Patent ID: 7902859

Claim:
Circuitry comprising: an output circuit comprising a first variable resistance block coupled between a first supply voltage and an output node, the first variable resistance block comprising a first 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 comprising: a second variable resistance block coupled between said 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 second 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 said first and second plurality of selectable resistive elements of the first and second variable resistance blocks are selected based on a voltage level at said first node of the external resistor; and a control block adapted to test individually at least two of the selectable resistive elements of the second plurality of selectable resistive elements in turn to determine whether each of the two selectable resistive elements should be on or off.