1. An integrated circuit comprising: an I/O driver having multiple selectable on-impedance values; a first reference driver having multiple selectable on-impedance values, at least some of the selectable on-impedance values substantially identical to corresponding selectable on-impedance values for the I/O driver; reference circuitry to select one of the reference driver on-impedance values to approximate a given driver impedance; a comparator to compare a signal level at the I/O driver with a signal level generated at the reference driver and output a comparison result; and calibration circuitry to select an I/O driver on-impedance substantially identical to the selected reference-driver on-impedance in a calibration mode, and to use the comparator result in that mode to perform calibration of a remote device based on a signal coupled from that remote device to the I/O driver.