Abstract:
A method in an integrated circuit for implementing a reduced voltage repeater circuit on a signal line having thereon reduced voltage signals. The reduced voltage signals has a voltage level that is below V DD . The reduced voltage repeater circuit is configured to be coupled to the signal line and having an input node coupled to a first portion of the signal line for receiving a first reduced voltage signal and an output node coupled to a second portion of the signal line for outputting a second reduced voltage signal. The method includes coupling the input node to the first portion of the signal line. The input node is coupled to an input stage of the reduced voltage repeater circuit. The input stage is configured to receive the first reduced voltage signal on the signal line. The input stage is also coupled to a level shifter stage that is arranged to output a set of level shifter stage control signals responsive to the first reduced voltage signal. A voltage range of the set of level shifter stage control signals is higher than a voltage range associated with the first reduced voltage signal. There is further included coupling the output node to the second portion of the signal line. The output node also is coupled to an output stage of the reduced voltage repeater circuit. The output stage is configured to output the second reduced voltage signal on the output node responsive to the set of level shifter stage control signals. A voltage range of the second reduced voltage signal is lower than the voltage range of the set of level shifter stage control signals.

Description:
This application is a continuation in part of U.S. patent application Ser. No. 09/037,289 entitled “Reduced voltage input/reduced voltage output ti-state buffers and methods therefor,” filed Mar. 9, 1998 U.S. Pat. No. 6,181,165, which is incorporated herein by reference. 
    
    
     RELATED APPLICATIONS 
     This application is related to the following applications, which are filed on the same date herewith and incorporated herein by reference: 
     Application entitled “MIXED SWING VOLTAGE REPEATERS FOR HIGH RESISTANCE OR HIGH CAPACITANCE SIGNAL LINES AND METHODS THEREFOR” filed by inventors Gerhard Mueller and David R. Hanson on the same date. 
     Application entitled “FULL SWING VOLTAGE INPUT/FULL SWING VOLTAGE OUTPUT BI-DIRECTIONAL REPEATERS FOR HIGH RESISTANCE OR HIGH CAPACITANCE B-DIRECTIONAL SIGNAL LINES AND METHODS THEREFOR” filed by inventors Gerhard Mueller and David R. Hanson on the same date. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to repeater circuits for high resistance and/or high capacitance signal lines on an integrated circuit. More particularly, the present invention relates to reduced voltage input/reduced voltage output repeaters which, when employed on a high resistance and/or high capacitance signal line, reduces the signal propagation delay, power dissipation, chip area, electrical noise, and/or electromigration. 
     In some integrated circuits, there exist signal lines which span long distances and/or coupled to many circuits. In modern dynamic random access memory circuits, for examples, certain unidirectional signal lines such as address lines may be coupled to many circuits and may therefore have a high capacitive load and/or resistance associated therewith. Likewise, certain bi-directional lines such as read write data (RWD) lines may also be coupled to many circuits and may therefore also have a high capacitive load and/or resistance associated therewith. The same issue also applies for many signal lines in modern microprocessors, digital signal processors, or the like. By way of example, the same issue may be seen with loaded read data lines and write data lines of memory circuits, clock lines of an integrated circuit, command lines, and/or any loaded signal carrying conductor of an integrated circuit. The propagation delay times for these signal lines, if left unremedied, may be unduly high for optimal circuit performance. 
     To facilitate discussion, FIG. 1 illustrates an exemplary signal line  100 , representing a signal conductor that may be found in a typical integrated circuit. Signal line  100  includes resistors  102  and  104 , representing the distributed resistance associated with signal line  100 . Resistors  102  and  104  have values which vary with, among others, the length of signal line  100 . There are also shown capacitors  106  and  108 , representing the distributed capacitance loads associated with the wire or signal bus and the circuits coupled to signal line  100 . 
     The resistance and capacitance associated with signal line  100  contribute significantly to a signal propagation delay between an input  110  and an output  112 . As discussed in a reference entitled “Principles of CMOS VLSI design: A Systems Perspective” by Neil Weste and Kamran Eshraghian, 2nd ed. (1992), the propagation delay of a typical signal line may be approximately represented by the equation 
     
       
           t   delay =0.7( RC )( n )( n+ 1)/2  Eq. 1  
       
     
     wherein n equals the number of section, R equals the resistance value, C equals the capacitance value. For the signal line of FIG. 1, the propagation delay is therefore approximately 2.1 RC (for n=2). 
     If the resistance value (R) and/or the capacitance value (C) is high, the propagation delay with signal line  100  may be significantly large and may unduly affect the performance of the integrated circuit on which signal line  100  is implemented. For this reason, repeaters are often employed in such signal lines to reduce the propagation delay. 
