1. An impedance control circuit comprising: a reference voltage output circuit for outputting one of a plurality of reference voltages; a variable resistor; a comparator that has a first input terminal supplied with a reference voltage from said reference voltage output circuit and a second input terminal connected to said variable resistor, and compares the voltages supplied to the first and second input terminals; and a control circuit for controlling said variable resistor and for establishing a plurality of impedances corresponding to the plurality of reference voltages in response to a signal output from said comparator.