Abstract:
Circuitry and methods are provided for reducing rise time associated with signals on an open-drain or open-collector signal line. Signal line voltage is monitored to determine if the signal line is being pulled LOW. If the signal line is not being pulled LOW, as indicated by signal line voltage exceeding a threshold level, additional pullup current is provided. The additional current may be provided gradually in relation to the signal line voltage, or may be provided in full whenever voltage exceeds the threshold. Circuitry may also be provided to monitor voltage slew rate on the signal line, and to enable the additional pullup current only when the slew rate exceeds a positive threshold level.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to methods and circuitry for increasing communication speeds in systems employing open-drain or open-collector circuitry for driving a signal line, and more particularly methods and circuitry for providing slew rate sensitive, hysteretic, active pullup. 
     “Open-drain” terminology is used extensively throughout the description which follows. Although “open-drain” may imply the use of field effect transistors, such as MOSFETsf one of skill in the art will recognize that other types of transistors, such as bipolar transistors may be used as well. Thus, it is to be understood that the open-drain terminology is used herein as a matter of convenience, and that the terminology specifically includes open-collector type circuits. Furthermore, it is to be understood that the invention may be practiced using other than MOSFET transistors and that “transistor” specifically includes such other suitable types of transistors. 
     Open-drain circuitry is commonly used to interconnect electronic devices by way of a common bus or signal line. The Inter-Integrated Circuit bus (I 2 C), the System Management Bus (SMBus), ACCESS.bus, and Apple Desktop Bus (ADB) are a few of the inter-device communication protocols that use an open-drain architecture. Open-drain signals are also used within computer systems for lines that may be driven by more than one source, e.g., an interrupt input of a microprocessor. 
     A device sends signals on an open-drain signal line by controlling a transistor coupled between the open-drain signal line and ground. Typically the transistor, which is used as a switch, is an N-channel MOSFET, but other types of transistors are also suitable for this purpose. In addition, the transistor may be internal to the device, or the device may have a terminal for controlling an external transistor. 
     When the device causes the transistor to be ON, the signal line is coupled to ground, causing its voltage to be pulled down to a LOW state or level, e.g., less than about 0.4 volts. Conversely, when all devices cause their corresponding driver transistors to be OFF, the signal line is biased to a HIGH state, e.g., 5 volts, by pullup circuitry connected between the signal line and a power supply rail. 
     The speed at which signals may be transmitted on an open-drain signal line is dependent on how rapidly the signal line can be cycled between LOW and HIGH levels. Because of parasitic capacitance associated with the signal line, the rate at which it may be switched is determined by how rapidly the parasitic capacitance may be charged and discharged. Other factors being equal, an increase in parasitic capacitance slows the charge and discharge rates, and lowers the maximum signaling rate. Therefore, many open-drain based interconnection standards specify a maximum signal line capacitance, typically a few hundred picofarads, to ensure adequate performance. 
     Another factor determining the rate at which the parasitic capacitance is charged and discharged is the resistance in the charge and discharge current paths. Since the resistance of an output transistor is typically very small when the transistor is ON, the parasitic capacitance may be discharged very rapidly, and the transition from HIGH to LOW occurs quickly. However, the parasitic capacitance is charged by pullup current provided by some form of pullup circuitry. 
     In a typical application employing an open-drain signal line, the pullup circuitry is simply a pullup resistor coupled between the&#39;signal line and a positive supply rail. Because the resistance of a pullup resistor is typically much larger than the ON resistance of a driver transistor, the rate at which the parasitic capacitance may be charged is much slower than the rate at which it may be discharged. The signal rise time is, therefore, much slower than the signal fall time. 
     One technique for shortening signal rise time is to use a smaller valued pullup resistor. Using a smaller resistance increases the available pullup current, so that any parasitic capacitance may be charged more quickly when all driver transistors are OFF. However, reducing pullup resistance may have adverse effects on circuit operation. 
