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 down counting.
The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe
bits in Normal mode. The OC0x Registers keep their values even when changing between Waveform
Generation modes.
Be aware that the TCCR0A.COM0x[1:0] bits are not double buffered together with the compare value.
Changing the TCCR0A.COM0x[1:0] bits will take effect immediately.
16.6. Compare Match Output Unit
The Compare Output mode bits in the Timer/Counter Control Register A (TCCR0A.COM0x) have two
functions:
• The Waveform Generator uses the COM0x bits for defining the Output Compare (OC0x) register
state at the next compare match.
• The COM0x bits control the OC0x pin output source
The figure below shows a simplified schematic of the logic affected by COM0x. 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 COM0x bits are shown, namely PORT and DDR.
On system reset the OC0x Register is reset to 0x00.
Note: 'OC0x state' is always referring to internal OC0x registers, not the OC0x pin.
Figure 16-4. Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx[1]
COMnx[0]
0
1
DATA BUS
FOCnx
clk
I/O
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 (OC0x) from the Waveform Generator
if either of the COM0x[1:0] bits are set. However, the OC0x pin direction (input or output) is still controlled
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
132