     FIG. 2 depicts a signal line  200 , representing a signal line having thereon a repeater to reduce its propagation delay. Signal line  200  is essentially signal line  100  of FIG. 1 with the addition of a repeater  202  disposed between an input  210  and an output  212 . In the example of FIG. 2, repeater  202  is implemented by a pair of cascaded CMOS inverter gates  204  and  206  as shown. For ease of discussion, repeater  202  is disposed such that it essentially halves the distributed resistance and capacitance of signal line  200 . 
     In this case, the application of Eq. 1 yields a propagation delay of 0.7 (RC)+t DPS +t DPS +0.7 (RC) or 1.4 (RC)+2t DPS , wherein t DPS  represents the time delay per inverter stage. Since t DPS  may be made very small (e.g., typically 250 ps or less in most cases), the use of repeater  202  substantially reduces the propagation delay of the signal line, particularly when the delay associated with the value of R and/or C is relatively large compared to the value of t DPS.    
     Although the use of CMOS repeater  202  proves to be useful in reducing the propagation delay for some signal lines, such an CMOS inverter-based repeater approach fails to provide adequate performance in reduced voltage input/reduced voltage output applications. Reduced voltage input refers to input voltages that are lower than the full V int  or V DD , the internal voltage at which the chip operates. By way of example, if V int  is equal to 2 V, reduced voltage signal may swing from 0-1 V or −0.5 V to 0.5 V. In some cases, the reduced voltage may be low enough (e.g., 1 V) that it approaches the threshold voltage of the transistors (typically at 0.7 V or so). Likewise, reduced voltage output refers to output voltages that are lower than the full V int , the internal voltage at which the chip operates. 
     To appreciate the problems encountered when reduced voltage signals are employed in the inverter-based repeater, which is operated at V int  or V DD , consider the situation wherein the input of the inverter is logically high but is represented by a reduced voltage signal (e.g., around 1 V). In this case, not only does the n-FET of the CMOS inverter stage conduct as expected but the p-FET, which is in series thereto, may also be softly on, causing leakage current to traverse the p-FET. The presence of the leakage current significantly degrades the signal on the output of the repeater circuit (and/or greatly increasing power consumption). 
     Despite the fact that CMOS inverter-based repeaters do not provide a satisfactory solution in reduced voltage applications, chip designers continue to search for ways to implement repeaters in the reduced voltage integrated circuits. Reduced voltage signals are attractive to designers since reduced voltage signals tend to dramatically reduce the power consumption of the integrated circuit. Further, the use of reduced voltage signals leads to decreased electromigration in the conductors (e.g., aluminum conductors) of the integrated circuit. With reduced electromigration, the chance of developing voids or shorts in the conductors is concomitantly reduced. Further, the reduction in the power consumption also leads to decreased electrical noise since less charge is dumped on the ground and power buses of the integrated circuit at any given time. 
     As can be appreciated from the foregoing, there is a desire for improved techniques for implementing reduced voltage input/reduced voltage output repeaters on the high resistance and/or high capacitance signal lines of an integrated circuit. 
     SUMMARY OF THE INVENTION 
     The invention relates, in one embodiment, to a method in an integrated circuit for implementing a reduced voltage repeater circuit on a signal line having thereon reduced voltage signals. The reduced voltage signals has a voltage level that is below V DD . The reduced voltage repeater circuit is configured to be coupled to the signal line and having an input node coupled to a first portion of the signal line for receiving a first reduced voltage signal and an output node coupled to a second portion of the signal line for outputting a second reduced voltage signal. The method includes coupling the input node to the first portion of the signal line. The input node is coupled to an input stage of the reduced voltage repeater circuit. The input stage is configured to receive the first reduced voltage signal on the signal line. The input stage is also coupled to a level shifter stage that is arranged to output a set of level shifter stage control signals responsive to the first reduced voltage signal. A voltage range of the set of level shifter stage control signals is higher than a voltage range associated with the first reduced voltage signal. 
     There is further included coupling the output node to the second portion of the signal line. The output node also is coupled to an output stage of the reduced voltage repeater circuit. The output stage is configured to output the second reduced voltage signal on the output node responsive to the set of level shifter stage control signals. A voltage range of the second reduced voltage signal is lower than the voltage range of the set of level shifter stage control signals. 
