ATmega640/1280/1281/2560/2561
16.9 Register Description
16.9.1
TCCR0A – Timer/Counter Control Register A
Bit
7
COM0A1
R/W
6
COM0A0
R/W
5
COM0B1
R/W
4
COM0B0
R/W
3
–
2
–
1
WGM01
R/W
0
0
WGM00
R/W
0
TCCR0A
0x24 (0x44)
Read/Write
Initial Value
R
0
R
0
0
0
0
0
• Bits 7:6 – COM0A1:0: Compare Match Output A Mode
These bits control the Output Compare pin (OC0A) behavior. If one or both of the COM0A1:0
bits are set, the OC0A 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 OC0A pin
must be set in order to enable the output driver.
When OC0A is connected to the pin, the function of the COM0A1:0 bits depends on the
WGM02:0 bit setting. Table 16-2 shows the COM0A1:0 bit functionality when the WGM02:0 bits
are set to a normal or CTC mode (non-PWM).
Table 16-2. Compare Output Mode, non-PWM Mode
COM0A1
COM0A0
Description
0
0
1
1
0
1
0
1
Normal port operation, OC0A disconnected.
Toggle OC0A on Compare Match
Clear OC0A on Compare Match
Set OC0A on Compare Match
Table 16-3 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM
mode.
Table 16-3. Compare Output Mode, Fast PWM Mode(1)
COM0A1
COM0A0
Description
0
0
Normal port operation, OC0A disconnected.
WGM02 = 0: Normal Port Operation, OC0A Disconnected.
WGM02 = 1: Toggle OC0A on Compare Match.
0
1
1
1
0
1
Clear OC0A on Compare Match, set OC0A at BOTTOM,
(non-inverting mode).
Set OC0A on Compare Match, clear OC0A at BOTTOM,
(inverting mode).
Note:
1. A special case occurs when OCR0A equals TOP and COM0A1 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 124 for more details.
Table 16-4 on page 130 shows the COM0A1:0 bit functionality when the WGM02:0 bits are set
to phase correct PWM mode.
129
2549L–AVR–08/07
ATMEL [ ATMEL ]
