METHOD AND APPARATUS FOR CALIBRATING A MULTI-LEVEL CURRENT MODE DRIVER AND FOR GENERATING A MULTI-LEVEL REFERENCE VOLTAGE IN SYSTEMS USING EQUALIZATION OR CROSSTALK CANCELLATION

Abstract: A current controller for a multi-level current mode driver. The current controller includes a multi-level voltage reference and at least one source calibration signal. A comparator is coupled by a coupling network t the multi-level voltage reference and the at least one source calibration signal. A selected voltage is applied from the multi-level voltage reference and a selected source calibration signal is applied from the at least one source calibration signal to the comparator. Further, system and method are shown for generation of at least one reference voltage level in a bus system. A reference voltage generator on a current driver includes at least one reference voltage level, at least one control signal, and an active device. The active device is coupled to the at least one control signal, such as a current signal, and a selected reference voltage of the at least one reference voltage level. The active device is arranged to shift the at least one reference voltage level based on the at least one current control signal such as an equalization signal, a crosstalk signal, or the combination thereof, employed on the current driver. Further, low-latency equalization mechanisms for multi-PAM communication systems are disclosed that reduce delay and complexity in signal correction mechanisms. The equalization mechanisms tap into input signals for a multi-PAM signal driver, and compensate for attenuation along a signal transmission line, crosstalk between adjacent lines, and signal reflections due to impedance discontinuities along the line.

Description: METHOD AND APPARATUS FOR CALIBRATING A MULTI-LEVEL CURRENT MODE DRIVER AND FOR GENERATING A MULTILEVEL REFERENCE VOLTAGE IN SYSTEMS USING EQUALIZATION OR CROSSTALK CANCELLATION CROSS REFERENCE TO RELATED APPLICATION(S) This application is a continuation-in-part of U.S. Patent Application Serial No. 09/478,916, entitled "Low Latency Multi-Level Communication Interface," filed on January 6, 2000, which claims priority to U.S. Provisional Patent Application Serial No. 60/158,189, entitled "A Method and Apparatus for Receiving High Speed Signals with Low Latency," filed on October 19, 1999, the contents of each of which are incorporated herein by reference. FIELD OF THE INVENTION The present invention relates generally to the field of electrical buses. More particularly, the present invention relates to a current driver for a high-speed bus, techniques to equalize or compensate for errors that may be present in a multi-level, multi-line signaling system, and, further, it relates to a reference voltage generation for an electrical bus with equalization or crosstalk. BACKGROUND OF THE INVENTION Computer systems and other electronic systems typically use buses for interconnecting integrated circuit components so that the components may communicate with one another. The buses frequently connect a master, such as a microprocessor or controller, to slaves, such as memories and bus transceivers. Generally, a master may send data to and receive data from one or more slaves. A slave may send data to and receive data from a master, but not another slave. Each master and slave coupled to a prior bus typically includes output driver circuitry for driving signals onto the bus. Some prior bus systems have output drivers that use transistor-transistor logic ("TTL") circuitry. Other prior bus systems have output drivers that include emitter-coupled logic ("ECL") circuitry. Other output drivers use complementary metal-oxide-semiconductor ("CMOS") circuitry or N-channel metal- oxide-semiconductor ("NMOS") circuitry. While many prior buses were driven by voltage level signals, it has become advantageous to provide buses that are driven by a current mode output driver. A benefit associated with a current mode driver is a reduction of peak switching current, hi particular, the current mode driver draws a known current regardless of load and operating conditions. A further benefit is that the current mode driver typically supresses noise coupled form power and ground supplies. A known current mode driver is shown in U.S. Patent No. 5,254,883 (the '"883 patent"), which is assigned to the assignee of the present invention and incorporated herein by reference. The '883 patent discusses an apparatus and method for setting and maintaining the operating current of a current mode driver. The driver in the '883 patent includes an output transistor array, output logic circuitry coupled to the transistor array and a current controller coupled to the output logic circuitry. For one embodiment, the current controller in the '883 patent is a resistor reference current controller. The current controller receives two input voltages, V TER and V REF , the latter of which is applied to an input of a comparator. V TERM is coupled by a resistor to a node, which is in turn coupled to a second input of the comparator. The voltage at the node is controlled by a transistor array, which is in turn controlled in accordance with an output of the comparator. When the transistor array is placed in the "off state, i.e. there is no current flowing through the transistors of the array to ground, the voltage at the node is equal to V TERM - hi addition, by using the output of the comparator to adjustably activate the transistor array, the '883 patent shows that the voltage at the node may be driven to be approximately equal to the reference voltage, V REF - Knowing the value of v REF and VTERM, the current mode driver of the '883 patent therefore provides a binary signaling scheme utilizing a symmetrical voltage swing about V REF - Specifically, in a first current state (the "off state), the current mode driver is not sinking current and the signal line (or bus line) is at a voltage, v 0 = V TERM , representing a logical "0." hi a second current state (the "on" state), the current mode driver is sinking current to drive the voltage on the signal line (or bus line) to be: V 0 = VTERM - 2 (VTERM - VREF)- The second state therefore representing a logical "1." While the above techniques have met with substantial success, end users of data processing systems, such as computers, continue to demand increased throughput. Whether throughput is expressed in terms of bandwidth, processing speed or any other measure, the bottom line is the desire to get a block of data from point A to point B faster. At the same time, however, it is desirable to achieve such increases without requiring deviation from known semiconductor fabrication techniques. Long-distance uses for multi-PAM signaling include computer or telecommunication systems that employ Gigabit Ethernet over optical fiber (IEEE 802.3z) and over copper wires (IEEE 802.3ab), which use three and five signal levels, respectively, spaced symmetrically about and including ground. Equalization techniques for long-distance multi-level signaling systems such as those used in radio or telecommunication networks commonly include adaptive filters, which can change equalization characteristics adaptively to improve equalization accuracy or in response to changing conditions. The complexities of these equalization techniques can delay signaling by slight but tolerable amounts for these long-distance systems. Multi-PAM is not traditionally used for communication between devices in close proximity or belonging to the same system, such as those connected to the same integrated circuit (IC) or printed circuit board (PCB). One reason for this may be that within such a system the characteristics of transmission lines, such as buses or signal lines, over which signals travel are tightly controlled, so that increases in data rate may be achieved by simply increasing data frequency. At higher frequencies, however, receiving devices may have a reduced ability to distinguish binary signals, so that dividing signals into smaller levels for multi-PAM is problematic. Multi-PAM may also be more difficult to implement in multi-drop bus systems (i.e., buses shared by multiple processing mechanisms), since the lower signal-to-noise ratio for such systems sometimes results in bit errors even for binary signals. Moreover, complex equalization techniques such as employed for long distance communication systems may not be feasible in a bus system for which low latency is a performance criterion. Further, while the binary signal levels are commonly used, the use of multi-level signals is a known technique for increasing the data rate of a digital signaling system. Such multi-level signaling is sometimes known as multiple pulse amplitude modulation or multi-PAM, and has been implemented with radio or other long-distance wireless signaling systems. Other long-distance uses for multi-PAM signaling include computer or telecommunication systems that employ Gigabit Ethernet over optical fiber and over copper wires, which use three and five signal levels, respectively. Additionally, multi-PAM signaling may be used for communication between devices in close proximity or belonging to the same system, such as those connected to the same integrated circuit ("IC") or printed circuit board ("PCB"). hi such systems, the characteristics of transmission lines, such as buses or transmission lines, over which the signals travel are tightly controlled, so that the increase in the data rate may be increased by increasing the transmit frequency. However, at higher frequencies, receiving devices may have a reduced ability to distinguish binary signals. Further, for cases in which attenuation of a signal exists between transmission and reception, different amounts of signal loss may occur depending upon the magnitude of transition between logic states. To compensate for the attenuation of the received signal, different equalization signals may be added to the main signal when driving different transitions in order to add predetermined high-frequency components to the transition signals that raise the slope of the edge of the transition. However, a difficulty with this approach for a typical multi- PAM system is that the voltage can only be pulled down from the VT ERM , unless negative current could flow through current sources. To allow overdriving a transition with the equalization signals, the highest logic state may, therefore, be reduced below V T E RM - This may cause the respective reference voltages to be no longer centered on the shifted data eyes. Further, in a system that has numerous closely spaced signal lines, such as a bus for a computer device or a similar device, crosstalk may exist between nearby lines. As is known in the art, crosstalk is a disturbance caused by the electric or magnetic fields of one telecommunication signal and impairs signals on adjacent signal lines. Crosstalk characteristics on a bus may be based upon how many lines are between a crosstalk creator and a signal line being affected by crosstalk. One method for crosstalk cancellation has been described in the co-pending U.S. Patent Application entitled "Low Latency Equalization in Multi-Level, Multi-Line Communication Systems," identified above. Similarly to the equalization mechanism, the crosstalk cancellation provides high frequency component signal and, thus, the highest logic state of the system is typically reduced below V TER M ? causing reference voltage levels to be no longer centered on the shifted data eyes. Thus, it is still desirable to develop a method and system for reference voltage generation that would track the logic state shifts due to equalization or crosstalk. SUMMARY OF THE INVENTION A multi-level driver uses multiple pulse amplitude modulation (multi-PAM) output drivers send multi-PAM signals. A multi-PAM signal has more than two voltage levels, with each data interval now transmitting a "symbol" at one of the valid voltage levels, hi one embodiment, a symbol represents two or more bits. The multi-PAM output driver drives an output symbol into a signal line. The output symbol represents at least two bits that include a most significant bit (MSB) and a least significant bit (LSB). A multi-PAM receiver receives the output symbol from the signal line and determines the MSB and the LSB. In accordance with a first aspect of the invention, a current controller for a multi¬ level current mode driver is provided. The current controller includes a multi-level voltage reference and at least one source calibration signal. A comparator is coupled by a coupling network to the multi-level voltage reference and the at least one source calibration signal. The current controller further includes a circuit for applying a selected voltage from the multi-level voltage reference and a selected source calibration signal from the at least one source calibration signal to the comparator. In accordance with a second aspect of the invention, a method of calibrating a multi-level current mode driver is provided. The method includes two current sinks, each capable of sinking current from a termination voltage though a resistor. The first transistor array drives a known amount of current through its resistor producing a first input signal. Similarly, the second current sink is turned on to produce a second input signal. An average value of the first input signal and the second input signal is calculated. The average value of the first input signal and the second input signal is compared to a first known reference voltage. And, the second current sink, and thereby the second input signal, is adjusted until the average value equals the known reference voltage. hi accordance with another aspect of the invention, a reference voltage generator for a driver, such as a current driver, is provided. The current driver includes at least one reference voltage level, at least one current control signal, and at least one active device coupled to a selected reference voltage level of the at least one reference voltage level and the at least one current control signal. The active device shifts the at least one reference voltage level based on the at least one current control signal such as an equalization signal or a crosstalk cancellation signal, hi one embodiment, the current driver is arranged to operate in a 2-PAM mode, a 4-PAM mode, or an N-PAM mode. i accordance with another aspect of the invention, a method for generating a plurality of reference voltage levels for a driver is provided. The method includes providing at least one reference voltage level, providing at least one current control signal, and adjusting the at least one reference voltage level based on the at least one current control signal. In one embodiment, the at least one reference voltage level is generated on a resistive voltage divider or a reference voltage driver. Further, the at least one current control signal may include, for example, an equalization current control signal or a crosstalk cancellation signal. Further, another embodiment of the invention is directed to low-latency equalization mechanisms for short-range communications systems. Such equalization mechanisms may compensate for signal attenuation along a transmission line, reflections due to impedance discontinuities along such a line, and crosstalk between adjacent lines. The equalization mechanisms may be particularly advantageous for multi-PAM communications systems These as well as other aspects and advantages of the present invention will become more apparent to those of ordinary skill in the art by reading the following detailed description, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a memory controller, bus and memories utilizing an output driver in accordance with a preferred embodiment of the present invention. Figure 2 illustrates a preferred encoding scheme utilizing a multi-level voltage reference for use with a multi-level output driver. Figures 3A and 3B are schematic diagrams of a first and a second multi-level output driver in accordance with embodiments of the present invention. Figure 4A is a graph showing g S distortion in a transistor. Figures 4B and 4C illustrate the effect of ga s distortion on the output voltage of a four-level output driver encoding in binary and gray code, respectively. Figure 5A is an electrical schematic of a multi-level output driver, having a binary generator, that corrects for g ds distortion. Figure 5B is an electrical schematic of an alternate embodiment of the binary generator shown in Figure 5 A. Figure 6 is an electrical schematic of a circuit to reduce switching noise at an output pin. Figure 7 is an electrical schematic of a multi-level driver, such as the driver shown in Figure 5A, that further incorporates a circuit to reduce switching noise, such as the circuit shown in Figure 6. Figure 8 is an electrical schematic of another alternative g dS compensated, multilevel output driver. Figure 9A is