Datasheet

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
Table 18-2 and “Modes of Operation” on page 104.
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. Table
18-3 shows the COM21:0 bit functionality when the WGM21:0 bits are set to a normal or CTC mode (non-PWM).
Bit 76543210
FOC2 WGM20 COM21 COM20 WGM21 CS22 CS21 CS20 TCCR2
Read/Write W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
Table 18-2. Waveform Generation Mode Bit Description
Mode
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode of
Operation
(1)
TOP
Update of
OCR2
TOV2 Flag
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
Table 18-3. Compare Output Mode, Non-PWM Mode
COM21 COM20 Description
0 0 Normal port operation, OC2 disconnected.
0 1 Toggle OC2 on Compare Match
1 0 Clear OC2 on Compare Match
1 1 Set OC2 on Compare Match