Abstract:
A method for dynamically balancing a serial data link is disclosed. The serial data link includes a first transmission line and a second transmission line. The method includes the steps of creating a DC offset voltage between the first and second transmission lines when the serial data link is in an idle state. When the serial data link is in use to carry data, the DC offset voltage between the first and second transmission lines is removed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to bidirectional, differential, high speed serial data links, and more particularly, to a line balancing scheme for such data links. 
     2. Background Information 
     The Universal Serial Bus (USB) is a cable bus that supports data exchange between a host computer (USB host) and a wide range of simultaneously accessible peripherals (USB devices). The USB physical interconnect is a tiered star topology. A hub is at the center of each star. Each wire segment is a point-to-point connection between the USB host and a hub or a USB device, or a hub connected to another hub or USB device. The USB host contains host controllers that provide access to the USB devices in the system. FIG. 1 shows a schematic view of the USB architecture. For more detailed information on USB, the reader is invited to review the “Universal Serial Bus Specification—Version 1.1” published Sep. 23, 1998. 
     A new, high speed mode being defined for USB can be classified as a bi-directional, differential, high speed, serial data link. One characteristic of bi-directional, differential, high speed, serial data links is that data packets are separated by intervals of “silence” where signals are not being transmitted on the data link. In such links, there is an inherent conflict between two opposing goals. On the one hand it is desirable to maintain a DC voltage balance between the two conductors which form the link in order to achieve the highest possible signal noise margin during transmission. However, on the other hand, having the lines at the same voltage during transmission of packets can potentially lead to unwanted noise pick up or oscillation in the receivers during silent intervals. 
     In the prior art, attempts to solve this problem were to permanently apply a DC voltage offset onto the two signaling lines. While the DC voltage offset reduces unwanted noise pick up and oscillation, the voltage offset causes reduced noise margin in the data link. 
     Another solution is the inclusion of a hysteresis loop in the receivers with the hysteresis window greater than the peak noise. Once again, although this reduces noise pick up and oscillation in the receivers, the noise margin is reduced. 
     In yet another prior art approach, the receivers are disabled during inter-packet transmission times. This approach is effective, but presents the problem of having to enable the receiver when the transmitter is about to transmit. In tightly timed systems this is possible, but in systems with loose or indeterminate timing, this is difficult. 
     Thus, what is needed is a method and apparatus for increasing the noise margin in a serial data link while reducing unwanted noise pick up and oscillation in the receivers. 
     SUMMARY OF THE INVENTION 
     A method for dynamically balancing a serial data link is disclosed. The serial data link includes a first transmission line and a second transmission line. The method includes the steps of creating a DC offset voltage between the first and second transmission lines when the serial data link is in an idle state. When the serial data link is in use to carry data, the DC offset voltage between the first and second transmission lines is removed. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will be described in conjunction with the accompanying Figures, wherein: 
     FIG. 1 is a schematic diagram of the tiered starred topology used in the USB architecture; 
     FIG. 2 is a schematic diagram of a circuit formed in accordance with the present invention; and 
     FIG. 3 is a flow diagram illustrating the method of the present invention utilizing the circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a schematic diagram of a circuit formed in accordance with the present invention. The diagram includes two transceivers: a first transceiver T 1  designated by reference numeral  103  and a second transceiver T 2  designated by reference numeral  105 . The transceivers  103  and  105  can be any device that can transmit and receive data packets over the data link. In the preferred embodiment, the data link conforms with the USB specification. However, the present invention may be used with any data link. In the preferred embodiment, the transceivers may be USB devices, hub controllers, hub hosts, and any other device that will use the USB data link. 
     The data link itself is represented schematically by a first signaling line  107  and a second signaling line  109 . Each termination of each of the lines  107  and  109  are terminated by a resistor. In FIG. 2, the resistors are designated R 4 , R 5 , R 6 , and R 7 . These resistors are generally of equal value and are selected to correctly terminate the differential link between transceivers  103  and  105 . Typically, the resistor values are on the order of 45 ohms. The above described portion of the circuit of FIG. 2 is known in the art. In accordance with the present invention, additional switches, resistors, and a pull up voltage are added to the circuit to implement the present invention. 
     In particular, a voltage source V+ 111  (also referred to as a pull-up voltage) is selectively applied to the two transmission lines  107  and  109 . The voltage source V+ 111  is continuously applied through a resistor R 2  to the first transmission line  107 . Additionally, the voltage source V+ 111  can be selectively applied to the second transmission line  109  through a resistor R 1  and a switch SW 1 . Further, the voltage source V+ 111  can be selectively applied to the second transmission line  109  through a resistor R 3  through a switch SW 2 . 
