ATMEGA8_08 Datasheet, PDF (117/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

ATmega8(L)
8-bit
Timer/Counter
Register
Description
Timer/Counter Control
Register – TCCR2
Bit
Read/Write
Initial Value
7
FOC2
W
0
6
WGM20
R/W
0
5
COM21
R/W
0
4
COM20
R/W
0
3
WGM21
R/W
0
2
CS22
R/W
0
1
CS21
R/W
0
0
CS20
R/W
0
TCCR2
• Bit 7 – FOC2: Force Output Compare
The FOC2 bit is only active when the WGM bits specify a non-PWM mode. However, for ensur-
ing 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. Therefore 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 Table 42 and “Modes of Operation” on
page 110.
Table 42. Waveform Generation Mode Bit Description
WGM21 WGM20 Timer/Counter Mode
Mode (CTC2) (PWM2) of Operation⁽¹⁾
TOP
Update of TOV2 Flag
OCR2
Set
0
0
0
Normal
0xFF Immediate MAX
1
0
1
PWM, Phase Correct
0xFF TOP
BOTTOM
2
1
0
CTC
OCR2 Immediate MAX
3
1
1
Fast PWM
0xFF BOTTOM MAX
Note:
1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 definitions.
However, the functionality 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. Table 43 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a
normal or CTC mode (non-PWM).
2486T–AVR–05/08
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