Compensated Drive Impedance and Terminations

Resistors are used to match the impedance of the trace. However, adding resistors close to device pins increases the size of the board area and component count, and can in some cases be physically impossible. To address these issues, a reference controller between the VDDI power supply, and pad signal is used to control the source and sink drivers between the pad and the ground. This compensation happens at power up, and on-demand by the user logic. The I/O compensation adjusts the impedances inside the GPIO or HSIO bank by comparing to the internal reference. The impedance change in I/O compensation is due to process variation. The compensation logic adjusts the impedance of the GPIO or HSIO by selectively turning the transistors ON or OFF in the I/Os. The impedance is adjusted to match the internal reference by doing an initial adjustment when the power-on detector for VDDI and VDDAUX gets to a minimal value. The change in impedance also compensates for Temperature variation and Supply Voltage fluctuations.