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ATmega165A/PA/325A/PA/3250A/PA/645A/P/6450A/P [DATASHEET]
8285E–AVR–02/2013
18.11 Register description
18.11.1
TCCR2A – Timer/Counter Control Register A
Bit 7 – FOC2A: Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode. However, for ensuring compatibility
with future devices, this bit must be set to zero when TCCR2A is written when operating in PWM mode. When writ-
ing a logical one to the FOC2A bit, an immediate compare match is forced on the Waveform Generation unit. The
OC2A output is changed according to its COM2A[1:0] bits setting. Note that the FOC2A bit is implemented as a
strobe. Therefore it is the value present in the COM2A[1:0] bits that determines the effect of the forced compare.
A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2A as TOP.
The FOC2A bit is always read as zero.
Bit 6, 3 – WGM2[1:0]: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum (TOP) counter value, and
what type of waveform generation to be used. Modes of operation supported by the Timer/Counter unit are: Normal
mode, Clear Timer on Compare match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes. See
Note:
1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM2[1:0] definitions. However, the func-
tionality and location of these bits are compatible with previous versions of the timer.
Bit 5:4 – COM2A[1:0]: Compare Match Output Mode A
These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A[1:0] 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 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 WGM2[1:0] bit setting.
Table 18-3 shows the COM2A1:0 bit functionality when the WGM2[1:0] bits are set to a normal or CTC mode (non-
PWM).
Bit
7
6
5
4
3
210
(0xB0)
FOC2A
WGM20
COM2A1
COM2A0
WGM21
CS22
CS21
CS20
TCCR2A
Read/Write
W
R/W
Initial Value
0
Table 18-2.
Waveform Generation Mode Bit Description
Mode
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode of
Operation
Update of
OCR2A at
TOV2 Flag
Set on
0
Normal
0xFF
Immediate
MAX
10
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
2
1
0
CTC
OCR2A
Immediate
MAX
31
1
Fast PWM0xFF
BOTTOM
MAX
Table 18-3.
Compare Output Mode, non-PWM Mode.
COM2A1
COM2A0
Description
0
Normal port operation, OC2A disconnected.
0
1
Toggle OC2A on compare match.
1
0
Clear OC2A on compare match.
1
Set OC2A on compare match.