     In another embodiment, the invention relates to a method, in an integrated circuit, for implementing a reduced voltage repeater circuit on a signal line having thereon reduced voltage signals. The reduced voltage signals has a voltage level that is below V DD . The reduced voltage repeater circuit is configured to be coupled to the signal line and has an input node coupled to a first portion of the signal line for receiving a first reduced voltage signal and an output node coupled to a second portion of the signal line for outputting a second reduced voltage signal. The method includes receiving the first reduced voltage signal using an input stage of a reduced voltage repeater circuit, the input stage being coupled to the input node. Additionally, there is included forming, using a level shifter stage of the reduced voltage repeater circuit, a set of control signals responsive to the first reduced voltage signal, a voltage range of the set of control signals being higher than a voltage range associated with the first reduced voltage signal. Furthermore, there is included outputting, using an output stage of the reduced voltage repeater circuit, a second reduced voltage signal responsive to the set of control signals, a voltage range associated with the second reduced voltage signal being lower than the voltage range of the control signals. 
     In another embodiment, the invention relates to a reduced voltage bi-directional repeater circuit configured to be coupled to a reduced voltage bi-directional repeater circuit on a signal line having thereon reduced voltage signals. The reduced voltage signals has a voltage level that is below V DD . The reduced voltage bi-directional repeater circuit is configured to be coupled to the signal line and has a first data port configured to be coupled to a first portion of the signal line and a second data port configured to be coupled to a second portion of the signal line. The repeater circuit includes a first enable node configured to receive a first repeater enable signal at the reduced voltage bi-directional repeater circuit. The first repeater enable signal indicates a direction of signal transmission from the first data port to the second data port. The repeater circuit further includes a second enable node configured to receive a second repeater enable signal at the reduced voltage bi-directional repeater circuit. The second repeater enable signal indicates a direction of signal transmission from the second data port to the first data port, wherein the first data port is coupled to both an input stage of a first reduced voltage repeater circuit and an output stage of a second reduced voltage repeater circuit. 
     These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates an exemplary signal line, representing a signal conductor that may be found in a typical integrated circuit. 
     FIG. 2 depicts the signal line of FIG. 1 having thereon a repeater to reduce its propagation delay FIG. 3A illustrates, in accordance with one embodiment of the present invention, a simplified reduced voltage signal tri-state buffer circuit, representing a circuit that may be employed as a reduced voltage signal unidirectional repeater. 
     FIG. 3B illustrates, in accordance with one embodiment of the present invention, a simplified reduced voltage bi-directional repeater. 
     FIG. 4A illustrates, in greater detail and in accordance with one embodiment of the present invention, a tri-state buffer circuit that is capable of passing reduced voltage signals. 
     FIG. 4B illustrates, in greater detail and in accordance with one embodiment of the present invention, a reduced voltage bi-directional repeater. 
     FIGS. 5-12 illustrate, in accordance with various embodiments of the present invention, various alternative configurations of the reduced voltage input/reduced voltage output tri-state buffer circuit that may be employed for a unidirectional repeater or a bi-directional repeater application. 
     FIG. 13 illustrates, to facilitate discussion, a diagrammatic representation of an exemplary DRAM architecture, including a RWD line. 
     FIGS. 14 a ,  14   b  and  14   c  illustrate a representation of the DRAM architecture of FIGS. 13 a ,  13   b  and  13   c , including a bi-directional repeater implemented on the RWD line in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known structures and/or process steps have not been described in detail in order to not unnecessarily obscure the present invention. 
     The invention relates, in one embodiment, to a technique for improving performance in reduced voltage integrated circuits. In accordance with one aspect of the present invention, various reduced voltage tri-state buffer configurations are disclosed as being suitable candidates for unidirectional or bi-directional repeater applications. In accordance with one aspect of the present invention, reduced voltage unidirectional repeaters are employed on high resistance and/or high capacitance unidirectional line(s) of an integrated circuit to reduce the signal propagation delay, power dissipation, chip area, electrical noise, and/or electromigration. In accordance with another aspect of the present invention, reduced voltage bi-directional repeaters are employed on high resistance and/or high capacitance bi-directional line(s) of an integrated circuit to reduce the signal propagation delay, power dissipation, chip area, electrical noise, and/or electromigration of the integrated circuit. 
     The features and advantages of the present invention may be better understood with reference to the figures that follow. FIG. 3A illustrates, in accordance with one embodiment of the present invention, a simplified tri-state buffer circuit  200 , including input stage  202 , level shifting stage  204 , and output stage  206 . Tri-state buffer circuit  200  represents a repeater circuit suitable for use in a unidirectional low voltage input/low voltage output application. As shown, the buffer enable signal is optionally coupled to input stage  202  to control transistors therein, which pass the reduced voltage input signal on terminal  208  to level shifting stage  204 . As will be shown later herein, the buffer enable signal is also employed in some embodiments to control the passage of signals within level shifter stage  204  and/or the output stage  206 . 