an electrical schematic of a g ds compensated, multi-level output driver with current control circuitry. Figure 9B is an electrical schematic of a set of stacked transistor pairs for a current drive block, such as the current drive blocks shown in Figure 9A. Figure 9C is an electrical schematic of a preferred g dS compensated, multi-level output driver. Figure 10 is an electrical schematic of a circuit for calibrating a g s compensated output driver with current control circuitry. Figures 11A and 11B are a flowchart of a method for calibrating the current control circuitry using the setup of Figure 10 for the output driver shown in Figure 9 A. Figure 12 is an electrical schematic of an on-chip, multi- level reference voltage generator utilizing a resistive voltage divider. Figures 13 A and 13B are electrical schematics of a first preferred alternative to the current control calibration circuit of Figure 10. Figure 13C is a timing diagram for the circuits of Figures 13 A and 13B. Figure 13D illustrates alternative embodiments for the differential comparator of Figure 13B. Figure 13E illustrates an electrical schematic of a charge coupled comparator using PMOS capacitors. Figures 14A and 14B are electrical schematics of a second preferred alternative to the current control calibration circuit of Figure 10. Figures 14C and 14D are timing diagrams for the circuits of Figures 14A and 14B. Figure 15A is an electrical schematic of a linear transconductor. Figure 15B is a schematic of a comparator using a transconductor stage. Figure 16 is a signaling device that may be used to create the multi-level voltage reference of Figure 2. Figure 17 is an electrical schematic of an on-chip, multi-level reference voltage generator utilizing a resistive voltage divider. Figure 18 is an alternative electrical schematic of an on-chip, multi-level reference voltage generator utilizing a resistive voltage divider. Figure 19 illustrates attenuation affecting the transitory level obtained by a step. Figure 20 illustrates addition of equalization signal to the logic state transitions affected by signal attenuation in Figure 19. Figure 21 is an encoding scheme utilizing a multi-level voltage reference where the logic levels are shifted to allow for overdrive signals. FIG. 22A shows initial, transition and final states of a signal that may represent a transition illustrated in Table 3. FIG. 22B shows initial, transition and final states of a main component of the signal shown in FIG. 22A. FIG. 22C shows initial, transition and final states of an auxiliary component of the signal shown in FIG. 22A. Figure 23 illustrates a device that can provide overdrive signals to compensate for signal attenuation. Figure 24 shows a finite impulse filter (FIR) filter representation of the device in Figure 23. Figure 25 illustrates a general equalization system for a signal line, including a self-equalization FIR filter and a number of crosstalk equalization FIRs; Figure 26A illustrates a signal transition that may create crosstalk on adjacent signal lines. Figure 26B illustrates crosstalk noise from the signal transition of Figure 26A, imposed on a signal in an adjacent line. Figure 26C illustrates an equalization signal that compensates for the crosstalk noise shown in Figure 26B. Figure 26D illustrates a first component of the equalization signal shown in Figure 26C. Figure 26E illustrates a second component of the equalization signal shown in Figure 26C. Figure 27 is a signal driver, self-equalization and crosstalk equalization communication device. FIG. 28 shows an equalization mechanism that inputs MSB and LSB signals to provide compensation. FIG. 29 shows a general FIR filter having a number of self-equalization and crosstalk-equalization mechanisms tapped at various delay times. FIG. 30 illustrates a mechanism for adjusting equalization parameters such as the magnitude, timing and sign of various equalization signals. FIG. 31 shows an encoder that converts MSB and LSB signals into input signals for the output driver and equalization mechanisms. FIG. 32 shows receiver that receives the multi-level signals sent by the main and auxiliary drivers and decode the signals into MSB and LSB components. FIG. 33 A shows a signal transition that has a high-frequency noise spike. FIG. 33B shows an equalization signal designed to compensate for a DC component of the spike of FIG. 32 A. FIG. 33C shows a smoothing of the signal of FIG. 33B by integration. FIG. 34 shows a system with a number of adjacent signal lines that may create crosstalk, and standardized FIR filters coupled between the lines. FIG. 35 shows a system with a number of adjacent signal lines grouped in pairs with a ground wire disposed between the pairs of lines and standardized FIR filters coupled between the lines. FIG. 36 illustrates a memory system in which embodiments of the present invention may be applied. Figure 37 is an electrical schematic of an on-chip, multi-level reference voltage generator utilizing a resistive voltage divider, where the multi-level reference voltage reflects logic state shifts. Figure 38 is an electrical schematic of a multi-level reference voltage generator utilizing op-amp drivers, where the multi-level reference voltage reflects logic state shifts. Figure 39 is an electrical schematic of an on-chip multi-level reference voltage generator utilizing on-chip drivers, where the multi-level reference voltage reflects logic state shifts. Figure 40 is a flow chart illustrating an exemplary method for generating at least one reference voltage level for a driver using equalization and crosstalk cancellation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) i Figure 1, a bus 320 interconnects a memory controller 321 and memories 322. The bus 302 is formed of signal lines 320-1, 320-2 that transmit address, data and control signals. Physically, on each integrated circuit 321, 322, the address, data and control signals are supplied to and output from external connections, called pins, and the bus 320 interconnects respective pins. The bus 320 may be implemented as traces on a printed circuit board, wires or cables and connectors. Each of these integrated circuits 321, 322 has bus output driver circuits 323 that connect to the pins to interface with the bus 320 to transmit signals to other ones of the integrated circuits, hi particular, the bus output drivers 323 in the memory controller 321 and in the memories 322 transmit data over the bus 320. Each bus output driver 323 drives a signal line of the bus 320. For example, bus output driver 323-1 in the memory controller 321 drives bus line 320-1. The bus 320 supports signaling with characteristics that are a function of many factors such as the system clock speed, the bus length, the amount of current that the output drivers can drive, the supply voltages, the spacing and width of the wires or traces making up the bus 320, the physical layout of the bus itself and the resistance of a terminating resistor Zo attached to each bus. At least a subset of the signal lines connect to pull-up resistors Zo that connect to a termination voltage VTERM. In some systems, all signal lines connect to pull-up resistors Zo that connect to the termination voltage VTERM. The termination voltage VTERM can be different from the supply voltage VDD or not. Further, the termination resistors can be off-chip or on-chip, hi one embodiment, the supply voltage VDD is equal to 2.5 volts, the termination voltage VTERM is equal to 1.8 volts, the bus voltage for a signal at low levels VOL is equal to 1.0 volts, and the voltage swing is 0.8 volts. The resistance of the terminating resistors Zo is equal to twenty-eight ohms. The output drivers 323 are designed to drive the bus 320 with a predetermined amount of current; and the bus receivers 324 are designed to receive the signals sent by the bus drivers 323 on the bust 320. hi a device, each bus receiver 324 receives signals from one signal line of the bus 320. The bus receivers 324 are integrating receivers according to the present invention. In one embodiment, the memories are random access memories (RAMs). hi an alternative embodiment, the memories are read-only memories (ROMs). Alternatively, the bus output drivers 323 and bus receivers 324 of the present invention are implemented in other semiconductor devices that use a bus to interconnect various types of integrated circuits such as microprocessors and disk controllers. hi yet another alternative embodiment, the output drivers are implemented in a point-to-point system. Although a bus that uses current mode signaling has been described with respect to Figure 1, the apparatus and method of the present invention may be used in any signaling system where it is desirable to distinguish between signals having different voltage levels. Multi-Level Signaling Referring back to Figure 1, in previously known implementations of the bus system, signals transmitted on each signal line of the bus have either of two voltage levels representing a binary zero or one for binary digital communication. For example, an output voltage equal to the voltage level VTERM set by the voltage source at one end of the termination resistor Zo may represent a binary zero. And, an output voltage level equal to VTERM - (I*Zo) may represent a binary one, where the output driver circuit 323 sinks an amount of current equal to I. In this way, the bus driver circuits 323 can be implemented as switched current sources that sink current when driving binary one's onto the signal lines. When receiving data, the receiver circuits 324 detect whether the voltage on the signal line is greater than or less than VTERM- 0.5(I*Zo), i.e. the midpoint between a logical zero and a logical one, to determine whether the data is a binary zero or one, respectively, i one embodiment, data is transmitted and received on each edge of the system clock to achieve a data rate equal to twice the frequency of the system clock, i an alternative embodiment, data is transmitted once per clock cycle of the system clock. As used herein, the term multi-level signaling refers to signaling schemes utilizing two or more signal levels. Multi-level signaling may also be referred to herein as multiple level pulse amplitude modulation, or multi-PAM, signaling, because the preferred coding methods are based upon the amplitude of the voltage signal. Although the multi-level signaling of the preferred embodiments will be described with respect to a current mode bus, multi-level signaling can also be used with a voltage mode bus. h various embodiments of the present invention, the data rate on a bus is increased without increasing either the system clock frequency or the number of signal lines. Output drivers generate, and receivers detect, multi-PAM signals that allow multiple (k) bits to be transmitted or received as one of 2 k possible voltages or data symbols at each clock edge or once per clock cycle. For example, one preferred embodiment is a 4-PAM system in which two bits are represented by 2 2 or four voltage levels, or data symbols, and the two bits are transferred at every clock edge by transferring an appropriate one of the four voltage levels. Therefore, the data rate of a 4-PAM system is twice that of a binary or 2-PAM system. Multi-PAM is not traditionally used in multi-drop bus systems due, at least in part, to the lower signal-to-noise ratio that is realized when the voltage range is divided into multiple levels. Prior art memory systems have been implemented as only binary systems. A preferred embodiment allows such systems to be implemented using more than two signal levels. hi Figure 2, a graph shows one embodiment utilizing a 4-PAM signaling scheme. Specifically, the multi-PAM voltage levels are associated with two-bit binary values or symbols such as 00, 01, 10 and 11. hi the embodiment of Figure 2, the binary values are assigned to voltage levels using Gray coding, i.e. the symbol sequence from the highest voltage level to the lowest voltage level is 00, 01, 11, 10. Gray coding provides the advantage of reducing the probability of dual-bit errors because only one of the two bits changes at each transition between voltage levels. If a received 4-PAM voltage symbol is misinterpreted as an adjacent symbol, a single-bit error will occur. The y-axis of the graph in Figure 2 shows the associated 4-PAM output voltages VOUT for each symbol. To provide the appropriate voltage to transmit a 4-PAM symbol, the output driver sinks a predetermined amount of current for that symbol. In particular, each symbol is associated with different amount of current. To transmit the symbol "00", the output driver 323 sinks no current and the signal line is pulled up to VTERM. TO transmit the symbol "01", the bus output driver 323 sinks a predetermined amount of current 101 to cause the output voltage VOUT to equal VTERM (I • Zo), where 101 is equal to 1/3 I. To transmit the symbol "11", the bus output driver 323 sinks a predetermined amount of current II 1 to cause the output voltage VOUT to equal VTERM 2/3 (I • Zo), where II 1 is equal to 2/3 I. To transmit the symbol "10", the bus output driver 323 sinks a predetermined amount of current I to cause the output voltage VOUT to equal VTΈRM (I • Zo). Further details regarding preferred embodiments of the output driver 323 are provided below. hi one embodiment the communication system is employed for a memory bus, which may for instance include random access memory (RAM), like that disclosed in U.S. Patent Number 5,243,703 to Farmwald et al., which is incorporated herein by reference. The multi-PAM communication and low-latency signal correction techniques disclosed herein may also be used for other contained systems, such as for communication between processors of a multiprocessor apparatus, or between a processor and a peripheral device, such as a disk drive controller or network interface card over an input/output bus. A 4-PAM receiver identifies a received symbol based on a voltage range or range of voltages associated with that symbol. A set of reference voltages VREFLO, VREFM and VREFHI function as thresholds to define ranges of voltages associated with each 4-PAM symbol, hi accordance with a preferred embodiment, the reference voltages VREFLO, VREFM and VREFHI are set at the midpoint voltage between neighboring symbols. For example, the symbol "00" is associated with voltages greater than VREFHI. The symbol "01" is associated with voltages falling within the range between VREFHI and VREFM. The symbol "11" is associated with a range of voltages from VREFM to VREFLO. The symbol "10" is associated with a range of voltages less than VREFLO. The reference voltages VREFHI, VREFM and VREFLO are threshold voltages from which a multi-PAM data symbol is determined to be one of the four possible data symbols. 