     For example, reducing the value of the pullup resistor increases current flow from V cc  to ground when a driver transistor is ON. This increased current represents wasted electrical power, which may be an important consideration inflow power applications such as battery powered devices. The increased current also increases the voltage drop across the driver transistor, thereby raising the signal line voltage and reducing the noise margin associated with a LOW signal line level. 
     In view of the foregoing it would, therefore, be desirable to improve data signaling speeds in communication systems employing an open-drain architecture, by reducing the rise time associated with open-drain signals, without compromising noise margin or power efficiency. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to improve data signaling speeds in communication systems employing an open-drain architecture, by reducing the rise time associated with open-drain signals, without compromising noise margin or power efficiency. 
     These and other objects and advantages of the present invention are realized by methods and circuitry in which pullup current is provided by a variable current source in which the available pullup current is a function of a voltage level on the signal line. In particular, the available pullup current is increased when the signal line voltage indicates that the signal line is not being pulled LOW. 
     In a first embodiment, the additional pullup current is provided whenever a signal on the signal line exceeds a threshold level. In a preferred embodiment, circuitry is provided to monitor the slew rate (dV/dt) of the signal, and higher pullup current is provided only when the signal exceeds the threshold and the slew rate is positive, such as during a LOW-to-HIGH signal transition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the present invention will be apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIGS. 1A through 1C are simplified schematic diagrams depicting three previously known types of pullup circuitry; 
     FIGS. 2A and 2B are, respectively, a graph of signal line voltage as a function of time, and pullup current as a function of signal line voltage, for the pullup circuits of FIGS. 1A-C; 
     FIG. 3 is a simplified schematic diagram of a first illustrative embodiment of pullup circuitry in accordance with principles of the present invention; 
     FIG. 4 is a representative graph of pullup current as a function of signal line voltage for the pullup circuitry of FIG. 3; 
     FIG. 5 is a simplified schematic diagram of a second illustrative embodiment of pullup circuitry in accordance with principles of the present invention; 
     FIG. 6 is a representative graph of pullup current as a function of signal line voltage for the pullup circuitry of FIG. 5; and 
     FIG. 7 is a simplified schematic diagram of a third illustrative embodiment of active pullup circuitry employing principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A through 1C are simplified schematic diagrams of previously known circuitry for implementing signal lines using an open-drain architecture. Device  16  represents a device coupled to signal line  11  and may be anything from an integrated circuit to a computer peripheral. Device  16  includes driver transistor  14  which may be turned ON or OFF by additional circuitry within device  16  (not shown). Alternatively, device  16  may include a terminal for controlling an external driver transistor. It should be noted that in the schematics of FIGS. 1,  3 , and  5  only one device is shown connected to signal line  11 ; however, one skilled in the art will understand that there may be more than one such device. 
     Capacitor  18  represents the parasitic capacitance associated with signal line  11 , including stray capacitance associated with signal line  11  itself, as well as with the drivers and receivers coupled to signal line  11 . The main effect of parasitic capacitance  18 , which is typically on the order of a few hundred picofarads, is to limit the rate at which data may be sent on signal line  11 . Specifically, the data rate on signal line  11  is limited by the rate at which the parasitic capacitance may be charged and discharged. For this reason, most communication protocols employing an open-drain architecture specify a maximum signal line capacitance. For example, the I 2 C specification allows a maximum signal line capacitance of 400 pF. 
     Driver transistor  14  is connected between signal line  11  and ground so that device  16  may actively pull signal line  11  LOW by turning driver transistor  14  ON. Since any similar device connected to signal line  11  is capable of pulling it LOW, the signal line can only be HIGH when driver transistor  14  associated with each device is turned OFF. 
     Thus, any device connected to signal line  11  may selectively drive the signal line LOW by turning ON the driver transistor associated with the device. Conversely, when transistor  14  is OFF in all devices connected to signal line  11 , pullup circuitry connected to the signal line biases the signal line HIGH. 
     In FIG. 1A, pullup circuitry  10  consists of pullup resistor  12  connected between VCC and signal line  11 . When transistor  14  is switched OFF, current flows through pullup resistor  12  to signal line  11 , pulling it up to V cc . Typically, pullup resistor  12  has a value on the order of a few thousand ohms. 
