a pull-up calibration circuit configured to generate a p-channel count signal for calibrating p-channel devices in the variable impedance output driver, the p-channel count signal being an average of a plurality of count signals in response to a respective plurality of input and output configurations on a first comparator; and

a pull-down calibration circuit configured to generate an n-channel count signal for calibrating n-channel devices in the variable impedance output driver, the n-channel count signal being an average of a plurality of count signals in response to a respective plurality of input and output configurations on a second comparator.