Abstract:
Logic circuits and methods for designing logic circuits are described herein. In one aspect, the logic circuit comprises complimentary logic units. Examples of various complimentary logic circuits, including driver circuits and core logics, are described herein. In another aspect, complimentary logic is implemented in the method of configuring integrated circuits.

Description:
RELATED APPLICATION 
   This application a divisional of application Ser. No. 10/887,363, filed Jul. 7, 2004, now U.S. Pat. No 7,102,380 the content of which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to digital communication interface and more specifically to high speed circuit designs. 
   BACKGROUND OF THE INVENTION 
   A signal line is a conductor used to transmit electrical signals between various devices in an electronic system or between devices located in two separate electronic systems. Output driver circuits contained on each device are used to buffer signals originating from the device so that they may be driven onto the signal lines. 
   There are well known single-ended output driver circuits (e.g., TTL drivers) that are simple to use. However, most of these previously disclosed driver circuits are not suitable for high speed signals due to their low maximum operating frequency and high noise. For example, the maximum operating frequency of a single-ended CMOS driver circuit IDT74FCT3807D/E, which is available from Integrated Device Technology, Inc. of Santa Clara, Calif., is 166 Mhz. 
   For driving high speed signals, differential drivers are often used. A typical differential driver  10  is schematically illustrated in  FIG. 1 . The differential driver  10  includes data inputs  12   a - 12   b  for inputting a differential data signal, and data outputs  14   a - 14   b  for providing the differential signal to a differential receiver  16  via signal lines. The arrangement of  FIG. 1  is well known to have high operating frequency. However, differential interface designs have disadvantages as well. First, every differential signal requires two or more signal lines. Therefore, a differential I/O interface will require at least twice the number of pins than a single-ended I/O interface, resulting in a larger chip. Furthermore, high speed systems generally require careful matching of the electrical length of the signal lines such that synchronous signals may be received with a common clock and a common phase. This design requirement is sometimes known as “length matching” or “delay matching.” A wide differential interface will require a large number of signal lines, necessarily complicating the length matching effort and increasing the cost of manufacture. In some instances, length matching many signal lines may be impossible on tightly packed circuit boards. Thus, at least in some electronic systems, it is not desirable to use differential interfaces. 
   Accordingly, a single-ended output interface design that communicates single-ended signals at a performance level that is comparable to that of a differential interface may be desirable. 
   SUMMARY OF ASPECTS OF THE INVENTION 
   An embodiment of the invention is a single-ended output interface that uses a differential driver as a design backbone. Unlike a conventional differential interface, which typically has two or more outputs for providing an output signal and its complement, the differential driver of the present embodiment has one of its outputs coupled to drive a signal onto a signal line, while a complementary output is not used for signal transmission. Rather, the complementary output is considered logically redundant and is terminated, for example, by coupling to package ground or system ground via a capacitor. A result of terminating a logically redundant output is that the performance of the output interface may be significantly improved over conventional designs. 
   In one embodiment of the invention, multiple differential drivers are implemented within an integrated circuit that has a package ground plane. According to this embodiment, each “unused” output of the differential drivers may be terminated at the package ground plane through a capacitor. The package ground plane itself may be coupled to one or more GND pins. In this way, very few pins are needed by the “unused” outputs. Furthermore, only one signal line is needed for each single-ended output signal. In comparison to conventional differential interfaces, where two pins and two signal lines are required for each differential signal, the number of pins and signal lines used by the present embodiment may be significantly smaller. 
   Another embodiment of the invention is an integrated circuit having a single-ended input and multiple single-ended outputs, for instance a clock driver. Within the integrated circuit, the input signal is first converted into a differential signal. The differential signal is distributed to the multiple differential drivers. Each differential driver may have an output for providing a single-ended output signal and an “unused” output, which terminates one component of the differential signal. Each “unused” output may be coupled to package ground or system ground via a capacitor for the purpose of improving the performance of the other output. 
   Another embodiment of the invention is an integrated circuit having single-ended inputs and single-ended outputs. The single-ended outputs are implemented using differential drivers each having one output that is “unused.” Within the integrated circuit, differential signals may be originated, processed and distributed to the multiple differential output driver circuits. Each differential driver may have an output for providing a single-ended output signal and an “unused” output. The “unused” output of each differential driver may be coupled to package ground or system ground via a capacitor for the purpose of improving the performance of the other output. Circuits that process differential signals within the integrated circuits may be implemented with differential standard cells in accordance with some embodiments of the invention. 
