Low swing voltage mode driver

Abstract

An output driver includes control logic configured to switch on a pull-up circuit and a pull-down circuit to provide an output impedance for a logic low on a transmission line. The output driver includes a variable pull-up resistor. The control logic is configured to switch on the pull-up circuit to a first value of impedance to drive a logic high on the transmission line. The control logic is configured to switch on the pull-up circuit to a second value of impedance and to switch on the pull-down circuit to provide the output impedance to drive a logic low on the transmission line. The system could alternatively be configured for the inverse to switch on a combination of pull-up and pull-down circuits for a logic high, where the pull-down circuit is switched on for a logic low.

What is claimed is: 
     
         1 . An output driver comprising:
 a pull-up circuit including a variable pull-up resistor to pull up a transmission line driven by the output driver;   a pull-down circuit including a pull-down resistor to pull down the transmission line; and   driver control logic coupled to the pull-up circuit and the pull-down circuit, configured to switch on the pull-up circuit to a first value of impedance to drive a logic high on the transmission line, and configured to switch on the pull-up circuit to a second value of impedance and to switch on the pull-down circuit to drive a logic low on the transmission line, where the second value of impedance in parallel with impedance of the pull-down circuit provides an effective output impedance for the output driver when driving the logic low.   
     
     
         2 . The output driver of  claim 1 , wherein the pull-up circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein the variable pull-up resistor is generated by switching on a number of the multiple legs to result in a desired impedance. 
     
     
         3 . The output driver of  claim 1 , wherein the pull-down circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein an impedance of the pull-down circuit is adjustable based on switching on a number of the multiple legs. 
     
     
         4 . The output driver of  claim 1 , wherein the driver control logic comprises driver pre-equalization logic. 
     
     
         5 . The output driver of  claim 1 , wherein the pull-up circuit includes multiple legs, and the pull-down circuit includes multiple legs, where the driver control logic is configured to switch on a ratio of pull-up and pull-down legs to generate the effective output impedance. 
     
     
         6 . The output driver of  claim 5 , wherein the ratio of pull-up and pull-down legs generates an effective output impedance approximately equal to the first value of impedance. 
     
     
         7 . The output driver of  claim 1 , further comprising:
 reference voltage logic to adjust a center reference voltage (Vref) to reduce a voltage swing on the transmission line.   
     
     
         8 . An electronic device comprising:
 an output driver in a memory subsystem to exchange signals between components of the memory subsystem, the output driver including
 a pull-up circuit including a variable pull-up resistor to pull up a transmission line driven by the output driver; 
 a pull-down circuit including a pull-down resistor to pull down the transmission line; and 
 driver control logic coupled to the pull-up circuit and the pull-down circuit, configured to switch on the pull-up circuit to a first value of impedance to drive a logic high on the transmission line, and configured to switch on the pull-up circuit to a second value of impedance and to switch on the pull-down circuit to drive a logic low on the transmission line, where the second value of impedance in parallel with impedance of the pull-down circuit provides an effective output impedance for the output driver when driving the logic low; and 
   a touchscreen display device configured to provide a user display based on data accessed by the memory subsystem.   
     
     
         9 . The electronic device of  claim 8 , wherein the pull-up circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein the variable pull-up resistor is generated by switching on a number of the multiple legs to result in a desired impedance. 
     
     
         10 . The electronic device of  claim 8 , wherein the pull-down circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein an impedance of the pull-down circuit is adjustable based on switching on a number of the multiple legs. 
     
     
         11 . The electronic device of  claim 8 , wherein the driver control logic comprises driver pre-equalization logic. 
     
     
         12 . The electronic device of  claim 8 , wherein the pull-up circuit includes multiple legs, and the pull-down circuit includes multiple legs, where the driver control logic is configured to switch on a ratio of pull-up and pull-down legs to generate the effective output impedance. 
     
     
         13 . The electronic device of  claim 12 , wherein the ratio of pull-up and pull-down legs generates an effective output impedance approximately equal to the first value of impedance. 
     
     
         14 . The electronic device of  claim 8 , the output driver further comprising:
 reference voltage logic to adjust a center reference voltage (Vref) to reduce a voltage swing on the transmission line.   
     
     
         15 . A method comprising:
 receiving an input signal to output with an output driver on a transmission line driven by the output driver;   configuring the output driver to drive the transmission line in accordance with the input signal, including,
 for a logic high, switching on a pull-up circuit of the output driver with driver control logic to a first value of impedance; and 
 for a logic low, switching on the pull-up circuit to a second value of impedance and switching on the pull-down circuit, where the second value of impedance in parallel with impedance of the pull-down circuit provides an effective output impedance for the output driver; and 
   driving the transmission line with the configured output driver.   
     
     
         16 . The method of  claim 15 , wherein the pull-up circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein an impedance of the pull-up circuit is adjustable based on switching on a number of the multiple legs. 
     
     
         17 . The method of  claim 15 , wherein the pull-down circuit includes multiple legs, each leg including a switching transistor and a resistor element, wherein an impedance of the pull-down circuit is adjustable based on switching on a number of the multiple legs. 
     
     
         18 . The method of  claim 15 , wherein configuring the output driver comprises configuring the output driver with pre-equalization logic coupled to the output driver. 
     
     
         19 . The method of  claim 15 , wherein the pull-up circuit includes multiple legs, and the pull-down circuit includes multiple legs, where configuring the output driver comprises switching on a ratio of pull-up and pull-down legs to generate the effective output impedance. 
     
     
         20 . A method comprising:
 receiving an input signal to output with an output driver on a transmission line driven by the output driver;   configuring the output driver to drive the transmission line in accordance with the input signal, including,
 for a logic low, switching on a pull-down circuit of the output driver with driver control logic to a first value of impedance; and 
 for a logic high, switching on the pull-down circuit to a second value of impedance and switching on a pull-up circuit, where the second value of impedance in parallel with impedance of the pull-up circuit provides an effective output impedance for the output driver; and 
   driving the transmission line with the configured output driver.