ATmega16(L)
Figure 30. Compare Match Output Unit, Schematic
COMn1
Waveform
Generator
COMn0
FOCn
D
Q
Q
1
0
OCn
Pin
OCn
D
PORT
D
Q
DDR
clkI/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 direc-
tion (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.
The design of the output compare pin logic allows initialization of the OC0 state before
the output is enabled. Note that some COM01:0 bit settings are reserved for certain
modes of operation. See “8-bit Timer/Counter Register Description” on page 77.
Compare Output Mode and
Waveform Generation
The Waveform Generator uses the COM01:0 bits differently in normal, CTC, and PWM
modes. For all modes, setting the COM01:0 = 0 tells the waveform generator that no
action on the OC0 Register is to be performed on the next compare match. For compare
output actions in the non-PWM modes refer to Table 39 on page 78. For fast PWM
mode, refer to Table 40 on page 78, and for phase correct PWM refer to Table 41 on
page 79.
A change of the COM01:0 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 FOC0 strobe bits.
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 (WGM01:0) and
Compare Output mode (COM01:0) bits. The Compare Output mode bits do not affect
the counting sequence, while the Waveform Generation mode bits do. The COM01:0
bits control whether the PWM output generated should be inverted or not (inverted or
non-inverted PWM). For non-PWM modes the COM01:0 bits control whether the output
should be set, cleared, or toggled at a compare match (See “Compare Match Output
Unit” on page 70.).
For detailed timing information refer to Figure 34, Figure 35, Figure 36 and Figure 37 in
“Timer/Counter Timing Diagrams” on page 75.
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2466E–AVR–10/02
ATMEL [ ATMEL ]