   Yet another embodiment of the invention is an integrated circuit having a logic core and a plurality of output pads or I/O pads coupled to the logic core. The output pads or I/O pads may include circuits for receiving single-ended signals from the logic core, converting the single-ended signals into differential signals, and providing one component of each differential signal as a single-ended output signal. Another component of each differential signal is terminated, for example, by coupling to package ground or system ground via a capacitor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings which illustrate various example embodiments of the invention. Throughout the description, similar reference names may be used to identify similar elements. 
       FIG. 1  depicts a differential driver. 
       FIG. 2  depicts an output driver circuit that uses a differential driver as a backbone according to an embodiment of the invention. 
       FIGS. 3A-3F  depict examples of various embodiments of the invention. 
       FIGS. 4A-4D  depict an example implementation of a circuit in accordance with an embodiment of the invention. 
       FIG. 5  depicts simulation results of the output interface design of  FIGS. 4A-4B . 
       FIG. 6  depicts a schematic of a known clock driver circuit. 
       FIG. 7  depicts a schematic of a clock driver circuit according to an embodiment of the invention. 
       FIG. 8  depicts an integrated circuit package where unused outputs of the differential drivers are coupled to the common ground plane, in accordance with an embodiment of the invention. 
       FIG. 9A-9D ) depict integrated circuits according to embodiments of the invention. 
       FIG. 10  depicts a ring oscillator circuit implemented according to an embodiment of the invention. 
       FIG. 11  depicts a crystal oscillator circuit implemented according to an embodiment of the invention. 
       FIG. 12A-12G  depict example differential standard cells according to embodiments of the invention. 
       FIG. 13  depicts two comparator circuits that may be used in another example implementation of an output driver circuit in accordance with an embodiment of the invention. 
       FIG. 14  depicts a high speed serial bus system that may be implemented according to an embodiment of the invention. 
       FIG. 15  depicts a high speed wireless communication system that may be implemented according to an embodiment of the invention. 
       FIG. 16  depicts an example half-adder circuit implemented according to an embodiment of the invention. 
       FIG. 17  depicts an example 4-to-1 multiplexer circuit implemented according to an embodiment of the invention. 
       FIG. 18  is a diagram depicting an example gate-level implementation a differential NAND gate of  FIG. 12A . 
       FIG. 19  is a diagram depicting an example gate-level implementation a differential NOR gate of  FIG. 12B . 
       FIG. 20  is a flow diagram depicting an example IC design process according to one aspect of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Various features of the invention, including specific implementations thereof, will now be described. Throughout the description, the term “differential signal” refers to a signal that is carried by more than one signal lines, and thus a differential signal includes two or more component signals that may be complementary to each other. If the sum of two time-varying signals always approximately equals a constant value, such as zero, the signals are said to be “complementary” to each other. The term “single-ended signal” refers to a signal that is carried by a single signal line. Furthermore, the terms “driver” and “driver circuit” are used synonymously. 
   Throughout the description, the term “unused output” refers to an output of a differential output driver that is not used to provide a signal to a receiver, or one that is not used to drive a signal line. The term “unused output” may also refer to an output of a differential output driver that may be coupled to package ground, system ground, voltage source, etc., via a capacitor. Additionally, the term “unused output” may refer to an output of a differential output driver that drives a component of a differential signal to package ground, system ground, voltage source, etc., via a capacitor. An “unused signal” herein may refer to a signal that is provided by an unused output and that is not provided to a signal receiver. A more specific meaning for the above terms may be inferred by context. 
   Furthermore, the terms “couple” and “coupled” may describe a direct or an indirect connection. For example, a node may be connected to one end of a capacitor, and another end of the capacitor may be connected to system ground. The node is said to be “coupled” to system ground even though the connection is an indirect one. 
   The various features of the invention set forth herein may be embodied within a wide range of integrated circuits including, but not limited to, signal drivers, clock drivers, oscillators (e.g., ring oscillators, crystal oscillators), serial bus drivers, ethernet drivers, optical transmitters, memory controllers, memories, microprocessors, wireless transmitters, and power amplifiers, some of which may be found in computer systems and wireless devices (e.g., laptop computers, wireless telephones and personal digital assistants). Also, it should be understood that some implementations described herein may be specific to CMOS technology and that features of the invention may be applicable to other integrated circuit technologies as well. 
