Abstract:
Process voltage temperature compensation are used for a bus driver; specifically, a PCI-X 2.0 DDR Standard bus driver. Performance is improved by enhancing the speed of the PCI-X buffer by removing the statically controlled gate stages and providing for output signal slew control by dual use of on-resistance of signal pass transistors. Although directed to PCI-X technology, this circuitry may also be used in SCCI, controlled impedance drivers, and other buffers, where short propagation delay and signal integrity are of concern.

Description:
FIELD OF THE INVENTION 
     The present invention relates to bus driver circuitry and more particularly to driving high speed data lines with compensation for process voltage temperature effects. 
     BACKGROUND OF THE INVENTION 
     A PCI bus system typically interconnects a large number of electronic devices. The system must maintain, manage and communicate bi-directional data from one device to another device or several devices at once. Each device may output different voltage levels while maintaining capability to read data on the bus. One reason for the difficulty of continuously increasing bus speeds to match the continuously increasing processor speeds is that input/output buffers coupled to the busses must often operate across a wide variety of operating conditions. For instance, the performance of an input/output buffer changes with respect to conditions such as process, voltage and temperature. 
     A parallel data bus typically comprises a number of bus lines to which the components of a computer system may be connected for communicating information between one another. Each component coupled to the data bus typically includes a set of bus driver circuits for transmitting data via the bus lines by switching the voltages of the bus lines between voltages that correspond to logic states, however defined. The speed at which a bus driver circuit switches the voltages of the bus line between logic states is called the “slew rate,” and the slew rate of the bus driver circuit is an extremely important characteristic for ensuring proper operation of the bus driver circuit at the clock speed of the data bus. 
     FIG. 1 shows a simplified sketch of a prior art PCI-X driver utilizing the feature of controlled output impedance. The input signal is routed by a number of gates to appropriate driver&#39;s output devices. The device selection is determined by impedance controller to correct for Process/Voltage/Temperature (PVT) effects. The size of these devices (MP 1x , MP 2x , . . . MP 1x ) are weighted in a certain manner (binary or with other ratios) to achieve a desired output impedance in conjunction with discrete resistor R p  or R N . Control signals for selection of a specific device are labeled CTRL and are generated by impedance controller as static logic signal and are set periodically to make the PVT adjustments to the output impedance. 
     P-channel and N-channel output devices are selected by separate paths to allow enabling the particular device, dual power supply mode of operation and also for the power-down feature. 
     To select the output device MP 1x , the control signal CTRL 1P  is set low and CTRL 2P  is set high. This selection with channel the input signal IN through inverters I 1 , I 2 , NOR gate N 1  and AND gate A 1  to the gate of device MP 1x . The MP 1x  device provides drive for positive-going or rising output signal. 
     Similarly, for the falling edge of the output signal, MN 1x  is selected by setting CTRL 1N  high, CTRL 2N , low and CTRL 3N  also low. In the case, the input signal IN is channeled to the gate of N-channel MOSFET MN 1x  output device through inverters I 1 , I 2 , NAND gate N 2 , and NOR gates N 2  and N 3 . 
     The greatest shortcoming of this approach is the excessive propagation delay from input IN to the output OUT due to the number of stages that the input signal has to propagate. In this case, there are five stages of delays, although in other implementations there may be a greater number of stages for more sophisticated PVT controllers. Since logical components of the same kind have variances in their individual propagation times from input to output, the greater the number of stages, the greater the potential cumulative variances of the propagation time of the various output drivers of the bus. 
     Another problem is adequately controlling the rise and fall times in complex output buffers. Large output devices are required for the delivery of adequate signal to drive transmission lines such as back-plane printed circuit board traces. As a result, the output impedances of the buffers become much lower than the characteristic impedances of the drive transmission lines. Consequently, mismatches lead to signal reflections and ringing and negatively affect the signal integrity. Recent drivers offer controlled output impedances to minimize the impedance mismatches. 
     Yet another problem results from the circuit implementation of FIG. 1 is a lack of output signal slew rate control. The output signal transition may be too fast or too slow depending upon device sizes, the circuit parasitics, PVT conditions, and buffer load. 
     One disadvantage of operating a system at a high speed is that the system may not provide a desired slew rate at high operating speeds. In particular, the constraints on loading, bus length, and bus pitch in conjunction with block data transfer do not provide for a stable slew rate at several hundred MHz or higher. 