     Within level shifting stage  204 , transistors therein shift the received input signal to a higher voltage range to control gates of transistors within output stage  206 . The higher voltage control signals permit transistors within output stage  206  to be controlled with a higher overdrive voltage, thereby permitting transistors within output stage  206  to source/sink a greater amount of current, thus more rapidly drive the load coupled to the buffer output to the desired reduced voltage level. 
     FIG. 3B illustrates, in accordance with one embodiment of the present invention, a simplified bi-directional repeater circuit  250 , including two tri-state buffers  252  and  254 . Each of tri-state buffers  252  and  254  may be implemented by, for example, the tri-state buffer circuit discussed in connection with FIG.  3 A and offers the advantages thereof. 
     As seen in FIG. 3B, the output of tri-state buffer  252  is coupled to the input of tri-state buffer  254 , forming PORT A. Likewise, the output of tri-state buffer  254  is coupled to the input of tri-state buffer  252 , forming PORT B. Both tri-state buffers  252  and  254  are controlled by control signals ENABLER and ENABLE-W, which are either complementary signals or both equal to a logic level ‘0’ (ground). Depending on the states of the control signals, PORT A may function as either an input port or an output port for reduced voltage signals (with PORT B functioning as the respective output port or input port). These control signals, which are coupled to the stages of the two tri-state buffers in accordance with techniques of the present invention, allow bi-directional repeater circuit to be implemented in reduced voltage applications such as in RWD signal lines of DRAM ICs. 
     FIG. 4A illustrates, in greater detail and in accordance with one embodiment of the present invention, a tri-state buffer circuit  300 , representing a non-inverting tri-state buffer capable of accepting a reduced voltage input and driving a load with its reduced voltage output to function as a unidirectional repeater or a building block of a bi-directional repeater. Buffer circuit  300  includes an input stage  302 , a level shifter stage  304 , and an output stage  306 . Input shifter stage  302  includes two field effect transistors (FETs)  308  and  310 , whose gates are controlled by buffer enable signal ENp on conductor  312 . Note that buffer enable signal ENp and its complement ENc are optional and may be tied high and low respectively without impacting the ability of the circuit of FIG. 4A to function as a basic reduced voltage input/reduced voltage output unidirectional buffer/repeater. The reduced voltage input signal is received at buffer input node  314  and passed by FETs  308  and  310  to nodes  316  and  318  when the buffer enable signal is enabled (i.e., when signal ENp is high). 
     It should be noted that although FETs  308  and  310  are represented in the drawing as low-threshold n-FETs (the low threshold characteristic is represented by the circle surrounding the transistor symbol), such is not a requirement as long as the threshold voltage of these input transistors is lower than the input voltage range. Low threshold transistors are, however, preferred (but not required) for these transistors. In general, low threshold FETs may have a lower threshold voltage (e.g., about 0.4 V to about 0.5 V) than typical FETs (which may be around 0.6 V-0.7 V). 
     Level shifter stage  304  receives the signals from input stage  302  and shifts the received signals to a higher voltage range to control gates of FETs  320  and  322  in output stage  306 . Depending on the value of the reduced voltage input signal on input node  314 , output stage  306  outputs either a logical low (V SS ) or a logical high (the high value of the reduced voltage range, or V REDUCED  herein). Accordingly, a reduced voltage input/reduced voltage output buffer circuit is formed. 
     Like transistors  310  and  308 , output transistors  320  and  322  are represented in the drawing as low-threshold n-FETs (the low threshold characteristic is represented by the circle surrounding the transistor symbol). Although low threshold transistors are preferred for these output transistors for optimum performance, transistors which may have a more typical threshold voltage range may also be employed. 
     To facilitate further understanding, the operation of tri-state buffer  300  will now be explained in detail. Consider the situation wherein the buffer enable signal is disabled to permit tri-state buffer to enter the tri-state mode. In the circuit of FIG. 4A, the tri-state mode is entered when signal ENp on conductor  312  is low. With low signal ENp, n-type FETs  308  and  310  are off, thereby preventing the signal at input node  314  from being passed to level shifter stage  304 . Note that inverters  324  and  328  are operated with an upper power level equal to V DD . As the term is employed herein, V DD  represents the voltage level at which the integrated circuit operates, which is higher than the reduced voltage level V REDUCED  but may be equal to or lower than the voltage level supplied to the integrated circuit from externally. 
     Inverter  324  causes signal ENc (which is the inverse of signal ENp) to go high on conductor  326 , thereby putting tri-state inverter  328  in a high impedance state and decoupling the tri-state inverter output from its input. A high signal ENc also turns on n-FET  330  to pull node  332  low, thereby turning off n-type FET  320 . Thus, buffer output  334  is decoupled from voltage source V REDUCED    336 . 
