The Output Compare unit 3B and Output Compare unit 4B shares the PD2 port pin for output. The outputs of the Output Compare units (OC3B and OC4B) overrides the normal PORTD2 Register when one of them is enabled (that is, when COMnx[1:0] is not equal to zero). When both OC3B and OC4B are enabled at the same time, it will also enable this modulator.

The functional equivalent schematic of the modulator is shown in the figure below. The schematic includes part of the Timer/Counter units and the port D pin 2 output driver circuit. When the modulator is enabled the type of modulation (logical AND or OR) can be selected by setting the PORTD2 Register as ‘1’.
Note: The DDRD2 controls the direction of the port independent of the COMnx[1:0] bit setting.
Figure 1. Output Compare Modulator, Schematic