4-PAM symbols or signals also allow for direct compatibility with 2-PAM or binary signaling. When operating in 4-PAM mode, the received data bits are compared to the three reference voltages, VREFHI, VREFM and VREFLO to determine the 4-PAM symbol and the associated two bits. Because the most significant bit (MSB) is determined by comparing the received data bit to VREFM, i.e.the MSB is zero for voltages greater than VREFM and the MSB is one for voltages less than VREFM, the multi-PAM system can be used as a 2-PAM system by ignoring the least significant bit (LSB) and using the MSB. Alternatively, to transmit 2-PAM symbols using the gray code of Figure 2, the LSB is set equal to zero (low), while the MSB determines the output voltage. More information on 2-PAM and multi-PAM systems may be found in the co-pending U.S. Patent Application entitled "Low Latency Multi-Level Communication Interface," identified above and incorporated herein in its entirety and identified above. Multi-PAM signaling increases the data rate with a small increase in power consumption because the number of input/output (I/O) pins and the system clock frequency may be the same as that used for binary signaling. The major factor in the power consumption of CMOS circuits, for example, is the CV 2 F power, which depends directly on the system clock frequency. Therefore, increasing the system clock frequency to increase the data rate directly increases the power consumption. Although some additional power is used for the additional circuitry of the multi-PAM interface, described below, this increase in power is much less than the increase in power that would occur if either the number of I/O pins or the system clock frequency were increased to increase the data rate. Multi-PAM signaling also increases the data rate without a corresponding increase in the electro-magnetic interference (EMI). If the data rate were increased by increasing the number of I/O pins or by increasing frequency, the EMI would increase proportionally. Because multi-PAM signaling does not increase the number of I/O pins, the EMI does not increase if the total voltage amplitude of the multi-PAM 110 pins remains the same as that used in binary signaling. The total voltage amplitude may be increased to provide greater voltage margin to improve system reliability. Although the EMI would increase correspondingly, the increase would be small than that incurred by increasing the number of I/O pins with binary signaling. Although the circuits described below use 4-PAM signaling, the embodiments described can be expanded for use in 8-PAM, 16-PAM and, more generally, N-PAM signaling. Accordingly, it is to be understood that the preferred embodiments are not limited to 4-PAM signaling, but rather may be applied to the general, N-PAM signaling, case. In Figure 3A, a 4-PAM output driver circuit 950 is used with current control bits (CCtrl<6:0>) to produced desired output voltage levels over a set of on-chip process, voltage and temperature (PVT) conditions. In the output driver 950, a first driver circuit 952 and a second driver circuit 954 connect to an I/O pin 956. The first driver circuit 952 drives the LSB, while the second driver circuit 954 drives the MSB. The first driver circuit 952 and the second driver circuit 954 have a set of driver blocks 958 that are connected in parallel. Since the driver blocks have the same components, one driver block 958 will be described. Each driver block has a binary weighted driver transistor 960-0 with a width to length (W/L) ratio as shown. The driver transistors 960 of the second driver circuit 954 are preferably twice as large as the driver transistors of the first driver circuit 952 because the second driver circuit 954 drives the MSB while the first driver circuit 952 drives the LSB. hi other words, the MSB is driven with twice as much current as the LSB. i driver block 958, odd and even data bits are multiplexed onto the driver transistors 950 via passgates 962 and an inverter 964. In this embodiment, odd data is transmitted at the rising edge of the clock, while even data is transmitted at the falling edge of the clock. NAND gates 966, 968 connect to current control bit zero <0>, and the LSB Odd Data bit and LSB Even Data bit, respectively. When the respective current control bit zero <0> is high, the NAND gates 966, 968 are responsive to the odd and even data. When the respective control bit is low, the output of the NAND gates 966, 968 is low and driver block 958 does not respond to the data bit. The current control bits provide the specified amount of current to cause the desired voltage swing regardless of the PVT conditions. The circuit of Fig. 3A uses seven current control bits. Techniques for determining the setting of the current control bits are described below. The passgates 962 include two transistor pairs, each pair including a PMOS transistor 972, 974 connected in parallel with an NMOS transistor 976, 978. The clock and clock_b signals connect in an opposite manner to the gates of the transistors of the transistor pair. Although Figure 3A shows that the first driver circuit 952 drives the LSB and the second driver circuit drives the MSB 954, in an alternative embodiment, the first driver circuit 954 drives the MSB and the second driver circuit drives the LSB. Alternatively, any arbitrary coding scheme can be produced by placing combinational logic to combine the data bits before sending the combined data bit to the driver block 958. Table 1 below shows two 4-PAM encoding schemes that may be implemented using the output driver 950 of Figure 3 A. Table 1 : Encoding Schemes In another embodiment shown in Figure 3B, a 4-PAM output driver 980 uses current control bits to control switch transistors in series with the output current source transistors, resulting in the desired output voltage levels. Two sets 981-1 and 981-2 of binary weighted transistors 982-986 combines the current control bits with 4-PAM signal generation. The current control bits directly control current-control NMOS transistors 982-2, 984-2, 986-2 that are connected in series with the driver transistors 982-1, 984-1, 986-1, respectively, that receive the LSB and MSB data. For odd data, the driver transistors 982-1, 984-1, 986-1, cause current to flow to the I O pin 956 when the respective data bit and the clock signal are high, and the associated current control bit is high to place NMOS transistors 982-2, 984-2 and 986-2 in the active state. The circuit for even data is not shown, but a separate set of current control NMOS transistors connects in series with a set of driver transistors that respond to the logical "AND" of the respective data bit and the complement of the clock signal Clock_b for even data. The output voltages of the circuits of Figures 3A and 3B include gds distortion from the driver transistors. In Figure 4A, a graph shows gds distortion. The x-axis shows the drain-to-source voltage, and the y-axis shows the drain current. Specifically, gds of a MOS transistor is the change of drain current in response to a change in drain voltage. Figures 4B and 4C show the data bits, in binary and gray code respectively, and the effect of gds distortion on the output voltage VOUT. In particular, as the output voltage VOUT decreases, the incremental voltage difference between adjacent bits pairs decreases. Because of gds distortion, the voltage increments between the 4-PAM voltages are generally not equal. hi Figure 5 A, a 4-PAM output driver 1000 that corrects for gds distortion is shown. The output driver 1000 is two-way multiplexed, with the multiplexing occurring at the I/O pin 956. The output driver is of the open-drain type and operated in current- mode, with the output current set by a bias voltage on a current source device coupled in series with each of the transistors 1002, 1004 and 1006. For simplicity the current control transistors are not shown, hi accordance with a preferred embodiment, a new output symbol is generated on each rising and falling edge (referred to herein as "odd" and "even," respectively) of the clock. The gds distortion is eliminated by adjusting the width to length (W/L) ratio of transistors 1004 and 1006 by factors α and β, such that β> >l and the incremental voltage difference between adjacent 4-PAM levels is constant. Transistors 1002, 1004 and 1006 have a width to length ratio of W/L, (W/L), and β(W/L) respectively. Examples of encoding schemes that may be implemented using the output driver of Figure 5A are shown in Table 2 below, hi accordance with a preferred embodiment, input signals A, B, and C are derived from the MSB and LSB of a symbol to be transmitted to produce the 4-PAM levels as shown in Table 2 below. The encoder of the output driver 1000 uses combinational logic 1007 to produce the A, B and C inputs according to Table 2. Table 2: Mapping of Data Bits to ABC Inputs and Encoding Schemes A binary encoder 1007 is illustrated in Figure 5B. hi the encoder 1007, an OR gate 1008 generates the A signal by performing an OR operation between the LSB and MSB. The B input is the MSB. An AND gate 1009 generates the C signal by performing an AND operation between the LSB and MSB. hi Figure 5C, an alternative preferred encoder 1007 encodes the LSB and MSB using Gray code. The encoder 1007 of Figure 5C is the same as the encoder 1007 of Figure 5B except that, to generate the C signal, the AND gate 1009a receives the complement of the LSB rather than the LSB. In Figure 9C, an alternative preferred embodiment of the gds compensated output driver is shown. In this embodiment, the output driver has separate odd and even symbol encoders, with the encoder outputs being multiplexed at the gates of the output transistors. On-chip, single-ended output drivers, as shown in Figures 3A and 3B generate switching noise. For example, when the transistors in the output driver transition from sinking no current such as when driving the "00" symbol, to sinking maximum current such as when driving the gray-coded "10" symbol, the current surges through the I/O pin 956 and through a ground pin. The path between I/O pin 956 and ground has inherent inductance that opposes the current surge and produces significant switching noise (i.e., ground bounce). Because the voltage margins for multi-PAM signaling are less than the voltage margins for binary signaling, switching noise may cause decoding errors. To reduce sensitivity to switching noise, output drivers can provide a constant or semi-constant current to ground regardless of the output current being driven. As shown in Figure 6, each single-ended transistor branch 960 (Figure 3A) and 986 (Figure 3B) in the output drivers of Figures 3A and 3B is replaced with a differential pair 1010. When the output driver sinks output current from the I/O pin 956, current is steered through transistor NI 1012 to ground. When transistor NI 1012 is inactive, transistor N2 1014 becomes active to allow the same or substantially the same amount of current to flow to ground. In this way, a substantially constant amount of current continuously flows to ground to eliminate a large portion of the output driver switching noise and provide a quieter on-chip ground, thereby improving the performance of the 4- PAM signaling. The signal Vi, is the signal that drives transistor NI 1012. Alternatively, the signal VR that drives transistor N2 1014 is a reference voltage between ground and Vi. In response to an input voltage Vcntrl, the current source 1016 sinks a predetermined current Io to ground. Figure 7 is another embodiment of a multi-PAM output driver that combines the circuit of Figure 5 A, which eliminates gds distortion, with the circuit of Figure 6 to reduce sensitivity to switching noise. hi Figure 8, yet another gds compensated 4-PAM output driver is shown. In the 4- PAM output driver, the A, B, and C signals drive equal-sized NMOS transistors 1018, 1020, 1022 having width W. hi accordance with a preferred embodiment, signals B and C also drive NMOS transistors 1024, 1026 of width WB and Wc, respectively, to compensate for gds distortion. The widths of the NMOS transistors 1024 and 1026, WB and Wc, respectively, are chosen such that the difference between output levels for adjacent bits is substantially the same, such as 1/3 (I-Zo). The widths of the transistors 1018-1026 may therefore have the following relationship: W+Wc> W+Wb > W In Figure 9A, a 4-PAM output driver corrects the gds distortion and provides current control. As described above, the signals A, B and C preferably determine the output voltage or symbol in accordance with the gray-coded binary signaling shown in Table 2, above, hi addition, three sets of current control calibration bits, CC, CCB and CCC, respectively determine the amount of current supplied by the output driver for various combinations of A, B and C. The first set of control bits CC provides primary current control, while the second and third sets of current control bits, CCB and CCC, respectively, fine tune the amount of current. The first set of current control bits CC has N bits; the second set of current control bits CCB has nl bits; and the third set of current control bits CCC has n2 bits, hi one embodiment, the relationship between the number of current control bits is as follows: nl < n2 < N. There may be different relationships between N, nl and n2 in alternative embodiments. Each of the A, B and C signals is associated with a current drive block 1040 to drive a predetermined amount of current associated with the symbol. Each current drive block 1040 includes one or more sets of stacked resistor pairs 1042 that are associated with each set of current control bits for that current driver block 1040. For example, the current drive block 1040-1 that drives the A signal receives current control bits CC. The current drive block 1040-2 that drives the B signal receives current control bits CC and CCB. The amount of current supplied by current drive block 1040-2 is adjusted for gds distortion using the CCB bits. The current drive block 1040-3 that drives the C signal receives current control bits CC and CCC. The amount of current supplied by current drive block 1040-3 is adjusted for gds distortion using the CCC bits. Referring also to Figure 9B, a set of stacked transistor pairs 1042 is shown. Each stacked transistor pair 1042 includes two NMOS transistors 1046, 1048 connected in series. The lower NMOS transistor 1046 connects to the one of the A, B, or C signals associated with the current drive block 1040. The upper NMOS transistor 1048 connects to a current control bit. The lower NMOS transistor 1046 is preferably wider than the upper NMOS transistor 1048. Because there are N CC bits, there are N stacked transistor pairs. For example, the current control block 1040 has N stacked transitory pairs 1042-1 to 1042-N, and each stacked transistor pair connects to one of the current control bits, CC <0> to CC <N-l>. The transistors of the stacked transistor pairs are binary weighted with respect to minimum width of Wl for the upper transistors and W2 for the lower transistors. The widths Wl and W2 may be chosen to determine output characteristics such as output resistance and capacitance. Generally the widths Wl and W2 are chosen such that Wl is less than W2. Although drawn to illustrate the circuit for the CC current control bits, the circuit diagram of Figure 9B also applies to the sets of stacked transistor pairs associated with the CCB and CCC current control bits. As shown in Figure 10, a current control calibration circuit 1050 determines the settings for the current control bits CC, CCB and CCC by selecting a current control reference voltage, VREF, and comparing the current control reference voltage, VREF, to a voltage at a mid-point between two calibration output voltages, Vouτ-1 and Vouτ-2. The current calibration circuit 1050 determines settings for each of the sets of current control bits CC, CCB and CCC for each 4-PAM output voltage such that Vouτ-1 and Vouτ-2 provide each adjacent pair of voltage levels to the circuit. A multiplexor 1052 receives the three 4-PAM reference voltages VREFFΠ, VREFM and VREFLO. A select reference voltage signal, SelRef, selects one of the referenced voltages as the selected current control reference voltage, VREF. A comparator 1054 compares the selected current control reference voltage VREF to a mid-point voltage Vx and generates a comparison signal. To generate the mid-point Vx, output driver 1 1056 sinks a first amount of current to provide the first output voltage Vouτ-1 and output diver 2 1058 sinks a second amount of current to provide the second output voltage Vouτ-2. Two passgate pairs 1060, 1062, in response to a current control enable and its complementary signal, act as a resistor divider to provide the midpoint voltage, Vx, between the first output voltage, Vouτ-1, and the second output voltage, Vouτ-2. A state machine 1064 includes first, second and third counters, 1066-1, 1066-2 and 1066-3 that provide the first, second and third sets of current control bits, CC, CCB and CCC, respectively. If the comparison signal indicates that the midpoint signal Vx is greater than the reference voltage VREF, the state machine 1064 increments an associated set of current control bits by one to increase the amount of current that is sunk by the output driver, thereby decreasing the midpoint voltage. If the midpoint voltage signal Vx is less than the current control reference voltage, VREF, the state machine 1064 decrements the associated current control bits by one, thereby increasing the midpoint voltage. hi one embodiment, the current control bits are calibrated during a power-up sequence. The theory of operation for calibrating the current control bits is as follows. The first set of current control bits CC provide the primary amount of current control for each current control block 1040. To compensate for gds distortion, the CCB and CCC current control bits fine tune the amount of current associated with the Gray-coded "11" and "10" signals, respectively. The current control bits are calibrated in the following order: CC, CCB, then CCC. In alternative embodiments, the current control bits may be calibrated after power- up in response to triggering events, e.g., lapse of a period of time, a change in ambient temperature, a change in power supply voltage, or in response to a threshold number of errors. Referring also to Figure 4B, the first and main set of current control bits CC are set using the voltage differences between the "00" and "01" symbols. The first set of current control bits CC are set to provide a amount of current to provide the output voltage for the "01" symbol such that VREFHI is placed at the midpoint between the output voltage for the "00" symbol and the output voltage for the "01" symbol. As shown in Figure 4B, because of gds distortion, without compensation, the voltage difference between the "01" symbol and the "11" symbol is less than the voltage difference between the "00" symbol and the "01" symbol. To compensate for the gds