
143
ATmega64A [DATASHEET]
8160D–AVR–02/2013
The setup of the OC2 should be performed before setting the Data Direction Register for the port pin to output. The
easiest way of setting the OC2 value is to use the Force Output Compare (FOC2) strobe bits in Normal mode. The
OC2 Register keeps its value even when changing between Waveform Generation modes.
Be aware that the COM21:0 bits are not double buffered together with the compare value. Changing the COM21:0
bits will take effect immediately.
18.6
Compare Match Output Unit
The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses the COM21:0 bits
for defining the Output Compare (OC2) state at the next Compare Match. Also, the COM21:0 bits control the OC2
pin output source.
Figure 18-4 shows a simplified schematic of the logic affected by the COM21:0 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 COM21:0 bits are shown. When referring to the OC2 state, the
reference is for the internal OC2 Register, not the OC2 pin. If a System Reset occur, the OC2 Register is reset to
“0”.
Figure 18-4. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC2) from the Waveform Generator if either of
the COM21:0 bits are set. However, the OC2 pin direction (input or output) is still controlled by the Data Direction
Register (DDR) for the port pin. The Data Direction Register bit for the OC2 pin (DDR_OC2) must be set as output
before the OC2 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 OC2 state before the output is enabled. Note
PORT
DDR
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
D
ATA
B
U
S
FOCn
clk
I/O