Datasheet

156
2545T–AVR–05/11
ATmega48/88/168
Notes: 1. MAX= 0xFF
2. BOTTOM= 0x00
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.
Table 18-8. Waveform generation mode bit description.
Mode WGM2 WGM1 WGM0
Timer/counter
mode of
operation TOP
Update of
OCRx at
TOV flag
set on
(1)(2)
0 0 0 0 Normal 0xFF Immediate MAX
10 0 1
PWM,
phase correct
0xFF TOP BOTTOM
2 0 1 0 CTC OCRA Immediate MAX
3 0 1 1 Fast PWM 0xFF BOTTOM MAX
41 0 0 Reserved – – –
51 0 1
PWM,
phase correct
OCRA TOP BOTTOM
61 1 0 Reserved – – –
71 1 1 Fast PWMOCRABOTTOMTOP
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