     The low signal ENp on conductor  312  turns on p-type FET  338 , thereby pulling node  318  high to turn on n-FET  340 . When FET  340  conducts, node  342  is pulled to V SS , thereby turning on p-FET  344  of level shifter stage  304 . When FET  344  conducts, node  316  is pulled towards V DD  (by V DD  voltage source  346 ) to turn off p-FET  348 , thereby decoupling node  342  from V DD  voltage source  350  and keeping node  342  at the V SS  level (dueto the fact that FET  340  conducts). 
     Since node  342  is low, FET  322  is also off, thereby decoupling buffer output  334  from V SS . With FETs  320  and  322  off, buffer output  334  is decoupled from the remainder of the buffer circuit, V REDUCED , and V SS . In other words, buffer circuit  300  is tri-stated and decoupled from the load. 
     When the buffer enable signal is enabled (i.e., when signal ENp of FIG. 4A is high), buffer circuit  300  is taken out of the tri-state mode. Accordingly, the voltage value on buffer output  334  will vary within the range 0-V REDUCED  responsive to the voltage value on input node  314 . 
     Consider the situation when signal ENp is high and a V SS  voltage level appears on input node  314 . The high signal ENp causes FETs  308  and  310  to turn on, passing the V SS  voltage level to nodes  318  and  316  respectively. Since FET  310  conducts, node  316  goes low to turn on FET  348 , thereby pulling node  342  to V DD  (by V DD  voltage source  350 ). Since ENp is high and its inverted ENc signal is low, tri-state inverter  328  passes the value on node  342  to node  332 , causing node  332  to go low (since tri-state inverter  328  inverts its output relative to its input). The low signal ENc turns off FET  330 , thereby decoupling node  332  from V SS . Since node  332  is at V SS , FET  320  is turned off to decouple buffer output  334  from V REDUCED  voltage source  336 . 
     The low node  318  (p-FET  338  is turned off by the high ENp signal to ensure that node  318  stays low) turns off FET  340  to decouple node  342  from V SS  and ensuring that node  342  stays at the V DD  level (due to the fact that FET  348  conducts). With node  342  at the high V DD  level, this full V DD  voltage is applied to the gate of output FET  322 , allowing FET  320  to sink current from the load via buffer output  334  and to quickly pull buffer output  334  to the V SS  voltage level. Thus, the presence of level shifter stage  304  allows gates of transistors  320  and  322  to be controlled by control signals having the full voltage range from V SS -V DD . As can be appreciated from the foregoing, a V SS  input signal on input node  314  causes a V SS  output signal to appear on output node  334  when buffer circuit  300  is not tri-stated. 
     Consider the situation when signal ENp is high (i.e., buffer circuit  300  is not tri-stated) and a V REDUCED  voltage level appears on input node  314 . The high signal ENp causes FETs  308  and  310  to turn on, passing the V REDUCED  voltage level to nodes  318  and  316  respectively. Since FET  308  conducts, the V REDUCED  voltage level is passed to node  318 , thereby turning on FET  340  to pull node  342  to V SS  When node  342  is pulled to V SS , p-FET  344  is fully on to pull node  316  to about V DD  (by V DD  voltage source  346 ). Thus node  316  is at about V DD  although the conduction of FET  310  causes V REDUCED  to be passed to node  316  from input node  314 . 
     Since node  316  is at about V DD , this full V DD  voltage is applied to the gate of p-FET  348  to turn FET  348  off, thereby decoupling node  342  from V DD  voltage source  350  and ensuring that node  342  stays at the V SS  level. It should be appreciated that level shifter stage  304  also functions to stabilize the voltage at node  342  at the V SS  value to ensure that FET  322  stays fully off to decouple buffer output  334  from V SS . Otherwise, FET  348  may be softly on when V REDUCED  is passed to node  316  by FET  310 , pulling the voltage at node  342  above the desired V SS  value and degrading performance and/or causing the buffer circuit to malfunction and/or consuming an undue amount of power. 
     With signal ENp high and its inverted signal ENc low, the V SS  value on node  342  causes node  332  to go to V DD  (since tri-state inverter  328  outputs the inverted value of its input). The low signal ENc also turns off FET  330  to decouple node  332  from V SS . With node  332  at the high V DD  level, this full V DD  voltage is applied to the gate of output FET  320 , allowing FET  320  to source current to the load via buffer output  334  and to quickly pull buffer output  334  to the V REDUCED  voltage level (by V REDUCED  voltage source  336 ). Thus, the presence of level shifter stage  304  allows gates of transistors  320  and  322  to be controlled by control signals having the full voltage range from V SS -V DD . As can be appreciated from the foregoing, a V REDUCED  input signal on input node  314  causes a V REDUCED  output signal to appear on output node  334  when buffer circuit  300  is not tri-stated. 
