes, and compact disc read-only memories (CD-ROMs). The computer circuitry  602  is typically coupled to the memory device  601  through appropriate address, data, and control busses to provide for writing data to and reading data from the memory device. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.