21.8 Compare Match Output Unit

The Compare Output mode bits in the Timer/Counter Control Register A (TCCR1A.COM1x) have two functions:

  • The Waveform Generator uses the COM1x bits for defining the Output Compare (OC1x) register state at the next compare match.
  • The COM1x bits control the OC1x pin output source

The figure below shows a simplified schematic of the logic affected by COM1x. 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 that are affected by the COM1x bits are shown, namely PORT and DDR.

On system reset the OC1x Register is reset to 0x00.

Note: 'OC1x state' is always referring to internal OC1x registers, not the OC1x pin.
Figure 21-4. Compare Match Output Unit, Schematic
Note: The “n” in the register and bit names indicates the device number (n = 0 for Timer/Counter 0), and the “x” indicates Output Compare unit (A/B).

The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform Generator if either of the COM1x[1:0] bits are set. However, the OC1x pin direction (input or output) is still controlled by the Data Direction Register (DDR) for the port pin. In the Data Direction Register, the bit for the OC1x pin (DDR.OC1x) must be set as output before the OC1x 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 OC1x register state before the output is enabled. Some TCCR1A.COM1x[1:0] bit settings are reserved for certain modes of operation.

The TCCR1A.COM1x[1:0] bits have no effect on the Input Capture unit.