Abstract:
An integrated circuit device is discussed that includes an data output driver having two modes of operation for driving a data bus. The output driver includes a circuits to produce a fall drive output high signal, a partial drive output high signal, a full drive output low signal and a partial drive output low signal. The output driver is protected against negative voltages on the data bus. The output driver is selectable and adaptable to drive terminated loads and unterminated loads.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to semiconductor integrated circuits. More particularly, it pertains to enhancing outputting capability of a Synchronous Dynamic Random Access Memory (SDRAM) so as to allow the SDRAM to be adaptively compatible with different termination requirements of various types of circuits.  
         BACKGROUND  
         [0002]    Memory devices are integrated circuits in which information may be stored and from which information may be extracted when desired. Each memory device is built from a plurality of memory cells. Each memory cell memorizes a bit of data. Although a bit of data seems insignificant, it may determine whether the stored information is correct, such as an amount in a checking account.  
           [0003]    The process of memorizing the bit of data by the memory cell is an example of the ingenuity of a memory device. But also equally important is the process of extracting the memorized bit of data from the memory cell. The process of extracting outputs the memorized bit of data so that subsequent devices, which are coupled to the memory device, may make use of it.  
           [0004]    Certain devices require that the memory device powerfully drive the memorized bit of data from the memory device to the input of those certain devices. Yet other devices, especially those used in graphics products, require a less powerful drive of the memorized bit of data. Current memory devices are unable to adapt to the needs of various devices.  
           [0005]    Thus, what is needed are devices and methods to adapt to the needs of various devices coupled to the memory device so as to enhance the output operations of future generations of memory devices, such as synchronous DRAMs (SDRAMs), and double-data-rate SDRAMs (DDR SDRAMs).  
         SUMMARY  
         [0006]    The above-mentioned problems with memory devices and other problems are addressed by the present invention and will be understood by reading and studying the following specification. Devices and methods are described which accord these benefits.  
           [0007]    One illustrative embodiment includes an output driver having two modes of operation. The output driver includes a first circuit to support one of the two modes. One of the two modes drives a subsequent input stage of another circuit without a termination in between the output driver and the subsequent input stage of the other circuit. The output driver includes a second circuit to support the other of the two modes. The other of the two modes drives the subsequent input stage of the other circuit with the termination in between the output driver and the subsequent input stage of the other circuit.  
           [0008]    Another illustrative embodiment includes an output driver for driving data at an output. The output driver includes a full-drive pull-up circuit to drive data high to a full level, a reduced-drive pull-up circuit to drive data high at a reduced level, a full-drive pull-down circuit to drive data low to a full level, and a reduced-drive pull-down circuit to drive data low at a reduced level.  
           [0009]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver circuit that produces a mode selected from a group consisting of a full-drive mode and a reduced-drive mode. The circuit also includes an output driver coupled to the pre-driver circuit that outputs the data in accordance with the mode produced by the pre-driver circuit.  
           [0010]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver circuit that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a negative protection circuit. The circuit includes an output driver coupled to the pre-driver circuit that outputs a data signal in accordance with the mode produced by the pre-driver circuit. The negative protection circuit inhibits undesired turning-on of the output driver.  
           [0011]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a booting circuit that boots a boot-up signal to a voltage level. The voltage level is selected from a regular voltage supply level or a pumped voltage supply level. The circuit includes an output driver coupled to the pre-driver circuit that outputs a data signal in accordance with the mode produced by the pre-driver circuit.  
           [0012]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver circuit that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a reduced-drive pull-up driver so as to produce the reduced-drive mode at a reduced-drive node.  
           [0013]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver circuit that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a full-drive pull-up driver so as to produce the full-drive mode at a full-drive node. The circuit includes an output driver coupled to the pre-driver circuit that outputs a data signal in accordance with the mode produced by the pre-driver circuit.  
           [0014]    Another illustrative embodiment includes a circuit to output data in a memory device. The circuit includes a pre-driver circuit that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a full-drive pull-down driver so as to produce the full-drive mode at a full-drive node. The circuit includes an output driver coupled to the pre-driver circuit that outputs a data signal in accordance with the mode produced by the pre-driver circuit.  