     Note that although buffer circuit  300  is configured as a tri-state buffer circuit that is noninverting, such is not a requirement. Accordingly, the inventions herein are not necessarily limited to the inverting (or noninverting) feature of the reduced input voltage/reduced output voltage tri-state buffer circuit. 
     By using control signals having the full voltage swing (V SS -V DD ) to control gates of output FETs  320  and  322 , a higher overdrive voltage is obtained to turn on and off these FETs. If the reduced voltage V REDUCED  had been employed to control gates of these output FETs, the FETs would need to be larger to source/sink the same amount of current in the same amount of time. Because the invention employs control signals having the full voltage swing (V SS -V DD ) to control gates of output FETs  320  and  322 , these FETs may be made smaller, which reduces space usage on chip. 
     Reducing the size of the output FETs also reduces the capacitive load to which the buffer circuit is coupled. This is advantageous in applications wherein multiple buffer circuits are employed to assert signals on a common bus conductor and multiple buffer circuit output stages may be coupled to that same common bus. By reducing the size and capacitance associated with the output FETs of the output stage in each buffer circuit, less load capacitance is presented to the buffer circuit that actually drives the bus conductor. With reduced load capacitance, latency and power consumption is advantageously reduced. 
     FIG. 4B illustrates, in accordance with one aspect of the present invention, a bi-directional repeater which employs two tri-state buffer circuits  300   a  and  300   b  coupled in opposite directions. In one preferred embodiment, each of tri-state buffers  300   a  and  300   b  is implemented by the tri-state buffer circuit discussed in connection with FIG.  4 A. For ease of illustration and comprehension, the various components of these tri-state buffers are numbered using the same reference numbering system employed in FIG.  4 A.To distinguish the components belonging to the upper tri-state buffer  300   a  from the components belonging to the lower tri-state buffer  300   b , however, these reference numbers are appended with the letter “a” or “b”. 
     Control signal ENRp is coupled to the input stage of tri-state buffer  300   a  and more specifically to nFETs  310   a  and  308   a . Control signal ENRP is also coupled to inverter  324   a  of the level shifting stage of tri-state buffer  300   a . Control signal ENWp, which is the complementary signal of control signal ENRp is coupled to the input stage of tri-state buffer  300   b  and more specifically to nFETs  310   b  and  308   b . Control signal ENWp is also coupled to inverter  324   b  of the level shifting stage of tri-state buffer  300   b . Note that ENRp and ENWp can also both be equal to a logic level ‘0’ (ground). 
     In operation, when control signal ENRp is high, tri-state buffer  300   a  functions as a unidirectional repeater that passes a reduced voltage signal at port RWD 1  to RWD 0 . Reference may be made back to FIG. 4A for specific details pertaining to the operation of tri-state buffer  300   a  when control signal ENRp is high. At the same time, control signal ENWp goes low, essentially turning off nFETs  308   b  and  310   b  of tri-state buffer circuit  300   b . Thus, tri-state buffer circuit  300   b  is essentially tri-stated and decoupled from port RWD 0  and port RWD 1 . In this case, the entire bi-directional repeater circuit of FIG. 4B functions as a unidirectional repeater which passes a reduced voltage input signal at port RWD 1  to port RWD 0  (i.e., left to right of FIG.  4 B). 
     In the reverse direction, when control signal ENWp is high, tri-state buffer  300   b  functions as a unidirectional repeater which passes a reduced voltage signal at port RWD 0  to RWD 1 . Again, reference may be made back to FIG. 4A for specific details pertaining to the operation of tri-state buffer  300   b  when control signal ENWp is high. At the same time, control signal ENRP goes low, essentially turning off nFETs  308   a  and  310   a  of tri-state buffer circuit  300   a . Thus, tri-state buffer circuit  300   a  is essentially tri-stated and decoupled from port RWD 1  and port RWD 0 . In this case, the entire bi-directional repeater circuit of FIG. 4B functions as a unidirectional repeater which passes a reduced voltage input signal at port RWD 0  to port RWD 1  (i.e., right to left of FIG.  4 B). In general, the enable signal is preferably valid before the data arrives at the repeater to prevent signal transmission delay. 