     A typical signal on signal line  11  of FIG. 1A is shown by the solid trace in FIG.  2 A. Prior to time t 0 , transistor  14  is OFF, and signal line  11  is HIGH. At time t 0 , transistor  14  is turned ON by device  16 , providing a low resistance path between signal line  11  and ground. This rapidly discharges capacitance  18  to ground, pulling signal line  11  LOW at time t 1 . The interval between time t 0  and t 1 , i.e., the time needed for signal line  11  to reach a LOW level after transistor  14  is turned ON, is referred to as the fall time (t f ). 
     At time t 2 , transistor  14  is turned OFF by device  16 . Current through pullup resistor  12  charges capacitance  18  causing the voltage on signal line  11  to rise, pulling signal line  11  HIGH at time t 3 . The interval between time t 2  and t 3 , i.e., the time needed for signal line  11  to reach a HIGH level after transistor  14  is turned OFF, is referred to as the rise time (t r ). 
     In essence, the circuit of FIG. 1A is a resistor-capacitor (RC) circuit. The response of RC circuits exhibit a characteristic exponential waveform over a time determined by the time constant of the circuit, wherein the time constant is the product of circuit capacitance and the resistance in the current path. Circuits having a larger time constant have longer rise and fall times. 
     In a typical open-drain system, the value of pullup resistor  12  is much larger than the On-resistance of driver transistor  14 . This causes signal rise time (t r ) to be many times longer than the signal fall time (t f ) Since the rate at which data may be transmitted on signal line  11  is largely limited by the rise time (t r ) techniques for increasing data transmission rates have generally focused on shortening the rise time in open-drain systems. 
     As described in the background of the invention, rise time may be reduced by reducing the value of pullup resistor  12 . This would reduce the RC time constant of the circuit, thereby providing a shorter rise time. Since reducing pullup resistance may adversely affect power consumption and noise susceptibility, other techniques have been developed to reduce signal rise time. 
     One such previously known technique for reducing rise time is illustrated in the schematic diagram of FIG.  1 B. Open-drain circuitry  20  includes pullup resistor  12 , transistor  14 , and capacitance  18  which correspond to like elements of FIG.  1 A. Pullup circuitry  20  also includes additional pullup resistor  12   a,  which may be selectively connected in parallel with pullup resistor  12  by means of switch  13 . Switch  13 , which may be, for example, a CD4066 CMOS switch, is controlled by a level on control input  15 , such that a LOW signal at control input  15  causes switch  13  to be OFF, while a HIGH signal causes the switch to be ON. 
     In the circuitry of FIG. 1B, when transistor  14  is ON, signal line  11  is LOW and switch  13  is OFF. When transistor  14  is initially turned OFF, and assuming no other device is pulling signal line  11  LOW, pullup resistor  12  provides current to charge parasitic capacitance  18 , and signal line voltage begins to rise. When signal line voltage rises enough to turn switch  13  ON, typically about one-half V cc , resistor  12   a  is connected in parallel with pullup resistor  12 , effectively reducing the total pullup resistance and increasing the available pullup current. 
     The decrease in pullup resistance caused by turning on switch  13  is a function of the relative values of resistors  12  and  12   a.  For example, if the values of resistors  12  and  12   a  are equal, the available pullup resistance is effectively halved when switch  13  is turned ON. This reduces the RC time constant associated with pulling signal line  11  high, resulting in a shorter rise time (t r ). 
     The response of pullup circuitry  20  is shown in FIGS. 2A and 2B. From time t 0  to t 1 , the circuit response and waveform are nearly identical to those of FIG.  1 A. At time t 2  transistor  14  is turned OFF, and the voltage on signal line  11  begins to rise, following the same waveform as the solid trace corresponding to the circuit of FIG.  1 A. At time t 4  signal line  11  reaches a voltage of about one-half V cc  and switch  13  turns ON, greatly reducing pullup resistance. The reduced pullup resistance reduces the RC time constant and signal line voltage rises much faster, as shown by the dashed line in FIG.  2 A. The corresponding pullup current is shown by the dashed line in FIG.  2 B. 