distortion, the output voltage for the "11" symbol is decreased by increasing the amount of current sunk by the output driver. The second set of current control bits CCB are set to increase the current sunk by the output driver such that the output voltage becomes equal to the desired voltage level when the midpoint voltage between output voltage for the "01" and "11" is equal to VREFM. Finally, the third set of current control bits CCC is adjusted such that the midpoint voltage between output voltage for the "11" and "10" is equal to VREFL. Referring to Figures 10, 11A and 11B, the operation of the circuit 1050 including the state machine 1064 will be described. The flowchart of Figures 11A and 11B uses gray coded output voltages. In step 1070, the current control enable signal (ccen) and its complement (ccenb) are set to activate the passgate pairs 1060 and 1062 and output the midpoint voltage Vx, described above. Three major blocks of steps 1072, 1074 and 1076 set the current control bits, CC, CCB and CCC, respectively. hi block 1072, step 1078 sets the initial conditions for determining the settings for the first set of current control bits CC. The state machine 1064 outputs the select reference voltage signal (SelRef) which causes the multiplexor 1054 to output the reference voltage VREFHI to the comparator 1054. A "00" symbol is supplied to output driver 1 1056 by outputting multi-PAM bit selection signals Al, Bl and Cl with values of zero. A "01" symbol is supplied to output driver 2 1058 by outputting multi-PAM bit selection signals A2 with a value of one, and B2 and C2 with a value of zero. The initial state of the first, second and third current control bits is as follows: CC = {1 0 0 ...0}; CCB = {1 0 0 ...0}; and CCC = {1 0 0 ...0}. The current control bits are initially set such that the stacked transistor pair sinking the most current will be activated. hi step 1080, the output drivers 1 and 2 output the voltages corresponding to the symbols "00" (the Vterm refernce) and "01" (the drive level under calibration) and the midpoint voltage Vx is generated. In step 1082, the comparator 1054 compares the midpoint voltage Vx to the selected reference voltage VREFHI. When the midpoint voltage is within one least significant bit of the reference voltage VREFFΠ, the first set of current control bits have the proper setting. The state machine 1058 determines that the midpoint voltage Vx is within one least significant bit of the reference voltage VREFHI when the current control bits begin to dither between two settings. In other words, the output of the comparator will alternate between a zero and a one. In step 1084, when the midpoint voltage Vx is not within one least significant bit of the reference voltage VREFHI, the state machine 1064 augments the first set of current control bits depending on the result of the comparison. The term "augment" is used to indicate either incrementing or decrementing the current control bits. The process proceeds to step 1080. If, in step 1082, the state machine 1064 determines that the midpoint voltage Vx is within one least significant bit of the reference voltage, the process proceeds to step 1086 to calibrate the second set of current control bits, CCB. In step 1086, the initial conditions for calibrating the second set of current control bits CCB are set. The state machine 1064 outputs the select reference voltage signal (SelRef) which causes the multiplexor 1054 to output the reference voltage VREFM to the comparator 1054. A "01" symbol is supplied to output driver 1 1056 by outputting multi- PAM bit selection signals Al with a value of one, and Bl and Cl with values of zero. A "11" symbol is supplied to output driver 2 1058 by outputting multi-PAM bit selection signals A2 and B2 with a value of one, and C2 with a value of zero. The state of the first set of current control signals CC remains unchanged. The initial state of the second and third sets of current control bits, CCB and CCC, respectively, is as follows: CCB = [ 1 0 0 ... 0}; CCC = [ 1 0 0 ... 0}. hi step 1088, the output drivers 1 1056 and 2 1058 output the voltages corresponding to the symbols "01" (the level calibrated in step 1072) and "11" (the level now under calibration), and the passgate pairs 1060, 1062 output the midpoint voltage Vx. hi step 1090, the comparator 1054 compares the midpoint voltage Vx to the selected reference voltage VREFM. When the midpoint voltage is not within one least significant bit of the reference voltage VREFM, as described above with respect to VREFHI, in step 1092, the state machine 1064 augments the second set of current control bits CCB by one and the process repeats at steps 1086. When the midpoint voltage is within one least significant bit of the reference voltage VREFM, as described above with respect to VREFHI, the second set of current control bits CCB have the proper setting and the process proceed to step 1094 to calibrate the third set of current control bits, CCC. hi step 1094, the initial conditions for calibrating the third set of current control bits CCC are set. The state machine 1064 outputs the select reference voltage signal (SelRef), which causes the multiplexor 1054 to output the reference voltage VREFLO to comparator 1054. A "11" symbol (calibrated in step 1074) is supplied to output driver 1 1056 by outputting multi-PAM bit selection signals Al and Bl with a value of one, and Cl with a value of zero. A "10" symbol (the level now under calibration) is supplied to output driver 2 1058 by outputting multi-PAM bit selection signals A2, B2 and C2 with a value of one. The state of the first and second sets of current control signals CC and CCB, respectively, remains unchanged. The initial state of the third sets of current control bits CCC is as follows: CCC = { 1 0 0 ... 0}. i step 1096, the output drivers 1 1056 and 2 1058 output the voltages corresponding to the symbols "11" and "10" and the passgate pairs 1060, 1062 output the midpoint voltage Vx. In step 1098, the comparator 1054 compares the midpoint voltage Vx to the selected reference voltage VREFLO. When the midpoint voltage is not within one least significant bit of the reference voltage VREFLO, as described above with respect to VREFHJ, in step 1100, the state machine 1064 augments the third set of current control bits CCC by one and the process repeats at step 1094. hi step 1098, when the midpoint voltage is within one least significant bit of the reference voltage VREFLO, the appropriate settings for the first, second and third sets of current control bits, CC, CCB and CCC respectively are determined and the calibration is complete. For the foregoing embodiment, a sequential search is described: starting at an initial value and augmenting. It should be emphasized, however, that alternative search techniques known to those skilled in the art may be used. For example, without limiting the foregoing, successive approximation using a binary search may be used. As a further, although less desirable because it is hardware intensive, alternative, a direct flash conversion may be used. hi Figure 12, a 4-PAM reference voltage generator 1380 generators the multi- PAM reference voltages VREFHJ, VREFM and VREFLO from external voltages, VTERM and VREF, supplied on input pins 1382, 1384 respectively. Unity gain amplifiers 1386, 1388 receive and output the input voltages VTERM and VREF respectively. A voltage divider, including series-connected resistors Rl, R2 and R3, is coupled between the outputs of the unity gain amplifiers 1386 and 1388. The lowest voltage VREF is selected to drive V R E FLO via a power driver 1390. Power drivers 1392, 1394 are coupled between resistors R3, R2 and R2 to provide reference voltages VREFHI and VREFM respectively. The power drivers 1390-1394 are connected as unity gain amplifiers. In one embodiment, the resistor values are selected such that resistors R2 and R3 have twice the resistance of resistor Rl, and VREF, which is supplied externally, is equal to the desired VREFLO voltage. An electrical schematic of a first preferred alternative to the current control calibration circuit of Figure 10 is shown in Figures 13 A and 13B. hi Figure 13 A, a comparator 1500 is coupled by a multiplexor 1502 to a multi-level voltage reference 1504, which in this case includes three discrete levels. One of the three reference voltage levels, V R EFHI, VREFM or V REFL O, is selectively applied to two inputs of the comparator 1500, as further described below. The comparator 1500 is also coupled to receive source calibration signals 1506 and 1508, which are supplied by current mode drivers, such as the 4-PAM driver 1000 shown in Figure 5 A. The source calibration signals 1506 and 1508, for the embodiment shown, include a first driver output at a known, or previously calibrated, voltage level on the input line 1506 and an unknown driver output voltage level on the input line 1508, such that the signal on input line 1508 is the signal being calibrated. The comparator 1500 provides an output for adjusting or calibrating the output of the drivers on input line 1508, as further described below, so that the driver output can be reliably received and decoded. Figure 13B is an electrical schematic of the comparator 1500 shown in Figure 13 A. The two inputs from the multiplexor 1502 and the source calibration signals 1506 and 1508 are each coupled to an input of a switch 1510. The outputs of the switches 1510 are combined in pairs and each combined switch output is connected to a coupling capacitor 1512. The coupling capacitors 1512 are connected to opposing inputs 1514a and 1514b of a transistor comparator 1516. The output of the transistor comparator 1516 is the voltage across nodes 1518 and 1520. Two switches 1522 selectively couple the output nodes 1518 and 1520 to the inputs 1514a and 1514b, respectively. The output nodes 1518 and 1520 are coupled to a latching stage 1524. As illustrated in Figure 13B, the elements of the comparator 1500, including the switches 1510, the coupling capacitors 1512, the amplifier 1516 and the switches 1522, are preferably implemented as semiconductor devices in an integrated circuit. The coupling capacitors 1512 are preferably constructed using MOS transistors connected as capacitors but other embodiments may alternatively use other capacitor types. Those skilled in the art of integrated circuit design will appreciate that, as a result of process variation, there is likely to be a random offset voltage associated with the transistor comparator 1516. In other words, if the same voltage is applied at the inputs 1514a and 1514b, a finite voltage will appear across output nodes 1518 and 1520, rather than the ideal case in which the output nodes 1518 and 1520 are at the same potential. While the offset voltage is not typically significant for systems using binary or 2-PAM signaling, it is preferable to correct for the offset voltage in systems using four or more signal levels, such as a 4-PAM system. The comparator 1500 of Figure 13B therefore includes offset cancellation circuitry. Specifically, the coupling capacitors 1512 and the switches 1522 are operable to provide offset cancellation as follows. During the cancellation phase, which may also be referred to herein as the auto-zero phase, signal az, which is coupled to the gates of the transistor switches 1522, is high. Referring back to Figure 13A momentarily, the signal az is generated by a non-overlapping clock driver, which includes elements U29, U16, U18, etc. The non-overlapping clock driver produces skewed signals, with a delay period between transitions. Referring again to Figure 13B, when the signal az goes high, the amplifier 1516 is placed into unity gain mode by turning on switches 1522 and the offset voltage is stored on the coupling capacitors 1512. hi addition, during the auto-zero phase, the switches 1510 are set to apply, in this particular embodiment, the reference voltage supplied by the multiplexor 1502 and the known output driver voltage 1506 to the coupling capacitors 1512. Thus, during the auto-zero phase, the transistor comparator 1516 samples the difference between the two known voltages as modified by the offset voltage of transistor comparator 1516. At the end of the auto-zero phase, switches 1522 are opened, placing the amplifier 1516 into a high gain mode, and then there is a momentary delay followed by a compare phase. At the start of the compare phase, the state of the switches 1510 is changed to sample the reference voltage supplied by the multiplexor 1502 and the unknown output driver voltage 1508 onto the coupling capacitors 1512. Because the charge stored from the auto-zero phase is trapped on the coupling capacitors 1512, any change in the input voltages, such as the change to the unknown output driver voltage 1508, produces a voltage across the input nodes 1514a and 1514b of the transistor comparator 1516. This in turn produces an output voltage across the nodes 1518 and 1520 that is preferably latched into a latching stage 1524. The control logic enables strobing of the latching stage 1524. fn accordance with a preferred embodiment, the latch 1524 may be strobed multiple times during a single compare phase. Alternatively, the latch 1524 may be strobed only once during a single compare phase. h accordance with a preferred embodiment, a current control transistor in the current mode driver is adjusted, for example as described above with respect to Figures 10, 11 A and 11B or as described in U.S. Patent No. 5,254,883, based upon the output of the transistor comparator 1516. h accordance with a preferred embodiment, the unknown driver output voltage level on line 1508 is incrementally adjusted, such as by increasing or decreasing the amount of current sunk by the output driver, until the average value of the voltage levels on lines 1506 and 1508 is equal to the reference voltage supplied by the multiplexor 1502. Figure 13C is a timing diagram illustrating the relationship between several of the signals referenced above. The timing signals 1526 and 1528 drive the non-overlapping clock driver in Figure 13 A. The auto-zero and compare phases are defined in accordance with the signal 1526. The signal 1530 is the voltage output of the comparator, as shown at pin 1532 in Figure 13 A. The known voltage signal 1506 and the known voltage reference from the multiplexor 1502 are essentially constant. The unknown voltage signal 1508 is adjusted, in this example it is decreasing. When the unknown voltage signal 1508 reaches the point where the reference, voltage is equal to the average of the signals 1506 and 1508, the output of the comparator circuit 1530 goes high. The current control calibration circuit shown in Figures 13 A and 13B may be utilized as follows to calibrate a 4-PAM output driver, such as the driver of Figure 5 A. When the transistors 1002, 1004 and 1006 are in the "off state the voltage at the output of the current mode driver is VT ERM - This corresponds to the symbol 00, which is the zero current state and does not need to be calibrated. The known voltage, VT ERM , is applied to line 1506 of the comparator 1500 and an unknown voltage generated by turning "on" the transistor 1002 (from Figure 5A) is applied to line 1508 of the comparator 1500. The multiplexor 1502 causes the reference voltage, V R E FHI , to be applied to the comparator 1500. Using feedback from the output of the comparator 1500, a current control transistor (not shown) coupled in series with the transistor 1002 is adjusted until the average of the voltages on lines 1506 and 1508 is equal to the reference voltage, V RE F HI - The voltage on line 1508 is now calibrated to correspond with the 4-PAM symbol "01". At this point, the voltage corresponding to the 4-PAM symbol "01" is applied to line 1506, and an unknown voltage generated by turning "on" the transistors 1002 and 1004 is applied to line 1508. The multiplexor 1502 is activated to cause the reference voltage, V R E FM , to be applied to the comparator 1500. Using feedback from the output of the comparator 1500, a current control transistor (not shown) coupled in series with the transistor 1004 is adjusted until the average of the voltages on lines 1506 and 1508 is equal to the reference voltage, V RE F M - The voltage on line 1508 is now calibrated to correspond with the 4-PAM symbol "11". Next, the voltage corresponding to the 4-PAM symbol "11" is applied to line 1506, and an unknown voltage generated by turning "on" the transistors 1002, 1004 and 1006 is applied to line 1508. The multiplexor 1502 is activated to cause the reference voltage, V R E FLO , to be applied to the comparator 1500. Using feedback from the output of the comparator 1500, a current control transistor (not shown) coupled in series with the transistor 1006 is adjusted until the average of the voltages on lines 1506 and 1508 is equal to the reference voltage, V REF I O - The voltage on line 1508 is now calibrated