Data Sheet

158
8161D–AVR–10/09
ATmega48PA/88PA/168PA/328P
17.11 Register Description
17.11.1 TCCR2A – Timer/Counter Control Register A
• Bits 7:6 – COM2A1:0: Compare Match Output A Mode
These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A1:0
bits are set, the OC2A output overrides the normal port functionality of the I/O pin it is connected
to. However, note that the Data Direction Register (DDR) bit corresponding to the OC2A pin
must be set in order to enable the output driver.
When OC2A is connected to the pin, the function of the COM2A1:0 bits depends on the
WGM22:0 bit setting. Table 17-2 shows the COM2A1:0 bit functionality when the WGM22:0 bits
are set to a normal or CTC mode (non-PWM).
Table 17-3 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set to fast PWM
mode.
Note: 1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at BOTTOM. See ”Fast PWM Mode” on
page 150 for more details.
Bit 7 6 5 4 3210
(0xB0)
COM2A1 COM2A0 COM2B1 COM2B0 – – WGM21 WGM20 TCCR2A
Read/Write R/W R/W R/W R/W R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 17-2. Compare Output Mode, non-PWM Mode
COM2A1 COM2A0 Description
0 0 Normal port operation, OC0A disconnected.
0 1 Toggle OC2A on Compare Match
1 0 Clear OC2A on Compare Match
1 1 Set OC2A on Compare Match
Table 17-3. Compare Output Mode, Fast PWM Mode
(1)
COM2A1 COM2A0 Description
0 0 Normal port operation, OC2A disconnected.
01
WGM22 = 0: Normal Port Operation, OC0A Disconnected.
WGM22 = 1: Toggle OC2A on Compare Match.
10
Clear OC2A on Compare Match, set OC2A at BOTTOM,
(non-inverting mode).
11
Set OC2A on Compare Match, clear OC2A at BOTTOM,
(inverting mode).