           [0015]    Another illustrative embodiment includes a pre-driver circuit that produces a mode signal selected from a group consisting of a full-drive mode and a reduced-drive mode. The pre-driver circuit includes a reduced-drive pull-down driver so as to produce the reduced-drive mode at a reduced-drive node. The pre-driver circuit includes an output driver coupled to the pre-driver circuit that outputs a data signal in accordance with the mode produced by the pre-driver circuit.  
           [0016]    These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a block diagram of a memory device according to an embodiment of the invention.  
         [0018]    [0018]FIG. 2 is a block diagram of an output buffer circuit of a memory device according to an embodiment of the invention.  
         [0019]    [0019]FIG. 3 is a block diagram of a pre-driver circuit of a memory device according to an embodiment of the invention.  
         [0020]    [0020]FIG. 4 is a circuit diagram of a boot circuit according to an embodiment of the invention.  
         [0021]    [0021]FIG. 5 is a circuit diagram of a circuit to provide a full-drive pull-up mode according to an embodiment of the invention.  
         [0022]    [0022]FIG. 6 is a circuit diagram of a circuit to provide a reduced-drive pull-up mode according to an embodiment of the invention.  
         [0023]    [0023]FIG. 7 is a circuit diagram of a circuit to provide negative voltage protection according to an embodiment of the invention.  
         [0024]    [0024]FIG. 8 is a circuit diagram of a circuit to provide full-drive pull-down mode according to an embodiment of the invention.  
         [0025]    [0025]FIG. 9 is a circuit diagram of a circuit to provide reduced-drive pull-down mode according to an embodiment of the invention.  
         [0026]    [0026]FIG. 10 is a block diagram of a system containing a memory device according to one embodiment of the present invention.  
         [0027]    [0027]FIG. 11 is a block diagram of a system containing and integrated circuit having a multiple mode driver according to one embodiment of the present invention.  
         [0028]    [0028]FIG. 12 is an elevation view of a semiconductor wafer according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.  
         [0030]    The terms “high” and “low” as used herein refer to Vcc, the voltage supply, and ground, respectively. The term “external supply” as used herein refers to Vcc, the voltage supply.  
         [0031]    [0031]FIG. 1 is a block diagram of a memory device  100  according to an embodiment of the invention. The memory device  100  includes multiple memory cells, which are organized into an array  102 . Recall that each memory cell memorizes at least one bit of data. The array  102  allows easy read or write access to each memory cell in the array  102 . The memory device  100  also includes an output buffer circuit  104 . The output buffer  104  helps to strengthen the data signal coming out from the memory device  100  so that the strengthened data signal may enter subsequent devices without degradation. In some systems using memory devices, the output drive strength of the memory device must be substantial while in other systems, the output drive strength is less demanding. As illustrated in FIG. 1, output  108  does not require a termination impedance while output  106  requires a termination impedance  110 . As such, the required output drive strength of output  106  is significantly greater than the output drive strength of output  108 .  
         [0032]    As illustrated in FIG. 1, the termination impedance  110  is required in applications that subscribe to the Stub Series-Terminated Load specification (SSTL). The SSTL interface standard is produced by the Joint Electron Devices Engineering Councils (JEDEC) and is available to the public in publications nos. JC-16-97-04 (Stub Series-Terminated Logic for 3.3 Volts) and JC-16-97-58 (Stub Series-Terminated Logic for 2.5 Volts) which are hereby incorporated by reference. The SSTL interface standard is intended for high-speed memory interface applications and specifies switching characteristics such that operating frequencies up to 200 MHZ are attainable. The primary application for SSTL devices is to interface with Synchronous Dynamic Random Access Memory (SDRAM). Part of the SSTL specification specifies a bus termination impedance of 25 to 50 ohms and a termination voltage. The termination  110  of FIG. 1 is illustrative only since the SSTL standard specifies a more detailed and particular termination scheme with appropriate values for resistors and capacitors. For example, the terminator for an SSTL bus line includes two resistors in parallel which are used to establish a voltage level approximately mid-way between Vcc and ground such that differential voltage swings can be utilized on the order of ±380 millivolts to ±400 millivolts about the midpoint voltage.  
         [0033]    By way of example, according to the SSTL specification, in order to drive a data signal at the termination load  110 , one would drive the data signal to a desired voltage above the termination voltage, which is about mid-way between Vcc and ground, to approximately +380 millivolts above the midpoint voltage for a logical ONE signal or −380 millivolts below the midpoint bus voltage for a logical ZERO signal. In the present invention, the use of SSTL to drive a data signal is considered to be a full-drive mode because the memory device  100  would more powerfully drive the data signal out of the output buffer  104  to the subsequent devices.  