     Another disadvantage of operating the system at high speed is that the system incurs ringing in the power lines VDD and GND, resulting in signal distortion. Thus, the inductive/capacitive characteristics of the bus and signal lines are exaggerated at a higher frequency resulting in signal distortion. 
     Yet another disadvantage of operating the system at high speed is that the system cannot provide low error rates. In particular, at high operating frequencies, the clocking scheme of the system does not guarantee synchronization between transmitted data and the clocking scheme in the destination device. Thus, incorrect data can be captured in a destination device. 
     The prior art approaches exhibit unacceptable signal delays and uncontrolled output signal rise and fall times. 
     The prior art drive circuits do not meet the newer high speed driver requirements for interface communications. 
     In the prior art, the slew rate was asymmetric. 
     SUMMARY OF THE INVENTION 
     This invention presents a method and device for driving high speed data lines by reducing the number of logic stages, separating the control logic from the data logic, and controlling a variable impedance circuit according to process, voltage, temperature (PVT) variations. 
     This invention relates to a circuit comprising a driver circuit configured to output a signal; a first capacitance formed from a portion of the driver circuit from a combination of discrete and parasitic capacitances; and a variable impedance circuit forming a portion of the driver circuit and coupled to the first capacitance, the variable impedance circuit configured to adjust the slew rate in response to changes in fabrication process and operating conditions wherein the variable impedance circuit comprises a first resistor, a first NMOS transistor and a first PMOS transistor in parallel and coupled to the first capacitance, wherein the gate of the first PMOS transistor is coupled to a process, voltage, and temperature controller. 
     This invention relates to a variable impedance circuit for use in a bus driver circuit, comprising a first impedance, a first NMOS transistor and a first PMOS transistor in parallel, a second impedance, a second NMOS transistor and a second PMOS transistor in parallel, wherein the a first combination of the first impedance, the first NMOS transistor, and the first PMOS transistor have a first end and an opposing second end and the second combination of the second impedance, the second NMOS transistor, and the second PMOS transistor has a third end and a fourth end, wherein the second end of the first combination and the third end of the second combination are directly electrically connected, wherein the first end of the first combination receives an input signal. 
     This invention relates to a bus driver circuit, comprising a first CMOS inverter which includes a first PMOS transistor and a first NMOS transistor, the transistors being electrically connected to one another; a first set of logic gates for controlling the first PMOS transistor by a first control signal; a second set of logic gates for passing an first input signal; and a third set of logic gates for controlling the first NMOS transistor by a second control signal; wherein the first set of logic gates, the second set of logic gates, and the third set of logic gates have no logic gates in common. 
     This invention relates to a method of outputting a signal on a bus driver circuit, comprising inputting an initial input signal; providing process, voltage, temperature controller signals to a variable impedance circuit through which at least a portion of the initial input signal passes; and outputting an output signal corresponding to the initial input signal. 
     This invention relates to a method of outputting a signal on a bus driver circuit, comprising providing separate logic paths for the control signals and the data signal. 
     This invention improves the bus driver circuit&#39;s propagation delay and controls the output signal rise and fall times by using a lower number of logic stages. 
     This invention provides better performance over current devices. 
     This invention offers cost effectiveness as the circuitry requires a reduced silicon area. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating a prior art bus driver. 
     FIG. 2 is a schematic diagram illustrating a first embodiment of the present invention. 
     FIG. 3 is a schematic diagram illustrating a resistor chain useable in the first embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The continuous increase in performance of personal computers, telecommunications, etc., demands faster and faster circuitry. One such example is the PCI-X 2.0 DDR technology (second generation of the Peripheral Component Interface, Double Data Rate). It serves as an interface between the PC and peripheral components such as VGA controllers, etc. Presently, circuit implementations are very complex due to the large number of functions they perform in conjunction with other logic operations. The overall driver&#39;s propagation delay becomes a limiting factor of performance. 
     Technology advances in integrated circuit fabrication have led to more compact chip designs. Lower voltage processes come with the smaller size. CMOS devices can use low voltage power supplies to prevent damage to devices having small feature sizes, and to reduce the overall power consumption. For example, power supplies for CMOS devices are being reduced from 3.3 volts to 2.5 volts and lower. However, low voltage CMOS devices often interface with transistor—transistor logic (TTL) devices that operate at higher supply voltages, e.g., 5 volts. 