     Clearly, in the circuit of FIG. 1B, all signal line driver transistors must be OFF before the signal line voltage can rise enough to turn ON switch  13 . As a result, pullup resistor  12  may be made large enough to address the concerns about excess current, power consumption, and noise margin discussed hereinabove, and resistor  12   a  may be made small enough to provide adequate pullup performance. 
     A third alternative pullup scheme is shown in FIG. 1C, wherein pullup current for signal line  11  is provided by constant current source  32 . In the circuitry of FIGS. 1A, and  1 B, pullup current drops as the voltage on signal line  11  rises, giving the response waveform its characteristic exponential shape. Using a constant current source ensures that the pullup current, and hence the charging rate of capacitance  18 , remains nearly constant, resulting in a near linear increase in signal line voltage. This is illustrated by the dotted line in FIGS. 2A and 2B. Note, that as signal line voltages near the supply rail, pullup current begins to drop due to reduced operating headroom for constant current source  32 . 
     Although the circuitry of FIGS. 1B and 1C are effective at reducing signal rise times in open-drain circuits, maximum signaling rates are still limited to less than about 1 MHz using these types of pullup circuits. In addition, care must be taken to keep stray capacitance to a very small value, for example, by limiting the length of signal line  11 , or the number of devices connected to signal line  11 . 
     Referring now to FIG. 3, a first illustrative embodiment of pullup circuitry in accordance with principles of the present invention is described. Pullup circuitry  40  includes transistors  41 - 44 , and resistors  45 - 48 . Transistors  41  and  42  are connected to form a current mirror such that collector current I 2  of transistor  42  is approximately proportional to collector current I 1  of transistor  41 . If signal line  11  is LOW, transistor  43  is biased OFF, and the current I 1  is determined by the values of resistors  45  and  46 . 
     When all open-drain driver transistors connected to signal line  11 , e.g., transistor  14 , are OFF, the collector current of transistor  42  begins to charge parasitic capacitance  18 , and the voltage on signal line  11  increases. When the signal line voltage exceeds the base-emitter voltage drop of transistor  43 , it begins conducting, sending current I 3  through resistor  47 . The sum of currents I 1  and I 3  flows through current mirror transistor  41 , consequently increasing current I 2 , and making additional current available to charge parasitic capacitance  18 . As the voltage on signal line  11  continues to rise, current I 3  also continues to increase, resulting in a continued increase in current I 2 . Thus, the pullup current is a direct function of the signal line voltage. 
     Eventually, current I 2  is large enough that the voltage drop across resistor  48  begins to forward bias the base-emitter junction of transistor  44  causing it to begin conducting current I 4 . Current I 4  tends to offset any further increase in current I 3  caused by the rising signal line voltage, thereby providing an upper limit on current I 2 . Finally, as the voltage on signal line  11  begins to approach V cc , pullup current I 2  begins to drop off due to saturation of transistor  42  and reduction of the voltage across resistor  48 . 
     The reverse sequence of events occurs when signal line  11  is pulled LOW by turning ON an open-drain driver connected to signal line  11 , e.g., transistor  14 . First, dropping signal line voltage increases current mirror headroom, and pullup current increases up to the limit set by transistor  44 . Pullup current is still much less than the current through driver transistor  14 , so signal line voltage continues to drop. Eventually, signal line  11  voltage is low enough that transistor  43  turns OFF, eliminating current I 3 , and consequently reducing available pullup current I 2  to the level set by resistors  45  and  46 . An exemplary graph of pullup current versus signal line voltage for the circuitry of FIG. 3 is shown in FIG.  4 . 