         [0034]    In other systems utilizing memory devices but not using SSTL, devices were connected directly to the output  108  of the memory device  100  without any termination. This is known as a point-to-point configuration and in some applications known as Transistor-Transistor Logic (TTL) and Low-Voltage Transistor-Transistor Logic (LVTTL). Graphics products typically use this configuration and the voltage swings on the output  110  typically swing from Vcc to Ground. This configuration, however, does not need the data signal to be as powerfully driven as in SSTL configurations. In fact, in certain cases, data that is driven powerfully in a point-to-point configuration may cause ringing and undesired noise on the bus lines.  
         [0035]    The output buffer  104  of the present invention solves this problem by adaptively switching between full-drive mode for devices that require a more powerful drive of the data signal (such as SSTL configurations) and a reduced-drive mode for devices that require a less powerful drive of the data signal (such as LVTTL and TTL configurations). Subsequent figures discussed below expand on this in more detail.  
         [0036]    [0036]FIG. 2 is a block diagram of an output buffer circuit for a single output data pin of a memory device according to an embodiment of the invention. An output buffer  200  may be organized into two separate circuits: a pre-driver circuit  202  and an output driver  204 .  
         [0037]    The pre-driver circuit  202  receives the data from the memory array. The pre-driver circuit  202  also receives drive information on whether full-drive or reduced-drive is desired on the output DQPAD. Based on the level of the data (high or low) and the drive information, the pre-driver circuit  202  produces data signals for the output driver  204  to process. The output driver  204  drives the data signals according to the appropriate mode out of the memory device.  
         [0038]    Output driver circuit  204  of FIG. 2 includes is a simplified DQ pad schematic which illustrates the operation of the dual-drive output. The output driver circuit  204  receives four main signals from the pre-driver circuit  202 . The FULLDRIVE_PUP signal is used to indicate when the output is to produce a full-drive pull-up signal for an SSTL load. When used alone, the PUP signal is used to produce a reduced-drive or half-drive signal for a non-SSTL load such as LVTTL. If an SSTL load is to be driven high, the output buffer circuit  204  receives both the active drive signals from FULLDRIVE_PUP and PUP. If only a conventional reduced-drive load is to be driven high, the output buffer circuit  204  receives only the active drive signal from PUP.  
         [0039]    The FULLDRIVE_PDNx signal is used to indicate when the output is to produce a full-drive pull-down signal for an SSTL load. When used alone, the PDNx signal is used to drive only a reduced-drive or half-drive signal for a non-SSTL load such as LVTTL. If an SSTL load is to be driven low, the output buffer circuit  204  receives both the active drive signals from FULLDRIVE_PDNx and PDNX. If only a conventional reduced-drive load is to be driven low, the output buffer circuit  204  receives only the active drive signal from PDNx.  
         [0040]    The four drive transistors of the output driver circuit  204  are sized to produce the needed drive current according to the load on the output DQPAD. One skilled in the art will recognize that each output data pin of an SDRAM will have a separate set of pre-driver  202  and driver  204  circuits driving each data output pin.  
         [0041]    [0041]FIG. 3 is a block diagram of a pre-driver circuit  202  of a memory device according to an embodiment of the invention as part of a data output path  300  of a memory device. The output data path  300  includes an output enabler  302  which controls whether the data path as a whole would produce a meaningful data signal. If the output enabler  302  enables the data output path such as in a read-data condition of the memory device, then an output latch  304 , which is coupled to the output enabler  302 , latches the data for the period of time in which valid data is required to be held on a data output pin..  
         [0042]    By holding the data bit for a predefined period of time, the output latch  304  frees up internal resources of the memory device, such as multiple direct-current sense amplifiers (not shown), to get additional data for outputting through the output buffer. This is especially important in Double Data-Rate (DDR) memory devices which produce a new data word on the data output pins with every clock edge or clock transition. The output latch  304  produces two primary signals: DQHO* and DQLO. The DQHO* signal represents an active-low data output high signal. This means that whenever this signal is at a low level (active level) the data signal ought to be output at a high level. The DQLO signal represents an active-high data output low signal. This means that whenever this signal is at a high level (active high) the data signal ought to be output at a low level. The output latch  304  presents the DQHO* and DQLO signals to the pre-driver circuit  202 .  