     The Peripheral Component Interconnect bus standard requires a minimum of 2.4 volts on the bus to identify a high transition. Typically, there are a large number of buffers and drivers tied to the bus, any of which can be a TTL device. Therefore, each device must be capable of driving at least 2.4 volts, and be able to withstand voltage levels as high as 6.5 volts. 
     Another issue with multiple supply voltages is that the different voltages have different characteristics. Some voltages may be stable before others. In a worst case scenario, the highest voltage, e.g., 5 volts, may stabilize first, and already be at its highest level while the other voltages, e.g., 3.3 volts and 1.8 volts, are still at ground or low level. Such an initial condition at power-up could expose low voltage CMOS devices to the full 5 volts. This can cause damage to the device or a shortened life. For example, this could damage the gate oxide in the transistors that form the devices. This situation can be exacerbated by the PCI specification, which requires some of the PIN&#39;s to power up at 5 volts. 
     The present invention provides a method and an apparatus for adjusting both the slew rate and the impedance of buffer circuits in order to compensate for variations in conditions such as process, voltage and temperature in drivers. Such a method and apparatus is readily implemented in drivers with minimal area to maximize the performance of buffer circuits over a variety operating conditions. To minimize problems with ringing and propagation delays, shorter length conductors are used. 
     FIG. 2 depicts the key features of this invention. The first key feature is the selection of the desired devices by a logic function outside of the signal path. That is, the signal path and the digital control logic are arranged as separate circuits which interface with the output drivers. The device selection signals called CTRL are all static signals, determined by the PVT impedance controller during the period of absent data transfers. To achieve this task, only the resulting decoded enable signal is routed to the specific device. 
     For simplicity, only the 1x devices are shown. It is understood that the actual circuitry would include all the 2x to ix devices. For selection of the output device driver MP 1x , the PVT controller sets the appropriate static control signal levels (CTRL 0P , CTRL 1P , CTRL 2P ) in such a way that node voltage V G1x  is high. This will turn off the P-channel MP 1xP , and turn on the N-channel MN 1xP . The N-channel MN 1xP  serves as a pass gate for the input signal IN which is fed through serial resistor R ONI . The P-channel MP 1xP  is used to tri-state the output device MP 1x  by shorting its gate to VDD. 
     Two predriver inverters I 1  and I 2  are sized for the minimum delay of the entire PCI-X driver. The output device is very large to supply large output current of a few hundred mA. Removal of three or more control gates from the input signal path results in a substantially faster buffer. In the present invention, the input signal propagates only through one serial device MN 1xp . 
     In many high speed circuits, there is a requirement to control the slew rates of signals, the rate at which the signal changes in volts/second, for various reasons including the minimization of electromagnetic interference. For signals of a given magnitude, slew rate values can be converted to rise and fall transition times. The need to control slew rates is particularly true in high speed interface circuits. 
     The second key feature of the invention is the dual utilization of the serial device MN 1xP  for slew rate control of the output signal. The on-resistance of the pass device MN 1xP  with an optional discrete resistor R ON1  constitutes an RC circuit in conjunction with the capacitor C 1 . Output driver P-channel MP 1x  gate to drain overlap capacitance in conjunction with any additional parallel discrete capacitance will form the capacitance C 1 . During the output transition, the equivalent capacitance is C M= (1−A M *C 1 ), where A M =−g m (MP 1x )*{[r DS (MP 1x )+R p1 ]∥[r DS (MN 1x )+R N1 ]}. This gain A M  is known as Miller gain and the C M  as Miller capacitance. It is desirable to have the Miller gain A M  as low as possible for wide bandwidth linear amplifiers. In the case of digital output drivers, the high gain is desirable to have a large output current in a single stage. The variable g m  (MP 1x ) represents the value of the transconductance of device MP 1x ; the variable r DS (MP 1x ) represents the value of the drain to source resistance of device MP 1x ; and the variable r DS (MN 1x ) represents the drain to source resistance of device MN 1x . In a FET device such as MP 1x , transconductance is the ratio of the change in output current to the initiating change in input voltage. 
