Datasheet

115
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Be aware that the COM2[1:0] bits are not double buffered together with the compare value. Changing the
COM2[1:0] bits will take effect immediately.
18.6 Compare Match Output Unit
The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses the COM2[1:0] bits
for defining the Output Compare (OC2) state at the next compare match. Also, the COM2[1:0] bits control the OC2
pin output source. Figure 17-4 shows a simplified schematic of the logic affected by the COM2[1:0] 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 COM2[1:0] bits are shown. When referring to the OC2 state,
the reference is for the internal OC2 Register, not the OC2 pin.
Figure 18-4. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC2) from the waveform generator if either of
the COM2[1:0] bits are set. However, the OC2 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 OC2 pin (DDR_OC2) must be set as output
before the OC2 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 OC2 state before the output is enabled. Note
that some COM2[1:0] bit settings are reserved for certain modes of operation. “Register Description” on page 132
18.6.1 Compare Output Mode and Waveform Generation
The waveform generator uses the COM2[1:0] bits differently in Normal, CTC, and PWM modes. For all modes, set-
ting the COM2[1:0] = 0 tells the Waveform Generator that no action on the OC2 Register is to be performed on the
next compare match. For compare output actions in the non-PWM modes refer to Table 17-3 on page 133. For fast
PWM mode, refer to Table 17-4 on page 134, and for phase correct PWM refer to Table 17-5 on page 134.
A change of the COM21:0 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 FOC2 strobe bits.
18.7 Modes of Operation
The mode of operation, that is, the behavior of the Timer/Counter and the output compare pins, is defined by the
combination of the Waveform Generation mode (WGM2[1:0]) and Compare Output mode (COM2[1:0]) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM2[1:0] bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PORT
DDR
DQ
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
DATA BU S
FOCn
clk
I/O