1.  Field of the Invention
The present invention relates generally to controlling electrical characteristics associated with input/output (I/O) circuits.  More particularly, the present invention relates to developing I/O circuits having electrical characteristics, such as operating frequencies, that are independent of variations in fabrication process, supply-voltage, and temperature (PVT) conditions. 
2.  Related Art
I/O circuits are used to interface traditional integrated circuits (ICs) with electrical environments external to the IC.  The I/O circuit acts as a driver for signals generated by the IC and provides these signals to a pad, which in-turn interfaces with the external electrical environment.  The I/O circuit may also receive signals from the external electrical environment through the pad.  A critical challenge in the design, fabrication, and operation of these I/O circuits is that their electrical characteristics may vary depending on the particular PVT conditions. 
In order to create independence between the electrical characteristics of the I/O circuits and PVT conditions, it is desirable that the Slew-rate (change in pad-voltage Vpad with rise time/fall time) should be relatively constant.  In other words, the transient current drive [I=(dVpad/dt)/Cload=Slew-rate/Cload, where Cload=load capacitance] of the I/O circuit should be independent of the PVT conditions. 
Traditional approaches for ensuring that the electrical characteristics of I/O circuits remain independent of PVT conditions include complicated switching arrangements.  These switching arrangements, for example, switch the number of fingers between the pre-driver and the output driver devices.  These traditional approaches, however, consume unacceptable amounts of the IC's real estate and are therefore less than optimal. 
What is needed, therefore, is an efficient technique to ensure that the electrical performance of I/O circuits remains substantially stable and independent from PVT variations.