User Manual

146
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
Be aware that the COM2x1:0 bits are not double buffered together with the compare value. Changing the
COM2x1:0 bits will take effect immediately.
18.6 Compare Match Output Unit
The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Generator uses the COM2x1:0
bits for defining the Output Compare (OC2x) state at the next compare match. Also, the COM2x1:0 bits control the
OC2x pin output source. Figure 18-4 shows a simplified schematic of the logic affected by the COM2x1:0 bit set-
ting. 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 COM2x1:0 bits are shown. When referring to the OC2x
state, the reference is for the internal OC2x Register, not the OC2x pin.
Figure 18-4. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC2x) from the Waveform Generator if either
of the COM2x1:0 bits are set. However, the OC2x pin direction (input or output) is still controlled by the Data Direc-
tion Register (DDR) for the port pin. The Data Direction Register bit for the OC2x pin (DDR_OC2x) must be set as
output before the OC2x value is visible on the pin. The port override function is independent of the Waveform Gen-
eration mode.
The design of the Output Compare pin logic allows initialization of the OC2x state before the output is enabled.
Note that some COM2x1:0 bit settings are reserved for certain modes of operation. See ”Register Description” on
page 155
18.6.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM2x1:0 bits differently in normal, CTC, and PWM modes. For all modes,
setting the COM2x1:0 = 0 tells the Waveform Generator that no action on the OC2x Register is to be performed on
the next compare match. For compare output actions in the non-PWM modes refer to Table 18-5 on page 156. For
fast PWM mode, refer to Table 18-6 on page 156, and for phase correct PWM refer to Table 18-7 on page 157.
A change of the COM2x1:0 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 FOC2x strobe bits.
PORT
DDR
DQ
DQ
OCnx
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
DATA BU S
FOCnx
clk
I/O