112
ATmega8A [DATASHEET]
8159E–AVR–02/2013
18.11 Register Description
18.11.1
TCCR2 – Timer/Counter Control Register
Bit 7 – FOC2: Force Output Compare
The FOC2 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 TCCR2 is written when operating in PWM mode. When writing a
logical one to the FOC2 bit, an immediate Compare Match is forced on the waveform generation unit. The OC2
output is changed according to its COM21:0 bits setting. Note that the FOC2 bit is implemented as a strobe. There-
fore it is the value present in the COM21:0 bits that determines the effect of the forced compare.
A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2 as TOP.
The FOC2 bit is always read as zero.
Bit 6,3 – WGM21:0: 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 WGM21:0 definitions. However, the function-
ality and location of these bits are compatible with previous versions of the timer.
Bit 5:4 – COM21:0: Compare Match Output Mode
These bits control the Output Compare Pin (OC2) behavior. If one or both of the COM21:0 bits are set, the OC2
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 OC2 pin must be set in order to enable the output driver.
When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0 bit setting.
Table18-3 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a normal or CTC mode (non-PWM).
Bit
765
43210
FOC2
WGM20
COM21
COM20
WGM21
CS22
CS21
CS20
TCCR2
Read/Write
W
R/W
Initial Value
000
00000
Table 18-2.
Waveform Generation Mode Bit Description
Mode
WGM21
WGM20
(PWM2)
Timer/Counter Mode of
TOP
Update of
OCR2
TOV2 Flag
Set
0
Normal
0xFF
Immediate
MAX
1
0
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
2
1
0
CTC
OCR2
Immediate
MAX
3
1
Fast PWM
0xFF
BOTTOM
MAX
Table 18-3.
Compare Output Mode, Non-PWM Mode
COM21
COM20
Description
0
Normal port operation, OC2 disconnected.
0
1
Toggle OC2 on Compare Match
1
0
Clear OC2 on Compare Match
1
Set OC2 on Compare Match