to correspond with the 4-PAM symbol "10". Those skilled in the art of circuit design will appreciate that the comparator 1500 may take other forms. Figure 13D illustrates alternative embodiments for the differential comparator of Figure 13B. Referring again to Figure 13B, it will be appreciated that if, for example, the comparator 1500 is implemented as an integrated circuit, then the coupling capacitors 1512 may be implemented using a PMOS FET topology as shown in Figure 13E. Such capacitors operate linearly when the applied voltage, V DC , is greater than the magnitude of the threshold voltage, V T , of the PMOS FET. The averaging and offset cancellation f nctions of the comparator 1500 are not optimally realized when the capacitors are operated in the non-linear range. It is therefore preferred that the applied voltage be kept within the linear range, hi accordance with a preferred embodiment, the applied voltage is within the range of approximately 1.0 volts to 1.8 volts. The auto-zero voltage, VA Z , may be approximately 0.6 volts. An electrical schematic of another preferred alternative to the current control calibration circuit of Figure 10 is shown in Figures 14A and 14B. As shown in Figure 14 A, this embodiment includes a comparator 1500, a multiplexor 1502, multi-level voltage reference 1504, and source calibration signals 1506 and 1508, which carry a known voltage signal and an unknown (to be calibrated) voltage signal, respectively, hi comparison to Figure 13 A, the circuit of Figure 14A differs in that it includes a resistive voltage combiner 1532 that is coupled to provide the average of the signals on lines 1506 and 1508 to the comparator 1500. In addition, for the embodiment of Figure 14A, the non-overlapping clock driver is replaced by an inverter delay chain 1534. As shown in Figure 14B, the comparator 1500 differs from that of Figure 13B. Notably, a different offset cancellation technique is utilized. For the embodiment of Figure 14B, a switch 1536 and feedback amplifier 1538 are used to compensate for the offset voltage associated with a differential amplifier 1540. The operation of the embodiment shown in Figures 14A and 14B will now be described. The timing of the offset cancellation phase and the compare phase are controlled by the inverter delay chain 1534. The inverter delay chain 1534 produces skewed signals evb, evb2, evbό, etc, shown in Figures 14C and 14D. The delay between these signals is approximately the delay of one or more logic gates. The delay period may be augmented by loading the gate outputs with additional capacitance. During the cancellation phase, the feedback amplifier 1538 senses the offset voltage associated with the differential amplifier 1540 as follows. When timing signal evb2 goes low, the inputs 1542 and 1544 of the amplifier 1540 are shorted together by a switch 1546. At the same time, a switch pair 1548 couples the outputs of the amplifier 1540 to the inputs of the feedback amplifier 1538. With the inputs 1542 and 1544 of the amplifier 1540 being shorted together, any voltage appearing at the output of the amplifier 1540 may be characterized as an output offset voltage. The feedback amplifier 1538 produces output current in the drains of transistors 1550 and 1552 in an amount that is proportional to the output offset voltage. The current supplied by the feedback amplifier 1538 works to drive the output offset voltage to zero, thereby balancing the amplifier 1540 when its inputs 1542 and 1544 are shorted. The resultant voltage required to produce the balancing current in the feedbac amplifier 1538 is stored on the capacitors 1554 and 1556 at the end of the cancellation phase when the switches 1548 are opened. As shown in Figures 14C and 14D, shortly after the cancellation phase ends on the falling edge of the signal evb, the switches 1546 and 1548 are opened, disconnecting the feedback amplifier 1538 and coupling the inputs 1542 and 1544 to the amplifier 1540, as the signal evb2 goes high. The transition of evb2 to high starts the compare phase. Momentarily after the compare phase starts, the signal evb6 goes high, activating the latching stage of the comparator 1500. When the latching stage is active, the output voltage of the differential amplifier 1540 is latched. The current control calibration circuit shown in Figures 14A and 14B may be utilized to calibrate a 4-PAM output driver in the same manner as described above with respect to Figures 13 A and 13B. Figure 15A is an electrical schematic of a linear transconductor. In a linear region of operation, the output voltage, V O U T , is proportional to the difference between the input voltages, vi and v . Thus, the output of the linear transconductor is balanced, i.e. vouτ= 0, when Vi - v Re f = v Ref - v 2 , or (v_ + v 2 )/2 = v Ref . In accordance with yet another alternative embodiment, therefore, the comparator comprises a transconductor stage, as shown in Figure 15B. For this embodiment, an offset canceling amplifier, such as the amplifier 1538 of Figure 14B, is preferably utilized. Reference Voltage Generator in Systems Using Equalization or Crosstalk Cancellation Referring back to Figure 1, the output drivers 323 and receivers 324 can operate, for example, in either 2-PAM or 4-PAM mode, hi one embodiment, the control, address and data signals use the same multi-PAM mode, such as 4-PAM. However, because 4- PAM may be more susceptible to errors from noise than 2-PAM, to improve system reliability, in another embodiments, signals on the bus may use the 2-PAM mode. Additionally, data may alternate between the 2-PAM and 4-PAM mode. For example, a pattern generator may be coupled to the memory controller 321 and may be used to periodically determine whether to operate the system at 2-PAM or 4-PAM. In one embodiment, the pattern generator may be arranged to periodically determine an error rate in the system, and if the error rate is above a predetermined threshold, 2-PAM signaling may be used. Hereinafter, systems and method for generation reference voltages will be described in reference to multi-PAM systems. However, it should be understood that the methods and systems are equally applicable in 2-PAM systems. As used herein, the term multi-level signaling refers to signaling schemes utilizing two or more signal levels. Multi-level signaling may also be referred to herein as multiple level pulse amplitude modulation, or multi-PAM, signaling, because the preferred coding methods are based upon the amplitude of the voltage signal. Other known multi-level signaling techniques may alternatively be used. Further, although the multi-level signaling of the preferred embodiments will be described with respect to current source drivers, different or equivalent drivers could also be used. Output drivers 323 generate, and receivers 324 detect, multi-PAM signals that allow multiple (k) bits to be transmitted or received as one of 2 k possible voltages or data symbols at each clock edge or once per clock cycle. For example, one preferred embodiment is a 4-PAM system in which two bits are represented by 2 2 or four voltage levels, or data symbols, and the two bits are transferred at every clock edge by transferring an appropriate one of the four voltage levels. Therefore, the data rate of a 4-PAM system is twice that of a binary or 2-PAM system. Figure 16 shows a representation of a communication system that may be used to create voltage levels of Figure 2. An output driver 2420 drives signals to an output pad 2418 and over a transmission line 2416, which may, for example, be a memory bus or a different interconnection between devices affixed to a circuit board, to be received at a pad 2425. The transmission line 2416 has a characteristic impedance Z 0 2427 that is substantially matched to a terminating resistor Zo 2429 to minimize reflections. The output driver includes a first transistor current source 2421, a second transistor current source 2422, and a third transistor current source 2423, which collectively produce a current I when all active, pulling the voltage at the pad 2425 down from V TERM by I • Zo, signaling the logical state 10 under the Gray code system. To produce voltage V O U T - V TERM , signaling the logical state 00, current sources 2421, 2422, and 2423 are all turned off. To produce voltage V OUT equal to V TERM - 1/3 (I • Zo), signaling the logical state 01, one of the current sources is turned on, and to produce voltage V O U T equal to V TERM - 2/3 (I • Zo), two of the current sources are turned on. The logical level 00 is chosen to have zero current flow to reduce power consumption for the situation in which much of the data transmitted has a MSB and LSB of zero. The reference levels are set halfway between the signal levels, so that VREFH=VTERM-(l/6) I-Zo, VREFM=VTERM-(l/2) I-Zo and VREFL=VTERM(5/6) I-Zo. Related disclosure of a multi-PAM signaling system can be found in U.S. Patent Application Serial Number 09/478,916, entitled Low Latency Multi-level Communication Interface, filed on January 6, 2000, which is incorporated by reference herein. Also incorporated by reference herein is a Provisional U.S. Patent Application entitled, "A 2Gb/s/pin 4-PAM parallel bus interface with transmit crosstalk cancellation & equalization and integrating receivers," filed on even date herewith, by Express Mail Label No.EK533218494US, inventors Pak Shing Chau et al. Figure 17 illustrates an exemplary 4-PAM reference voltage generator 2500 that generates the multi-PAM reference voltages VREF H I, V REFM and V REFLO from external voltages, VTERM and VR E F, supplied on input pins 2502 and 2504 respectively. Unity gain amplifiers 2506, 2508 receive and output the input voltages V TER M and VREF respectively. A voltage divider, including series-connected resistors Rl, R2 and R3, is coupled between the outputs of the unity gain amplifiers 2506 and 2508. The lowest voltage V R EF is selected to drive V REFLO via a power driver 2514. Power drivers 2510, 2512 are coupled between resistors Rl, R2 and R3 to provide reference voltages V REF HI and V R EFM respectively. The power drivers 2510-2514 are connected as unity gain amplifiers. h one embodiment, the resistor values are selected such that resistors R2 and R3 have twice the resistance of resistor Rl, and V R E F , which is supplied externally, is equal to the desired V RE FL O voltage. Figure 18 illustrates an alternative embodiment of a 4-PAM reference voltage generator 2600 for generating the multi-PAM reference voltages V REFHI , V REFM , and V REFLO from the external voltage V T E RM supplied on an input pin 2602. A voltage divider, including series-connected resistors Rl, R2, R3, and R4, is coupled between the V TER M and a ground voltage supplied on an input pin 2602. In one embodiment, the resistor values are chosen so that the compromise between the static power consumption and noise immunity is achieved. Thus, the resistor values may be selected so that not too much static power is burned, and, further, so that reference voltages are not susceptible to noise injection. However, it should be understood that the reference voltage generation is not limited to the use of the voltage generators illustrated in Figure 17 or Figure 18, and different or equivalent reference voltage generators may alternatively be used. For cases in which attenuation of a signal exists between the signal's reception and transmission, different amounts of signal loss may occur depending upon the magnitude of the transition between logic levels. Figure 19 illustrates attenuation levels associated with three transition states. For instance, transitioning between the 10 state and the 00 state may have a greater signal deficiency than a transition between the 11 state and the 00 state, which in turn may have a greater signal deficiency than a transition between the 01 state and the 00 state, while maintaining the same logic state over plural bit periods may have no attenuation error. Thus, while the correct dc-level will eventully be achieved, each transition between states may have a different error associated with it. Figure 20 illustrates the addition of a different equalization signal 3S, 2S, or S to the main signal when driving different transitions to compensate for the attenuation of the received signal. The equalization signal in this embodiment are transitory, so that each signal may, for example, have a duration less than or equal to one bit signal, after which the equalization signal is terminated, allowing the initially overdriven signal to maintain a steady state logic level. In other words, the equalization signals S, 2S or 3S add predetermined high-frequency components to the transition signals that raise the slope of the edge of transition. However, a difficulty with this approach for a system such as shown in Figure 16 is that the voltage can only be pulled down from the V TERM , unless negative current could flow through the current sources 2421, 2422, and 2423. In other words, with the 00 level set at the V TE R M , as illustrated in Figure 2, it is difficult for the equalization signals S, 2S or 3S to add to the transition above the V TER M- Figure 21 depicts a multi-level (4-PAM) signaling system in which the 00 logic is reduced below V TERM by a predetermined amount in order to allow overdriving a transition, for example, from the 10 state to the 00 state by a predetermined equalization signal such as 3S via release of the pulldown current. This 00 logic level may be provided by having none of the main current drivers turned on and three equalization drivers turned on, to produce a voltage level of 0M+3S. The logical state 10 can be pulled lower, if necessary, to overdrive a transition, and it is characterized by having three main drivers turned on, for a total voltage level of 3M+0S. The logical state 01 is pulled down and may have a voltage of 1M+2S, while the state 11 may have a voltage of 2M+1S. Table 3 illustrates drive signal levels involved in transitioning between the logical states of Figure 21. Table 3. hi order to transition from the initial state 00 (0M+3S) to the final state 10 (3M+3S), for example, an overdriven transition drive signal of 3M+3S is provided. Conversely, changing from the initial state 10 (3M+0S) to the final state 00 involves a drive signal of 0M+0S. The transition drive signal in this example has an overdriven duration that is equal to that of one bit (or dual-bit) signal, although in general an overdriven period of a drive signal may have a duration that is less than or greater than that of a bit signal. For example, a drive signal may be overdriven for a first portion of a bit signal time, with a second portion of that signal not being overdriven. Several characteristics are apparent from the Table 3. First note that for the received signals, the M multiplier is the complement of the S multiplier. Also note that the drive signal S multiplier is equal to the initial S multiplier, while the transition M multiplier is equal to the final M multiplier. Also note that Table 3 illustrates just one set of transition drive signals specific to a 4-PAM signaling system with a single error correction, and may be extrapolated to be used with communication systems having additional signal levels and additional error corrections. For instance, a similar table may be constructed to deal with signal reflections, although such reflections may be either positive or negative and may have a delay of more than one bit signal duration, as measured at the signal receiver. Table 3 illustrates one set of transition drive signals specific to a 4-PAM signaling system with a single error correction. However, it should be understood that the illustrated set of transition signals may be extrapolated to be used with communication systems having additional signal levels and additional error corrections. Further, it should be understand that similar transition drive signals could also be used in 2-PAM systems with equalizaiton. Figure 22A-22C show drive signals that may, for example, represent the transition from state 10 ("D") to 00 ("A"), which for clarity of illustration is then shown to remain at state A for the next bit signal. FIG. 22 A shows a signal 5000 with an overdriven transition during period 1 between an initial voltage during period 0 and a final voltage during period 2. The signal 5000 can be made from a combination of a step function signal 5200, shown in FIG. 22B, and a another step function signal 5500, shown in FIG. 22C, that is a scaled, inverted and delayed function of the signal 5200. hi this example, the voltage change of signal 5500 is a small fraction of that of signal 5200, delayed by one period, which may be equal to a bit signal duration. Figure 23 is a schematic block diagram illustrating an exemplary device that may provide the transition drive signals shown in Figure 21. An encoder 2702 receives binary MSB and binary LSB signals and converts pairs of