     FIG. 4 also shows a dashed line which represents a load-line corresponding to the ON resistance of driver transistor  14 . This is an indication of how much current transistor  14  can sink at any given signal line voltage, i.e., the available “pulldown” current. In designing a pullup circuit such as that in FIG. 3, it is important that the pullup current always remain less than the current transistor  14  can sink. Otherwise, transistor  14  cannot sink enough current to pull signal line  11  LOW. 
     An illustrative schematic diagram of a more preferred embodiment of pullup circuitry is shown in FIG.  5 . In accordance with the principles of the present invention, pullup circuitry  60  provides additional pullup current only when signal line  11  is not being pulled LOW. 
     Pullup circuitry  60  functions in a manner analogous to the circuitry of FIG.  3 . Transistors  61  and  62  form a current mirror, wherein the current through transistor  62  provides pullup current to signal line  11 . Transistor  63  causes an increase in pullup current I 2  as signal line voltage increases, and transistor  64  limits the maximum pullup current to an acceptable level. However, pullup circuitry  60  includes additional circuitry to create hysteresis in the current-voltage characteristic of the pullup circuit as is shown in FIG.  6 . 
     Operational amplifier  67 , in conjunction with capacitor  68  and resistor  69  form a differentiator that monitors the change in voltage on signal line  11 . The output of operational amplifier  67  is a signal indicative of how fast the signal line voltage is changing, i.e., the slew rate . When the signal corresponds to a positive slew rate that exceeds a threshold level, comparator  53  outputs a signal turning ON transistor  54 . The threshold level is provided at the ‘+’ input of comparator  53  by current source  65  and diodes  51  and  52 . Turning transistor  54  ON enables current I 3  to flow through transistor  63 , providing increased pullup current in a manner analogous to that described in connection with FIG.  3 . 
     However, when the voltage slew rate is below the threshold because the signal line voltage is constant or falling, comparator  53  keeps transistor  54  turned OFF, and pullup current I 2  is limited to a value set by current source  66 . Transistor  54  and the associated slew rate circuitry introduce hysteresis into the current-voltage characteristic of pullup circuitry  60 . That is, the pullup current provided by pullup circuitry  60  depends on whether the signal voltage is rising or falling. A representative current-voltage characteristic is shown in FIG.  6 . 
     Because pullup circuitry  60  provides additional pullup current only when the voltage on signal line  11  is rising, the pullup current may exceed the pull-down current load line represented by a dashed line in FIG.  6 . This enables the rise in pullup current to be very rapid. Indeed, as long as the increased current is only provided when signal line  11  is not being pulled down, the change in pullup current may be an instantaneous step change. 
     Referring now to FIG. 7, exemplary pullup circuitry for providing a hysteretic, non-linear pullup current is described in more detail. Pullup circuitry  70  includes four basic sections of circuitry: voltage level detection circuitry  71  for monitoring the voltage level on signal line  11 ; slew rate detection circuitry  77  for monitoring the rate at which the signal line voltage is changing; nominal pullup current circuitry  88  for providing pullup current when the signal line is stable or being pulled down; and high pullup current circuitry  95  for providing increased pullup current when needed. In addition, pullup circuitry  70  includes circuitry for implementing a low power mode suitable for use in battery powered systems. 
     Additional voltages and signals are provided to the circuitry of FIG. 7 by circuitry not shown therein. For example, voltage regulating circuitry (not shown) provides voltages to BIASH and BIASL for biasing, respectively, the high-side and low-side MOSFET current sources of FIG. 7, and provides a voltage reference to VREF. Additional circuitry provides a shutdown signal to −SHDN. SGNL is connected to the signal line, e.g., signal line  11  of FIG.  5 . 
     Taking each section of FIG. 7 in turn, voltage level detection circuitry  71  includes a differential amplifier  72  which splits current I 1  into currents I 1a  and I 1b  according to the voltage at SGNL relative to the voltage at VREF (a voltage reference). Current I 1a  is mirrored by current mirror  73  to provide current I 2  tending to pull node  74  to ground. Similarly, current I 1b  is mirrored by current mirrors  75  and  76  to provide current I 3  tending to pull node  74  up to V cc . 
