Datasheet

ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
140
18.11.2 TCCR2B – Timer/Counter Control Register B
• 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 TCCR2B is written when operating
in PWM mode. When writing 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 COM2A1:0 bits setting. Note that the FOC2A bit is
implemented as a strobe. Therefore it is the value present in the COM2A1:0 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 – FOC2B: Force Output Compare B
The FOC2B 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 TCCR2B is written when operating
in PWM mode. When writing a logical one to the FOC2B bit, an immediate compare match is forced on the waveform
generation unit. The OC2B output is changed according to its COM2B1:0 bits setting. Note that the FOC2B bit is
implemented as a strobe. Therefore it is the value present in the COM2B1:0 bits that determines the effect of the forced
compare.
A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2B as TOP.
The FOC2B bit is always read as zero.
• Bits 5:4 – Reserved
These bits are reserved bits in the Atmel
®
ATmega48PA/88PA/168PA and will always read as zero.
• Bit 3 – WGM22: Waveform Generation Mode
See the description in Section 18.11.1 “TCCR2A – Timer/Counter Control Register A” on page 137.
• Bit 2:0 – CS22:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see Table 18-9 on page 140.
If external pin modes are used for the Timer/Counter0, transitions on the T0 pin will clock the counter even if the pin is
configured as an output. This feature allows software control of the counting.
Bit 7 6 5 4 3 2 1 0
(0xB1) FOC2A FOC2B – – WGM22 CS22 CS21 CS20 TCCR2B
Read/Write W W R R R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 18-9. Clock Select Bit Description
CS22 CS21 CS20 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clk
T2S
/(No prescaling)
0 1 0 clk
T2S
/8 (From prescaler)
0 1 1 clk
T2S
/32 (From prescaler)
1 0 0 clk
T2S
/64 (From prescaler)
1 0 1 clk
T2S
/128 (From prescaler)
1 1 0 clk
T
2
S
/256 (From prescaler)
1 1 1 clk
T
2
S
/1024 (From prescaler)