     FIG. 3 shows the circuit for the realization of time constant τ=RC. Discrete resistors R 1 , R 2 , R 3 , etc. in series with R on  (MN 1xp ) form a programmable resistor chain. The number of discrete resistors may be chosen as needed for fine resolution for slew rate control. The value of discrete resistors is in a range of kΩ. Each of the resistors such as R 1  can be bypassed by a pair of devices M nsw1 /M psw1  which are controlled by signals S n1 /S p1  respectively. The PVT controller can be utilized to set the desired value of resistance. For example, for the worst case conditions (lowest output drive current), the PVT controller will select the lowest resistance combination of R to yield the smallest time constant τ. This will augment the overall buffer delay for less variation and maintain the buffer slew rate across the PVT worst case values. 
     The output device MP ix  and NM ix  are of different sizes. The W/L of the P-channel device is larger than the W/L of the N-channel device by a factor of μ n /μ p =2.5 (ratio of electron and hole mobilities). The resulting P-channel gate-to-drain capacitance is larger that of the N-channel device. Therefore, R ON1  and R ON2  will be adjusted for this difference to make the rise and fall time (+/− slew rate) symmetric. In the prior art, slew rate symmetry was nearly impossible to achieve. 
     The preferred embodiment uses CMOS technology. Alternatively, the driver transistors could be implemented with bi-polar technology. The device sizes of the pre-drivers and output transistors can be varied, thus providing finer control over slew rate and drive strength during varying PVT conditions. 
     An intended advantage of an embodiment of the invention is to provide high speed bus communication between two devices. 
     Another intended advantage of an embodiment of the invention is to reduce error rates in high speed communication busses. In particular, the slew rate controlled driver circuitry reduces ringing on power lines and ensure a consistent rise and fall time for data signals, thus reducing data distortion which in turn reduces error rates by matching transmission line impedances. 
     Another intended advantage of an embodiment of the invention is to provide an interface circuitry for multiple devices coupled to a bus. 
     Another intended advantage of the invention is to provide a uniform and quick response from a bank of driver circuits. 
     In operation, as shown in FIG. 2, an input signal IN passes through the two inverters I 1  and I 2  causing a delay of up to a few nanoseconds for gate propagation. The inverters I 1  and I 2  buffer the large input capacitance of output devices driven from the core of the chip. If all control signals CTRL are enabled, the input signal from the inverters passes through resistances R ON1  and R ON2  before passing on the CMOS inverter formed from devices MP 1x  and MN 1x  to generate output signal OUT. 
     The two devices MP 1x  and MN 1x  are designed to have matching characteristics. Thus, they are complementary to each other. When off, their resistance is effectively infinite; when on, their channel resistance is about 200Ω. Since the CMOS gate is essentially an open circuit, it draws no current and the output voltage is equal to either V SS  or V DD , depending on which device is conducting. The parallel resistance of the on-resistance of the P-channel driver with R p1  is desirably equal to 75Ω so as to be equal to the driven transmission line characteristic impedance Z o . 
     When the delayed input signal from the inverters I 1  and I 2  at the gates of the devices MP 1x  and MN 1x  is grounded (logic 0), the N-channel device MN 1x  is unbiased, and therefore has no channel enhanced within itself. It is an open circuit, and therefore leaves the output line disconnected from ground. At the same time, the P-channel device MP 1x  is on, so it has a channel enhanced within itself. This channel has a resistance of about 200Ω, connecting the output line to V DD . This pulls the output signal OUT up to a logic  1  level. The parallel resistance of the on-resistance of the N-channel driver with R N1  is desirably equal to 75Ω so as to be equal to the driven transmission line characteristic impedance Z o . 
     When the delayed input at the gates of the devices MP 1x  and MN 1x  is at a logic  1  level, the P-channel device MP 1x  is off and the N-channel device MN 1  is on, thus pulling the output signal OUT down to ground (logic 0). Thus, this circuit performs logic inversion, and simultaneously active pull-up and pull-down, according to the output state. 
     As an alternative embodiment, the incorporation of FIG.  3 &#39;s resistor chain in place of resistor R ON1  allows for fine control of the slew rate. 
     Resistors R ON1  and R ON2  serve to augment the total output resistance to characteristic impedance Z o . 
     V SS  is ground or a negative supply voltage. V DD  is a positive supply voltage. 
     In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.