         [0043]    The output latch  304  presents the DQHO* signal to a boot circuit  308  of the pre-driver circuit. The boot circuit  308  provides a voltage level that is appropriate for a full-drive pull-up mode or a reduced-drive pull-up mode. The boot circuit  308  presents the desired voltage to both the reduced-drive pull-up circuit  306  and the full-drive pull-up  310 .  
         [0044]    The pre-driver circuit  202  includes a negative protection circuit  316 . The negative protection circuit  316  prevents a negative voltage data signal, which is driven from devices outside of the memory device, from inadvertently turning on the output buffer. In other words, several external devices and memory devices share the same bus as the present memory. When those external devices are sending signals on the bus wires, some of those signals may create a momentary negative voltage (below ground level) in response to voltage undershoot or ringing on the bus wires. These spurious voltages may cause the output buffer to inadvertently turn on if there isn&#39;t some protection against the negative voltages. As will be described more fully below, the negative voltage protection circuit disables the output drivers and pre-drivers from inadvertently turning on.  
         [0045]    The pre-driver circuit  202  includes a reduced-drive pull-down circuit  312  and a full-drive pull-down circuit  314 . Both of these pull-down circuits receive the DQLO signal from the output latch  304 . Thus, the combination of the pull-up circuits  306  and  310  and the pull-down circuits  312  and  314  allow the output data signal to be driven to a desired logic voltage level, high or low, at an appropriate drive mode, full-drive or reduced-drive.  
         [0046]    [0046]FIG. 4 is a circuit diagram of a boot circuit  308  according to an embodiment of the invention. The boot circuit  308  includes an inverter circuit  430  that receives the DQHO* signal and produces three signals: DQHO, DQH and DQH*. The DQH* signal has the same information content as the DQHO* signal except that it is strengthened by the inverter circuit. The DQH signal has the same information content as the DQHO* or the DQH* signal except that its level is inverted with respect to the level of the DQHO* or the DQH* signal. The third signal is simply another inverted DQHO* signal labeled DQHO.  
         [0047]    The boot circuit  308  includes a level shifter  402  which will be described by way of its operation. When the DQH* signal is high, this indicates that the reduced-drive mode is desired. With DQH* at a high level, the n-channel transistor T 418  pulls the gate of the p-channel transistor T 412  to ground, and transistor T 412  is turned on. Transistor T 414  is turned off because transistor T 410  is turned-on due to the high DQHO* signal at the gate of the transistor T 410 .  
         [0048]    With transistor T 412  turned on, the node labeled KEEP* is coupled to the supply source VCCP DQ, which is a pumped voltage bus dedicated for powering the high-current demands of the output driver circuits. The VCCP_DQ pumped voltage level in greater than the Vcc supply levels and is produced in well known fashion using an on-chip voltage pump circuit (not shown). Because KEEP* is high, the transistor T 420  remains turned off but transistor T 426  is turned on. Because transistor T 426  is turned on, the transistor T 426  couples the node BOOT_UP to the regular supply voltage Vcc. Thus, in reduced-drive pull-up mode, the BOOT_UP node is coupled to the level of the regular supply voltage Vcc. Also, the set of transistors  428  are wired in a diode configuration to form a diode stack to clamp the voltage level on the BOOT_UP node from rising above Vcc.  
         [0049]    Each transistor in the set of transistors  424  has its source coupled to its drain to form a capacitor configuration. When the DQHO* signal is high, the node BOOT_UP is coupled to the supply voltage. Thus, one side of the transistors T 424 A, T 424 B, and T 424 C is coupled to the BOOT_UP supply voltage, but the other side of these capacitor-coupled transistors is coupled to a low voltage because the high DQHO* signal is inverted due to the inverter  430 . As will be discussed, when the DQHO* signal goes low, the transistors T 424 A, T 424 B, and T 424 C act as a voltage pump to pump the BOOT_UP node to a pumped voltage.  