   Referring to  FIG. 2 , there is schematically illustrated an output driver circuit in accordance with an embodiment of the invention. The output driver circuit includes inputs  22   a - 22   b  for receiving a differential signal, and drivers  23   a - 23   b  for providing the differential signal through outputs  24   a - 24   b . According to an embodiment of the invention, the differential signal includes two complementary signal components. Note that driver  23   a  drives one of the complementary signals as a single-ended output signal to receiver  29  via a signal line. The other one of the complementary signals is unused and is terminated, for instance by coupling the output  24   b  to system ground (GND) via a capacitor  25 . As a result of terminating the unused signal, which is considered logically redundant to and inverse of the “used” signal, the performance of the output driver circuit may be significantly better than those of conventional single-ended driver designs. 
   In a preferred embodiment, the driver  23   a  and driver  23   b  are connected to the same voltage source and the same ground. In one embodiment of the invention, the circuit in  FIG. 2  may be implemented using TTL-CMOS, which may minimize static current requirement and provide high power output. For example, a TTL-CMOS circuit according to the invention may have a static current that is close to zero (e.g., 0.1 μA) and may have a power output of 3 V or more. A power output of 3 V or more is significantly higher than the power output of an LVDS (Low Voltage Differential Signaling) differential driver, which is typically about 350 mV. Thus, the invention may allow one to achieve high frequency without compromising performance for low static current and high output power. 
   Also depicted in  FIG. 2  are die  21 , package  27 , and inductors  26  representative of the inductance associated with the bonding wires of the package  27 . Also shown in  FIG. 2  is a decoupling capacitor  28 . The decoupling capacitor  28  may be located on the die  21 , outside the die  21  but inside the package  27 , or outside the package  27 . 
   According to one embodiment of the invention, the unused output of the driver  23   b  may be terminated inside or outside the package, and the capacitor  25  may be placed inside the die  21 , outside the die  21  but within the package  27 , or outside the package  27 . Furthermore, the capacitor  25  may be coupled to a voltage source, such as Vcc, or any pre-determined voltage. 
     FIGS. 3A-3F  depict several ways of terminating the unused output. In light of the disclosure herein, one of ordinary skill in the art would appreciate that many other ways of terminating unused outputs are within the scope of the principles of the invention disclosed herein. For instance, in embodiments where a capacitor is illustrated, one of ordinary skill in the art would appreciate that an inductor and/or resistor may be used in combination with or in lieu of the capacitor, depending on the application and loading. Many other combinations and permutations of resistance, capacitance and inductance values and their locations are possible. 
     FIG. 3A  schematically depicts an output driver circuit according to an embodiment of the invention. The output driver circuit includes a differential driver  30  that is configured to receive a differential signal. Unlike outputs of conventional differential drivers, one output of the differential driver  30  provides a single-ended output signal to a signal line, and another output  32  is unused and is terminated. As shown in  FIG. 3A , a capacitor  34  couples the unused output  32  to GND. In one embodiment, the capacitor  34  may have the same capacitance as the load, which is represented by capacitor  38  and which is typically a signal I/O receiver. In one implementation, the capacitance of capacitor  34  may be approximately half-way between the maximum loading capacitance and minimum loading capacitance of the integrated circuit, and the capacitance may vary depending on application. In another implementation where the output load capacitor  38  has a maximum value of about 15 pf, the capacitance of the capacitor  34  is preferably between approximately 5 pf to approximately 13 pf. In the embodiment shown in  FIG. 3A , the capacitor  34  is implemented outside the die  35  and the chip package  31 , for instance on a printed circuit board (PCB). Also shown in  FIG. 3A  are inductors  36   a - 36   b , which represent the inductance within the package  31 . 
     FIG. 3B  schematically depicts a differential driver  30  whose unused output  32  is terminated outside the package  31  via a capacitor  34   a  and an inductor  36   b . Note that in this embodiment the capacitor  34   a  is located on the same die  35  as the differential driver  30 . As in the embodiment shown in  FIG. 3A , the capacitor  34   a  may have the same capacitance as the load. In one implementation, its capacitance may be approximately 5-13 pf. Note that this capacitance may vary depending on the application. 
     FIG. 3C  schematically depicts a differential driver  30  whose unused output  32  is terminated inside the package  31 , in accordance with an embodiment of the invention. In this embodiment, the unused output  32  is terminated at a ground plane of the package  31 . The ground plane is in turn coupled to an external ground (e.g., system ground) via a connector or pin  39 . 
     FIG. 3D  schematically depicts another embodiment of the invention. In this embodiment, the unused output  32  of the differential driver  30  is coupled to an external voltage source Vcc via a capacitor  34 . Note that in this embodiment, the load is coupled to Vcc as well. 
     FIG. 3E  schematically depicts yet another embodiment of the invention. In this embodiment, the unused output  32  of the differential driver  30  is terminated at a pre-determined voltage via a capacitor  34 . Note that in this embodiment, the load is coupled to the same pre-determined voltage as well. 
     FIG. 3F  schematically depicts yet another embodiment of the invention. In this embodiment, capacitor  34   a  and resistor  37  are located on die  35 . Preferably the resistor  37  may have approximately the same resistance as the series resistor R on the signal line. The series resistor R may be implemented to suppress the reflection signal on the signal line. 
