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ATmega64A [DATASHEET]
8160D–AVR–02/2013
16.7.3
Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all Compare Matches for one timer clock cycle, there are
risks involved when changing TCNTn when using any of the Output Compare channels, independent of whether
the Timer/Counter is running or not. If the value written to TCNTn equals the OCRnx value, the Compare Match will
be missed, resulting in incorrect waveform generation. Do not write the TCNTn equal to TOP in PWM modes with
variable TOP values. The Compare Match for the TOP will be ignored and the counter will continue to 0xFFFF.
Similarly, do not write the TCNTn value equal to BOTTOM when the counter is downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe bits in Normal
mode. The OCnx Register keeps its value even when changing between waveform generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the
COMnx1:0 bits will take effect immediately.
16.8
Compare Match Output Unit
The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses the COMnx1:0
bits for defining the Output Compare (OCnx) state at the next Compare Match. Secondly the COMnx1:0 bits con-
trol the OCnx pin output source.
Figure 16-5 shows a simplified schematic of the logic affected by the COMnx1:0
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 COMnx1:0 bits are shown. When referring to the
OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a System Reset occur, the OCnx
Register is reset to “0”.
Figure 16-5. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform Generator if either
of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled by the Data Direc-
PORT
DDR
DQ
OCnx
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCnx
clk
I/O