Datasheet

Table Of Contents
97
2570N–AVR–05/11
ATmega325/3250/645/6450
Note: 1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.
However, the functionality and location of these bits are compatible with previous versions of
the timer.
Bit 5:4 – COM01:0: Compare Match Output 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
WGM01:0 bit setting. Table 15-3 shows the COM0A1:0 bit functionality when the WGM01:0 bits
are set to a normal or CTC mode (non-PWM).
Table 15-4 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to fast PWM
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 TOP. See “Fast PWM Mode” on page 92
for more details.
Table 15-5 shows the COM0A1:0 bit functionality when the WGM01:0 bits are set to phase cor-
rect PWM mode.
Table 15-2. Waveform Generation Mode Bit Description
(1)
Mode
WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter
Mode of Operation TOP
Update of
OCR0A at
TOV0 Flag
Set on
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR0A Immediate MAX
31 1Fast PWM 0xFFBOTTOMMAX
Table 15-3. Compare Output Mode, non-PWM Mode
COM0A1 COM0A0 Description
0 0 Normal port operation, OC0A disconnected.
0 1 Toggle OC0A on compare match
1 0 Clear OC0A on compare match
1 1 Set OC0A on compare match
Table 15-4. Compare Output Mode, Fast PWM Mode
(1)
COM0A1 COM0A0 Description
0 0 Normal port operation, OC0A disconnected.
01Reserved
1 0 Clear OC0A on compare match, set OC0A at BOTTOM,
(non-inverting mode)
1 1 Set OC0A on compare match, clear OC0A at BOTTOM,
(inverting mode.)