   It should be noted that the output driver circuits and the receivers may not necessarily be implemented within the same system. In other words, the signal lines connecting the output driver circuits and the receivers are not limited to signal traces of a printed circuit board (PCB). The output driver circuits according to the present invention may be used to drive signals across cables (e.g., CAT-6 cables) or other types of electrical connections. According to one embodiment, the output driver circuit may drive signals that have a large voltage swing. Thus, the signals may be carried for a large distance. Furthermore, in some embodiments, the signal lines may not be strictly electrical connections. Rather, a signal line may be any signal path, which may include electrical connections, optical connections, wireless connections, and/or any other type of conduits, and/or any combination thereof. 
   Referring now to  FIGS. 4A-4D , there is shown schematically an example implementation of a circuit according to an embodiment of the invention. In  FIGS. 4A-4D  and other drawings, “gg” indicates chip ground, and “vv” indicates chip voltage Vdd. This illustrated implementation may be sub-divided generally into three stages. The first stage  410 , which includes inverter  412  and transmission gate  414 , converts the input signal into a differential signal. Naturally, the inverter  412  causes a small signal propagation delay. A function of the transmission gate  414  is to provide sufficient delay such that the resulting differential signal has complementary components. In an alternate embodiment, the transmission gate  414  may be replaced by an appropriate RC circuit. In that embodiment, the RC circuit may have RC characteristics that generally match those of the inverter  412 . 
   With reference still to  FIG. 4A , the first stage  410  may be coupled to an electrostatic discharge (ESD) circuit  416  that protects the input circuit from electrostatic discharge. Also note that in this variation the ESD circuit  416  utilizes the transmission gate  414  to provide the ESD protection function. The ESD circuit  416  further provides a 5V I/O tolerant function when the overall circuit is driven by 3 V to 3.6 V. Furthermore, the first stage  410  may include a comparator, an example of which is shown in  FIG. 13  (described further below), for receiving differential signals. 
   The second stage  420  includes two inverter circuits  422   a - 422   b  coupled to inverter  412  and transmission gate  414 , respectively, to receive the differential signal. Note that the second stage  420  is optional. In another embodiment of the invention, outputs of the first stage  410  may connect directly to inputs of the third stage  430 . In other embodiments, the second stage  420  may include any logic circuit. For instance, the second stage  420  may include latches, flip-flops, etc., in place of the inverter circuits  422   a - 422   b.    
   According to an embodiment of the invention, the second stage  420  may include circuits capable of processing differential or complementary signals. These circuits may be implemented with a plurality of differential standard cells that have differential inputs and differential outputs. Examples of some differential standard cells of the invention are illustrated in  FIGS. 12A-12G , which are described further below. 
   It should be appreciated by one skilled in the art having the benefit of the present disclosure that the differential standard cells of the invention are different from previously disclosed differential circuits such as differential current mode logic. For instance, current mode logic circuits have static currents (and current sources), and thus they are not suitable for VLSI implementation. In contrast, circuits built according to the differential standard cells of the invention may not have static currents (except for leakage current), and thus they are suitable for VLSI implementation. It should also be appreciated by one skilled in the art having the benefit of the present disclosure that the differential standard cells shown in  FIGS. 12A-12G  and  FIG. 13  (described below) do not represent an exhaustive list, and that many other differential standard cell designs consistent with the principles of the invention are possible. Since the differential standard cells are not using current source, the term “voltage mode” is used herein to describe the differential standard cells and to distinguish them from current mode logic. 
   The third stage  430 , which is shown in  FIG. 4B , may include multiple inverter circuits although only two inverter circuits  432   a - 432   b  are illustrated. In this embodiment, the inverter circuits  432   a - 432   b  are coupled to the inverter circuits  422   a - 422   b  ( FIG. 4A ), respectively. In other embodiments, the connections may be swapped. That is, the inverter circuit  432   a  may be coupled to inverter circuit  422   b , and inverter circuit  432   b  may be coupled to inverter circuit  422   a.    
   The third stage  430  further includes transistor  442 , which acts as a capacitor, and ESD Diodes  444 . According to an embodiment of the invention, the inverter circuit  432   a  provides the “unused output” of the differential driver of  FIGS. 4A-4B . According to one embodiment of the invention, the output  446  is coupled to ground plane of an integrated circuit package such that the output  446  may be coupled to GND when the integrated circuit is in operation. The output  448  may be coupled to an output pin of the integrated circuit such that the output  448  may drive a signal line when the integrated circuit is in operation. 
