Rainbow Electronics ATmega3290P_V - Manual (Page 93)

93
ATmega329/3290/649/6490
2552H–AVR–11/06
Figure 31. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0A) from the
Waveform Generator if either of the COM0A1:0 bits are set. However, the OC0A 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 OC0A pin (DDR_OC0A) must be set as
output before the OC0A 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 OC0A state
before the output is enabled. Note that some COM0A1:0 bit settings are reserved for
certain modes of operation. See “8-bit Timer/Counter Register Description” on page 99.
Compare Output Mode and
Waveform Generation The Waveform Generator uses the COM0A1:0 bits differently in Normal, CTC, and
PWM modes. For all modes, setting the COM0A1:0 = 0 tells the Waveform Generator
that no action on the OC0A Register is to be performed on the next compare match. For
compare output actions in the non-PWM modes refer to Table 56 on page 100. For fast
PWM mode, refer to Table 57 on page 100, and for phase correct PWM refer to Table
58 on page 101.
A change of the COM0A1:0 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 FOC0A strobe bits.
PORT
DDR
D Q
D Q
OCn
Pin OCnx
D Q
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCn
clk
I/O