8-bit AVR Microcontrollers

Compare Match Output Unit

The Compare Output mode (COM2x[1:0]) bits have two functions. The waveform generator uses the COM2x[1:0] bits for defining the Output Compare (OC2x) state at the next compare match. Also, the COM2x[1:0] bits control the OC2x pin output source. The following figure shows a simplified schematic of the logic affected by the COM2x[1:0] bit setting. The I/O registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the general I/O Port Control registers (DDR and PORT) that are affected by the COM2x[1:0] bits are shown. When referring to the OC2x state, the reference is for the internal OC2x register, not the OC2x pin.

Figure 1. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2x) from the waveform generator if either of the COM2x1:0 bits are set. However, the OC2x pin direction (input or output) is still controlled by the Data Direction Register (DDR) for the port pin. The DDR bit for the OC2x pin (DDR_OC2x) must be set as output before the OC2x value is visible on the pin. The port override function is independent of the Waveform Generation mode.

The design of the output compare pin logic allows initialization of the OC2x state before the output is enabled. Note that some COM2x[1:0] bit settings are reserved for certain modes of operation. See Register Description.