         [0050]    Recalling that DQH* is a low-active signal such that when the signal DQH* is low, this indicates that output data high is desired. It also means the signal DQH is high. If signal DQH* is low, the transistor T 418  is turned off and the transistor T 416  is turned on. The transistor T 416 , being on, couples the node KEEP* to ground. With the node KEEP* at a low level, the transistor T 420  turns on. Recall that with the DQH* signal low, the transistors T 424 A, T 424 B, and T 424 C act to pump the BOOT_UP node to a pumped voltage. As the transistor T 420  turns on, the node BOOT_UP is clamped to the pumped voltage VCCP_DQ without further assistance from the transistors T 424 AD, T 424 B, and T 424 C.  
         [0051]    What has been discussed hereinbefore is that the boot circuit  400  allows the node BOOT_UP to be booted up to a regular supply voltage when output data low is desired or to a pumped supply voltage when output data high is desired. The node BOOT_UP will be shared with other circuits in the pre-driver circuit  202  as discussed hereinbelow.  
         [0052]    [0052]FIG. 5 is a detailed schematic circuit diagram of the full-drive pull-up circuit  310  of FIG. 3 and operates to source the output-high signal in full-drive pull-up mode. The circuit  310  is receptive to the FULL-DRIVE signal which indicate the drive mode of the output drivers. If the level of the FULL-DRIVE signal is high, this indicates that full-drive mode on the output pins is desired. If the level is low, then full-drive mode is not desired. The source of the FULL_DRIVE signal can be many things such as an external pin on the memory device, or an internal programmable register. The FULL_DRIVE mode signal may be factory-programmable or programmed in the field. The FULL_DRIVE mode signal may be manually set or may be selected by automatic sensing of the type of connection or termination on the output pins and setting the drive mode accordingly. Those skilled in the art will readily recognize that the FULL_DRIVE signal can be set though a wide variety of manual or automatic means.  
         [0053]    When the FULL-DRIVE signal is low, transistor T 522  is turned off. Inverter  526  inverts the low FULL-DRIVE signal to present a high signal at the gate of the transistor T 524  which will turn on transistor T 524 . Transistor T 524  then pulls the gate of the transistor T 518  to ground which will turn on transistor T 518 . Transistor T 518  then couples the node FD_BOOT* to the supply node BOOT_UP, which is high. With FD_BOOT* being high, the set of p-channel transistors  504  will not be turned on. Transistor T 530  will be turned on to pull the node FULL-DRIVE PUP to ground thereby disabling the full-drive mode.  
         [0054]    Note that transistors T 528  and T 508  are connected to a NEG signal at the gate. The NEG signal is generated by a negative voltage protection circuit to inhibit the circuit  310  from turning on because of undesired manipulation of an output path by devices external to a memory device. This is described in more detail below in conjunction with the description of the negative voltage protection circuit shown in FIG. 7.  
         [0055]    When the FULL-DRIVE signal is high, transistor T 522  turns on and transistor T 524  turns off. Transistor T 522  couples the node FD_BOOT* to ground. With FD_BOOT* at a low level, the set of p-channel transistors  504  turns on to transfer the voltage at the supply node BOOT_UP to each source of each p-channel transistor in the set of p-channel transistors  504 .  
         [0056]    To enable the full-drive pull-up mode, the pumped voltage at the supply node BOOT_UP should be transferred to the node FULL_DRIVE PUP. For this transfer to be possible, the set of p-channel transistors  506  are turned on and the signal DQHO* is at a low level. A low DQHO* signal will pass through the pair of invertors  502  unchanged in level to turn on p-channel transistors T 506 A and T 506 B. When the set of p-channel transistor  506  is turned on, the voltage of the node BOOT_UP at each drain of each transistor  506  is transferred to the node FULL_DRIVE PUP at the source of each transistor. A high DQHO* signal will turn on transistor T 510  which couples the node FULL_DRIVE PUP low indicating that no full drive on the output pin is needed.  
         [0057]    [0057]FIG. 6 is a schematic diagram of the reduced drive pull up circuit  306  to provide a reduced-drive pull-up mode signal. The reduced drive pull up circuit  306  is receptive to the DQHO* signal and the BOOT_UP supply voltage. If the DQHO* signal is low, the set of invertors  608  allows the signal to pass unchanged. When a low DQHO* signal is presented at each gate of each transistor of the set of transistors  602 , the voltage at the supply node BOOT_UP is presented to the node PUP.  