     FIG. 4C  depicts a decoupling capacitor  440 , and  FIG. 4D  depicts an ESD protection circuit  450 . Both the decoupling capacitor  440  and the ESD protection circuit  450  may be part of the same integrated circuit as the output driver circuits. The decoupling capacitor  440  is for providing a clean voltage source and ground within the die, and the ESD protection circuit  450  is for protecting the circuits from electrostatic damage. Other circuitry may be implemented as part of the integrated circuit as well. The capacitance of the decoupling capacitor  440  can be very small or very large and may vary from one implementation to another as long as it is capable of providing a clean voltage source and ground within the die. 
   According to an embodiment of the invention, the circuits of  FIGS. 4A-4D  are implemented using CMOS technology. PMOS transistors shown in  FIGS. 4A-4B  have the following device parameters: m=4, w=80 μm, L=0.35 μm (except PMOS transistors  442 ). NMOS transistors shown in  FIGS. 4A-4B  have the following device parameters: m=4, w=40 μm, L=0.35 μm. PMOS transistor  442  has the following device parameters: m=3, w=46.5 μm, L=12.9 μm. The NMOS transistor  440  ( FIG. 4C ) has the following device parameters: m=3000, w=30 μm, L=20 μm. The NMOS transistor  450  ( FIG. 4D ) has the following device parameters: m=8, w=40 μm, L=0.35 μm. These implementation details are provided for completeness purposes only and such details should not be construed to limit the scope of the invention. Embodiments of the present invention may be implemented in many other ways using different technologies, different types of transistors and different device parameters. 
   Referring now to  FIG. 13 , there is shown a differential comparator  130  that may be used as an alternative to circuits  412  and  414  of  FIG. 4A . The circuits  412  and  414  are configured to receive a single-ended input signal and to convert the single-ended input signal into a differential signal. Unlike circuits  412  and  414 , the differential comparator  130 , which includes comparator circuits  130   a - 130   b , is configured to receive a differential signal and provide the comparison result and its complement (or inverse) to other circuits, for instance circuits  422   a  and  422   b . According to an embodiment of the invention, the differential comparator circuit  130  may be used for receiving differential signals originated from another portion of the integrated circuit or outside of the integrated circuit. The differential comparator circuit  130  may be used also for receiving LVDS, LVPECL, HSTL and other differential signals that have a small voltage swing. In some embodiments where the differential signals have large voltage swings, the differential signals may be fed directly to circuits of the second stage  420  or the third stage  430 . 
   Attention now turns to  FIG. 5 , which depicts simulation results of the output driver circuit design of  FIGS. 4A-4D . The simulation results are obtained by using TSMC 0.35 μm BSIM-3 spice model. The output frequency of approximately 1 Ghz is achievable with a 5 pf load. 
   Referring now to  FIG. 6 , there is shown a schematic of a known CMOS clock driver integrated circuit  60 , an example of which is an integrated circuit bearing model number IDT74FCT3807D/E, which is available from Integrated Device Technology, Inc. of Santa Clara, Calif. As shown, this clock driver circuit has an input for receiving a clock signal, and ten outputs for distributing the clock signal to ten devices. A maximum operating frequency of the clock driver circuit is 166 Mhz. In many applications, an operating frequency higher than 166 Mhz is often desired. 
     FIG. 7  depicts a schematic of a clock driver integrated circuit  70  according to an embodiment of the invention. As shown the clock driver circuit includes an input inverter  72  and a transmission gate  73  for receiving an input signal, and output drivers  74   a - 74   j  for providing multiple output signals. Note that, although the input signal and the output signals are single-ended signals, differential signals are communicated within the integrated circuit to the output drivers  74   a - 74   j . As shown in  FIG. 7 , the input inverter  72  and the transmission gate  73  convert the input signal into a differential signal and provide the differential signal to the output drivers  74   a - 74   j . Furthermore, output drivers  74   a - 74   j  each have an unused output such that one component of each output differential signal is not transmitted. According to the present embodiment, the clock driver integrated circuit may achieve an operating frequency of 1 Ghz by using a 0.35 μm CMOS process technology. This performance level is significantly higher than the maximum performance level of the conventional CMOS clock driver shown in  FIG. 6 . In light of the disclosure herein, one of ordinary skill in the art would appreciate that the circuit shown in  FIG. 7  may be implemented with other semiconductor technologies, such as 0.25, 0.18, 0.09 μm processes and/or GaAs, BiCMOS, and BJT processes, which may further enhance the frequency performance of the circuit. 
   In one embodiment of the invention, multiple differential drivers are implemented within in an integrated circuit. In this embodiment, the unused output of each differential driver may be coupled to an external ground (e.g., system ground) via individual GND pins. However, in some applications having an individual GND pin for each output driver circuit may be undesirable because the increased number of pins may increase the size and cost of the integrated circuit. 
   In another embodiment of the invention, multiple unused outputs may be coupled together to a package ground plane of the integrated circuit. The package ground plane is coupled to one or more GND pins, which are designated to be coupled to an external ground (e.g., system ground). In other words, one or more GND pins may be shared by all the unused outputs of the output driver circuits. In this way, a single GND pin may support a wide output interface. 
