Balanced impedance method for differential signaling

Abstract

A system and method for implementing a differential signaling driver with a common-mode voltage not equal to one half the power supply voltage using voltage-mode techniques. Embodiments of the present invention maintain balanced impedance at the signal output. In an embodiment, a driver may have multiple operating modes for each potential supply voltage or common-mode voltage. In an embodiment, each potential mode may involve configuring the driver by activating or deactivating switches or resistors in the driver and each potential mode may have different resistor values.

We claim: 
     
         1 . A system comprising:
 a differential signaling driver having a plurality of variable resistors and a supply voltage, wherein the driver may be configured to develop a plurality of potential common-mode voltages such that at least one common-mode voltage deviates from one-half the supply voltage; and   a controller to select one of the potential common-mode voltages and to set the variable resistors in accordance with the selected common-mode voltage.   
     
     
         2 . The system of  claim 1  wherein the controller activates at least a subset of the resistors in accordance with the selected common-mode voltage. 
     
     
         3 . The system of  claim 1  further comprising:
 a set of registers to store a plurality of resistor values; 
 wherein each potential common-mode voltage corresponds to at least one of the plurality of resistor values and the controller sets the variable resistors in accordance with the value from the register corresponding to the selected common-mode voltage. 
 
     
     
         4 . The system of  claim 1  wherein the controller sets the variable resistors such that the driver has output terminals with balanced impedance. 
     
     
         5 . A system comprising:
 an analog source;   an analog signal processor to convert an analog signal from the analog source into a digital signal and to transmit the digital signal using a voltage-mode differential signaling driver; and   a digital signal processor to process the digital signal received from the voltage-mode differential signaling driver;   wherein the voltage-mode differential signaling driver has a common-mode voltage that deviates from one-half of a supply voltage;   wherein the resistor values are set such that the driver has balanced output impedance.   
     
     
         6 . A method of configuring a voltage-mode differential driver comprising:
 selecting with a controller a supply voltage and a common-mode voltage, wherein the selected common-mode deviates from one half the supply voltage;   activating with the controller a plurality of resistors in the driver to achieve the selected common-mode voltage; and   selecting with the controller, values for the plurality of resistors to achieve the selected common-mode voltage;   wherein the resistor values are set such that the driver has balanced output impedance.   
     
     
         7 . The method of  claim 6  further comprising:
 activating at least one of a plurality of switches in the driver to achieve the selected common-mode voltage. 
 
     
     
         8 . The method of  claim 6  further comprising:
 detecting with the controller a change in the common-mode voltage; and 
 adjusting with the controller the values for the plurality of resistors to maintain the selected common-mode voltage. 
 
     
     
         9 . The method of  claim 6  further comprising:
 detecting with the controller a change in the supply voltage; and 
 adjusting with the controller the values for the plurality of resistors to maintain the selected common-mode voltage. 
 
     
     
         10 . A method comprising:
 selecting one of a plurality of modes for a differential signaling driver;   activating at least one of a plurality of switches in the driver in accordance with the selected mode;   activating at least one of a plurality of resistors in the driver in accordance with the selected mode;   wherein the selected mode specifies a supply voltage and a common-mode voltage, the common-mode voltage deviates from one half the supply voltage; and   wherein the selected mode specifies values for the plurality of resistors.