140
8023F–AVR–07/09
ATmega325P/3250P
The general I/O port function is overridden by the Output Compare (OC2A) from the Waveform
Generator if either of the COM2A1:0 bits are set. However, the OC2A pin direction (input or out-
put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction
Register bit for the OC2A pin (DDR_OC2A) must be set as output before the OC2A value is vis-
ible 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 OC2A state before the
output is enabled. Note that some COM2A1:0 bit settings are reserved for certain modes of
16.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM2A1:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COM2A1:0 = 0 tells the Waveform Generator that no action on the
OC2A Register is to be performed on the next compare match. For compare output actions in
A change of the COM2A1:0 bits state will have effect at the first compare match after the bits are
written. For non-PWM modes, the action can be forced to have immediate effect by using the
FOC2A strobe bits.
16.7
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 (WGM21:0) and Compare Output
mode (COM2A1:0) bits. The Compare Output mode bits do not affect the counting sequence,
while the Waveform Generation mode bits do. The COM2A1:0 bits control whether the PWM
output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM
modes the COM2A1:0 bits control whether the output should be set, cleared, or toggled at a
16.7.1
Normal Mode
The simplest mode of operation is the Normal mode (WGM21:0 = 0). In this mode the counting
direction is always up (incrementing), and no counter clear is performed. The counter simply
overruns when it passes its maximum 8-bit value (TOP = 0xFF) and then restarts from the bot-
tom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV2) will be set in the same
timer clock cycle as the TCNT2 becomes zero. The TOV2 Flag in this case behaves like a ninth
bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt
that automatically clears the TOV2 Flag, the timer resolution can be increased by software.
There are no special cases to consider in the Normal mode, a new counter value can be written
anytime.
The Output Compare unit can be used to generate interrupts at some given time. Using the Out-
put Compare to generate waveforms in Normal mode is not recommended, since this will
occupy too much of the CPU time.