those 2-PAM signals into digital input signals along lines Cl, C2 and C3 for a main driver 2705 having three current sources 2705, 2712, and 2713, to convert the binary data of the MSB and LSB into 4-PAM data at an output pad 2715. As illustrated in Figure 23, a delay mechanism 2718 also receives the input signals from the encoder 2702. The delay mechanism 2718 inverts and delays the input signals by a one-bit signal period to feed signals El, E2, and E3 to an auxiliary driver 2720 including auxiliary driver current sources 2721, 2722, and 2723 that are similar to the respective main driver current sources 2711, 2712 and 2713. However, each auxiliary driver current source 2721, 2722, and 2723 has a gain that is a fraction of the respective main driver current sources 2711, 2712, and 2713. Thus, when the main driver 2705 outputs an output signal, the auxiliary driver 2720 outputs a signal that is delayed, inverted and proportional to the output signal of the driver 2705. The signals generated by the driver 2705 and 2720 combine at the line 2728 to form a desired signal. Although current sources 2711, 2712, 2713, 2721, 2722 and 2723 are shown as having a single transistor, each of these current sources may include multiple transistors that may differ in power or number from the other current sources. The output voltages of the device 2700 of FIG. 23 would have a distortion, for example, if each of the current sources 2711, 2712 and 2713 were identical, since the drain to source voltage drop for each depends in part on whether the others are active, affecting the current. To compensate for this gds distortion the current sources can differ in number or transistor gain, e.g., by adjusting channel width. Current sources 2711, 2712, 2713, 2721, 2722 and 2723 may also receive additional input signals, for example, to adjust signal strength due to process, voltage and temperature (PVT) conditions, hi addition, data may be transmitted at both rising and falling clock edges, so a pair of substantially identical main drivers and auxiliary drivers may exist for driving multi-PAM signals from input signals generated from MSB and LSB odd and even signals. Auxiliary driver current sources 2721, 2722 and 2723, which may also suffer from gds distortion unless adjusted as described above, drive much less current than the corresponding main driver current sources 2711, 2712 and 2713. For example, each of main driver current sources 2711, 2712 and 2713 may be formed of more current sources than respective auxiliary driver current sources 2721, 2722 and 2723. Alternatively, main driver current sources 2711, 2712 and 2713 may be formed of transistors having wider channels than those of respective auxiliary driver current sources 2721, 2722 and 2723. Thus the output of auxiliary driver current sources 2721, 2722 and 2723 are substantially proportional to the main driver current sources 2711, 2712 and 2713. hi FIG. 24, the drivers 5105 and 5120 and delay element 5118 are represented as a FIR filter 5130. Input signals from the encoder are sent to the filter, which operates on the signals with function M and delays, inverts and operates on the signals with function S, to output a filtered signal. Referring to Table 3, the main driver 2705 can be represented with the letter M and the auxiliary driver 2720 can be represented with the letter S, with the numeral proceeding each letter describing how many of the respective current sources are active. A ratio of the gain of the auxiliary driver, S, to that of the main driver, M, is termed k, the equalization coefficient of the auxiliary driver. The ratio k is less than one, and may range from about one percent to about fifty percent. Thus, the driving device on the bus know the equalization current based on the equalization coefficient set on the auxiliary driver. In one embodiment, the drivers 2705 and 2720 and the delay element 2718 of Figure 23 may be represented as an FIR filter, where the input signals from the encoder 2702 are sent to the filter, which operates on the signals to output a filtered signal. Equalization techniques similar to those described above for the attenuation can be used to compensate for crosstalk and reflection errors. Figure 25 and Figure 26 including Figures 26A, 26B, 26C, 26D and 26E, show a general system using a self equalization FIR filter 2806 and crosstalk (XTK) equalization FIR 804 and crosstalk equalization FIR filter 2802 for filtering errors on a line V(n) due to crosstalk from adjacent lines Ai(n) through Aj(n), respectively, to produce equalized output V OUT (N). Figure 26A shows a step function signal 2900 on the line A. that generates, as shown in Figure 26B, a transient signal 2902 in line V due to inductive coupling. Figure 26C shows an equalization signal 2904 that is generated by inputting the signal 2900 to the crosstalk equalization FIR filter 2804 to compensate for transient signal 2902. Capacitive crosstalk generated in line V from lines A \ through Aj can be equalized in a similar fashion. Both inductive and capacitive crosstalk may generate a transient signal having a polarity dependent upon the location of the receiver compared to the location at which the transient signal was generated. Figure 26D shows a first signal component 2906 that may be used to compensate for such an inductive or capacitive crosstalk error, which is a scaled and inverted function of the signal 2900 that generated the error. Figure 26E shows a second signal component 2908 that may be added to the signal 2906 to create the crosstalk equalization signal 2904 of Figure 26C. The signal 2908 can be scaled or a fractional function of signal 2900 that is delayed one bit period. Figure 27 illustrates an exemplary electrical schematic 2950 that may be coupled to a device illustrated in Figure 23 and can be used to equalize inductive or capacitive errors created by signals in line 2728 and sensed in lines that are adjacent to the line 2728. A first crosstalk equalization driver 2905 receives signals along lines Pl, P2 and P3 that have been inverted by inverters 2910 controlling current sources 2711, 2712, and 2713. The combined current from the current sources 2911, 2912, and 2913 is output on a crosstalk equalization line 2920 that is connected to an adjacent line, not shown, that is subjected to crosstalk errors from the line 2728. The output on the line 2920 from the driver 2905 is thus scaled and inverted compared to the signal produced by the main driver 2705, much like the signal 2906 of Figure 26D is a scaled and inverted compared to the signal 2900 in Figure 26A. A second crosstalk equalization driver 2925 receives input signals along lines XI, X2, and X3 that have been delayed by the delay 2715, where the lines XI, X2 and X3 control the current sources 2921, 2922 and 2923. Thus, the output of the second crosstalk equalization driver 2925 on the line 2920 is delayed and scaled compared to the signals produced by the main driver 2705, much like the signal 2908 of Figure 26E is scaled and delayed compared to the signal 2900 in Figure 26 A. The combined output of the crosstalk equalization driver 2905 and 2925 on the line 2920 is much like the signal 2904 of Figure 26C, which can be used to compensate for the transient signal 2902 of Figure 26B. Thus, the crosstalk equalization drivers 2905 and 2925 can compensate for the crosstalk inflicted by the line 2728 on nearby lines. Similarly to the equalization cancellation, the crosstalk cancellation employs a predetermined crosstalk cancellation current based on the crosstalk coefficients used in the system. An advantage of the circuit shown in FIG. 12 is that cost and space effective circuits are implemented that generate equalization signals by tapping into the input signals for the main signal driver, instead of generating equalizing signals based upon the output signals that are transmitted. Similar equalization mechanisms can be used to compensate for reflections in a transmission line due to impedance discontinuities in the line. These reflections may be positive or negative, and may occur at various times relative to the main signal. Thus for example a known reflection may occur at a receiver with a delay of two clock cycles, for which plural unit delay elements may be provided between the input signals and the compensation circuits. Moreover, although a 4-PAM signaling system is shown, additional signal levels are possible with the provision of additional current sources. Unit delay elements may be provided for example by flip-flops, whereas delay elements that generate delays of less than a bit period may be provided for instance by inverters. FIG. 28 shows another mechanism for equalizing multi-PAM signals, however the equalization mechanism inputs, on two lines, the MSB and LSB signals, rather than inputting the three lines of thermometer code inputs. A unit delay element 5230 receiving the MSB and LSB outputs those signals with a delay substantially equal to one signal bit duration. Unit delay 5230 allows comparison at transition mapping device 5233 of the MSB and LSB with the previous MSB and LSB, to gauge the multi-PAM transition between the prior and current MSB and LSB. Based upon this transition, transition mapping device 5233 outputs a three bit signal to current digital-analog converter (ID AC) 5235, which drives a compensation signal on line 5237, which in this example connects with a multi-PAM signal, not shown in this figure, to compensate for crosstalk on that other line. FIG. 29 shows such a general equalization system that can be used to compensate for various signal imperfections, whether the imperfections are due to attenuation, crosstalk, or reflections. Input signals on lines C(N) that control a main driver M(N) pass through a number J of delay elements Dl through DJ, each of which delay the input signals by one bit signal period. Various self-equalization drivers S(N) through S(N+J), each of which is proportional to main driver M(N), are tapped off the input signals at various delay times. The equalization drivers S(N) through S(N+J) may each have a gain that is a different fraction of that of the main driver, and may or may not invert the input signals, depending upon the particular equalization desired. The output of the main driver and each of the equalization drivers are then combined to produce equalized signal V'(N). Various crosstalk-equalization drivers X(N) through X(N+J), each of which is proportional to main driver M(N), are also tapped off the input signals at various delay times, to provide crosstalk compensation signal X'(N) to an adjacent signal line. FIG. 30 illustrates a mechanism 5300 for adjusting equalization parameters such as the magnitude, timing and sign of the equalization signal. An encoder 5302 provides input signals on lines C(N) to a main signal driver, not shown in this figure, the input signals also encountering delay elements Dl and D2 that delay the input signals C(N) by one unit delay each, to provide delayed signals on lines C(N+1) and C(N+2). Signals on lines C(N), C(N+1) and C(N+2) are input to multiplexers MUX 5307 and MUX 5308, which choose, based upon selection lines SI and S2, which of input signals on lines C(N), C(N+1) and C(N+2) to input to equalization drivers 5310 and 5320. As discussed, the equalization drivers may have different current gains. Exclusive OR gates 5315 and 5325 can be controlled to invert or not to invert the input signals selected by multiplexers MUXl and MUX2. This system can be expanded to provide multiple equalization drivers, XOR gates, multiplexers and delay elements, which together afford a variety of combinations of scaling, timing and inversion of the input signals to produce a desired equalization. FIG. 31 shows the encoder 5102 that converts MSB and LSB signals into input signals that are output on lines Cl, C2 and C3 to control the output driver 105. Data may be transmitted and read at both rising and falling clock edges, so a pair of identical encoders may exist for translating MSB and LSB odd and even signals, with the encoders' output multiplexed on lines Cl, C2 and C3. The MSB is input to a latch 5352 that also receives a transmit clock signal (TCLK) and complementary transmit clock signal (TCLK_B) for differentially latching the MSB as an input signal on line C2. The MSB is also input to an OR gate 5340 that receives as the other input LSB. The output of OR gate 5340 is latched at 5350, which outputs another input signal on line Cl. The LSB passes through inverter 5348 to become LSB_B, which is input to an AND gate 5344 that receives as the other input the MSB. The output of AND gate 5344 is clocked at 5355 and output on line C3 as a third input signal. Thus for MSB=0 and LSB=0, all the input signals are off. For MSB=0 and LSB=1, the OR gate 5340 outputs on so that input signal Cl is on, but C2 and C3 are still off. When both MSB=1 and LSB=1, input signals Cl and C2 are on, but AND gate 5344 inputs LSB_B, which is low so the input signal on line C3 is off. When MSB=1 and LSB=0, input signals on all the lines Cl, C2 and C3 are turned on. hi this fashion the MSB and LSB may be combined as Gray code and translated to thermometer code input signals on lines Cl, C2 and C3 that control the current sources to drive 4-PAM signals. FIG. 32 shows an integrating receiver 5400 that may be used to receive the multilevel signals sent by drivers such as those described above, and decode the signals into MSB and LSB components. As mentioned above, the data may be transmitted at twice the clock frequency, and a pair of receivers 5400 may read even and odd data. An MSB receiver 5402 of the 4-PAM receiver 5400 in this example receives and decodes a 4-PAM input signal VIN by determining whether the signal VF is greater or less than VREFM. hi the MSB receiver 5402, a latching comparator 5404 compares the value of the voltage of the received input signal VIN to the reference voltage VREFM and latches the value of the result of the comparison B in response to a receive clock signal. In one embodiment, data is taken at both rising and falling clock edges, hi an LSB receiver 5408 two latching comparators 5410 and 5414 compare the value of the voltage of the received input signal VIN to the reference voltages VREFH and VREFL, and latch the value of the result of the comparison A and C, respectively, in response to the receive clock signal. To decode the LSB, the signals from the comparator outputs B, A, and C are then passed through combinational logic 5420. The latching comparators 5404, 5410 and 5414 are implemented as integrating receivers to reduce the sensitivity of the output signal to noise. This can be accomplished by integrating a comparison of whether the input signal is above or below the reference voltages over most or all of the bit cycle, and then latching the integrated result as the outputs A, B and C. Examples of integrating receivers can be found in above-referenced U.S. Patent Application Serial Number 09/478,916. Errors generated by crosstalk or capacitive coupling may be attenuated by the time they reach a receiver, which may in circuit board implementations be located up to about a foot away from the signal drivers, much as the transitions are attenuated over that distance. For crosstalk errors this attenuation may actually be beneficial as it tends to smooth out a high-frequency transient signal into a transient signal that may be compensated with an equalization having a duration of one bit. hi a situation for which a neighbor's signal still has a high-frequency component when received, an integrating receiver can also smooth the data, so that compensating for a DC component of the crosstalk can be accomplished. FIG. 33A shows a signal 450 undergoing a transition that may generate crosstalk in an adjacent line having a signal 5452 shown in FIG. 33B. The crosstalk is apparent in FIG. 33B as a spike or high frequency component that is disturbing a step function transition. FIG. 33C shows an equalization signal 5455 designed to compensate for a DC component of the spike of FIG. 33B. Combination of signal 5452 and compensating signal 5455 is shown in FIG. 33D as signal 5457, which may leave a dipole component of the spike uncompensated. FIG. 33E shows that after integration by the receiver of FIG. 32 the dipole component has been removed, leaving a smooth transition signal 5459. As illustrated in FIG. 34, in a system that has numerous closely spaced signal lines SI, S2, S3 and S4, such as a bus for a computer or similar device, crosstalk may exist between each line and each nearby line. This crosstalk may have characteristics, based upon how many lines are between the crosstalk creator and the crosstalk victim, which can be equalized with an FIR having corresponding equalization characteristics, such as magnitude, timing and sign. Since these crosstalk characteristics have a regular pattern, crosstalk FIRs for each line may have a number