     FIGS. 5-12 depict various alternative embodiments, showing the various exemplary manners in which input stage, the level shifter stage, and/or output stage may be configured. One of ordinary skills in the art will readily appreciate that any of the exemplary embodiments discussed in these figures may be employed as a unidirectional repeater (e.g., for address lines in DRAMs and/or other loaded unidirectional signal carrying conductors in integrated circuits) or as a bi-directional repeater stage (e.g., for RWD lines in DRAMs and/or other loaded bi-directional signal carrying conductors in integrated circuits). In the case of a bi-directional repeater, any of the tri-state buffers shown in FIGS.  4 A and  5 - 12  may be substituted for either of tri-state buffers  252  and  254  of FIG.  3 B. 
     In each of these FIGS. 5-12, the level shifter stage is employed to boost the reduced voltage input signal into control signals having a greater voltage range to control the output transistors in the output stage. The output transistors are connected in series between V REDUCED  and V SS  to output signals in this reduced voltage range. With the output transistors turned on and off by the higher voltage control signals from the level shifter stage, these transistors can advantageously source or sink a greater amount of current to drive the load with reduced latency. 
     In FIG. 5, the level shifter stage is implemented by a NOR gate  392  instead of a tri-state inverter as in the case of FIG.  4 A. In FIG. 6, a transmission gate  402  is employed instead in the level shifter stage. Transmission gate  402  functions to pass the voltage between its two nodes, i.e., between nodes  404  and node  406 , responsive to control signals  408  and  410 . Again, the level shifter stage comprising transmission gate  402 , transistors  412 ,  414 , and  416  ensures that node  404  stays low when a logical high signal having a reduced voltage (e.g., 1 V) appears at the buffer input. The remainder of the buffer of FIG. 6 functions roughly in an analogous manner to the buffer of FIG. 4A, and the operation of the buffer of FIG. 6 is readily understandable to one skilled in the art in view of this disclosure. 
     In FIG. 7, an inverter  502  is employed in the level shifter stage to furnish control signals having the voltage range between V SS  and V DD  to the output transistor  502 . Two inverters are shown coupled to the gate of transistor  504  to source sufficient current for properly controlling transistor  504 . However, they may be omitted if the buffer enable signal can sufficiently control transistor  504 . There are three output transistors in the output stage, of which transistor  504  acts to quickly decouple the V REDUCED  voltage source from the output when signal ENp is low. As a tradeoff, however, each of output transistors  504  and  506  may be required to be larger to reduce the resistance in series between the V REDUCED  voltage source and the output. The larger transistor  506  may contribute to a higher capacitive load, especially when multiple tri-state buffers are coupled to the same output. In FIG. 8, output transistor  602  is added to ensure that V SS  is also quickly decoupled from the output when the ENp signal is low. Again, the tradeoff results in larger transistors  602  and  604  to overcome the series resistance. The remainder of the buffers of FIGS. 7 and 8 function roughly in an analogous manner to the buffer of FIG. 4A, and the operation of these buffers is readily understandable to one skilled in the art in view of this disclosure. 
     In FIG. 9, a tri-state inverter  702  is employed in the level shifter stage. Tri-state inverter  702  operates in an analogous manner to tri-state inverter  328  of FIG.  4 A. In FIG. 10, transistors  802  and  804  in the output stage are coupled to signal ENpx (generated by inverters  806  and  808  of the level shifter stage) to facilitate fast decoupling of the output from both V SS  and V REDUCED . However, the presence of four transistors in series in the output stage may require larger devices to be employed to overcome the series resistance. In FIG. 11, decoupling of the output from V SS  is performed in the same manner as was done in the buffer of FIG.  4 A. Decoupling of the output from V REDUCED  is accomplished by transistor  902 , albeit at the potential cost of requiring larger devices to be employed for transistors  902  and  904 . In FIG. 12, decoupling of the output from V REDUCED  is performed in the same manner as was done in the buffer of FIG.  4 A. Decoupling of the output from V SS  is accomplished by transistor  1002 , albeit at the potential cost of requiring larger devices to be employed for transistors  1002  and  1004 . The remainder of the buffers of FIGS. 9-12 function in a roughly analogous manner to the buffer of FIG. 4A, and the operation of these buffers are readily understandable to one skilled in the art in view of the remainder of this disclosure. 
     As mentioned earlier, any of the buffers disclosed herein may be employed as a reduced voltage input/reduced voltage output repeater for a unidirectional signal line (such as an address line in a DRAM, a microprocessor, a DSP, or the like). Likewise, any of the buffers disclosed herein may be employed as either the upper half or the lower half of a bi-directional repeater to reduce, among others, the propagation delay associated with high capacitance and/or high resistance bi-directional signal lines. 