     If the voltage at SGNL is lower than VREF, which is preferably about 0.6 volts, current I 1a  is smaller than current l 1b , and consequently, current I 2  is smaller than I 3 . This results in node  74  being pulled up to a high level. Conversely, if the voltage at SGNL is higher than VREF, current I 1a  is larger than current l 1b  and current I 2  is greater than I 3 , resulting in node  74  being pulled low. Thus, node  74  is LOW when SGNL voltage exceeds VREF, and HIGH otherwise. 
     Turning now to slew rate detection circuitry  77 , constant current I 4  is provided by transistors  78 ,  79 , and  80  in conjunction with current mirror  82 . Current I 4  is mirrored by current mirrors  81  and  82  to provide currents I 5  and I 6 , respectively. Preferably, current mirror  81  has a gain of about twice that of current mirror  82 , so that current I 5  is normally about twice as large as I 6 , and node  83  is pulled high. 
     Capacitor  84  blocks any DC component of SGNL voltage, but passes the AC component through to current mirror  82 . Specifically, an increasing SGNL voltage adds to the current flowing into current mirror  82 , thereby increasing current I 6 . At the same time, the current flowing through current mirror  81  is reduced, thereby decreasing current I 5 . A sufficiently rapid positive change in SGNL voltage causes current I 6  to be larger than I 5 , pulling node  83  low. Capacitor  84  and resistor  85  are selected to provide adequate sensitivity to slew rate without being overly sensitive to noise on the signal line (SGNL). Suitable values for capacitor  84  and resistor  85  are about 2pF and about 187 Ω, respectively. 
     Nominal pullup current circuitry  88  provides pullup current when the SGNL line is stable or being pulled down. Circuitry  88  includes current mirror  89  having an output current coupled back to SGNL, and an input current set by transistors  90  and  91 . Transistor  92  may be turned OFF by a low level on the −SHDN terminal, isolating transistor  90 , and thereby reducing the input current to current mirror  89 . 
     This circuit architecture provides a means of reducing pullup current to a shutdown level when appropriate. For example, when the signal line is high, and has been high for an extended period of time, pullup current may be reduced to a low level to conserve power in a battery powered device. Preferably, pullup circuitry  88  is designed such that normal pullup current is about 250 μA when −SHDN is HIGH, and low power pullup current is about 100 μA when −SHDN is LOW. 
     Lastly, pullup current boost circuitry  95  provides additional pullup current when the voltage at SGNL is above a threshold voltage, as determined by voltage level detection circuitry  71 , and exceeds a minimum positive slew rate, as determined by slew rate detection circuitry  77 . The inputs to gate  96  are coupled to node  74 , the output of voltage level detector  71 , and node  83 , the output of slew rate detector  77 . As described hereinabove, node  74  is pulled LOW whenever the voltage level at SGNL exceeds VREF, and node  83  is pulled LOW whenever the voltage slew rate at SGNL becomes sufficiently large. The output of gate  96  is HIGH only when both inputs are LOW. Thus, the output of gate  96  is high when the conditions for supplying boosted pullup current are satisfied. 
     A HIGH output of gate  96  turns transistor  97  OFF and transistor  98  ON, thereby enabling a constant current source comprising transistor  99  and current mirror  100 . The output of current mirror  100  is connected in parallel with the output of current mirror  89 , so as to provide boosted pullup current. Preferably, the output current of current mirror  100  is about 1.7 mA. 
     In addition, a HIGH at the output of gate  96  turns ON transistor  101 . Transistor  101  provides an additional source of input current for current mirror  89 , increasing its output current. Preferably, turning ON transistor  101  increases the output of current mirror  89  to about 300 μA. Thus, when the voltage level and slew rate conditions are satisfied, i.e., during LOW-to-HIGH transitions, pullup current is boosted from about 250 μA to about 2 mA, thereby significantly reducing signal rise time. 
     One skilled in the art will appreciate that the present invention may be practiced by other than the described embodiments, which are presented for purposed of illustration and not of limitation, and that the present invention is limited only by the claims which follow: