Abstract:
A multiport device is configured to recognize each active segment on a bus, and to selectively propagate signals within the device depending upon whether the segment is active. Optimal signal propagation is achieved by invoking the control of the propagation of signals only after a first active-transition on the bus. Initial transitions are propagated unconditionally, to minimize propagation delay, and subsequent signal propagations are conditionally controlled, to avoid latch-up. A latch is associated with each port. The latch is set each time the port is actively driven by a device on that port. The latch is reset when all the devices are in the quiescent state, or when another port remains active after the currently active port becomes inactive. The state of each port&#39;s latch controls the propagation of internally generated signals to the port. If the latch is set, internally generated signals are not propagated to the port, thereby preventing latch-up. If the latch is not set, both internally generated signals and externally generated signals are propagated to the port, thereby minimizing propagation delays.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to the inventor&#39;s U.S. Pat. No. 6,362,654, “BIDIRECTIONAL REPEATER USING HIGH AND LOW THRESHOLD DETECTION”, issued Mar. 26, 2002, and U.S. Pat. No. 6,433,622, “VOLTAGE STABILIZED LOW LEVEL DRIVER”, issued Aug. 13, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of electronics, and in particular to a device that facilitates the interconnection of devices using an I 2 C interface. 
     2. Description of Related Art 
     The Inter Integrated Circuit (I 2 C) bus developed by Philips Corporation allows integrated circuits to communicate directly with each other via a simple bi-directional 2-wire (plus ground) bus. A device connects to each of the two wires on the bus, one (SData) for the communication of data, and the other (SClk) for the control and synchronization of the communication of data between the devices. Each device is connected in parallel to each of the other devices, and each of the bus lines, SData and SClk, function as a wired-AND of all the lines on the bus. The output of each device is configured as an open-collector/open-drain device, and one or more pull-up resistors maintain a ‘soft’ logic high value on the bus while the bus is in the quiescent state. When a device desires access to the bus, the device pulls the bus to a logic low value, via the open-collector/open-drain device that is placed in a conductive state to ground potential. 
     To facilitate a high speed data transfer, the I 2 C specification limits the maximum capacitive loading on the bus, thereby limiting the maximum length of the bus. The specified maximum capacitive loading is 400 pf, which limits the bus length to a few meters. The 82B715 integrated circuit from Philips Semiconductors is an I 2 C bus extender that allows for routing of the bus beyond the specified maximum length. The 82B715 offers a current gain of ten from the input to the output, thereby providing a capacitive reduction of ten to the bus on the input side of the bus extender. By placing a bus extender at each end of a long length of wire, only one tenth of the capacitance of the wire appears as a load to the I 2 C bus or device at each end of the long length of wire. The current buffering also preserves the bi-directional, open-collector/open-drain characteristics of the I 2 C bus. 
     A hub is distinguished from an extender in that a hub is conventionally defined as a device that distributes a signal to multiple devices. For example, on a printed circuit board, a hub may be centrally placed among a variety of devices that communicate via the bus. From this central location, the bus is distributed to each device, preferably via a driver on each port of the bus that drives a corresponding segment of the bus. In this manner, the length of each segment of the bus, from the hub to the devices, is reduced, relative to one continuous length of wire extending to all devices, and the bus loading is distributed among the hub drivers. 
     In a bi-directional configuration, wherein the same wire is used to both transmit and receive, a hub appears as a collection of input ports that each fan out to a driver at every other port. Because the output of the driver at a port shares the same node as the input from the devices on that port&#39;s segment of the bus, and the input fans out to every other driver, a bi-directional hub must be configured to distinguish an externally generated event on the bus segments from an internally generated event, to preclude a latch-up. Such a latch-up is caused when an input event that is propagated from one port is interpreted as an input event from another port, and re-propagated to the originating port from the other port. When the originator of the event ceases the event, this cessation must be propagated to all the ports. 
     Conventional bi-directional hubs typically include control logic that determines which port is receiving a driving signal, and thereafter blocks any input from any other port from being propagated to any port. That is, the conventional hub selectively enables only one port as a receiving port, and forces all other ports to a transmit state, wherein these other ports transmit the data received at the receiving port to their corresponding bus segments. Because only one port is enabled for receiving and propagating a signal at any one time, the possibility of a latch-up is avoided. Note, however, that this selective enabling of one port at a time presupposes that only one port may be active at any point in time, or, if two ports are active, the first port to be active prevails. 
     Preferably, a hub is “transparent” to devices on the bus. That is, although a hub allows for improved bus performance, its presence on the bus should be undetectable. If the bus without the hub is a wired-AND bus, for example, when any device pulls the bus low, the entire length of the bus is brought to the low state. If multiple devices pull the bus low, the bus will remain low until the last device ceases to assert the low voltage. A conventional hub that only reacts to one port at a time will often introduce transition behavior as it disables the current receiving port, and then enables another port as the receiving port. Because only one port at a time is enabled as a receiving port, the hub is unaware that another segment of the bus is also being driven low until it attempts to bring the bus to a high level. The segment being driven low will remain low, but other segments will be allowed to rise to the high level until the port that is being driven low is enabled as the sole receiving port. Some specifications, and in particular the I 2 C specification, rely upon a consistent performance in the presence of a simultaneous assertion of the active state by more than one device. In the I 2 C specification, for example, the synchronization of the clock signal among multiple devices is determined by the time that a first device pulls the bus low, and the time that a last device ceases pulling the bus low. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a multiport device that facilitates a multiple fanout of an open-collector/open-drain bus while also providing for a high-speed data transfer. It is a further object of this invention to provide a multiport device that maintains the bus control protocol of an I 2 C bus. It is a further object of this invention to provide a multiport device that minimizes propagation delays. 
     These objects and others are achieved by providing a multiport device that is configured to recognize each active segment on the bus. The propagation of signals from each port to each other port is controlled by state of each segment. Optimal signal propagation is achieved by invoking the control of the propagation of signals only after a first active-transition on the bus. Initial transitions are propagated unconditionally, to minimize propagation delay, and subsequent signal propagations are conditionally controlled, to avoid latch-up. A latch is associated with each port. The latch is set each time the port is actively driven by a device on that port. The latch is reset when all the devices are in the quiescent state, or when another port remains active after the currently active port becomes inactive. The state of each port&#39;s latch controls the propagation of internally generated signals to the port. If the latch is set, internally generated signals are not propagated to the port, thereby preventing latch-up. If the latch is not set, both internally generated signals and externally generated signals are propagated to the port, thereby minimizing propagation delays. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
     FIG. 1 illustrates an example block diagram of an I 2 C system with a multiport hub in accordance with this invention. 
     FIG. 2 illustrates an example block diagram of a multiport hub in accordance with this invention. 
     FIG. 3 illustrates an example block diagram of a multiport hub controller in accordance with this invention. 
     FIG. 4 illustrates an example block diagram of a coupling control cell of a multiport hub controller in accordance with this invention. 
     FIG. 5 illustrates an example block diagram of a control block for determining an aggregate hub logic state of a multiport hub in accordance with this invention. 
    
    
     Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an example block diagram of an I 2 C system  100  with a multiport hub (4-port hub)  150  in accordance with this invention. Although the I 2 C is used as a paradigm for a bi-directional bus system, and although the I 2 C bus uses a wired-AND logic structure for communicating on the bus, one of ordinary skill will recognize that this invention is not limited to a wired-AND configuration nor to an I 2 C bus structure. Complementary structures, for use in a wired-OR configuration will be evident to one of ordinary skill in the art. 
     In the I 2 C bus structure, external devices  110  communicate via two buses, a data bus, and a clock bus. In a quiescent state, the buses are maintained at a “high” voltage value, nominally +3 to +10 volts. This high, or inactive, or quiescent, voltage is provided by “pullup” resistors  120  that connect the bus to a positive supply voltage  125 . FIG. 1 illustrates four segments of the bus connected to the 4-port hub  150 , each segment being identified as ports A, B, C, and D on the hub  150 . A 4-port hub is presented herein for illustrative purposes; any number of ports may be embodied in a multiport hub in accordance with the principles of this invention. The devices on the bus are identified as devices  110   a, b, c , and  d , corresponding to the port to which they are attached. Communication via the bus is effected by driving the bus to a low voltage state to signify the active state, and releasing the bus, allowing it to rise to the high voltage state, to signify the inactive state. Each of the devices on the bus is configured as open-collector or open-drain devices, that provide a high impedance path (preferably infinite impedance) to the positive voltage  125 , and a switchable low impedance path to ground. If any device  110  pulls the bus to a low voltage, all of the devices on the same segment of the bus will be brought to this same low voltage. The hub  150  is designed to propagate this low voltage to the devices on the other segments of the bus. If and only if all devices  110  are in an in-active state, wherein they are not driving the bus low, will all the segments of the bus be permitted to rise to the high voltage  125 . In effect, the hub  150  is configured to propagate the active state from any device to every other device, and to allow the bus to return to the inactive state when no device asserts the active state. 
     FIG. 2 illustrates an example block diagram of a multiport hub  200  in accordance with this invention. In an I 2 C environment, a pair of hubs  200  form the hub  150  of FIG. 1, to independently provide a hub function to each of the data bus and the clock bus. 
     Each port of the multiport hub  200  has an associated port handler. Four port handlers are illustrated in FIG. 2, although any number of ports can be provided. Each port handler  210  includes a driver  220  that drives an associated segment of the bus, commonly via a pad or other connection means to the hub  200 . The driver  220  is also coupled, via the port&#39;s bi-directional connection to external devices ( 110  in FIG.  1 ), to a detector  240  that senses the logic state of the associated segment of the bus, regardless of whether the bus is being driven by the driver  220  or an external device. Because internally generated signals (from the driver  220 ) are communicated via the same media as externally generated signals (from devices  110 ), the detector  240  must distinguish between internally and externally generated signals, to avoid latch-up when internally generated signals are propagated back to the originating driver, as discussed above. 
     The detector  240  at each port distinguishes between an internally generated signal, and an externally generated signal. A variety of techniques, common in the art, are available for distinguishing between internally and externally generated signals. The aforementioned 82B715 bus extender, for example, uses current sinking and sensing to distinguish the source of a signal. U.S. Pat. No. 6,362,654 “BIDIRECTIONAL REPEATER USING HIGH AND LOW THRESHOLD DETECTION”, issued Mar. 26, 2002 to Alma Anderson and Paul Andrews, discloses a technique for distinguishing between internally and externally generated signals based on a distinguishing threshold voltage, and is incorporated by reference herein. In accordance with this copending application, the driver  220  is configured to limit its output voltage level to a low value that corresponds to a logic-low state, but is above the voltage level that external devices  10  are configured to produce. A threshold voltage is set to be between the voltage level of the driver  220  at the logic-low state, and the voltage level of the external devices  110  at the logic-low state. The detector  240  in this example provides an LState that corresponds to the logic-state of the port, and an EState that corresponds to a logic-low state if and only if the voltage at the port is below the threshold. In this manner, an LState of logic-low and an EState of logic-high signifies that the low voltage at the port is caused by the internal driver  220 , and not an external device  110 . Correspondingly, an EState logic-low signifies that the low voltage at the port is caused by the external device  110 . U.S. Pat. No. 6,433,622 “VOLTAGE STABILIZED LOW LEVEL DRIVER”, issued Aug. 13, 2002 to Alma Anderson and Paul Andrews, discloses an embodiment for a driver  220  that provides the required controlled low-voltage at a voltage that is stable over a wide range of conditions, and is incorporated by reference herein. 
     In each of the known techniques for distinguishing an internally driven event from an externally driven event, the time required to ascertain that the event is truly from an external device is longer than the time required to determine the logic value at a node. For example, in a wired-AND configuration, as soon as the voltage at a node begins to fall from the quiescent voltage, it can be assumed that the decrease represents either a change of logic-state, or noise. Assuming a maximum magnitude of expected noise, as soon as the voltage drops below the quiescent voltage by the maximum noise magnitude, a logic-low value can be assumed. Conversely, the difference between an internally generated signal and an externally generated signal, either of which may be producing the logic-low state, is typically determined by waiting until the node exhibits a particular distinguishing behavior, or not. Therefore, the minimum delay time for determining whether a node is being driven internally or externally is typically equal to or greater than the maximum transition time of either the internal or external device. Further exacerbating the problem, to assure a proper determination of whether the node is driven internally or externally, a worst-case maximum transition time is typically used as a design constraint. 
     As noted above, if the internally generated signal is fed back to the node that originated the propagation of the internally generated signal, a latch-up can result. Propagating only the externally generated signal eliminates the likelihood of latch-up, but, as discussed above, introduces a substantial propagation delay, to allow time for the detector  240  to distinguish between internally and externally generated signals. In accordance with this invention, to minimize propagation delay, the determined logic-state is propagated as soon as a first logic-low is detected, regardless of the source of this logic-low signal. Thereafter, the port that originated the first logic-low controls the logic state of the hub HLState, and the coupling of this logic-state to the originating port is broken, thereby preventing a latch-up. 
     A controller  300  controls a switch  230  in each port handler  210 , to control the coupling to the corresponding driver  220 . In the quiescent state, the control to each of the switches  220  de-asserted, and each driver  220  is coupled to the hub logic state, HLState. The controller  300  determines this hub logic state from the logic state LState of each port handler  210 , such that, as soon as a logic-low-state is detected at any port, that change is propagated, via HLState, to all of the port drivers  220 . The controller  300  also asserts the control signal for that port&#39;s switch  230 , thereby decoupling the hub logic state HLState from that port&#39;s driver  220 . By decoupling the hub logic state HLState from the originating port, latch-up is avoided. 
     In accordance with another aspect of this invention, when the control signal for a port&#39;s switch  230  is asserted, the driver  220  is coupled to a hub external-state HEState. In a wired-AND configuration, the hub external-state HEState is the logical-AND of each of the port&#39;s external state EState. By coupling the hub external-state HEState to the originating port, a true wired-AND configuration is maintained. If another external device ( 110  in FIG.  1 ), for example, drives another bus segment low while the originating port is being driven low, the originating port will be held at a low state for as long as either one of the ports is being driven low. In like manner, as discussed further below, the controller will also assert the control signal for the port that is being driven by the other external device, so that its driver  220  will be coupled to the hub external state HEState, thereby holding that port at a logic-low state until both ports cease to be driven by external devices. 
     FIG. 3 illustrates an example block diagram of a multiport hub controller  300  in accordance with this invention. The logic blocks  310  and  320  determine the hub logic and external states HLState, HEState, respectively, based on the logic and external state LState, EState, respectively, of each port. The hub logic control block  310  is discussed further below with regard to FIG.  5 . As mentioned above, in a wired-AND configuration, the logic block  320  is a logical-AND of the individual external states EState(i) of each of the ports(i) in the hub. As would be evident to one of ordinary skill in the art, if a wired-OR configuration is used, for a non-I 2 C embodiment, the AND gate  320  would be replaced by an OR gate. 
     A coupling control block  350  receives each of the LState(i) and Estate(i) signals, as well as the hub signals HLState and HEState, and uses these signals to set or reset control latches  330  that are associated with each port. Each latch  330  determines the corresponding control signal that are used to control the switch ( 230  in FIG. 2) at each port handler ( 210  in FIG. 2) to couple or decouple the appropriate hub logic-state or extemal-state to the corresponding driver ( 220  in FIG.  2 ), as discussed above. 
     FIG. 4 illustrates an example block diagram of a coupling control cell  400  and corresponding controlled latch  330  of a multiport hub controller ( 300  in FIG. 3) in accordance with this invention. The coupling control block  350  of FIG. 3 functionally comprises a coupling control cell  400  for each of the ports, although some of the logic elements, such as the OR-gate  430 , discussed below, may be common to multiple ports. 
     As discussed above, the control signal Control(i) associated with each port(i) is de-asserted when the bus is in the quiescent state. This is effected by resetting, or clearing, the control latch(i) of each port when the hub logic-state is a logic-high value, via the HLState input to the OR-gate  460 . 
     Also discussed above, whenever it is determined that a port is being driven low by an external device, the control signal Control(i) is asserted. This is effected by setting the control latch(i) of a port(i) whenever the port&#39;s extemal-state EState(i) indicates a logic-low value, via the NAND gate  410 . 
     The control signal Control(i) is also de-asserted when it is determined that the particular port is no longer being driven low by an external source ( 110  in FIG.  1 ). If this port is the only port being externally driven low, this de-assertion is effected when the port&#39;s logic state changes and the hub logic-state becomes a logic-high, clearing the latch(i)  330  as discussed above. Alternatively, if the port&#39;s extemal-state EState(i) becomes a logic-high while the hub external-state remains at a logic-low state, this indicates that another device is holding the bus low, and this port&#39;s latch(i) can be reset. This reset is effected via the logic gate  470  and the OR gate  460 . 
     As discussed above, a transition from the quiescent state at any port(i) is propagated to all ports from the first port that detects this transition. To avoid latch-up, this first port(i) is configured to decouple the hub logic state HLState from its driver, via the control signal Control(i). This is effected by setting the latch(i)  330  whenever the detected logic state LState(i) of the port(i) becomes low, and none of the other ports have yet been set, via the OR gates  430  and  420 , and NAND gate  410 , indicating that this is the first port to have detected the logic-low signal, and thus must be externally driven. 
     FIG. 5 illustrates an example block diagram of the logic control block  310  for determining the aggregate hub logic state HLState of the multiport hub  200  of FIG. 2 in accordance with this invention. The logic gates  510  each determine whether the associated port is a controlling port, and the AND gate  520  provides a hub logic-state that corresponds to the one or more controlling port&#39;s logic state LState. In the quiescent, logic-high state, the logic state LState of each port is high, and thereby the hub logic state HLState is high. When the HLState is high, each of the control latches  330  of FIGS. 3 and 4 are reset, via the OR-gate  460  in FIG. 4, bringing each of the coupling control lines associated with each port to a logic-low level. This logic-low level on the inverted control input to the logic gate  510  desensitizes each gate  510  to the logic state LState of each corresponding port by forcing the output of the gate  510  to a logic-high state regardless of the LState input. 
     When a first logic-low event occurs at a port(i), the port&#39;s detector  240  (FIG. 2) provides a logic-low LState(i) output. This logic-low LState(i) sets the corresponding port control signal Control(i) to a logic-high state, via the gates  420  and  410  of FIG.  4 . Note that as noted above, in the quiescent state, all of the control signals are reset to a logic-low, and therefore the output of the logic gate  430  is also at a logic-low value, thereby sensitizing the gate  420  to LState(i). The initial logic-low LState(i) and the subsequent logic-high Control(i) corresponding to the originating port(i) cause the corresponding hub logic control gate  510  to go to a logic-low state, forcing the hub logic state HLState to a logic-low state, via the gate  520 . This logic-low hub logic state HLState is propagated to all ports, except the originating port, via the controlled switch  230  and driver  220  of each port. 
     Note that although the detector  240  at each port will subsequently detect a logic-low LState(i), these logic states will have no effect on the hub logic state HLState, because each of the gates  510  are desensitized to the corresponding port&#39;s LState until the corresponding control signal for that port is asserted. As noted above, if a second port is subsequently driven to a logic-low value via an external source ( 110  in FIG.  1 ), that port&#39;s external state EState will reflect this logic-low state, and the port&#39;s control latch will be set, via the gate  410  in FIG.  4 . Thereby, the corresponding gate  510  will be sensitized to the port&#39;s logic value LState, and the gate  520  will hold in the hub logic state HLState at logic-low until all of the driven ports return to the logic-high state. In this manner, the hub logic state HLState corresponds to a true wired-AND of each of the driven ports&#39; logic values LState. A corresponding complement of the logic of FIGS. 3-5, well within the skill of one of ordinary skill in the art, will provide for a wired-OR embodiment, as required. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, the particular logic configuration and parameters may be modified without affecting the principles of operation of the invention. For example, the detector  240  of each port handler  210  may be configured to provide an indication of whether or not the logic-state LState is provided by an external device, rather than indicating the external-state directly. Similarly, the controller  300  is illustrated as a separate block from the port handlers, for ease of understanding, whereas its function could be distributed among each of the port handlers  210 . That is, for example, each port handler  210  could include the control logic cell  400  and latch  330  associated with each port. These and other system configuration and optimization features will be evident to one of ordinary skill in the art in view of this disclosure, and are included within the scope of the following claims.