     To facilitate discussion of the application of the bi-directional repeater of the present invention in a modern high density integrated circuit, FIGS. 13 a ,  13   b  and  13   c  (referred to collectively herein as FIG. 13) illustrates, a diagrammatic representation of an exemplary DRAM architecture, which shows a RWD line  1302  coupled to a driver/receiver pair  1304  and to each of the sixteen abstract driver/receiver pairs  1306 ( a )-( p ). In FIG. 13, the tri-state buffers within outline  1340  represent the generalized driver/receiver circuit. In this example, each of driver/receiver pairs  1306 ( a )-( p ) represents the driver/receiver pair associated with a second sense amplifier, i.e., the sense amplifier that is employed to further amplify the signal from a cell after that signal has been amplified once by a first sense amplifier. 
     Data lines D 0 -D 15  from each of the cells represents the data to be read from or written to the cells, or more specifically to the first sense amplifier associated with the cell depending on the state of the signals that control drivers  1308  and  1310  associated with each of these driver/receiver pairs  1306 . If data is to be written to the cell that is coupled to data line D 12 , for example, the bit of data may be received by driver/receiver pair  1304  and driven onto RWD line  1302 . Driver  1304  (or more specifically driver  1312  therein) is turned on to pass the data to  1308  which then drives the data onto data line D 12  to be written to the cell. If data is to be read from the cell that is coupled to data line D 12 , for example, the bit of data may be received by driver/receiver pair  1306 ( a ) and driven onto RWD line  1302 . Driver/receiver pair  1304  (or more specifically driver  1313  therein) is turned on to pass the data from data line D 12  to a FIFO or off-chip driver circuit. 
     As can be seen, RWD line  1302  is a bi-directional line that is employed to pass data from off chip to one of the cells or from one of the cells to a FIFO or off-chip driver circuit and ultimately off chip. Note that for simplicity the FIFO and/or off-chip driver circuits have been omitted. With reference to FIG. 13, each driver/receiver pair  1306  has associated with it a capacitor  1320 , representing the capacitive load of that driver/receiver pair  1306  as seen from RWD line  1302  and includes the input capacitance of driver  1308  as well as the output capacitance of driver  1310 . RWD line  1302  then has a capacitive load distributed along its length that includes the capacitance associated with each of the driver/receiver pair  1306  as well as the capacitance of the RWD line itself. Furthermore, RWD line  1302  is a long signal line and tends to have a significant resistance along its length, particularly between driver/receiver pair  1306  (such as driver/receiver pair  1306 ( p )) and driver/receiver pair  1304 . The large resistance and capacitance associated with RWD line  1302  degrades performance both when writing to a cell and when reading therefrom. 
     FIGS. 14 a ,  14   b  and  14   c  (referred to collectively as FIG. 14) shows, in accordance with one embodiment of the present invention, the DRAM circuit portion of FIG. 13, including a bi-directional repeater  1402  disposed in between driver/receiver pair  1304  and the driver/receiver pairs of the cell array. 
     Bi-directional repeater  1402  is preferably disposed such that it is positioned on RWD line  1302  between driver/receiver pair  1304  and all reduced voltage driver/receiver pairs  1306 . That is, it is preferable that any data written to or read from a driver/receiver pair  1306  via the RWD line traverses the bi-directional repeater. When so disposed, bi-directional repeater  1402  serves to decouple a portion of the capacitance associated with RWD line  1302  to improve performance during reading and writing. Note that FIG. 14 is not drawn to scale, e.g., in DRAMS the resistance Rx representing the resistance of a spine RWD can be substantial, i.e., R 1 +R 2 +R 3 . Further, the presence of bi-directional repeater  1402  reduces the amount of resistance seen by driver  1310  of driver/receiver pair  1306  when reading data and reduces the amount of resistance seen by driver  1312  of driver/receiver pair  1304  when writing data to the cell. 
     As can be seen from the foregoing, the use of the repeater of the present invention advantageously reduces the propagation delay associated with high capacitance, high resistance load lines. Furthermore, the use of the repeater of the present invention at strategic locations on the high capacitive load, high resistance lines advantageously improves signaling, i.e., improving the rise and fall edges to counteract the attenuation effects and/or propagation delay of the signal line. The improvement of the rise and fall times is essential to realize high bandwidth data transfer. Without this improvement, the timing window for which the transmitted data is valid is reduced and consequently the frequency at which the bus can be run is limited. If a reduced voltage unidirectional or bi-directional repeater is implemented on an integrated circuit (such as a DRAM, a microprocessor, a DSP chip, or the like) that also employs reduced voltage signals, further advantages in terms of power dissipation, electrical noise, electromigration, and chip area usage is also realized. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.