     The operation of switch SW 1 , is controlled by a first control signal provided by the first transceiver  103 . The first control signal is carried by a first control signal line  113 . The operation of second switch SW 2  is controlled by a second control signal provided by the second transceiver  105 . The second control signal is carried by a second control signal line  115 . 
     Generally, it is preferred that the resistor values R 1 , R 2 , and R 3  are all the same value and preferably on the order of 1.5 Kohms. By having the resistors R 1 , R 2 , and R 3  all have substantially the same value, as will be seen below, the voltage differential applied to the first and second transmission lines  107  and  109  during operation will be the same. Further, the values of resistors R 1 , R 2 , and R 3  are generally considerably higher than the resistors R 4 -R 7 . 
     The present invention seeks to introduce a DC offset between the two transmission lines  107  and  109  such that one transmission line is at a sufficiently higher voltage potential than the other transmission line to overcome noise and oscillation problems associated with a voltage balanced condition. Thus, when neither of the two transceivers  103  or  105  are transmitting, and the data link is idle, the data link is in a deterministic state. 
     For example, the imposition of a DC offset is accomplished by the present invention by having the resistor R 2  always connected to the pull up voltage source V+ 111 . This will result in the voltage on the first transmission line  107  to be pulled up. The magnitude of the pull up voltage would be a function of the combination of the pull up voltage source V+ 111  and the values of resistors R 2 , R 5  and R 7 . In a preferred embodiment the DC offset resulting from the pull up voltage source V+ 111  would be on the order of 50 to 100 millivolts. 
     However, when either of the two transceivers  103  or  105  are about to transmit data, it is desirable to eliminate the DC offset between the two transmission lines  107  and  109 . Thus, when a transceiver  103  or  105  is preparing to transmit, the transceiver causes an equal DC voltage to be placed onto the second transmission line  109  that overcomes the offset provided by the voltage source V+ 111  and the resistor R 2  onto the first transmission line  107 . A brief interval is allowed prior to starting transmission of data to allow the line to balance and stabilize. 
     Specifically, if the transceiver  103  is about to transmit data, it would send a control signal through the first control signal line  113  to switch SW 1  causing it to close. Because resistors R 1  and R 2  are nominally equal, the pull up voltage applied onto the lines  107  and  109  will be nominally equal. This provides an ideal noise margin situation. Because data is transmitted differentially, an improved noise margin is present because the DC offset is now absent. Following the completion of transmission of data, the transceiver  103  sends a control signal along first control signal line  113  causing switch SW 1 , to open, thereby allowing the second transmission line  109  to return to its deterministic DC voltage offset quiescent state relative to the first transmission line  107 . 
     Similarly, if the transceiver  105  is about to transmit data, it would send a control signal through the second control signal line  115  to switch SW 2  causing it to close. Because resistors R 3  and R 2  are nominally equal, the pull up voltage applied onto the lines  107  and  109  will be nominally equal. This provides an ideal noise margin situation. Because data is transmitted differentially, an improved noise margin is present because the DC offset is now absent. Following the completion of transmission of data, the transceiver  105  sends a control signal along second control signal line  115  causing switch SW 2  to open, thereby allowing the second transmission line  109  to return to its deterministic DC voltage offset quiescent state relative to the first transmission line  107 . 
     Turning to FIG. 3, a flow diagram illustrating the steps of the present invention is illustrated. First, at a box  201 , a transceiver is selected to transmit. Next, at box  203 , a control signal is sent from the transmitting transceiver along its control signal line which closes its associated switch and applies the pull up voltage V+ 111  to the second transmission line  109 . For example, if transceiver  103  wishes to transmit data, a control signal would be sent along first control signal line  113  to switch SW 1  closing the switch and applying the pull up voltage V+ 111  through resistor R 1  to the second transmission line  109 . After this has been accomplished, at box  205 , using conventional methods, the data is transmitted over the transmission lines  107  and  109  to the receiving transceiver  105 . Finally, at box  207 , once the data has been completely transmitted, a control signal is sent along first control signal line  113  by the transceiver  103  to the switch SW 1 , instructing the switch SW 1 , to open, thereby removing the pull up voltage V+ 111  from the second transmission line  109 . This places the two transmission lines  107  and  109  in a DC voltage offset state which reduces noise pick up and oscillations in the receivers. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.