   An integrated circuit package  84  where unused outputs of the output driver circuits are coupled to a package ground plane is depicted in  FIG. 8 . As illustrated, multiple bond wires connect the bond pads that correspond to the unused outputs of the output driver circuits to the Ground Plane  80 , which is itself connected to GND Pins  82   a - 82   e  via other bond wires. Note that GND Pins  82   a - 82   e  are not designated for signal transmission purposes but are designated to be coupled to ground. 
   In another embodiment of the invention, unused outputs of the output driver circuits may be coupled together to a common node within the die or within the chip package. The common node may be coupled to ground node, a voltage source, or a node with a pre-determined voltage so as to terminate the unused signals. 
   Attention now turns to  FIG. 9A , which depicts schematically an integrated circuit  90   a  according to an embodiment of the invention. The integrated circuit  90   a  includes core logic  94   a , which may include, for instance, CMOS logic circuits such as a central processing unit (CPU) core, and/or a memory core (e.g., a DRAM core). The integrated circuit  90   a  further includes output drivers (or “output pads”)  20   a  for providing output signals. 
   According to the embodiment shown in  FIG. 9A , an output driver  20   a  receives a single-ended signal from the core logic  94   a  via input  22   a . The output driver  20   a , which may include circuits shown in  FIGS. 4A-4B , converts the single-ended signal into a differential signal, provides one of the component of the differential signal as an output signal via output  24   a , and terminates the other component signal via output  24   b  and capacitor  34   a.    
   According to an embodiment, the output  24   a  may be coupled to a signal pin designated to provide an output signal, whereas the output  24   b  may be coupled to a GND pin that is designated to be coupled to system ground. In another embodiment, the output  24   b  may be coupled to a package ground plane, which is in turn coupled to a GND pin that is designated to be coupled to system ground. In other embodiments, the output  24   b  may be terminated using other techniques. 
   Preferably, the output drivers  20   a  share the same chip voltage “vv” and the same chip ground “gg”. However, it should be understood that in other variations the output drivers  20   a  may or may not share the same chip voltage “vv” or the same chip ground “gg”. For instance, one of the output drivers may be coupled to a first chip voltage vv and a first chip ground gg 1 , while another one of the output drivers may be coupled to a second chip voltage vv 2  and the chip ground gg 1 . Furthermore, one of the output drivers may be coupled to a second chip voltage vv 2  and a second chip ground gg 2 . Many other variations may be apparent to those of ordinary skill in the art having the benefit of this disclosure. 
     FIG. 9B  depicts schematically an integrated circuit  90   b  according to another embodiment of the invention. The integrated circuit  90   b  includes core logic  94   b  and output drivers (or “output pads”)  20   b  for providing output signals. According to the embodiment shown in  FIG. 9B  the core logic  94   b , which may include CMOS logic circuits and/or circuits similar to those shown in  FIG. 4A ,  FIGS. 12A-12G  and  FIG. 13 , provides differential signals to the output drivers  20   b . The output drivers  20   b , which may include circuits shown in  FIG. 4B , each provide one component of the received differential signal as an output signal via output  24   a , and terminates the other component signal via output  24   b  and capacitor  34   a . In the illustrated embodiment, the output driver  20   b  may include circuits shown in  FIG. 4B , for instance an inverter coupled to the input  22   a , and another inverter coupled to the input  22   b.    
   Preferably, the output drivers  20   b  share the same chip voltage “vv” and the same chip ground “gg”. However, it should be understood that in other variations the output drivers  20   b  may or may not share the same chip voltage “vv” or the same chip ground “gg”. 
     FIG. 9C  depicts schematically an integrated circuit  90   c  according to an embodiment of the invention. The integrated circuit  90   c  includes core logic  94   c  and input and output (I/O) drivers (or “I/O pads”)  20   c  for receiving input signals or providing output signals. The core logic  94   c  may include CMOS logic circuits and/or circuits similar to those shown in  FIG. 4A ,  FIGS. 12A-12G  and  FIG. 13 . According to the embodiment shown in  FIG. 9C , an I/O driver  20   c  includes an input driver for receiving a signal-ended signal from an external source, and a differential signal driver for receiving a differential signal from the core logic  94  via inputs  22   a - 22   b . The I/O driver  20   c  may further include a control input (not shown) for receiving a mode selection signal from the core logic  94  that dictates whether the I/O driver  20   c  should be in an input mode or an output mode. 
   In the output mode, the I/O driver  20   c , which may include circuits shown in  FIG. 4B , for instance an inverter coupled to the input  22   a  and another inverter coupled to the input  22   b , provides one of the component signal of the differential signal as an output signal via output  24   a , and terminates the other component signal via output  24   b  and capacitor  34   a . The I/O driver  20   c  may include circuits, for instance like those shown in  FIG. 4A , for receiving a single-ended signal via the I/O pin when the driver is in input mode, and for converting the single-ended signal into a differential signal, which may be provided to the core logic  94   c  via connections  44   a - 44   b.    
   Preferably, the I/O drivers  20   c  share the same chip voltage “vv” and the same chip ground “gg”. However, it should be understood that in other variations the I/O drivers  20   c  may or may not share the same chip voltage “vv” or the same chip ground “gg”. 