         [0058]    The gate of the transistor  604  couples to a signal NEG. The signal NEG is normally at a high level to turn on the transistor T 604 . The signal NEG becomes low to turn off the transistor T 604  only when a negative voltage was detected on the output pin as described below. If the signal DQHO* is high, the transistor T 606  will turn on and couple the node signal PUP to ground.  
         [0059]    [0059]FIG. 7 is a circuit diagram of a circuit to provide negative voltage protection according to an embodiment of the invention. Negative voltage protection circuit  316  includes a negative voltage protection subcircuit  702  for the reduced-drive pull-up circuit  306  and a negative voltage protection subcircuit  704  for the full-drive pull-up circuit  310 . The wire DqxPAD is a sense line connected to the data output pin on the memory device to which other external devices are connected via a databus. The data bus may be driven by other memory devices operating as drivers or sources of voltage while devices such as memory controllers or microprocessors may be connected to the data bus to receive or sink voltage. The possibility exists, therefore, for the voltage on a data bus line to momentarily become negative from ringing or undershoot on a lowgoing signal driver. Without proper protection against such negative voltages, the circuits internal to the memory device may inadvertently turn on. The protection against these negative voltages is shown in FIG. 7.  
         [0060]    In the negative voltage protection subcircuit  702 , when the signal DQXPAD becomes negative it will turn on transistors T 708  and T 714 . Transistor T 708  allows the signal PUP to follow the signal DQxPAD when it is negative enough to turn on transistor T 708 . A negative voltage PUP signal will turn off an output driver as will be discussed. The transistor T 714  will also allow the NEG signal to follow the negative DQXPAD signal. The NEG signal will turn off various circuits internal to the pre-driver circuit of the memory device so as to inhibit undesired results. Transistor T 712  normally would couple the NEG signal high when the DQxPAD signal is not negative enough to turn on transistor T 714 .  
         [0061]    In the negative voltage protection subcircuit  704 , when the signal DQxPAD becomes negative, it will turn on transistor T 718  to allow the FULL_DRIVE PUP signal to follow the negative DQxPAD signal. A negative FULL_DRIVE PUP signal would turn off a portion of the output buffer.  
         [0062]    [0062]FIG. 8 is a schematic diagram of the full-drive pull-down circuit  314  used to indicate the full-drive pull-down mode according to an embodiment of the invention. When both the FULL_DRIVE signal and the DQLO signal are at a high level, the node A will be pulled to ground. More specifically, when the FULL_DRIVE signal is high, transistor T 814  is turned off and the set of n-channel transistors  806  will be turned on. When the DQLO signal is also high, transistor T 802  is turned off and the set of n-channel transistors  804  will be turned on. With both sets of n-channel transistors  804  and  806  turned on, the node A is pulled to ground. With node A at ground, the n-channel transistor T 812  is turned off and the p-channel transistor  810  is turned on. Transistor T 810  couples the node PDN to the supply voltage, which is at a high level.  
         [0063]    When either the FULL_DRIVE signal or the DQLO signal is low, the node A is at a high level. More specifically, when the FULL_DRIVE signal is low, the transistor T 814  is turned on and the set of n-channel transistors  806  is turned off. Transistor T 814  couples the node A to the voltage supply Vcc. When the DQLO signal is low, transistor T 802  is turned on and the set of n-channel transistors  804  is turned off. Transistor T 802  couples the node A to the voltage supply Vcc. When the node A is at a high level as being coupled to the voltage supply, transistor T 812  is turned on and the p-channel transistor  810  is turned off. The transistor T 812  couples the node PDN to ground.  
         [0064]    What has been shown hereinbefore is that to enable the full-drive pull-down mode, both the FULL_DRIVE signal and the DQLO signal should be at a high level. To disable the full-drive pull-down mode, either the FULL-DRIVE signal or the DQLO signal can be at a low level.  
         [0065]    [0065]FIG. 9 is a schematic diagram of the reduced drive pull-down circuit  312  which operates on reduced-drive pull-down mode according to an embodiment of the invention. Circuit  312  is receptive to the DQLO signal and produces the PDN signal.  
         [0066]    When the DQLO signal is low, transistor T 902  is turned on and the set of n-channel transistors  904  is turned off. Transistor T 902  will then couple the node B to the voltage supply Vcc. With node B at a high level, transistor T 908  is turned on and the p-channel transistor  906  is turned off. When transistor T 908  is turned on, the node PDN couples to ground.  
