Abstract: A passive processing circuit for processing a ground side switch closure to a CMOS input (20) of an MCU, particularly useful in a D.C. electrical system where ground offsets may occur, such as in an automotive vehicle. Three resistors (R1, R2, R3) are connected between supply and ground potentials of a D.C. power supply voltage (10). The junction of two (R1, R2) connects to an output (16) of a grounded input switch (12). The junction of two (R2, R3) connects to the CMOS input (20). A fourth resistor (R4) connects between the bi-directional input/output (22) of the MCU and the junction of the second and third resistors. The MCU executes an algorithm that selectively allows and disallows the resistor (R4) to coact with the second and third resistors. The circuit provides a cost-efficient solution for extending the range of supply line voltage over which a CMOS input can correctly read a switch closure.