AT90CAN128
Compare Match Output
Unit
The Compare Output mode (COM2A1:0) bits have two functions. The Waveform Gener-
ator uses the COM2A1:0 bits for defining the Output Compare (OC2A) state at the next
compare match. Also, the COM2A1:0 bits control the OC2A pin output source. Figure 65
shows a simplified schematic of the logic affected by the COM2A1:0 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 COM2A1:0
bits are shown. When referring to the OC2A state, the reference is for the internal OC2A
Register, not the OC2A pin.
Figure 65. Compare Match Output Unit, Schematic
COMnx1
Waveform
Generator
COMnx0
FOCnx
D
Q
1
0
OCnx
Pin
OCnx
D
Q
PORT
D
Q
DDR
clkI/O
Compare Output Function
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 output) 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 visible on the pin. The port override function is indepen-
dent 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 operation. See “8-bit Timer/Counter Register Description” on page 151
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 com-
pare output actions in the non-PWM modes refer to Table 67 on page 152. For fast
PWM mode, refer to Table 68 on page 152, and for phase correct PWM refer to Table
69 on page 153.
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.
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
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4250E–CAN–12/04
ATMEL [ ATMEL ]
