Calculation of Characteristic Impedance for given IO standard

Consider 3.3V CMOS, that provides drive capability of 24mA
Max value of VCC is 3.6V.
VOH is 2.4V.
Maximum allowed drop in the transmission line for proper operation is
VCCmax – VOH = 1.2V.
 Maximum allowed impedance of the line is 1.2/0.024 = 50 ohms