Circuitry including an output circuit having a first variable resistance block coupled between a first supply voltage and an output node, the first variable resistance block having a plurality of selectable resistive elements coupled in series with at least one resistor between the first supply voltage and the output node, the output circuit having an output impedance determined by the resistance of the first variable resistance block; and a compensation circuit for regulating the impedance of the first variable resistance block of the output circuit, the compensation circuit having a second variable resistance block coupled between the first supply voltage and the first node of an external resistor, the second node of the external resistor being coupled to a second supply voltage, wherein the second variable resistance block comprises a plurality of selectable resistive elements coupled in series with at least one resistor between the first supply voltage and the first node of the external resistor, and wherein the plurality of selectable resistive elements of the first and second variable resistance blocks are selected based on a voltage level at the first node of the external resistor.