         [0067]    When the DQLO signal is high, transistor T 902  is turned off and the set of n-channel transistors  904  is turned on. The set of n-channel transistors  904  couples the node B to ground. When the node B is at ground, transistor T 908  is turned off and the p-channel transistor  906  is turned on. The p-channel transistor  906  couples the node PDN to the voltage supply, which is at a high level.  
         [0068]    What has been shown hereinbefore is that to enable the reduced-drive pull-down mode, the DQLO signal should be high. To disable the reduced-drive pull-down mode, the DQLO signal should be low.  
         [0069]    [0069]FIG. 10 is a block diagram of a system according to one embodiment of the present invention. Computer system  1000  contains a processor  1010  and a memory system  1002  housed in a computer unit  1005 . Computer system  1000  is but one example of an electronic system containing another electronic system, e.g., memory system  1002 , as a subcomponent. The memory system  1002  includes a memory device that includes the multiple-mode output driver as discussed in various embodiments of the present invention. Computer system  1000  optionally contains user interface components. These user interface components include a keyboard  1020 , a pointing device  1030 , a monitor  1040 , a printer  1050 , and a bulk storage device  1060 . It will be appreciated that other components are often associated with computer system  1000  such as modems, device driver cards, additional storage devices, etc. It will further be appreciated that the processor  1010  and memory system  1002  of computer system  1000  can be incorporated on a single integrated circuit. Such single-package processing units reduce the communication time between the processor and the memory circuit. Any of these components of the system may contain a memory device that includes the multiple-mode output driver of the present invention. This is particularly true of a graphics subsystem  1070  of FIG. 10 utilizing SGRAM that includes the multiple-mode output driver as discussed in various embodiments of the present invention.  
         [0070]    [0070]FIG. 11 is a block diagram of a system according to one embodiment of the present invention. Memory system  1900  contains one or more memory modules  1902  and a memory controller  1912 . Each memory module  1902  includes at least one memory device  1910 . Memory controller  1912  provides and controls a bidirectional interface between memory system  1900  and an external system bus  1920 . Memory system  1900  accepts a command signal from the external bus  1920  and relays it to the one or more memory modules  1902  on a command link  1930 . Memory system  1900  provides for data input and data output between the one or more memory modules  1902  and external system bus  1920  on data links  1940 . At least one of the memory devices  1910  includes the multiple-mode output driver as discussed in various embodiments of the present invention.  
       CONCLUSION  
       [0071]    It will be understood by those skilled in the art that the circuits and signals described above in the various embodiments of the present invention are illustrative and that their implementation can take on many variations without departing from the spirit and scope of the claimed invention. For example, although some signals such as DQHO* have been described as active-low or low-active (they indicate an active or an ON state with a low or zero voltage), one skilled in the art will readily recognize that this is a design convention which is a matter of choice and convenience and that active-high signals may be substituted therefor. In a similar fashion, active-high signals (where the active or the ON state is indicated by a high or non-zero voltage) are also readily substitutable for active-low signals. Thus, negative or positive logic conventions, active-high or active-low signals, positive or negative voltages, p-channel or n-channel transistors, and many other complementary systems may be used with the present invention without departing from the claimed invention or limiting the equivalents of the present invention.  
         [0072]    It will further be understood that the above description of a SDRAM (Synchronous Dynamic Random Access Memory) is intended to provide a general understanding of the memory device and is not a complete description of all the elements and features of a SDRAM. Further, the invention is equally applicable to any size and type of memory circuit and is not intended to be limited to the SDRAM described above. Other alternative types of devices include DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory) or Flash memories. Additionally, the memory device could be SGRAM (Synchronous Graphics Random Access Memory), SDRAM II, and DDR SDRAM (Double Data Rate SDRAM), as well as Synchlink or Rambus DRAMs and other emerging memory technologies.  
         [0073]    As recognized by those skilled in the art, memory devices of the type described herein are generally fabricated as an integrated circuit containing a variety of semiconductor devices as shown in FIG. 12. The integrated circuit is supported by a substrate  1500 . Integrated circuits are typically repeated multiple times on each substrate  1510 . The substrate is further processed to separate the integrated circuits into dies as is well known in the art.  
         [0074]    Although the specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used. Accordingly, the scope of the invention should only be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.