of standard equalization drivers each designed to compensate for crosstalk a certain number of lines away. Thus self FIR 5500, self FIR 5502, self FIR 5505 and self FIR 5508 may each have equal ratios k which can be used to compensate for attenuation and reflections. Similarly XTK 5510, XTK FIR 5512, XTK FIR 5515 and XTK FIR 5518 may each have equal ratios k' that depend upon whether a line being equalized is a first adjacent line, second adjacent line, third adjacent line or further spaced from the crosstalk generating and equalizing line. As shown in FIG. 35 spaced signal lines SI, S2, S3 and S4 may be grouped in pairs or other arrangements, for example with a ground wire disposed between the pairs of lines. In this case, crosstalk equalization FIRs 5560, 5562, 5565 and 5568 may distinguish whether a signal line subjected to crosstalk from and adjacent signal line is spaced apart by a ground wire, and adjust the equalization parameters accordingly. Similarly, lines that are adjacent in one area, such as at signal pads for a circuit, may be separated in another area, for example due to routing in separate layers of a circuit board. In this situation, alternately spaced lines, such as SI and S3, may induce more crosstalk in each other than adjacent lines, such as SI and S2, and the equalization parameters may be adjusted accordingly. FIG. 36 illustrates a memory system 5600 in which embodiments of the present invention may be applied. The system 5600 includes a printed circuit board 5601 (sometimes called a motherboard) to which a memory controller 5603, a signaling path 5605 and connectors 5607 A, 5607B are affixed. Memory modules 5609 A, 5609B, each containing one or more memory devices 5611, are affixed to the printed circuit board 5601 by being removably inserted into the connectors 5607 A, 5607B. Though not shown in FIG. 36, the memory modules 5609A, 5609B include traces to couple the memory devices 5611 to the signaling path 5605 and ultimately to the memory controller 5603. In the embodiment of FIG. 36, the signaling path 5605 constitutes a multi-drop bus that is coupled to each memory module. The individual memory devices of a given module may be coupled to the same set of signaling lines within signaling path 5605, or each memory device of the module may be coupled to a respective subset of the signaling lines, hi the latter case, two or more memory devices 5611 on a module may be accessed simultaneously to read or write a data value that is wider (i.e., contains more bits) than the data interface of a single memory device. In an alternative embodiment (not shown), each of the memory modules may be coupled to the memory controller via a dedicated signaling path (i.e., a point-to-point connection rather than a multi-drop bus), hi such an embodiment, each of the memory devices on the memory module may be coupled to a shared set of signaling lines of the dedicated path, or each memory device may be coupled to respective subsets of the signaling lines. The signaling path 5605 may include a single, multiplexed set of signal lines to transfer both data and control information between the memory controller 5603 and memory devices 5611. Alternatively, the signaling path 5605 may include a set of signaling lines for transferring data between the memory devices 5611 and the memory controller 5603, and a separate set of signaling lines for transferring timing and control information between the memory devices 5611 and the memory controller 5603 (e.g., clock signals, read/write commands, and address information). Preferably the control information is uni-directional, flowing from the memory controller 5603 to the memory devices 5611, but status information may also be communicated from the memory devices 5611 to the memory controller. Also, the timing information may be generated within the memory controller 5611, or by external circuitry (not shown). In one embodiment, selected signaling lines within signaling path 5605 are used to carry multi-level signals (e.g., 4-PAM signals) between the memory devices 5611 and the memory controller 5603. For transfers from the memory controller 5603 to a given memory device 5611, a set of driver circuits within the memory controller 5603 output multi-level signals onto the selected traces of the signaling path, with equalization and/or cross-talk cancellation being applied to each such signal as described above. Receiver circuits within the addressed memory device (or memory devices) receive the multi-level signals from the memory controller 5603 and convert the signals into corresponding binary representations. Conversely, for transfers from the memory device 5611 to memory controller 5603 (e.g., in response to a read command issued to the memory device 5611 by the memory controller 5603), driver circuits within the memory device 5611 output equalized and cross-talk-canceled multi-level signals onto the selected set of traces of the signaling path for receipt by receiver circuits within the memory controller 5603. Such multi-level signaling may be used to transfer both data and control information between the memory controller 5603 and the memory devices 5611, or multi¬ level signaling maybe used for one type of information transfer (e.g., data or control), but not the other. Further, the signaling technique used to transfer either data or control information (or both) may be dynamically switched between binary and multi-level signaling according to detected events, such as bandwidth demand, detection of a threshold error rate, and so forth. Although a memory system that includes connectors for removable insertion of memory modules is depicted in FIG. 36, other system topologies may be used. For example, the memory devices need not be disposed on memory modules, but rather may be individually coupled to the printed circuit board 5601. Also, the memory devices, the memory controller and the signaling path may all be included within a single integrated circuit along with other circuitry (e.g., graphics control circuitry, digital signal processing circuitry, general purpose processing circuitry, etc.). The system of FIG. 36 can be used in any number of electronic devices, including without limitation, computer systems, telephones, network devices (e.g., switch, router, interface card, etc.), handheld electronic devices, intelligent appliances. As mentioned in reference to Figure 21, in order to compensate for attenuation, crosstalk or reflection errors using, for example, the equalization drivers illustrated in Figure 25, the highest output logic state is reduced below the V T E RM and the lower logic states are shifted respectively based on the equalization and/or crosstalk currents that are used on a device to compensate for signal errors. In such an embodiment, the V REFHI , V REFM and V REFLO are no longer centered on the shifted data eyes, and, thus do not track errors associated with the current control. Typically, an electronic device on a bus, such as the bus 302 illustrated in Figure 1, does not set its equalization or crosstalk coefficients until it is assigned to a predetermined signal line, or a signal channel, such as one of the signal lines 320-1 or 320-2 illustrated in Figure 1. However, when a device is put on a signal line, the device adapts a predetermined set of equalization and/or crosstalk coefficients based on the characteristics of the signal line. Thus, any device on the bus, once put on a signal line, has a direct knowledge of its default and crosstalk currents that shift the logic states down in order to allow equalization and crosstalk signals to overdrive logic state transitions. In accordance with a preferred embodiment, the reference voltage levels are shifted correspondingly to the logic state shifts so that the reference voltage levels provide accurate threshold levels. Exemplary embodiments for reference voltage generation will be described hereinafter in reference to a 4-PAM system. However, it should be understood that the exemplary embodiments are equally applicable in 2-PAM or N-PAM systems. Figure 37 illustrates a 4-PAM voltage generator 3000 that generates the multi- PAM reference voltages VREFHI, VREFM and VREFLO from the external voltage V TERM supplied on a voltage pin 3002 using a voltage divider, where the reference voltage levels reflect the shifts in the logic state level shifts in a system employing equalization and/or crosstalk signals. The voltage divider, including series-connected resistors Rl, R2, R3, and R4, is coupled between the voltage pin 3002 supplying the V TER M and a voltage pin 3006 supplying a VG R OUND- According to an embodiment illustrated in Figure 37, one or more active devices on a channel, such as an active device 3004, level-shifts the reference voltage levels based on the logic state shifts. As mentioned in the preceding paragraphs, the logic states are shifted down based on predetermined equalization and crosstalk currents applied by a device on a bus. As illustrated in Figure 37, the active device 3004 includes an active current source generating an offset current ("I OF F S E T ") that is controlled by one or more current control signals ("CCS") 3016. hi one embodiment, the CCS 3016 is controlled by a set of equalization and crosstalk coefficients used by a device on the bus so that the active device 3004 draws a scaled amount of the current used for the highest logic state equalization and crosstalk cancellation, hi an embodiment, a scale factor associated with the equalization and crosstalk current corresponds to a ratio of the termination resistor Zo and the first resistor Rl of the voltage divider, hi such an embodiment, the I OFFS E T is equal to I EQ , XTC • (Zo/Rl), where the I EQ , XT C corresponds to a combined equalization and crosstalk current used by a device on a bus. Thus, the active device 3004 pulls down the reference voltage levels V RE FHI, V REFM and V R EFLO according to the voltage shifts of the logic states. Table 4 illustrates reference voltage levels for the 4-PAM device that may be implemented using the active device illustrated in Figure 37. Table 4. According to the output voltage levels illustrated in Table 4, the active device 3004 pulls a predetermined amount of current from the V R E FHI level based on the equalization and crosstalk settings used on a device. Further, as illustrated in Table 4, the lower reference voltage levels V REFM and VREF LO are shifted based on the VRE FHI level so that the new levels for the V REF M and V R E FLO provide accurate threshold levels for the logic states below the highest logic state. However, it should be understood that more than one active device could also be used, and the present invention is not limited to the use of a single active device. In a bi¬ directional or multi-drop bus, e.g. a memory bus, having a number of individual drivers, each with different equalization or crosstalk coefficients, each driver on a channel may have an active device to shift the reference voltage levels, hi such an embodiment, the amount of current to make the V REF shift may be equal to a ratio of that current to the equalization/crosstalk component so that the current is properly ratioed. Further, it should be understood that the reference voltage generation according to an exemplary embodiment is not limited to using the voltage generator 3000 illustrated in Figure 37. Figure 38 is an electrical schematic 3050 of a multi-level voltage generator that reflects logic state shifts due to equalization or crosstalk cancellation and utilizes the op- amp drive references illustrated in Figure 17. As illustrated in Figure 38, one or more active current sources, such a current source 3052, generates an offset current ("I O F FSET ") that is controlled by one or more CCS 3054. The current source 3052 shifts the reference voltage levels based on equalization or crosstalk cancellation parameters. In the illustrated embodiment, the offset current pulls down the op-amp reference voltage levels as discussed in greater detail in reference to Figure 37. Figure 39 illustrates an alternative embodiment of an electronic circuit 3100 that generates level-shifted reference voltages VREFHI, VREFM, and VREFLO via on-chip VREF drivers. An output driver 3144 drives signals to an output pad 3146 and over a transmission line having a characteristic impedance Zo 3118 that is substantially matched to a terminating resistor R TERM to minimize reflection. The output driver 3144 may include three current sources, as illustrated in Figure 16, which together produce a current 1 3130 when all are active, pulling the voltage at the pad 3106 down from V T E RM by I- Zo, signaling the logical state 10 under the Gray code system, as illustrated in Figure 2. To produce the remaining logical states, some of the transistors in the current drivers are deactivated, as described in reference to Figure 2. Figure 39 illustrates three additional drivers for generating the V R E FHI , V REF M, and V REFLO - For example, the current driver 3156 drives into a terminating resistor R TERM 3112 half of the current that would normally be driven to generate the logical state 01 under the Gray code system and, further, shifts the V REFHI level according to the equalization and crosstalk currents applied on the chip. Thus, the current driver 3134 drives a current 1/6 to generate the V REFHI level that is further shifted by the current driver 3136 to compensate for the equalization and crosstalk. Similarly, to generate the V REF M voltage level, a current driver 3158 drives into a terminating resistor R TE R M 3114 three-fourths of the current that would normally be driven to generate the logical state 11 under the Gray code system, and, further, shifts the V REFM level according to the equalization and crosstalk current applied on the chip. Thus, the current driver 3140 drives a current 1/2 to generate the V REFM level that is further shifted by the current driver 3142 to compensate for the equalization and crosstalk. Finally, as illustrated in Figure 39, to generate the V REF L O voltage level, a current driver 3160 drives into a terminating resistor R TER M 114 five-sixth of the current that would normally be driven to generate the logical state 10 under the Gray code system and, further, shifts the V REFLO level according to the equalization and crosstalk currents applied on the chip. Thus, the current driver 3140 drives a current 5/6 • I to generate the V REFLO level that is further shifted by the current driver 3146 to compensate for the equalization and crosstalk. According to one embodiment, the equalization currents 3136, 3142 and 3146 are equal to the voltage shift used to compensate for crosstalk and equalization. Two reference current generation techniques have been described in reference to Figures 37, 38 and 39. However, any one of those could be eliminated, and an off-chip voltage divider could be used instead, hi such an embodiment, for example, the V REFM could be used from a direct-compatible-style board. Referring to Figure 40, a method 3200 for generating at least one reference voltage level for a driver employing equalization and crosstalk cancellation will be described. At step 3202, a reference voltage generator generates at least one reference voltage. For example, in a 2-PAM system, the voltage generator generates one reference voltage, and in a 4-PAM system, the voltage generator generates three reference voltage levels. In one embodiment, the voltage generator may include the reference voltage generators described in reference to Figure 37, 38 or 39; however, different voltage generators could also be used. At step 3204, at least one current control signal is determined. The at least one current control signal may include an equalization current control signal, a crosstalk current control signal, or the combination of both. At step 3206, at least one reference voltage level generated by the reference voltage generator is adjusted based on the at least one current control signal determined at step 3204. According to an exemplary embodiment, the at least one reference voltage level is shifted down based on the shift of the highest logic state required by the use of the control signals. It should be understood that the method 3200 is only an exemplary method, and different or equivalent methods may be used to generate reference voltages based on at least one control signal such as an equalization current control signal or a crosstalk signal. While the invention has been described in connection with a number of preferred embodiments, the foregoing is not intended to limit the scope of the invention to a particular form, circuit arrangement, or semiconductor topology. To the contrary, the invention is intended to be defined by the appended claims and to include such alternatives, modifications and variations as may be apparent to those skilled in the art upon reading the foregoing detailed description.

