Datasheet

Figure 20-5 Compare Match Output Unit, Schematic
PORT
DDR
D Q
D Q
OCnx
Pin
OCnx
D Q
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BUS
FOCnx
clk
I/O
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 Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin
(DDR_OCnx) must be set as output before the OCnx value is visible on the pin. The port override function
is generally independent of the Waveform Generation mode, but there are some exceptions. Refer to
tables Table 20-2 Compare Output Mode, non-PWM on page 159, Table 20-3 Compare Output Mode,
Fast PWM on page 160 and Table 20-4 Compare Output Mode, Phase Correct and Phase and
Frequency Correct PWM on page 160 for details.
The design of the Output Compare Pin logic allows initialization of the OCnx state before the output is
enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See
Register Description on page 157.
The COMnx1:0 bits have no effect on the Input Capture unit.
20.8.1. Compare Output Mode and Waveform Generation
The waveform generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all
modes, setting the COMnx1:0 = 0 tells the waveform generator that no action on the OCnx Register is to
be performed on the next Compare Match. For compare output actions in the non-PWM modes refer to
Table 20-2 Compare Output Mode, non-PWM on page 159. For fast PWM mode refer to Table 20-3
Compare Output Mode, Fast PWM on page 160, and for phase correct and phase and frequency correct
PWM refer to Table 20-4 Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
on page 160.
A change of the COMnx1:0 bits state will have effect at the first Compare Match after the bits are written.
For nonPWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.
20.9. Modes of Operation
The mode of operation (i.e., the behavior of the Timer/Counter and the Output Compare pins) is defined
by the combination of the Waveform Generation mode (WGMn3:0) and Compare Output mode
(COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence, while the
Waveform Generation mode bits do. The COMnx1:0 bits control whether the PWM output generated
should be inverted or not (inverted or non-inverted PWM). For non-PWM modes the COMnx1:0 bits
Atmel ATmega64A [DATASHEET]
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