The Output Compare Modulator (OCM) allows generation of waveforms modulated with a carrier frequency. The modulator uses the outputs from the Output Compare Unit B of the 16-bit Timer/Counter3 and the Output Compare Unit of the 16-bit Timer/Counter4. When the modulator is enabled, the two output compare channels are modulated together as shown in the block diagram.
The Output Comparator 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 bit when one of them is enabled (that is, when COMnx1:0 is not equal to zero). When both OC3B and OC4B are enabled simultaneously, the modulator is automatically enabled.