     FIG. 9D  depicts another embodiment of the invention that is similar to one depicted in  FIG. 9C  except that the core logic  94   d  provides and receives single-ended signals to and from I/O drivers (or “I/O pads”)  20   d . In this embodiment, the I/O drivers  20   d  may include circuits for converting single-ended signals into differential signals in an output mode, and circuits for providing signals to the core logic  94   d  in an input mode. Preferably, the I/O drivers  20   d  share the same chip voltage “vv” and the same chip ground “gg”. However, it should be understood that in other variations the I/O drivers  20   d  may or may not share the same chip voltage “vv” or the same chip ground “gg”. 
   Principles of the present invention may be applied to implement various other types of circuits. For example, a ring oscillator  95  implemented according to an embodiment of the invention is shown in  FIG. 10 . The ring oscillator  95  includes components found in common ring oscillators. Unlike conventional ring oscillators, however, the ring oscillator  95  includes a transmission gate  101 , inverters  103 , and capacitor  99  that make up a current path to direct the unused signals to package ground or system ground through a coupling capacitor. The performance of the ring oscillator  95  may be significantly better than conventional designs. In one variation, the ring oscillator disclosed herein may be implemented as a clock for a computer or other electronic devices requiring high frequency clocks. 
   Referring now to  FIG. 11 , a crystal oscillator  97  implemented according to an embodiment of the invention is shown. The crystal oscillator  97  includes components found in common crystal oscillators. Unlike conventional crystal oscillators, however, the crystal oscillator  97  includes a current path (which includes transmission gate  101 , inverters  103 , and capacitor  99 ) to direct the unused signals to the system or package ground through a coupling capacitor. The performance of the crystal oscillator  97  may be significantly better than conventional designs. In one variation, the crystal oscillator disclosed herein may be implemented as a clock for a computer or other electronic devices requiring high frequency clocks. 
   In a computer network system implementation, an embodiment of the differential output driver of invention may be used to enhance performance of a network interface (e.g., an Ethernet adaptor, a DSL module, etc.) by improving the network communication speed and/or by improving the maximum driving distance. Referring now to  FIG. 14 , there is shown a high-speed serial bus system  140  (such as Ethernet or DSL) according to an embodiment of the invention. The bus system includes a host device  141 , a controller  142 , a transmitter  144  and a receiver  146 . In the illustrated embodiment, the transmitter  144  receives a signal from the controller  142 , generates a differential signal, and provides one component signal to the bus. In accordance with an embodiment of the invention, the other component of the differential signal is terminated via a coupling capacitor. An advantage of the serial bus of  FIG. 14  is that the output frequency of the transmitter  144  may be very high. If implemented using 0.35 μm TTL-CMOS or a similar technology, the output frequency may be 1 GHz or more. Furthermore, the output power of the transmitter  144  may be 3 V or more. An output power of 3 V or more may allow the signals to be carried by the signal line for a significantly longer distance than that is possible with a lower power output voltage. 
   Referring now to  FIG. 15 , there is shown a high-speed wireless communication device  150  implemented according to an embodiment of the invention. The wireless communication device includes a host device  151 , a controller  152 , a transmitter  154 , a receiver  156 , and an antenna  159 . In the illustrated embodiment, the transmitter  154  receives a differential signal from the controller  152 , provides one of the component signals to the antenna  159 , and terminates the other. In accordance with an embodiment of the invention, the other component of the differential signal is terminated via a capacitor. An advantage of the wireless device of  FIG. 15  is that the output frequency of the transmitter  154  and the power amplifier  158  may be implemented with low cost TTL-CMOS technology, as opposed to more expensive technologies such as GaAs currently used in high-speed wireless communication systems. 
   Attention now turns to another aspect of the invention. In this aspect of the invention, differential standard cells are used to implement at least part of the logic core of an integrated circuit such that very high speed can be achieved. For example, the second stage  420  of  FIG. 4A  and the core logic circuits  90   a - 90   d  of  FIGS. 9A-9D  may include differential standard cells of the invention. The differential standard cells may be implemented independently of the high-speed driver circuits described in this specification. Some differential standard cells according to one aspect of the invention are depicted in  FIGS. 12A-12G . The following Table 1 summarizes the description of these figures. 
   According to an embodiment of the invention, a differential standard cell includes at least in part a pair of logically complementary circuits one of which is for performing a logic function, and another of which is for performing a logically complementary function. For example, consider the differential NAND cell shown in  FIG. 12A . The differential NAND cell includes a NAND gate for performing a NAND operation on inputs A and B to produce an output value OUT. The differential NAND cell further includes a NOR gate for performing a NOR operation on inputs A_bar and B_bar to produce an output value OUT_bar that is inverse to OUT. Preferably, the differential cells share the same chip voltage “vv” and the same chip ground “gg.” However, it should be understood that in other variations the differential cells may or may not share the same chip voltage “vv” or the same chip ground “gg”. 
   
