Datasheet

Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT0 value equal to BOTTOM when the counter is downcounting.
The setup of the OC0 should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC0 value is to use the Force Output Compare (FOC0) strobe bit
in Normal mode. The OC0 Register keeps its value even when changing between waveform generation
modes.
Be aware that the COM01:0 bits are not double buffered together with the compare value. Changing the
COM01:0 bits will take effect immediately.
21.6. Compare Match Output Unit
The Compare Output mode (COM01:0) bits have two functions. The waveform generator uses the
COM01:0 bits for defining the Output Compare (OC0) state at the next Compare Match. Also, the
COM01:0 bits control the OC0 pin output source. The figure below shows a simplified schematic of the
logic affected by the COM01:0 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
COM01:0 bits are shown. When referring to the OC0 state, the reference is for the internal OC0 Register,
not the OC0 pin.
Figure 21-4 Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCn
Pin
OCn
D Q
Waveform
Gene rator
COMn1
COMn0
0
1
DATABUS
FOCn
clk
I/O
The general I/O port function is overridden by the Output Compare (OC0) from the waveform generator if
either of the COM01:0 bits are set. However, the OC0 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 OC0 pin
(DDR_OC0) must be set as output before the OC0 value is visible on the pin. The port override function is
independent of the Waveform Generation mode.
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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