Claims: We Claim: 1. A current controller for a multi-level current mode driver, comprising: a multi-level voltage reference; at least one source calibration signal; a comparator coupled by a coupling network to the multi-level voltage reference and the at least one source calibration signal; and means for applying a selected voltage from the multi-level voltage reference and a selected source calibration signal from the at least one source calibration signal to the comparator. 2. A current controller as claimed in claim 1, wherein the coupling network is capacitive. 3. A current controller as claimed in claim 1, wherein the coupling network comprises a resistive voltage divider. 4. A current controller as claimed in claim 1, wherein the at least one source calibration signal comprises a voltage associated with a predetermined data symbol. 5. A current controller as claimed in claim 4, wherein the predetermined data symbol represents two data bits. 6. A current controller as claimed in claim 5, wherein the multi-level voltage reference comprises three voltage levels. 7. A current controller as claimed in claim 4, wherein the predetermined data symbol represents more than two data bits. 8. A current controller as claimed in claim 7, wherein the multi-level voltage reference comprises more than three voltage levels. 9. A current controller as claimed in claim 1, wherein the coupling network comprises a charge-coupled device at an input to the comparator. 10. A current controller as claimed in claim 9, wherein the charge-coupled device stores an offset voltage associated with the comparator. 11. A current controller as claimed in claim 1, wherein the applying means comprises a plurality of semiconductor switches. 12. A current controller as claimed in claim 1, wherein the applying means comprises a multiplexer. 13. A current controller as claimed in claim 1, wherein the at least one source calibration signal comprises a first signal and a second signal, wherein the first signal is . associated with a known voltage level and the second signal is associated with an unknown voltage signal. 14. A current controller as claimed in claim 1, wherein the coupling network comprises: a first switch and a second switch, the first and second switches being operable to selectively connect and disconnect the comparator to at least one of the multi-level voltage reference and the at least one source calibration signal; a third switch that is operable to selectively short a first comparator input to a second comparator input; an amplifier having an input that is coupled to an output of the comparator; and a current source coupled to an output of the amplifier. 15. A current controller as claimed in claim 14, wherein the current source applies, at the output of the comparator, a compensating voltage in response to a signal supplied by the amplifier. 16. A current controller as claimed in claim 14, further comprising a switch and a capacitor coupled to the input of the amplifier, the switch being operable to store a voltage at the output of the comparator on the capacitor. 17. A method of calibrating a multi-level current mode driver, comprising the steps of: providing a first current sink and a second cunent sink; activating the first cunent sink to drive a known cunent through a load thereby producing a first input signal; activating the second cunent sink to produce a second input signal; calculating an average value of the first input signal and the second input signal; comparing the average value of the first input signal and the second input signal to a first known reference voltage; and adjusting the second cunent sink, and thereby the second input signal, until the average value equals the known reference voltage. 18. A method as claimed in claim 17, further comprising the steps of: applying the adjusted second input signal as a new first input signal; activating the first cunent sink and a second cunent sink, thereby producing a precalibration voltage as a new second input signal; calculating an average value of the new first input signal and the new second input signal; comparing the average value of the new first input signal and the new second input signal to a second known reference voltage; and adjusting the second cunent sink, and thereby the new second input signal, until the average value is equal to the second known reference voltage. 19. A method as claimed in claim 18, further comprising the steps of: applying the adjusted second input signal as a new first input signal; activating the first cunent sink, the second cunent sink, and a third current sink, thereby producing a third precalibration voltage as a new second input signal; calculating an average value of the new first input signal and the new second input signal; comparing the average value of the new first input signal and the new second input signal to a third known reference voltage; and adjusting the third cunent sink, and thereby the new second input signal, until the average value is equal to the third known reference voltage. 20. A reference voltage generator for a driver comprising: at least one reference voltage; at least one current control signal; and at least one active device coupled to a selected reference voltage level of the at least one reference voltage and the at least one current control signal, the at least one active device shifting the at least one reference voltage based on the at least one cunent control signal. 21. The reference voltage generator of claim 20, wherein the at least one reference voltage comprises three reference voltage levels. 22. The reference voltage generator of claim 20, wherein the at least one reference voltage comprises more than three reference voltage levels. 23. The reference voltage generator of claim 20, wherein the at least one reference voltage is generated on at least one reference voltage driver. 24. The reference voltage generator of claim 20, wherein the at least one reference voltage is generated on a voltage divider. 25. The reference voltage generator of claim 20, wherein the selected reference voltage is a highest voltage reference associated with the at least one voltage reference. 26. The reference voltage generator of claim 20, wherein the at least one cunent control signal comprises an equalization cunent control signal. 27. The reference voltage generator of claim 20, wherein the at least one cunent control signal comprises a crosstalk cunent control signal. 28. The reference voltage generator of claim 20, wherein the at least one current control signal comprises an equalization cunent control signal and a crosstalk current control signal. 29. The reference voltage generator of claim 20, wherein the at least one active device comprises a cunent source shifting the at least one reference voltage based on the at least one current control signal. 30. The reference voltage generator of claim 29, wherein the cunent source shifts the at least one reference voltage down. 31. The reference voltage generator of claim 20, wherein: the at least one reference voltage is generated on a resistive voltage divider coupled to a reference voltage; and the active device is coupled to a highest reference voltage generated on the voltage divider, the active device shifting the at least one reference voltage based on the at least one cunent control signal and resistor values associated with the resistive voltage divider. 32. The reference voltage generator of claim 20, wherein the at least one cunent control signal is based on a voltage shift of a highest level associated with the driver. 33. The reference voltage generator of claim 20, wherein the driver comprises a current mode driver. 34. The reference voltage generator of claim 33, wherein the cunent mode driver comprises multi-level cunent mode driver. 35. The reference voltage generator of claim 20, wherein the driver is ananged to operate in a 2-PAM mode or a 4-PAM mode. 36. A method for generating at least one reference voltage level for a driver, the method comprising: providing at least one reference voltage level; providing at least one cunent control signal, the cunent control signal based on a logic-state level shift associated with the driver; and adjusting the at least one reference voltage level based on the at least one current control signal. 37. The method of claim 36, wherein the step of providing the at least one reference voltage level comprises generating at least one reference voltage level on a resistive voltage divider. 38. The method of claim 36, wherem the level shift associated with the driver is based on an equalization signal applied to the driver. 39. The method of claim 38, wherein the level shift is further based on a crosstalk signal. 40. The method of claim 36, wherein the step of adjusting the at least one reference voltage level comprises shifting down the at least one reference voltage level based on the at least one cunent control signal. 41. A memory bus system comprising: a bus including a plurality of signal lines; at least one reference voltage; a plurality of drivers employing at least one control signal; and an active device associated with each of the plurality of drivers, the active device coupled to a highest reference voltage level of the at least one reference voltage and the at least one control signal, the active device ananged to shift the at least one reference voltage based on the at least one control signal. 42. The memory bus system of claim 41, wherein the memory system is ananged to operate as a 2-PAM system or a multi-PAM system. 43. The memory bus system of claim 41, wherein the at least one control signal comprises at least one cunent control signal. 44. The memory bus system of claim 43, wherein the at least one current control signal comprises an equalization signal. 45. The memory bus system of claim 44, wherein the at least one current control signal further comprises a crosstalk signal. 46. A communication system comprising: a first printed circuit board; a conductive path affixed to the printed circuit board; a driver circuit affixed to the first printed circuit board and coupled to the conductive path to output onto the conductive path a signal having a voltage level that varies in time between at least three distinct levels representative of at least three distinct digital values, the driver circuit including an equalization circuit to adjust the voltage level of the signal output by the driver circuit at a first time according to a digital value represented by the signal at a previous time; and a receiver circuit affixed to the first printed circuit board and coupled to receive the signal from the conductive path to determine which of the at least three distinct digital values is represented by the signal at a given time. 47. The system of claim 46, wherein the driver and receiver circuits are respective integrated circuits affixed to the first printed circuit board. 48. The system of claim 46, wherein the driver and receiver circuits and conductive path are incorporated within a common integrated circuit that is affixed to the first printed circuit board 49. The system of claim 46, wherein at least one of the driver and receiver circuits is coupled to a second printed circuit board that is affixed to the first printed circuit board. 50. The system of claim 49, wherein the second printed circuit board is removably affixed to the first printed circuit board. 51. The system of claim 46, wherein the driver circuit receives a plurality of input signals, and the equalization circuit receives and delays said plurality of input signals. 52. The system of claim 46, wherein the driver circuit receives a plurality of input signals, and the equalization circuit receives and inverts said plurality of input signals. 53. The system of claim 46, wherein the equalization circuit compensates for attenuation of the signal in the conductive path. 54. The system of claim 46, wherein the equalization circuit compensates for reflection of the signal in the conductive path. 55. The system of claim 46, wherein the equalization circuit compensates for crosstalk generated by the signal in a second conductive path. 56. The system of claim 46, wherein said signal has a fourth voltage level that varies in time between at least three distinct levels, the fourth level representative of a fourth digital value that is distinct from the at least three distinct digital values. 57. A communication system comprising: a signaling device configured to produce a set of N signal levels representing a set of logical states, said device including a main driver adapted to receive a plurality of input signals and to output a signal, based on said input signals, that shifts over time between said signal levels; and an equalization mechanism containing an auxiliary driver that is substantially proportional to said main driver, said equalization mechanism adapted to receive said input signals and generate a set of N equalization signals based on said input signals, wherein N is greater than two. 58. The system of claim 57, wherein for each of said signal levels, an activation level of said main driver and an activation level of said auxiliary driver sum to equal N. 59. The system of claim 57, wherein said equalization mechanism includes an element adapted to invert said input signals. 60. The system of claim 57, wherein said equalization mechanism includes an element adapted to delay said input signals by a time substantially equal to a bit period. 61. The system of claim 57, wherein said main driver and said auxiliary driver each include a cunent source, and said signal levels are voltages that are in a range between ground and a positive voltage. 62. The system of claim 57, wherein said equalization mechanism is configured to compensate for attenuation of said signal over a signal line. 63. The system of claim 57, wherein said equalization mechanism is configured to compensate for reflection of said signal over a signal line. 64. The system of claim 57, wherein said signal is transmitted over a first signal line and creates crosstalk in a second signal line, and said equalization mechanism is coupled between said first and second lines and configured to compensate for said crosstalk in said second line. 65. A communication system comprising: a first main driver adapted to receive a first plurality of input signals and to output on a first line a first signal that varies over time between N output levels based on said first plurality of input signals, and a second main driver adapted to receive a second plurality of input signals and to output on a second line a second signal that varies over time between N output levels based on said second plurality of input signals, said main drivers adapted to shift from a first of said signal levels to a second of said signal levels by a transition having N-1 possible values for each said first signal level, said transition being substantially equal to a multiple of a difference between adjacent signal levels, said signal levels representing a set of logical states; and a first equalization mechanism including a first auxiliary driver having a gain that is substantially proportional to and smaller than said first main driver, said first equalization mechanism configured to receive said first plurality of input signals and to output on said second line a third signal, whereby said third signal compensates for crosstalk on said second line generated by said first signal. 66. The system of claim 65, wherein said main drivers and equalization mechanism each include a cunent source, and said signal levels are voltages that are in a range between ground and a positive voltage. 67. The system of claim 65, wherein said equalization mechanism further comprises a delay element and a second auxiliary driver having a gain that is substantially proportional to and smaller than said first main driver, said second auxiliary driver configured to receive said first plurality of input signals after delay by said delay element, and to output on said first line a fourth signal. 68. The system of claim 65, wherein said equalization mechanism further comprises an inversion element and a second auxiliary driver having a gain that is substantially proportional to and smaller than said first main driver, said second auxiliary driver configured to receive said first plurality of input signals after inversion by said inversion element, and to output on said first line a fourth signal.

Inventors: BEST SCOTT, CHAU PARK, HOROWITZ MARK, KIM JUN, LIAW H J, SIDIROPOULOS STEFANOS, WERNER CARL, ZERBE JARED LEVAN, BEST, SCOTT, CHAU, PARK, HOROWITZ, MARK, KIM, JUN, LIAW, H., J., SIDIROPOULOS, STEFANOS, WERNER, CARL, ZERBE, JARED, LEVAN
Applicants: BEST SCOTT, CHAU PARK, HOROWITZ MARK, KIM JUN, LIAW H J, RAMBUS INC, SIDIROPOULOS STEFANOS, WERNER CARL, ZERBE JARED LEVAN, BEST, SCOTT, CHAU, PARK, HOROWITZ, MARK, KIM, JUN, LIAW, H., J., RAMBUS INC., SIDIROPOULOS, STEFANOS, WERNER, CARL, ZERBE, JARED, LEVAN
Application year: 2001
Main classification: H04L25
IPC codes: H04L25