     
       
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
           
           
             
                 
               FIG. 12A 
               Differential NAND cell 
             
             
                 
               FIG. 12B 
               Differential NOR cell 
             
             
                 
               FIG. 12C 
               Differential XOR cell 
             
             
                 
               FIG. 12D 
               Differential XNOR cell 
             
             
                 
               FIG. 12E 
               Differential NOT cell 
             
             
                 
               FIG. 12F 
               Differential latch cell 
             
             
                 
               FIG. 12G 
               Differential D flip flop cell 
             
             
                 
                 
             
           
        
       
     
   
   According to an embodiment of the invention, the differential standard cells are considered building blocks or “primitive cells” of an integrated circuit design, and they may be used by an automated electronic design process to produce an integrated circuit. A flow diagram depicting an IC design process  161  according to one aspect of the invention is shown in  FIG. 20 . The process  161  described with respect to this flow chart is implemented within a computer system in a CAD (computer automated design) environment. Within the process  161 , a circuit designer first generates a high-level description  162  of a circuit in a hardware description language such as Verilog. 
   A computer-implemented compiler program  165  processes this high-level description  162  and generates therefrom a detailed list of logic components and the interconnections between these components. This list is called a “netlist”  166 . The components of the netlist  166  can include primitive cells such as full-adders, NAND gates, NOR gates, XOR gates, latches, and D-flip flops, etc. According to an embodiment of the invention, the netlist  166  includes differential standard cells, such as those described above with reference to  FIGS. 12A-12G , as primitive cells. 
   In processing the high-level description, the compiler program  165  may first generate a netlist of generic primitive cells that are technology independent. According to one embodiment of the invention, the compiler  165  may then apply a Differential Standard Cell Library  164  and/or other cell libraries  163  to this generic netlist in order to generate a netlist  166  that contains differential standard cells. For example, if the generic netlist includes a NAND gate, then the compiler  165  may map a differential NAND cell to the NAND gate to produce a netlist that includes a NAND gate and a NOR gate. 
   The netlist  166 , however, does not contain any information with respect to the physical design of the circuit. For example, the netlist  166  does not specify where the cells are placed on a circuit board or silicon chip, or where the interconnects run. Determining this physical design information is the function of a computer controlled place-and-route process  167 . 
   The place-and-route process  167  first finds a location for each cell on a circuit board or silicon chip. The locations are typically selected to optimize certain objectives such as wire length, circuit speed, power consumption, and/or other criteria, and subject to the condition that the cells are spread evenly over the circuit board or silicon chip and that the cells do not overlap with each other. The place-and-route process  167  also generates the wire geometry based on the placement information for connecting the pins of the cells together. The output of the place-and-route process  167  includes cell placement data structures and wire geometry data structures that are used to make the final geometric database needed for fabrication of the circuit. The placement and wire geometry data structures of the design are sometimes referred to as a “layout”  168 . The layout  168  can be regarded as a template for the fabrication of the physical embodiment of the integrated circuit using transistors, routing resources, etc. 
   Due to the requirement of additional gates, it is expected circuits containing differential standard cells of the invention may require more die area than circuits implementing a similar logic function without using differential standard cells. An example half-adder circuit  160  according to an embodiment of the invention is illustrated in  FIG. 16 . Note that the half-adder circuit  160  includes two inputs for receiving A and B, and two inputs for receiving the complements of A and B. The half-adder circuit  160  further includes an output for providing C_out and another output for providing the complement or inverse of C_out. The circuit  160  may be implemented with a differential NAND cell, a differential XOR cell, and a differential NOT cell. Note that a portion of the circuit  160 , which is used for producing the inverse of C_out, is logically complementary to the portion that is responsible for generating C_out. Also note that in  FIG. 16 , a NAND gate, a XOR gate, and a NOT gate in one portion of the circuit are mirrored by a NOR gate, an XNOR gate, and a NOT gate, respectively, in the complementary portion of the circuit. 
   An example 4-to-1 multiplexer circuit  170  according to an embodiment of the invention is illustrated in  FIG. 17 . The multiplexer circuit  170  includes one set of inputs for receiving data and another set of inputs for receiving the complements of the data. Furthermore, the multiplexer circuit  170  includes two outputs for providing an output value and its complement. The circuit  170  may be implemented with differential NAND cells and a differential NOT cell. Note that a portion of the circuit  170 , which is used for producing out_b, is logically complementary to the portion that is responsible for generating “out.” Also note that in  FIG. 17 , NAND gates in one portion of the circuit are mirrored by NOR gates in the complementary portion of the circuit. 
   A diagram illustrating an example gate-level implementation a differential NAND cell of  FIG. 12A  is shown in  FIG. 18 . A diagram illustrating an example gate-level implementation a differential NOR cell of  FIG. 12B  is shown in  FIG. 19 . These implementation diagrams are shown for illustration purposes only. In light of the present disclosure, a person skilled in the art would realize that the differential standard cells may be implemented in many different ways. One of ordinary skill in the art having the benefit of the disclosure herein would appreciate that most logic circuits in the market may be reconfigured with the differential cells described herein such that complementary circuitry is provided to improve overall circuit performance. It should be understood that the differential circuits described herein may be used to implement various portions of an integrated circuit and that applications of the differential circuits should not be limited to the second stage  420 , or core logic  90   a - 90   d.    
   Embodiments of the invention have thus been disclosed. The foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and explanation. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein. Accordingly, it is the claims, not merely the foregoing illustration, that are intended to define the exclusive rights of the invention. 
   Furthermore, throughout this specification (including the claims), unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element or group of elements but not the exclusion of any other element or group of elements. The word “include,” or variations such as “includes” or “including,” will be understood to imply the inclusion of a stated element or group of elements but not the exclusion of any other element or group of elements. Claims that do not contain the terms “means for” and “step for” are not intended to be construed under 35 U.S.C. § 112, paragraph 6.